Author name code: velli
ADS astronomy entries on 2022-09-14
author:"Velli, Marco"
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Title: Magnetic field spectral evolution in the inner heliosphere
Authors: Sioulas, Nikos; Huang, Zesen; Shi, Chen; Velli, Marco;
Tenerani, Anna; Vlahos, Loukas; Bowen, Trevor A.; Bale, Stuart D.;
Bonnell, J. W.; Harvey, P. R.; Larson, Davin; Pulupa, arc; Livi,
Roberto; Woodham, L. D.; Horbury, T. S.; Stevens, Michael L.; Dudok
de Wit, T.; MacDowall, R. J.; Malaspina, David M.; Goetz, K.; Huang,
Jia; Kasper, Justin; Owen, Christopher J.; Maksimović, Milan; Louarn,
P.; Fedorov, A.
Bibcode: 2022arXiv220902451S
Altcode:
The radial evolution of the magnetic field fluctuations spectral
index and its dependence on plasma parameters is investigated using
a merged Parker Solar Probe ($PSP$) and Solar Orbiter ($SO$) dataset
covering heliocentric distances between $0.06 ~ \lesssim R ~\lesssim
1$ au. The spectrum is studied as a function of scale, normalized to
the radially dependent ion inertial scale $d_{i}$. In the vicinity of
the Sun, the magnetic spectrum inertial range is limited with a power
law exponent $\alpha_{B}$ consistent with the Iroshnikov-Kraichman
phenomenology of Alfvénic turbulence, $\alpha_{B} = -3/2$, independent
of plasma parameters. The inertial range of turbulence grows with
distance from the Sun, progressively extending to larger spatial
scales, while at the same time steepening towards a Kolomogorov
scaling, with a mean value of $\alpha_{B} =-5/3$. Highly alfvénic
intervals seem to retain their near-Sun scaling and only show a minor
steepening with distance. In contrast, intervals, where turbulence
is characterized by large magnetic energy excess and no dominance of
outwardly propagating Alfvénic fluctuations, appear to have spectra
that steepen significantly with distance from the Sun, resulting in
slightly anomalously steep inertial range slopes at $1~au$. Though
generically slower solar wind streams exhibit steeper spectra, the
correlation can be attributed to the underlying positive correlation
between solar wind speed and alfvénicity, i.e. to the relatively rare
occurrence of highly Alfvénic slow wind.
Title: Acceleration of polytropic solar wind: Parker Solar Probe
observation and one-dimensional model
Authors: Shi, Chen; Velli, Marco; Bale, Stuart D.; Réville, Victor;
Maksimović, Milan; Dakeyo, Jean-Baptiste
Bibcode: 2022arXiv220903508S
Altcode:
The acceleration of the solar coronal plasma to supersonic speeds is one
of the most fundamental yet unresolved problem in heliophysics. Despite
the success of Parker's pioneering theory on an isothermal solar corona,
the realistic solar wind is observed to be non-isothermal, and the
decay of its temperature with radial distance usually can be fitted to
a polytropic model. In this work, we use Parker Solar Probe data from
the first nine encounters to estimate the polytropic index of solar
wind protons. We show that the polytropic index varies between 1.25
and $5/3$ and depends strongly on solar wind speed, faster solar wind
on average displaying a smaller polytropic index. We comprehensively
analyze the 1D spherically symmetric solar wind model with polytropic
index $\gamma \in [1,5/3]$. We derive a closed algebraic equation
set for transonic stellar flows, i.e. flows that pass the sound point
smoothly. We show that an accelerating wind solution only exists in
the parameter space bounded by $C_0/C_g < 1$ and $(C_0/C_g)^2 >
2(\gamma-1)$ where $C_0$ and $C_g$ are the surface sound speed and one
half of the escape velocity of the star, and no stellar wind exists for
$\gamma > 3/2$. With realist solar coronal temperatures, the observed
solar wind with $\gamma \gtrsim 1.25$ cannot be explained by the simple
polytropic model. We show that mechanisms such as strong heating in
the lower corona that leads to a thick isothermal layer around the Sun
and large-amplitude Alfvén wave pressure are necessary to remove the
constraint in $\gamma$ and accelerate the solar wind to high speeds.
Title: Preferential Heating of Protons over Electrons from Coherent
Structures during the First Perihelion of the Parker Solar Probe
Authors: Sioulas, Nikos; Shi, Chen; Huang, Zesen; Velli, Marco
Bibcode: 2022ApJ...935L..29S
Altcode: 2022arXiv220610671S
The solar wind undergoes significant heating as it propagates away
from the Sun; the exact mechanisms responsible for this heating
remain unclear. Using data from the first perihelion of the Parker
Solar Probe mission, we examine the properties of proton and electron
heating occurring within magnetic coherent structures identified by
means of the Partial Variance of Increments (PVI) method. Statistically,
regions of space with strong gradients in the magnetic field, PVI ≥ 1,
are associated with strongly enhanced proton but only slightly elevated
electron temperatures. Our analysis indicates a heating mechanism in
the nascent solar wind environment facilitated by a nonlinear turbulent
cascade that preferentially heats protons over electrons.
Title: Magnetic Field Intermittency in the Solar Wind: Parker Solar
Probe and SolO Observations Ranging from the Alfvén Region up to 1 AU
Authors: Sioulas, Nikos; Huang, Zesen; Velli, Marco; Chhiber, Rohit;
Cuesta, Manuel E.; Shi, Chen; Matthaeus, William H.; Bandyopadhyay,
Riddhi; Vlahos, Loukas; Bowen, Trevor A.; Qudsi, Ramiz A.; Bale,
Stuart D.; Owen, Christopher J.; Louarn, P.; Fedorov, A.; Maksimović,
Milan; Stevens, Michael L.; Case, Anthony; Kasper, Justin; Larson,
Davin; Pulupa, Marc; Livi, Roberto
Bibcode: 2022ApJ...934..143S
Altcode: 2022arXiv220600871S
Parker Solar Probe (PSP) and SolO data are utilized to investigate
magnetic field intermittency in the solar wind (SW). Small-scale
intermittency (20-100 d i ) is observed to radially
strengthen when methods relying on higher-order moments are considered
(SF q ; SDK), but no clear trend is observed at larger
scales. However, lower-order moment-based methods (e.g., partial
variance of increments; PVI) are deemed more appropriate for examining
the evolution of the bulk of coherent structures (CSs), PVI ≥ 3. Using
PVI, we observe a scale-dependent evolution in the fraction of the data
set occupied by CSs, f PVI≥3. Specifically, regardless
of the SW speed, a subtle increase is found in f PVI≥3
for ℓ = 20 d i , in contrast to a more pronounced radial
increase in CSs observed at larger scales. Intermittency is investigated
in relation to plasma parameters. Though, slower SW speed intervals
exhibit higher f PVI≥6 and higher kurtosis maxima, no
statistical differences are observed for f PVI≥3. Highly
Alfvénic intervals display lower levels of intermittency. The
anisotropy with respect to the angle between the magnetic field
and SW flow, ΘVB is investigated. Intermittency is
weaker at ΘVB ≍ 0° and is strengthened at larger
angles. Considering the evolution at a constant alignment angle, a
weakening of intermittency is observed with increasing advection time
of the SW. Our results indicate that the strengthening of intermittency
in the inner heliosphere is driven by the increase in comparatively
highly intermittent perpendicular intervals sampled by the probes with
increasing distance, an effect related directly to the evolution of
the Parker spiral.
Title: Linking Small-scale Solar Wind Properties with Large-scale
Coronal Source Regions through Joint Parker Solar Probe-Metis/Solar
Orbiter Observations
Authors: Telloni, Daniele; Zank, Gary P.; Sorriso-Valvo, Luca;
D'Amicis, Raffaella; Panasenco, Olga; Susino, Roberto; Bruno, Roberto;
Perrone, Denise; Adhikari, Laxman; Liang, Haoming; Nakanotani, Masaru;
Zhao, Lingling; Hadid, Lina Z.; Sánchez-Cano, Beatriz; Verscharen,
Daniel; Velli, Marco; Grimani, Catia; Marino, Raffaele; Carbone,
Francesco; Mancuso, Salvatore; Biondo, Ruggero; Pagano, Paolo; Reale,
Fabio; Bale, Stuart D.; Kasper, Justin C.; Case, Anthony W.; de Wit,
Thierry Dudok; Goetz, Keith; Harvey, Peter R.; Korreck, Kelly E.;
Larson, Davin; Livi, Roberto; MacDowall, Robert J.; Malaspina, David
M.; Pulupa, Marc; Stevens, Michael L.; Whittlesey, Phyllis; Romoli,
Marco; Andretta, Vincenzo; Deppo, Vania Da; Fineschi, Silvano; Heinzel,
Petr; Moses, John D.; Naletto, Giampiero; Nicolini, Gianalfredo;
Spadaro, Daniele; Stangalini, Marco; Teriaca, Luca; Capobianco,
Gerardo; Capuano, Giuseppe E.; Casini, Chiara; Casti, Marta; Chioetto,
Paolo; Corso, Alain J.; Leo, Yara De; Fabi, Michele; Frassati,
Federica; Frassetto, Fabio; Giordano, Silvio; Guglielmino, Salvo L.;
Jerse, Giovanna; Landini, Federico; Liberatore, Alessandro; Magli,
Enrico; Massone, Giuseppe; Messerotti, Mauro; Pancrazzi, Maurizio;
Pelizzo, Maria G.; Romano, Paolo; Sasso, Clementina; Schühle, Udo;
Slemer, Alessandra; Straus, Thomas; Uslenghi, Michela; Volpicelli,
Cosimo A.; Zangrilli, Luca; Zuppella, Paola; Abbo, Lucia; Auchère,
Frédéric; Cuadrado, Regina Aznar; Berlicki, Arkadiusz; Ciaravella,
Angela; Lamy, Philippe; Lanzafame, Alessandro; Malvezzi, Marco;
Nicolosi, Piergiorgio; Nisticò, Giuseppe; Peter, Hardi; Solanki,
Sami K.; Strachan, Leonard; Tsinganos, Kanaris; Ventura, Rita; Vial,
Jean-Claude; Woch, Joachim; Zimbardo, Gaetano
Bibcode: 2022ApJ...935..112T
Altcode:
The solar wind measured in situ by Parker Solar Probe in the very
inner heliosphere is studied in combination with the remote-sensing
observation of the coronal source region provided by the METIS
coronagraph aboard Solar Orbiter. The coronal outflows observed near
the ecliptic by Metis on 2021 January 17 at 16:30 UT, between 3.5 and
6.3 R ⊙ above the eastern solar limb, can be associated
with the streams sampled by PSP at 0.11 and 0.26 au from the Sun,
in two time intervals almost 5 days apart. The two plasma flows
come from two distinct source regions, characterized by different
magnetic field polarity and intensity at the coronal base. It follows
that both the global and local properties of the two streams are
different. Specifically, the solar wind emanating from the stronger
magnetic field region has a lower bulk flux density, as expected,
and is in a state of well-developed Alfvénic turbulence, with low
intermittency. This is interpreted in terms of slab turbulence in the
context of nearly incompressible magnetohydrodynamics. Conversely,
the highly intermittent and poorly developed turbulent behavior of the
solar wind from the weaker magnetic field region is presumably due to
large magnetic deflections most likely attributed to the presence of
switchbacks of interchange reconnection origin.
Title: Conservation of Total Wave Action in the Expanding Solar Wind
Authors: Huang, Zesen; Shi, Chen; Sioulas, Nikos; Velli, Marco
Bibcode: 2022ApJ...935...60H
Altcode: 2022arXiv220601809H
The conservation of wave action in moving plasmas has been well known
for over half a century. However, wave action is not conserved when
multiple wave modes propagate and coexist close to the degeneration
condition (where the sound speed equals the Alfvén speed, i.e., plasma
β ~ 1). Here, we show that the violation of conservation is due to
wave mode conversion, and that the total wave action summed over the
interacting modes is still conserved. Though the result is general,
we focus on MHD waves and identify three distinctive mode-conversion
mechanisms, i.e., degeneracy, linear mode conversion, and resonance,
and provide an intuitive physical picture for the mode-conversion
processes. We use one-dimensional MHD simulations with the Expanding
Box Model to simulate the nonlinear evolution of monochromatic MHD
waves in the expanding solar wind. The simulation results validate the
theory; total wave action therefore remains an interesting diagnostic
for studies of waves and turbulence in the solar wind.
Title: Electron-Driven Instabilities in the Solar Wind
Authors: Verscharen, Daniel; Chandran, B. D. G.; Boella, E.; Halekas,
J.; Innocenti, M. E.; Jagarlamudi, V. K.; Micera, A.; Pierrard, V.;
Štverák, Š.; Vasko, I. Y.; Velli, M.; Whittlesey, P. L.
Bibcode: 2022FrASS...9.1628V
Altcode: 2022arXiv220610403V
The electrons are an essential particle species in the solar
wind. They often exhibit non-equilibrium features in their velocity
distribution function. These include temperature anisotropies,
tails (kurtosis), and reflectional asymmetries (skewness), which
contribute a significant heat flux to the solar wind. If these
non-equilibrium features are sufficiently strong, they drive kinetic
micro-instabilities. We develop a semi-graphical framework based
on the equations of quasi-linear theory to describe electron-driven
instabilities in the solar wind. We apply our framework to resonant
instabilities driven by temperature anisotropies. These include the
electron whistler anisotropy instability and the propagating electron
firehose instability. We then describe resonant instabilities driven by
reflectional asymmetries in the electron distribution function. These
include the electron/ion-acoustic, kinetic Alfvén heat-flux, Langmuir,
electron-beam, electron/ion-cyclotron, electron/electron-acoustic,
whistler heat-flux, oblique fast-magnetosonic/whistler, lower-hybrid
fan, and electron-deficit whistler instability. We briefly comment on
non-resonant instabilities driven by electron temperature anisotropies
such as the mirror-mode and the non-propagating firehose instability. We
conclude our review with a list of open research topics in the field
of electron-driven instabilities in the solar wind.
Title: Patches of Magnetic Switchbacks and Their Origins
Authors: Shi, Chen; Panasenco, Olga; Velli, Marco; Tenerani, Anna;
Verniero, Jaye L.; Sioulas, Nikos; Huang, Zesen; Brosius, A.; Bale,
Stuart D.; Klein, Kristopher; Kasper, Justin; de Wit, Thierry Dudok;
Goetz, Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina,
David M.; Pulupa, Marc; Larson, Davin; Livi, Roberto; Case, Anthony;
Stevens, Michael
Bibcode: 2022ApJ...934..152S
Altcode: 2022arXiv220603807S
Parker Solar Probe (PSP) has shown that the solar wind in the inner
heliosphere is characterized by the quasi omnipresence of magnetic
switchbacks ("switchback" hereinafter), local backward bends of
magnetic field lines. Switchbacks also tend to come in patches, with
a large-scale modulation that appears to have a spatial scale size
comparable to supergranulation on the Sun. Here we inspect data from
the first 10 encounters of PSP focusing on different time intervals
when clear switchback patches were observed by PSP. We show that the
switchbacks modulation, on a timescale of several hours, seems to be
independent of whether PSP is near perihelion, when it rapidly traverses
large swaths of longitude remaining at the same heliocentric distance,
or near the radial-scan part of its orbit, when PSP hovers over the
same longitude on the Sun while rapidly moving radially inwards or
outwards. This implies that switchback patches must also have an
intrinsically temporal modulation most probably originating at the
Sun. Between two consecutive patches, the magnetic field is usually
very quiescent with weak fluctuations. We compare various parameters
between the quiescent intervals and the switchback intervals. The
results show that the quiescent intervals are typically less Alfvénic
than switchback intervals, and the magnetic power spectrum is usually
shallower in quiescent intervals. We propose that the temporal
modulation of switchback patches may be related to the "breathing"
of emerging flux that appears in images as the formation of "bubbles"
below prominences in the Hinode/SOT observations.
Title: Interchange reconnection within coronal holes powers the fast
solar wind
Authors: Bale, S. D.; Drake, J. F.; McManus, M. D.; Desai, M. I.;
Badman, S. T.; Larson, D. E.; Swisdak, M.; Raouafi, N. E.; Phan, T.;
Velli, M.; McComas, D. J.; Cohen, C. M. S.; Mitchell, D.; Panasenco,
O.; Kasper, J. C.
Bibcode: 2022arXiv220807932B
Altcode:
The fast solar wind that fills the heliosphere originates from deep
within regions of open magnetic field on the Sun called coronal
holes. However the energy source responsible for accelerating the
outflowing plasma to such high speeds is still widely debated, although
there is broad evidence that it is ultimately magnetic in nature with
candidate mechanisms including Alfven wave heating and interchange
reconnection. The magnetic field near the solar surface within coronal
holes is structured on spatial scales associated with the boundaries of
meso-scale supergranulation convection cells, where descending flows
create intense bundles of magnetic field. The energy density in these
network magnetic field bundles is a likely candidate as an energy source
of the wind. Here we report measurements of two fast solar wind streams
from the Parker Solar Probe (PSP) spacecraft near its 10th perihelion
which provides strong evidence for the interchange reconnection
mechanism. Specifically, we show that supergranulation structure at
the coronal hole base remains imprinted in the near-Sun solar wind
resulting in asymmetric patches of magnetic 'switchbacks' and bursty
solar wind streams with corresponding energetic ions with power law-like
distributions extending to beyond 100 keV. Particle-in-cell simulations
of interchange reconnection between open and closed magnetic structures
support key features of the observations, including the energetic
ion spectra. Important characteristics of interchange reconnection
in the low corona are inferred from the PSP data including that the
reconnection is collisionless and that the rate of energy release is
sufficient to heat the ambient plasma and drive the fast wind.
Title: Understanding the Solar Wind: Parker Solar Probe in the
Inner Heliosphere
Authors: Velli, Marco; Bale, Stuart; Panasenco, Olga; Tenerani, Anna;
Shi, Chen; Verniero, Jaye
Bibcode: 2022cosp...44.1317V
Altcode:
The magnetic field is fundamental to solar activity and shapes
the interplanetary environment, as clearly shown by the full three
dimensional monitoring of the heliosphere provided by the measurements
of the Helios, Ulysses, SOHO, ACE, Wind, STEREO, Hinode, IRIS, SDO,
and Voyager spacecraft. Magnetic fields are also the source for coronal
heating and the very existence of the solar wind; produced by the
sun's dynamo and emerging into the corona, magnetic fields become a
conduit for waves, act to store energy, and then propel plasma into
the heliosphere in the form of Coronal Mass Ejections (CMEs). Parker
Solar Probe was launched to carry out the first in situ exploration of
the outer solar corona and inner heliosphere. Direct measurements of
the plasma in the closest atmosphere of our star have already produced
significant surprises including switchbacks, the predominance of Alfvén
wave turbulence, magnetic reconnection in the forming heliospheric
current sheets. Here I will review both models and observations,
including progress and pitfalls in the interpretation of PSP results.
Title: FIP fractionation in the turbulent solar chromosphere and
corona: incompressible and compressible models
Authors: Reville, Victor; Buchlin, Eric; Verdini, Andrea; Rouillard,
Alexis; Velli, Marco; Lavarra, Michael; Poirier, Nicolas
Bibcode: 2022cosp...44.2576R
Altcode:
Low first ionisation potential (FIP) elements show enriched abundances
in the slow solar wind and coronal loops compared to photospheric
values. Turbulence is likely to be a key physical mechanism to explain
these abundances. Turbulent mixing is indeed essential to prevent
gravitational settling of heavy elements. Moreover, the average
turbulent Lorentz force, the ponderomotive force, could explain
the preferential lifting of low FIP ions in the upper chromosphere
and transition region. In this talk, we use unidimensional models
of the solar atmosphere, to compute the turbulent properties around
the transition regions in several regimes. We use the incompressible
(or reduced) MHD formalism with the SHELL-ATM code, and show that the
turbulent field is consistent with both coronal heating and significant
FIP fractionation. Then, we use the compressible MHD code PLUTO, and
compare the turbulent properties of the two models. In particular,
we look at the effect of chromospheric shocks on the propagation of
Alfvén waves near the top of the chromosphere that may act to modify
wave properties in the ionisation region of heavy elements. This work
has been funded by the ERC project SLOW SOURCE - DLV-819189
Title: Investigating the solar sources and evolution of the Alfvénic
slow wind with a coordinated Parker Solar Probe - Solar Orbiter study
Authors: D'Amicis, Raffaella; Panasenco, Olga; Velli, Marco; Telloni,
Daniele; Perrone, Denise; Bruno, Roberto; de Marco, Rossana
Bibcode: 2022cosp...44.1335D
Altcode:
The launch of Parker Solar Probe (PSP) and Solar Orbiter (SO) started
a new era in the exploration of the inner heliosphere. Since both
missions will follow the ascending phase of the solar cycle, joint
studies will offer unprecedented opportunities to study the Alfvénic
slow wind in situ and to identify its solar source. This is extremely
important for improving the understanding of this solar wind regime,
with particular reference to its origin and evolution, and of the
general problem of solar wind acceleration. In this study, we focus on
a particular orbital configuration, occurring at the end of April 2021,
in which PSP was magnetically and then radially aligned with SO. A
Potential Field Source-Surface (PFSS) model was used to link in-situ
measurements with the corresponding solar wind source regions, mapping
PSP and SO measurements back to a pseudostreamer configuration in the
solar corona, which is of primary importance to set the conditions
for the development of the Alfvénic slow solar wind. One week after,
Earth was connected to the same pseudostreamer. This event represents
a good opportunity to study the evolution of the plasma coming from the
same source region with particular reference to its turbulent behaviour
and the Alfvénic content of the fluctuations from 0.074 AU to 1 AU.
Title: 4π Heliospheric Observing System - 4π-HeliOS: Exploring
the Heliosphere from the Solar Interior to the Solar Wind
Authors: Raouafi, Nour E.; Gibson, Sarah; Ho, George; Laming,
J. Martin; Georgoulis, Manolis K.; Szabo, Adam; Vourlidas, Angelos;
Mason, Glenn M.; Hoeksema, J. Todd; Velli, Marco; Berger, Thomas;
Hassler, Donald M.; Kinnison, James; Viall, Nicholeen; Case, Anthony;
Newmark, Jeffrey; Lepri, Susan; Krishna Jagarlamudi, Vamsee; Raouafi,
Nour; Bourouaine, Sofiane; Vievering, Juliana T.; Englander, Jacob A.;
Shannon, Jackson L.; Perez, Rafael M.; Chattopadhyay, Debarati; Mason,
James P.; Leary, Meagan L.; Santo, Andy; Casti, Marta; Upton, Lisa A.
Bibcode: 2022cosp...44.1530R
Altcode:
The 4$\pi$ Heliospheric Observing System (4$\pi$-HeliOS) is an
innovative mission concept study for the next Solar and Space
Physics Decadal Survey to fill long-standing knowledge gaps in
Heliophysics. A constellation of spacecraft will provide both remote
sensing and in situ observations of the Sun and heliosphere from a
full 4$\pi$-steradian field of view. The concept implements a holistic
observational philosophy that extends from the Sun's interior, to the
photosphere, through the corona, and into the solar wind simultaneously
with multiple spacecraft at multiple vantage points optimized for
continual global coverage over much of a solar cycle. The mission
constellation includes two spacecraft in the ecliptic and two flying as
high as $\sim$70$^\circ$ solar latitude. 4$\pi$-HeliOS will provide
new insights into the fundamental processes that shape the whole
heliosphere. The overarching goals of the 4$\pi$-HeliOS concept are
to understand the global structure and dynamics of the Sun's interior,
the generation of solar magnetic fields, the origin of the solar cycle,
the causes of solar activity, and the structure and dynamics of the
corona as it creates the heliosphere. The mission design study is
underway at the Johns Hopkins Applied Physics Laboratory Concurrent
Engineering Laboratory (ACE Lab), a premier mission design center,
fostering rapid and collaborative mission design evolutions.
Title: Photospheric and low coronal sources of different types
of solar wind and transients observed by Parker Solar Probe and
Solar Orbiter
Authors: Panasenco, Olga; Bale, Stuart; Velli, Marco; Tenerani, Anna;
Shi, Chen; D'Amicis, Raffaella; Verniero, Jaye; Sioulas, Nikos
Bibcode: 2022cosp...44.1532P
Altcode:
Initial Parker Solar Probe results have shown that slow Alfvénic
solar wind intervals appear to be a frequent, if not standard,
component of the nascent solar wind inside 0.5 AU. In addition to
the strong presence of Alfvénic fluctuations propagating away from
the Sun, such intervals also display the huge oscillations known as
switchbacks, where the Alfvénic fluctuation is accompanied by a fold
in the radial magnetic field and a corresponding forward propagating
radial jet. Switchbacks often come in patches, separated by short
intervals depleted with fluctuations, and periods without switchbacks
may also show a striking quiescence, with the magnetic field remaining
mostly radial and very small amplitude velocity and magnetic field
fluctuations. These observations pose a series of questions on the
origins of the solar wind and the role of coronal structure, as well
as of the evolution of fluctuations within the solar wind. Here
we discuss how the sources of the solar wind measured in situ are
related to photospheric magnetic network and large-scale solar coronal
magnetic structures. In this presentation we use a wealth of remote
sensing and in-situ measurements to pinpoint the sources of the solar
wind observed by PSP and Solar Orbiter. We then discuss the origin and
evolution of so called slow Alfvénic wind, the origins of switchbacks
and sub-Alfvénic wind patches observed in situ at 13.3 Rs and further
during PSP Encounters 1 - 11.
Title: Statistical study of MHD turbulence straddling the Alfven
surface.
Authors: Sioulas, Nikos; Bale, Stuart; Stevens, Michael; Kasper,
Justin; Panasenco, Olga; Velli, Marco; Reville, Victor; Tenerani,
Anna; Shi, Chen; Whittlesey, Phyllis; Livi, Roberto; Verniero, Jaye;
Bowen, Trevor; Huang, Zesen
Bibcode: 2022cosp...44.1474S
Altcode:
Driven by the internal dynamics of the Sun, the solar wind expands
into the interplanetary medium to fill the increasing volume of the
heliosphere. A point of fundamental physical significance during the
expansion is the locus at which the radial solar wind speed $V_{SW}$
equals the Alfven speed $V_{A}$, distinguishing the magnetically
dominated sub-Alfvenic, coronal flow $M_{A} \equiv V_{sw}/V_{A} \ll 1$
from the super-Alfvenic solar wind plasma by dynamical means, namely
the "Alfven region". During its latest perihelia, the Parker Solar
Probe mission has encountered several extended sub-Alfvenic regions
providing us with unprecedented in-situ measurements in the vicinity
of the Alfven-zone. These observations will ultimately enable us to
explore the consequences of the Alfven-zone in processes such as the
heating of the solar corona, as well as the generation and subsequent
acceleration of the solar wind. In this work, a statistical study
comparing the properties of turbulence straddling the Alfven surface
during encounters $ E_{8}$, $ E_{9}$ $&$ $ E_{10}$ of Parker
Solar Probe is presented. The degree of intermittency of the magnetic
and velocity field, as well as the Alfvenicity of the fluctuations,
magnetic compressibility, wavevector anisotropy, are examined.
Title: Frustrated relaxation and instabilities in coronal heating
and solar wind formation
Authors: Velli, Marco; Panasenco, Olga
Bibcode: 2022cosp...44.1502V
Altcode:
Coronal heating modeling includes the spontaneous formation of
quasi-singular current sheets from the smooth photospheric driving. In
this presentation we discuss the difference between the corona as
describable by "smooth" force-free extrapolations and a corona that is
in a state not of passive relaxation but of what we call a state of
"frustrated relaxation": a state where relaxation is incomplete and
condemned to continuous local dynamics - and coronal heating - due to
photospheric forcing. Such a state is continuously forced and slowly
marching to the catastrophic instability or loss of equilibrium of
CMEs. Our presentation explores frustrated relaxation in the corona via
modeling, numerical simulations and observations. We will in particular
describe the process of current sheet formation as one due to a state
of magnetically dominated turbulence. But we will also describe the
loss of coronal confinement caused by Rayleigh-Taylor and ballooning
-like instability in the heated plasma around complex magnetic field
topologies, a solar wind source that has been almost neglected in the
past. We will specifically demonstate that current sheet formation
is a nonlinear process that is not simply describable in terms of
discontinuities, and also provide a path for turbulence modeling of
the corona and accelerating solar wind.
Title: Sources of the Solar Wind and its Embedded Fluctuations as
Observed by Parker Solar Probe
Authors: Velli, Marco; Bale, Stuart; Panasenco, Olga; Reville,
Victor; Tenerani, Anna; Shi, Chen; D'Amicis, Raffaella; Sioulas,
Nikos; Huang, Zesen
Bibcode: 2022cosp...44.1413V
Altcode:
We discuss the sources of the solar wind observed by Parker Solar
Probe over its first 10 encounters (perihelia) with the Sun, and
relate their global properties to those of the embedded turbulence,
including characteristics such as overall magnitude, Alfvénicity,
relative magnitude of density fluctuations, prevalence of switchback
structures. We then describe what we have learned thanks to Parker
of the relationship of the turbulence properties to the solar wind
origins, before arguing how structures such as magnetic funnels,
isolated coronal holes, coronal hole boundaries, pseudostreamers,
helmet streamers and the nascent heliospheric current sheet contribute
to the structure of the inner heliosphere.
Title: Kinetic effects on the evolution of Alfvenic fluctuations
and switchbacks
Authors: Tenerani, Anna; Panasenco, Olga; Velli, Marco; Shi, Chen;
Sioulas, Nikos; Gonzalez, Carlos; Matteini, Lorenzo
Bibcode: 2022cosp...44.1422T
Altcode:
Alfvénic fluctuations represent the dominant contribution to turbulent
fluctuations in the solar wind, especially, but not limited to, the
fastest streams with velocity of the order of 600-700 km/s. Observations
from the inner heliosphere to the closest regions to the sun explored
by Parker Solar Probe show that such fluctuations are characterized
by a nearly constant magnetic field amplitude, a condition which
remains largely to be understood and that may be an indication of how
fluctuations evolve and relax in the expanding solar wind. Switchbacks,
an extreme case of Alfvenic fluctuation, display similar properties,
although Parker Solar Probe measurements have revealed a non-negligible
level of compressibility in some cases. Here we will address how
coupling of broadband Alfvenic fluctuations to compressible modes is
mediated by dispersive and kinetic effects. Emphasis will be given
to the role of dispersive and kinetic effects on the stability and
long-term evolution of switchbacks, with a focus on wave-particle
interactions at steepened wave fronts.
Title: Conservation of Total Wave Action and Magnetosonic Resonance
Broadening in Expanding Solar Wind
Authors: Huang, Zesen; Velli, Marco; Shi, Chen; Sioulas, Nikos
Bibcode: 2022cosp...44.1106H
Altcode:
The conservation of wave action for moving plasmas and magnetofluids
has been well-known for over half a century. However, wave action is
not conserved when multiple wave modes propagate and coexist close
to resonance conditions. At resonance we show that the violation is
mainly due to wave mode conversion, and that the total wave action
summed over interacting modes is a universally conserved quantity. The
model further reveals two distinctive mode conversion mechanisms,
i.e. magnetosonic resonance broadened by solar wind expansion and
eigen-mode degeneracy, providing an intuitive physical picture for
the mode conversion processes. In the solar wind, wave vectors are
refracted towards the radial, while the magnetic field rotates from
radial towards azimuth, as per the Parker spiral. In addition, in
the solar corona, the Alfvén speed is much greater than the sound
speed, but by about 0.1 AU the speeds are comparable. Therefore,
mode conversion can be expected to occur, and Expanding Box Model
simulations of the nonlinear evolution of monochromatic MHD waves in
the expanding solar wind confirm this. Therefore wave action remains
an interesting diagnostic for studies of waves and turbulence in the
solar wind. We are currently applying these ideas to Parker Solar
Probe observations in the inner heliosphere.
Title: Patches of magnetic switchbacks: hints of their origins
Authors: Shi, Chen; Bale, Stuart; Stevens, Michael; Kasper, Justin;
Panasenco, Olga; Velli, Marco; Whittlesey, Phyllis; Tenerani, Anna;
Livi, Roberto; Verniero, Jaye; Sioulas, Nikos; Huang, Zesen
Bibcode: 2022cosp...44.1475S
Altcode:
One of the most important findings made by Parker Solar Probe
(PSP) is the omni-presence of the magnetic switchbacks, which are
the local backward-bends of the magnetic field lines, in the young
solar wind. Although many studies were conducted on the properties
and dynamics of these switchbacks, how and where they are generated
are still not fully understood yet. In this study, we analyze
the data from the first seven encounters of PSP with a focus on
the properties of the switchback "patches", i.e., the large-scale
modulation of the switchbacks. We select the time intervals when clear
switchback patches were observed by PSP. We show that the appearance
of switchbacks is frequently modulated on a timescale of several hours
and this timescale seems to be independent on whether PSP is near the
perihelion or near the radial-scan part of its orbit, implying that the
patch of switchbacks likely corresponds to some transient phenomenon
in the solar corona. We find that between two consecutive patches,
the plasma and magnetic field are usually very quiescent with weak
fluctuations. We compare various parameters between the quiescent
intervals and the switchback intervals. The results show: (1) The
quiescent intervals are typically less Alfvenic than the switchback
intervals. (2) The magnetic power spectra are in general steeper in
the switchback intervals than the quiescent intervals.
Title: Features of magnetic field switchbacks similar to those
features of large-amplitude Alfvén waves: PSP and Wind Observations
Authors: Bourouaine, Sofiane; Bale, Stuart; Raouafi, Nour E.; Velli,
Marco; Perez, Jean Carlos; Chandran, Benjamin
Bibcode: 2022cosp...44.1435B
Altcode:
In this presentation, we show observations of magnetic switchback
(SB) features near 1 au using data from the Wind spacecraft. These
features appear to be strikingly similar to the ones observed by the
Parker Solar Probe mission (PSP) closer to the Sun. More generally, we
found that the SBs are mainly associated with large-amplitude Alfvénic
oscillations that propagate outward from the sun. We show that the SB
features near the Sun result from the propagation of large-amplitude
Alfven waves along a nearly radial parker field. We also demonstrate
that SBs may not appear always as one-sided spikes in $V$, especially
at larger heliocentric distances where the Parker Spiral increasingly
departs from the radial direction.
Title: Searching for a Solar Source of Magnetic-Field Switchbacks
in Parker Solar Probe's First Encounter
Authors: de Pablos, D.; Samanta, T.; Badman, S. T.; Schwanitz, C.;
Bahauddin, S. M.; Harra, L. K.; Petrie, G.; Mac Cormack, C.; Mandrini,
C. H.; Raouafi, N. E.; Martinez Pillet, V.; Velli, M.
Bibcode: 2022SoPh..297...90D
Altcode:
Parker Solar Probe observations show ubiquitous magnetic-field reversals
closer to the Sun, often referred to as "switchbacks". The switchbacks
have been observed before in the solar wind near 1 AU and beyond, but
their occurrence was historically rare. PSP measurements below ∼ 0.2
AU show that switchbacks are, however, the most prominent structures
in the "young" solar wind. In this work, we analyze remote-sensing
observations of a small equatorial coronal hole to which PSP was
connected during the perihelion of Encounter 1. We investigate whether
some of the switchbacks captured during the encounter were of coronal
origin by correlating common switchback in situ signatures with remote
observations of their expected coronal footpoint. We find strong
evidence that timescales present in the corona are relevant to the
outflowing, switchback-filled solar wind, as illustrated by strong
linear correlation. We also determine that spatial analysis of the
observed region is optimal, as the implied average solar-wind speed
more closely matches that observed by PSP at the time. We observe that
hemispherical structures are strongly correlated with the radial proton
velocity and the mass flux in the solar wind. The above findings suggest
that a subpopulation of the switchbacks are seeded at the corona and
travel into interplanetary space.
Title: Simulating the FIP effect in coronal loops using a
multi-species kinetic-fluid model.
Authors: Poirier, Nicolas; Buchlin, Eric; Verdini, Andrea; Rouillard,
Alexis; Velli, Marco; Reville, Victor; Lavarra, Michael; Blelly,
Pierre-Louis; Indurain, Mikel
Bibcode: 2022cosp...44.2577P
Altcode:
We investigate abundance variations of heavy ions in coronal loops. We
develop and exploit a multi-species model of the solar atmosphere
(called IRAP's Solar Atmospheric Model: ISAM) that solves for the
transport of neutral and charged particles from the chromosphere to
the corona. We investigate the effect of different mechanisms that
could produce the First Ionization Potential (FIP) effect. We compare
the effects of the thermal, friction and ponderomotive force. The
propagation, reflection and dissipation of Alfvén waves is solved
using two distinct models, the first one from Chandran et al. (2011)
and the second one that is a more sophisticated turbulence model called
Shell-ATM. ISAM solves a set of 16-moment transport equations for
both neutrals and charged particles with a comprehensive treatment of
particle interactions and ionization/recombination processes. Protons
and electrons are heated by Alfvén waves, which then heat up the heavy
ions via collision processes. We show comparisons of our results with
other models and observations, with an emphasis on FIP biases. This
work was funded by the European Research Council through the project
SLOW SOURCE - DLV-819189.
Title: The supergranulation-scale stream structure and underlying
acceleration profile of the emerging solar wind
Authors: Bale, Stuart; Moncuquet, Michel; Horbury, Tim; Drake, James;
Maksimovic, Milan; Kasper, Justin; Raouafi, Nour E.; Velli, Marco;
Badman, Sam; Romeo, Orlando; Chandran, Benjamin
Bibcode: 2022cosp...44.1415B
Altcode:
Near one astronomical unit (1 AU) and in the outer heliosphere,
the solar wind is observed to be a relatively homogeneous and highly
turbulent flow that is punctuated occasionally by large-scale transient
interplanetary structures such as coronal mass ejections (CMEs)
and corotating interaction regions (CIRs). As the wind expands and
accelerates away from the Sun, turbulent evolution destroys much of the
original source structure leaving a relatively uniform flow field. Here
we use measurements from the NASA Parker Solar Probe spacecraft to
demonstrate that within ~0.2 AU of the Sun, the solar wind is structured
into distinct 'streams' that are organized on angular scales of order 5
degrees longitude with respect to the solar surface. This angular scale
is comparable to that of solar supergranulation convection cells which
are also known to organize and concentrate the photospheric magnetic
field. We argue that the discrete solar wind streams have their origins
in the network magnetic field, which is also known to be associated
with coronal jets and plumes. As a way to identify individual streams,
we characterize a 'baseline' solar wind radial speed profile which
is apparently functionally similar to the classical Parker solar wind
model, after accounting for the discrete structure and high latitude
of the wind sources. We use a Potential Field Source Surface (PFSS)
instantiation to demonstrate supergranulation-scale mixed-polarity
magnetic field structure near the footpoints of discrete streams
measured by PSP during Encounter 06. We argue that the Parker Solar
Probe instruments are measuring the acceleration of the solar wind in
situ and we offer a comparison with the expectations of interchange
reconnection as a wind energization mechanism at its source.
Title: Investigating Alfvénic Turbulence in Fast and Slow Solar
Wind Streams
Authors: D'Amicis, Raffaella; Perrone, Denise; Velli, Marco;
Sorriso-Valvo, Luca; Telloni, Daniele; Bruno, Roberto; De Marco,
Rossana
Bibcode: 2022Univ....8..352D
Altcode:
Solar wind turbulence dominated by large-amplitude Alfvénic
fluctuations, mainly propagating away from the Sun, is ubiquitous
in high-speed solar wind streams. Recent observations performed in
the inner heliosphere (from 1 AU down to tens of solar radii) have
proved that also slow wind streams show sometimes strong Alfvénic
signatures. Within this context, the present paper focuses on a
comparative study on the characterization of Alfvénic turbulence in
fast and slow solar wind intervals observed at 1 AU where degradation
of Alfvénic correlations is expected. In particular, we compared
the behavior of different parameters to characterize the Alfvénic
content of the fluctuations, using also the Elsässer variables to
derive the spectral behavior of the normalized cross-helicity and
residual energy. This study confirms that the Alfvénic slow wind stream
resembles, in many respects, a fast wind stream. The velocity-magnetic
field (v-b) correlation coefficient is similar in the two cases as well
as the amplitude of the fluctuations although it is not clear to what
extent the condition of incompressibility holds. Moreover, the spectral
analysis shows that fast wind and Alfvénic slow wind have similar
normalized cross-helicity values but in general the fast wind streams
are closer to energy equipartition. Despite the overall similarities
between the two solar wind regimes, each stream shows also peculiar
features, that could be linked to the intrinsic evolution history that
each of them has experienced and that should be taken into account to
investigate how and why Alfvénicity evolves in the inner heliosphere.
Title: Constraining Global Coronal Models with Multiple Independent
Observables
Authors: Badman, Samuel T.; Brooks, David H.; Poirier, Nicolas;
Warren, Harry P.; Petrie, Gordon; Rouillard, Alexis P.; Nick Arge,
C.; Bale, Stuart D.; de Pablos Agüero, Diego; Harra, Louise; Jones,
Shaela I.; Kouloumvakos, Athanasios; Riley, Pete; Panasenco, Olga;
Velli, Marco; Wallace, Samantha
Bibcode: 2022ApJ...932..135B
Altcode: 2022arXiv220111818B
Global coronal models seek to produce an accurate physical
representation of the Sun's atmosphere that can be used, for example, to
drive space-weather models. Assessing their accuracy is a complex task,
and there are multiple observational pathways to provide constraints
and tune model parameters. Here, we combine several such independent
constraints, defining a model-agnostic framework for standardized
comparison. We require models to predict the distribution of coronal
holes at the photosphere, and neutral line topology at the model's outer
boundary. We compare these predictions to extreme-ultraviolet (EUV)
observations of coronal hole locations, white-light Carrington maps of
the streamer belt, and the magnetic sector structure measured in situ
by Parker Solar Probe and 1 au spacecraft. We study these metrics for
potential field source surface (PFSS) models as a function of source
surface height and magnetogram choice, as well as comparing to the more
physical Wang-Sheeley-Arge (WSA) and the Magnetohydrodynamic Algorithm
outside a Sphere (MAS) models. We find that simultaneous optimization
of PFSS models to all three metrics is not currently possible, implying
a trade-off between the quality of representation of coronal holes
and streamer belt topology. WSA and MAS results show the additional
physics that they include address this by flattening the streamer belt
while maintaining coronal hole sizes, with MAS also improving coronal
hole representation relative to WSA. We conclude that this framework
is highly useful for inter- and intra-model comparisons. Integral to
the framework is the standardization of observables required of each
model, evaluating different model aspects.
Title: Features of Magnetic Field Switchbacks in Relation to the
Local-field Geometry of Large-amplitude Alfvénic Oscillations:
Wind and PSP Observations
Authors: Bourouaine, Sofiane; Perez, Jean C.; Raouafi, Nour E.;
Chandran, Benjamin D. G.; Bale, Stuart D.; Velli, Marco
Bibcode: 2022ApJ...932L..13B
Altcode: 2022arXiv220409800B
In this Letter, we report observations of magnetic switchback (SB)
features near 1 au using data from the Wind spacecraft. These
features appear to be strikingly similar to the ones observed
by the Parker Solar Probe mission closer to the Sun: namely,
one-sided spikes (or enhancements) in the solar-wind bulk speed V
that correlate/anticorrelate with the spikes seen in the radial-field
component B R . In the solar-wind streams that we analyzed,
these specific SB features near 1 au are associated with large-amplitude
Alfvénic oscillations that propagate outward from the Sun along
a local background (prevalent) magnetic field B 0 that
is nearly radial. We also show that, when B 0 is nearly
perpendicular to the radial direction, the large-amplitude Alfvénic
oscillations display variations in V that are two sided (i.e., V
alternately increases and decreases depending on the vector Δ B =
B - B 0). As a consequence, SBs may not always appear as
one-sided spikes in V, especially at larger heliocentric distances
where the local background field statistically departs from the radial
direction. We suggest that SBs can be well described by large-amplitude
Alfvénic fluctuations if the field rotation is computed with respect
to a well-determined local background field that, in some cases,
may deviate from the large-scale Parker field.
Title: Parker Solar Probe Observations of Solar Wind Energetic Proton
Beams Produced by Magnetic Reconnection in the Near-Sun Heliospheric
Current Sheet
Authors: Phan, T. D.; Verniero, J. L.; Larson, D.; Lavraud, B.;
Drake, J. F.; Øieroset, M.; Eastwood, J. P.; Bale, S. D.; Livi, R.;
Halekas, J. S.; Whittlesey, P. L.; Rahmati, A.; Stansby, D.; Pulupa,
M.; MacDowall, R. J.; Szabo, P. A.; Koval, A.; Desai, M.; Fuselier,
S. A.; Velli, M.; Hesse, M.; Pyakurel, P. S.; Maheshwari, K.; Kasper,
J. C.; Stevens, J. M.; Case, A. W.; Raouafi, N. E.
Bibcode: 2022GeoRL..4996986P
Altcode:
We report observations of reconnection exhausts in the Heliospheric
Current Sheet (HCS) during Parker Solar Probe Encounters 08 and 07,
at 16 Rs and 20 Rs, respectively. Heliospheric
current sheet (HCS) reconnection accelerated protons to almost twice
the solar wind speed and increased the proton core energy by a factor
of ∼3, due to the Alfvén speed being comparable to the solar wind
flow speed at these near-Sun distances. Furthermore, protons were
energized to super-thermal energies. During E08, energized protons
were found to have leaked out of the exhaust along separatrix field
lines, appearing as field-aligned energetic proton beams in a broad
region outside the HCS. Concurrent dropouts of strahl electrons,
indicating disconnection from the Sun, provide further evidence
for the HCS being the source of the beams. Around the HCS in E07,
there were also proton beams but without electron strahl dropouts,
indicating that their origin was not the local HCS reconnection exhaust.
Title: Flux rope and dynamics of the heliospheric current sheet. Study
of the Parker Solar Probe and Solar Orbiter conjunction of June 2020
Authors: Réville, V.; Fargette, N.; Rouillard, A. P.; Lavraud,
B.; Velli, M.; Strugarek, A.; Parenti, S.; Brun, A. S.; Shi, C.;
Kouloumvakos, A.; Poirier, N.; Pinto, R. F.; Louarn, P.; Fedorov,
A.; Owen, C. J.; Génot, V.; Horbury, T. S.; Laker, R.; O'Brien, H.;
Angelini, V.; Fauchon-Jones, E.; Kasper, J. C.
Bibcode: 2022A&A...659A.110R
Altcode: 2021arXiv211207445R
Context. Solar Orbiter and Parker Solar Probe jointly observed the
solar wind for the first time in June 2020, capturing data from very
different solar wind streams: calm, Alfvénic wind and also highly
dynamic large-scale structures. Context. Our aim is to understand the
origin and characteristics of the highly dynamic solar wind observed by
the two probes, particularly in the vicinity of the heliospheric current
sheet (HCS).
Methods: We analyzed the plasma data obtained
by Parker Solar Probe and Solar Orbiter in situ during the month of
June 2020. We used the Alfvén-wave turbulence magnetohydrodynamic
solar wind model WindPredict-AW and we performed two 3D simulations
based on ADAPT solar magnetograms for this period.
Results:
We show that the dynamic regions measured by both spacecraft are
pervaded by flux ropes close to the HCS. These flux ropes are also
present in the simulations, forming at the tip of helmet streamers,
that is, at the base of the heliospheric current sheet. The formation
mechanism involves a pressure-driven instability followed by a fast
tearing reconnection process. We further characterize the 3D spatial
structure of helmet streamer born flux ropes, which appears in the
simulations to be related to the network of quasi-separatrices.
Title: Erratum: "The Role of Alfvén Wave Dynamics on the Large-scale
Properties of the Solar Wind: Comparing an MHD Simulation with Parker
Solar Probe E1 data" (2020, ApJS, 246, 24)
Authors: Réville, Victor; Velli, Marco; Panasenco, Olga; Tenerani,
Anna; Shi, Chen; Badman, Samuel T.; Bale, Stuart D.; Kasper, J. C.;
Stevens, Michael L.; Korreck, Kelly E.; Bonnell, J. W.; Case, Anthony
W.; Dudok de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; Larson,
Davin E.; Livi, Roberto; Malaspina, David M.; MacDowall, Robert J.;
Pulupa, Marc; Whittlesey, Phyllis L.
Bibcode: 2022ApJS..259...29R
Altcode:
No abstract at ADS
Title: Statistical Analysis of Intermittency and its Association
with Proton Heating in the Near-Sun Environment
Authors: Sioulas, Nikos; Velli, Marco; Chhiber, Rohit; Vlahos, Loukas;
Matthaeus, William H.; Bandyopadhyay, Riddhi; Cuesta, Manuel E.; Shi,
Chen; Bowen, Trevor A.; Qudsi, Ramiz A.; Stevens, Michael L.; Bale,
Stuart D.
Bibcode: 2022ApJ...927..140S
Altcode: 2022arXiv220110067S
We use data from the first six encounters of the Parker Solar Probe
and employ the partial variance of increments (PVI) method to study the
statistical properties of coherent structures in the inner heliosphere
with the aim of exploring physical connections between magnetic field
intermittency and observable consequences such as plasma heating and
turbulence dissipation. Our results support proton heating localized
in the vicinity of, and strongly correlated with, magnetic structures
characterized by PVI ≥ 1. We show that, on average, such events
constitute ≍19% of the data set, though variations may occur depending
on the plasma parameters. We show that the waiting time distribution
(WT) of identified events is consistent across all six encounters
following a power-law scaling at lower WTs. This result indicates that
coherent structures are not evenly distributed in the solar wind but
rather tend to be tightly correlated and form clusters. We observe
that the strongest magnetic discontinuities, PVI ≥ 6, usually
associated with reconnection exhausts, are sites where magnetic
energy is locally dissipated in proton heating and are associated
with the most abrupt changes in proton temperature. However, due to
the scarcity of such events, their relative contribution to energy
dissipation is minor. Taking clustering effects into consideration,
we show that smaller scale, more frequent structures with PVI between
1 ≲ PVI ≲ 6 play a major role in magnetic energy dissipation. The
number density of such events is strongly associated with the global
solar wind temperature, with denser intervals being associated with
higher T p .
Title: Influence of the Heliospheric Current Sheet on the Evolution
of Solar Wind Turbulence
Authors: Shi, Chen; Velli, Marco; Tenerani, Anna; Réville, Victor;
Rappazzo, Franco
Bibcode: 2022ApJ...928...93S
Altcode: 2022arXiv220102894S
The effects of the heliospheric current sheet (HCS) on the evolution of
Alfvénic turbulence in the solar wind are studied using MHD simulations
incorporating the expanding-box model. The simulations show that, near
the HCS, the Alfvénicity of the turbulence decreases as manifested
by lower normalized cross-helicity and larger excess of magnetic
energy. The numerical results are supported by a superposed-epoch
analysis using OMNI data, which shows that the normalized cross-helicity
decreases inside the plasma sheet surrounding HCS, and the excess of
magnetic energy is significantly enhanced at the center of HCS. Our
simulation results indicate that the decrease of Alfvénicity around the
HCS is due to the weakening of radial magnetic field and the effects
of the transverse gradient in the background magnetic field. The
magnetic energy excess in the turbulence may be a result of the loss
of Alfvénic correlation between velocity and magnetic field and the
faster decay of transverse kinetic energy with respect to magnetic
energy in a spherically expanding solar wind.
Title: Flux Rope Merging and the Structure of Switchbacks in the
Solar Wind
Authors: Agapitov, O. V.; Drake, J. F.; Swisdak, M.; Bale, S. D.;
Horbury, T. S.; Kasper, J. C.; MacDowall, R. J.; Mozer, F. S.; Phan,
T. D.; Pulupa, M.; Raouafi, N. E.; Velli, M.
Bibcode: 2022ApJ...925..213A
Altcode: 2021arXiv210904016A
A major discovery of Parker Solar Probe (PSP) was the presence of
large numbers of localized increases in the radial solar wind speed and
associated sharp deflections of the magnetic field-switchbacks (SBs). A
possible generation mechanism of SBs is through magnetic reconnection
between open and closed magnetic flux near the solar surface, termed
interchange reconnection, that leads to the ejection of flux ropes
(FRs) into the solar wind. Observations also suggest that SBs undergo
merging, consistent with an FR picture of these structures. The
role of FR merging in controlling the structure of SBs in the solar
wind is explored through direct observations, analytic analysis, and
numerical simulations. Analytic analysis reveals key features of the
structure of FRs and their scaling with heliocentric distance R, which
are consistent with observations and demonstrate the critical role of
merging in controlling the structure of SBs. FR merging is shown to
energetically favor reductions in the strength of the wrapping magnetic
field and the elongation of SBs. A further consequence is the resulting
dominance of the axial magnetic field within SBs that leads to the
observed characteristic sharp rotation of the magnetic field into the
axial direction at the SB boundary. Finally, the radial scaling of the
SB area in the FR model suggests that the observational probability
of SB identification should be insensitive to R, which is consistent
with the most recent statistical analysis of SB observations from PSP.
Title: First Solar Orbiter observation of the Alfvénic slow wind
and identification of its solar source
Authors: D'Amicis, R.; Bruno, R.; Panasenco, O.; Telloni, D.; Perrone,
D.; Marcucci, M. F.; Woodham, L.; Velli, M.; De Marco, R.; Jagarlamudi,
V.; Coco, I.; Owen, C.; Louarn, P.; Livi, S.; Horbury, T.; André,
N.; Angelini, V.; Evans, V.; Fedorov, A.; Genot, V.; Lavraud, B.;
Matteini, L.; Müller, D.; O'Brien, H.; Pezzi, O.; Rouillard, A. P.;
Sorriso-Valvo, L.; Tenerani, A.; Verscharen, D.; Zouganelis, I.
Bibcode: 2021A&A...656A..21D
Altcode:
Context. Turbulence dominated by large-amplitude, nonlinear Alfvén-like
fluctuations mainly propagating away from the Sun is ubiquitous
in high-speed solar wind streams. Recent studies have demontrated
that slow wind streams may also show strong Alfvénic signatures,
especially in the inner heliosphere.
Aims: The present study
focuses on the characterisation of an Alfvénic slow solar wind interval
observed by Solar Orbiter between 14 and 18 July 2020 at a heliocentric
distance of 0.64 AU.
Methods: Our analysis is based on plasma
moments and magnetic field measurements from the Solar Wind Analyser
(SWA) and Magnetometer (MAG) instruments, respectively. We compared
the behaviour of different parameters to characterise the stream
in terms of the Alfvénic content and magnetic properties. We also
performed a spectral analysis to highlight spectral features and
waves signature using power spectral density and magnetic helicity
spectrograms, respectively. Moreover, we reconstruct the Solar Orbiter
magnetic connectivity to the solar sources both via a ballistic
and a potential field source surface (PFSS) model.
Results:
The Alfvénic slow wind stream described in this paper resembles, in
many respects, a fast wind stream. Indeed, at large scales, the time
series of the speed profile shows a compression region, a main portion
of the stream, and a rarefaction region, characterised by different
features. Moreover, before the rarefaction region, we pinpoint several
structures at different scales recalling the spaghetti-like flux-tube
texture of the interplanetary magnetic field. Finally, we identify the
connections between Solar Orbiter in situ measurements, tracing them
down to coronal streamer and pseudostreamer configurations.
Conclusions: The characterisation of the Alfvénic slow wind stream
observed by Solar Orbiter and the identification of its solar source
are extremely important aspects for improving the understanding of
future observations of the same solar wind regime, especially as solar
activity is increasing toward a maximum, where a higher incidence of
this solar wind regime is expected.
Title: A solar source of Alfvenic magnetic field switchbacks: in
situ remnants of magnetic funnels on supergranulation scales
Authors: Bale, Stuart; Desai, Mihir; Halekas, Jasper; Horbury,
Timothy; McManus, Michael; Panasenco, Olga; Badman, Samuel; Bowen,
Trevor; Drake, James; Kasper, Justin; Laker, Ronan; Mallet, Alfred;
Matteini, Lorenzo; Raouafi, Nour; Squire, Jonathan; Velli, Marco;
Woodham, Lloyd; Woolley, Thomas
Bibcode: 2021AGUFMSH33B..04B
Altcode:
One of the more striking observations from the NASA Parker Solar Probe
(PSP) spacecraft is the prevalence in the inner heliosphere of large
amplitude, Alfvenic magnetic field reversals termed 'switchbacks'. These
dB/B~1 fluctuations occur on a range of timescales, are spherically
polarized, and occur in patches separated by intervals of more quiet,
radial solar wind magnetic field. We use measurements from the
FIELDS, SWEAP, and ISOIS instrument suites on PSP to demonstrate
that patches of magnetic field switchbacks are localized within
stable solar wind extensions of structures originating at the base
of the corona. These structures are characterized by an increase in
alpha particle abundance, Mach number, plasma beta and pressure, and
by depletions in the magnetic field magnitude and electron core and
strahl temperature. These intervals are in local pressure-balance,
which implies stationary spatial structure, and the central magnetic
field depressions are consistent with overexpanded flux tubes. The
structures are asymmetric in Carrington longitude with the leading
edge being steeper and with a small edge of hotter plasma and enhanced
magnetic field fluctuations. Some of the structures contain suprathermal
ions to ~85 keV. The structures are separated in longitude by angular
scales associated with supergranulation and chromospheric network
magnetic field. This implies both an origin of the streams and suggests
that these magnetic field switchbacks, hot plasma, alpha particles,
and suprathermal ions originate within and near the leading edge of
the diverging magnetic field funnels associated with the photospheric
network magnetic field.
Title: Adding a transition region in global MHD models of the
solar corona
Authors: Réville, V.; Parenti, S.; Brun, A. S.; Strugarek, A.;
Rouillard, A. P.; Velli, M.; Perri, B.; Pinto, R. F.
Bibcode: 2021sf2a.conf..230R
Altcode:
Global MHD simulations of the solar corona are an essential tool
to investigate long standing problems, such as finding the source
of coronal heating and the mechanisms responsible for the onset and
propagation of coronal mass ejections. The very low atmospheric layers
of the corona, are however, very difficult to model as they imply very
steep gradients of density and temperature over only a few thousand
kilometers. In this proceedings, we illustrate some of the benefits
of including a very simple transition region in global MHD models and
the differences in the plasma properties, comparing with in situ data
of the Parker Solar Probe.
Title: Radial evolution of switchbacks in the inner heliosphere:
observations from PSP to Ulysses
Authors: Tenerani, Anna; Sioulas, Nikos; Matteini, Lorenzo; Panasenco,
Olga; Shi, Chen; Velli, Marco
Bibcode: 2021AGUFMSH35C2092T
Altcode:
We have analyzed magnetic field data from the first six encounters
of Parker Solar Probe, three fast streams observed by Helios 1
and 2, and two Ulysses south polar passes to determine the radial
evolution of switchbacks in the range of heliocentric distances
0.1 < R < 3 au. We have compared the radial evolution of the
magnetic field variances with that of the mean square amplitudes
of switchbacks. In addition, we have calculated the occurrence rate
of switchbacks at various radial distances. We find that the radial
amplitudes of switchbacks decrease faster than that of the overall
turbulent fluctuations, following the radial decrease of the mean
(radial) magnetic field. This result is consistent with the expected
saturation of amplitudes, a condition that must be satisfied by
fluctuations like switchbacks that display a constant total magnetic
field strength. Furthermore, we find that the occurrence of switchbacks
in the solar wind is scale-dependent: the fraction of longer duration
switchbacks increases with radial distance, whereas the fraction of
shorter switchbacks decreases with radial distance. Our results show
that switchbacks decay and re-form in the inner heliosphere. We confirm
that they can be generated in-situ by the expansion, although other
types of switchbacks, forming closer to the sun, cannot be ruled out.
Title: On the validity of the Taylor Hypothesis in the inner
heliosphere as observed by the Parker Solar Probe
Authors: Chasapis, Alexandros; Chhiber, Rohit; Bandyopadhyay, Riddhi;
Qudsi, Ramiz; Malaspina, David; Short, Benjamin; Matthaeus, William;
Goldstein, Melvyn; Maruca, Bennett; Parashar, Tulasi; Ruffolo, David;
Usmanov, Arcadi; Bale, Stuart; Bowen, Trevor; Bonnell, John; Dudok de
Wit, Thierry; Goetz, Keith; Harvey, Peter; MacDowall, Robert; Pulupa,
Marc; Kasper, Justin; Korreck, Kelly; Case, Anthony; Stevens, Michael;
Whittlesey, Phyllis; Larson, Davin; Livi, Roberto; Klein, Kristopher;
Velli, Marco; Raouafi, Nour
Bibcode: 2021AGUFMSH15C2048C
Altcode:
The Taylor hypothesis is an essential tool in studying space plasma with
single point in situ observations. Its use and validity are generally
well established for observations in the near-Earth solar wind. However,
this is not the case for Parker Solar Probe observations in the
inner heliosphere. The very high orbital velocity of the spacecraft,
combined with the unusual plasma parameters very close to the sun,
impose significant challenges in its use. We examine the validity of
the Taylor hypothesis in throughout the Parker Solar Probe encounters
so far. Specifically, we examine the ratio of the Alfven velocity to
the apparent solar wind velocity, and the magnitude of the turbulent
fluctuations of the velocity of the solar wind, as observed by the
spacecraft in its own reference frame. We find that the necessary
conditions appear to be satisfied for most of the orbit, with both
these quantities remaining relatively small. However, at heliocentric
distances smaller than 50 solar radii, the values are observed to rise
above 0.1, and can consistently exceed 0.3, leading to the conclusion
that the Taylor hypothesis may begin to break down in these inner
regions. At larger distances, while both values remain generally low,
at times we observe some periods of much higher values, either due
to a change of the local plasma conditions or due to strong turbulent
fluctuations, suggesting that the Taylor hypothesis may break down in
such transient regions. An alternative formulation of the frozen-in
hypothesis, which would be valid for outward-propagating dominant
fluctuations, is also examined. Its conditions on the outward and inward
propagating Elsasser modes are found to be satisfied near perihelion
for encounters 1 and 2, and for parts of subsequent encounters. We
conclude that the conditions for the validity of the Taylor hypothesis
may not always to be satisfied in the inner heliosphere at distances
below 50 solar radii, and that in such cases, alternative formulations
may be successfully employed.
Title: First Solar Orbiter observation of an Alfvenic slow wind stream
Authors: D'Amicis, Raffaella; Bruno, Roberto; Panasenco, Olga;
Telloni, Daniele; Perrone, Denise; Marcucci, Maria Federica; Woodham,
Lloyd; Velli, Marco; De Marco, Rossana; Jagarlamudi, vamsee Krishna;
Coco, Igino; Owen, Christopher; Louarn, Philippe; Livi, Stefano;
Horbury, Timothy; Andre, Nicolas; Angelini, Virginia; Evans, Vincent;
Fedorov, Andrei; Genot, Vincent; Lavraud, Benoit; Matteini, Lorenzo;
Muller, Daniel; O'Brien, Helen; Pezzi, Oreste; Rouillard, Alexis;
Sorriso-Valvo, Luca; Tenerani, Anna; Verscharen, Daniel; Zouganelis,
Yannis
Bibcode: 2021AGUFMSH21A..10D
Altcode:
Alfvénic turbulence, dominated by large-amplitude Alfvénic
fluctuations mainly propagating away from the Sun, is a feature
characterizing not only the high-speed streams but also some slow
wind intervals. Within this framework, the present study focuses on
an Alfvénic slow solar wind stream observed by Solar Orbiter in July
2020 at a heliocentric distance of 0.64 AU. Using data collected from
the Solar Wind Analyzer (SWA) and the Magnetometer (MAG), we provide
a fully description of this stream from many respects identifying
different regions within the stream characterized by distinct features
using different indicators and including also a spectral analysis
to highlight spectral features and waves signature. In addition,
we pinpoint several structures at different scales recalling the
spaghetti-like flux-tube texture of the interplanetary magnetic field
and we reconstruct the Solar Orbiter magnetic connectivity to the
solar sources both via a ballistic and a potential field source surface
(PFSS) model. The characterization of the Alfvénic slow wind stream
observed by Solar Orbiter and the identification of its solar source
are extremely important for improving the understanding of future
observations of the same solar wind regime and the general problem of
solar wind acceleration. This is particularly relevant for upcoming
Solar Orbiter observations as solar activity is increasing toward a
maximum, where a higher incidence of this solar wind regime has been
observed over previous solar cycles.
Title: Ion and electron temperatures in the solar wind and their
correlations with the solar wind speed
Authors: Shi, Chen; Velli, Marco
Bibcode: 2021AGUFMSH34B..03S
Altcode:
It is observed that in the solar wind the ion temperature has a strong
positive correlation with the solar wind speed while on the contrary,
the electron temperature usually shows an anti-correlation with the
solar wind speed. These features have been confirmed by the Parker
Solar Probe data collected in the very young solar wind. However, a
theory that explains this different temperature-speed correlations for
ions and electrons still lacks. Here we propose that Alfvén waves,
which have been proven via numerical simulations to be an efficient
power source that accelerates the solar wind, could play an important
role. The idea is that, if the Alfvén waves are the major source that
accelerates the solar wind, and their energy dissipates mostly into
the internal energy of the ions, the positive correlation between ion
temperature and wind speed should be naturally reproduced. Meanwhile, as
the electron has the same velocity with the ion but is not heated much
by the waves, i.e., it adiabatically expands, its temperature may have
an anti-correlation with the wind speed under certain conditions. We
develop a 1D two-temperature solar wind model where the ions and
electrons have different temperatures but the same number density and
bulk velocity, and the solar wind is powered by the Alfvén waves which
evolve self-consistently with the wind through two transport equations
for the outward and inward propagating wave components. We explore
the evolution of the ion and electron temperatures under varying wave
properties, i.e., the wave amplitude and wave dissipation rate, etc.,
based on this model.
Title: Conservation of total wave action and the one dimensional
evolution of simple waves in the solar wind
Authors: Huang, Zesen; Shi, Chen; Velli, Marco
Bibcode: 2021AGUFMSH35C2071H
Altcode:
We investigate the evolution of outwardly propagating simple MHD waves
in a model of the expanding solar wind using MHD simulations. In
order to understand the different evolution of slow, Alfvén and
fast modes, the question of wave-action conservation is re-examined
theoretically. Using the fluctuation averaged Lagrangian, we discuss
the conservation of total wave action and Equi-partition of wave energy
for MHD waves. Results show that, even though the wave action for a
simple monochromatic wave is subject to loss under resonance/degeneracy
condition - conditions that can occur in the expanding solar wind
in the regions where plasma ß crosses one, the total wave action
possessed by all modes remains conserved, representing a wave action
exchange between different degrees of freedom. The Expanding Box
simulations demonstrate the results of the theoretical modeling,
and reveal further details about mode-mixing, Alfvén resonance and
wave steepening. All of these may help to understand the evolution of
fluctuations from the inner heliosphere out to Earth orbit and beyond.
Title: Statistical analysis of intermittent structures and their
implications on heating during the first six PSP encounters.
Authors: Sioulas, Nikos; Velli, Marco; Matthaeus, William; Vlahos,
Loukas; Qudsi, Ramiz; Chhiber, Rohit; Bandyopadhyay, Riddhi; Bowen,
Trevor; Stevens, Michael; Bale, Stuart
Bibcode: 2021AGUFMSH35C2098S
Altcode:
We use high-resolution Parker Solar Probe data from the first six
encounters to study the statistical properties of intermittent,
coherent structures and investigate the physical connections between
magnetic field intermittency and observable consequences such as solar
wind dissipation and plasma heating. More specifically, the Partial
Variance of Increments (PVI) method is employed to estimate the fraction
of coherent structures in our dataset. We find that coherent structures
constitute ~2.5 % of the entire dataset, roughly one-tenth of the value
reported in the near-earth environment, indicating in-situ formation
of intermittent magnetic field structures developed by the non-linear
turbulent cascade. We move on to analyze waiting time distributions
of identified events by imposing thresholds on the PVI time series. We
show that the shape of the waiting time distribution strongly depends
on the resolution of the magnetic field time series and the time-lag
used to estimate the PVI time series. We proceed to analyze the
contribution of coherent structures to the heating of the Solar Wind
(SW). We find a positive correlation between proton temperature and
PVI, indicating that proton heating is localized in the vicinity and
strongly correlated with intermittent structures. More precisely,
the strongest discontinuities in the magnetic field are associated
with the most abrupt changes in proton temperature . Still, due to the
scarcity of such events, their relative contribution to the dissipation
of energy in the solar wind is minor. We propose that smaller scale,
more frequent, magnetic field variations of PVI events in the range 2
< PVI< 6, determine the global solar wind temperature. Finally,
our results indicate that due to the low density of coherent structures
in the young solar wind environment, intermittent heating is not as
pronounced as in the outer part of the heliosphere.
Title: Flux conservation, radial scalings, Mach numbers, and
critical distances in the solar wind: magnetohydrodynamics and
Ulysses observations
Authors: Verscharen, Daniel; Bale, Stuart; Velli, Marco
Bibcode: 2021AGUFMSH12A..03V
Altcode:
One of the key challenges in solar and heliospheric physics is to
understand the acceleration of the solar wind. As a super-sonic,
super-Alfvénic plasma flow, the solar wind carries mass, momentum,
energy, and angular momentum from the Sun into interplanetary space. We
present a framework based on two-fluid magnetohydrodynamics to estimate
the flux of these quantities based on spacecraft data independent of
the heliocentric distance of the location of measurement. Applying
this method to the Ulysses dataset allows us to study the dependence of
these fluxes on heliolatitude and solar cycle. The use of scaling laws
provides us with the heliolatitudinal dependence and the solar-cycle
dependence of the scaled Alfvénic and sonic Mach numbers as well as the
Alfvén and sonic critical radii. Moreover, we estimate the distance
at which the local thermal pressure and the local energy density
in the magnetic field balance. These results serve as predictions
for observations with Parker Solar Probe, which currently explores
the very inner heliosphere, and Solar Orbiter, which will measure the
solar wind outside the plane of the ecliptic in the inner heliosphere
during the course of the mission.
Title: Patches of the magnetic switchbacks: hints of their origins
Authors: Shi, Chen; Velli, Marco; Panasenco, Olga; Tenerani, Anna;
Bale, Stuart; Larson, Davin; Bowen, Trevor; Whittlesey, Phyllis;
Livi, Roberto; Halekas, Jasper; Kasper, Justin; Stevens, Michael;
Malaspina, David
Bibcode: 2021AGUFMSH11A..01S
Altcode:
One of the most important findings made by Parker Solar Probe (PSP)
is the omni-presence of the magnetic switchbacks, which are the local
backward-bends of the magnetic field lines. Although many studies were
conducted on the properties and dynamics of these switchbacks, how and
where they are generated are still not fully understood yet. In this
study, we analyze the data from the first seven encounters of PSP with a
focus on the properties of the switchback patches, i.e., the large-scale
modulation of the switchbacks. We select the time intervals when clear
switchback patches are observed by PSP. We show that the appearance
of switchbacks is frequently modulated on a timescale of several hours
and this timescale seems to be independent on whether PSP is near the
perihelion or near the radial-scan part of its orbit, implying that the
patch of switchbacks likely corresponds to some transient phenomenon
on the Sun. We find that between two consecutive patches, the magnetic
field is usually very quiet with weak fluctuations. We compare various
parameters between the quiet intervals and the switchback intervals. The
results show: (1) The quiet intervals are typically less Alfvénic
than the switchback intervals. (2) The magnetic power spectra for the
switchback intervals usually show a shallower large-scale range and
a steeper small-scale range with a break frequency at around 10-2 Hz
while the power spectra for the quiet intervals typically do not show
such a break. (3) In some intervals, an anti-correlation between the
alpha-particle abundance and the switchback patches is observed. We
calculate the magnetic footpoints of PSP using the PFSS model and
discuss the possible correlation between the switchback patches and
the supergranules.
Title: Necessary Conditions for a Hot Quiet Sun Atmosphere:
Chromospheric Flares and Low Corona Twisted Flux Rope Eruptions
Authors: Amari, Tahar; Luciani, Jean-Francois; Aly, Jean-Jacques;
Canou, Aurelien; Mikic, Zoran; Velli, Marco
Bibcode: 2021AGUFMSH12B..05A
Altcode:
The issue of relevant scales involved in the heating of the solar
atmosphere is an important one. Since the temperature already reaches 1
MK a few megameters above the photosphere, observations made by Parker
Solar Probe will be able to explore those at larger heights but only
indirectly at those lower heights, where small scale coupling between
sub-photospheric, chromospheric and coronal structure and dynamics
occurs. While Solar Orbiter will be able to bring such observations,
modeling appears a complementary interesting approach to interpret
those observations Taking a sub-surface dynamo and a sharp realistic
VAL- like scale profile from photosphere to corona, with a fixed
temperature profile in time, we investigate the necessary conditions
implied on the structures and dynamics of the atmosphere to keep this
thermal structuration, as well as their implication in the energy
budget of the atmosphere. Under those hypothesis we show that :i)
the transverse photospheric field below 100km plays a major role;
ii) an associated scale of one megameter activity naturally results
to produce a zone above the photosphere with high confined electric
currents, which then expands into the chromosphere and releases energy(4
500 W/m2) through small-scale eruptions driving sonic motions; iii)
meso scale structuration, leads to the formation of larger coherent
twisted flux ropes, and associated eruptive like activity in a way
similar to large scale eruptive phenomena, as result of cancellation,
emergence, and convergence motions. Finally a wave dynamics is also
naturally driven in core corona associated to above 300 W/m2.
Title: Deciphering Solar Magnetic Activity: 140 Years of the `Extended
Solar Cycle' - Mapping the Hale Cycle
Authors: McIntosh, Scott W.; Leamon, Robert J.; Egeland, Ricky;
Dikpati, Mausumi; Altrock, Richard C.; Banerjee, Dipankar; Chatterjee,
Subhamoy; Srivastava, Abhishek K.; Velli, Marco
Bibcode: 2021SoPh..296..189M
Altcode: 2020arXiv201006048M
We investigate the occurrence of the "extended solar cycle" (ESC) as it
occurs in a host of observational data spanning 140 years. Investigating
coronal, chromospheric, photospheric, and interior diagnostics, we
develop a consistent picture of solar activity migration linked to the
22-year Hale (magnetic) cycle using superposed epoch analysis (SEA)
and previously identified Hale cycle termination events as the key
time for the SEA. Our analysis shows that the ESC and Hale cycle,
as highlighted by the terminator-keyed SEA, is strongly recurrent
throughout the entire observational record studied, some 140
years. Applying the same SEA method to the sunspot record confirms
that Maunder's butterfly pattern is a subset of the underlying Hale
cycle, strongly suggesting that the production of sunspots is not
the fundamental feature of the Hale cycle, but the ESC is. The ESC
(and Hale cycle) pattern highlights the importance of 55∘
latitude in the evolution, and possible production, of solar magnetism.
Title: Solar Orbiter's first Venus flyby: Observations from the
Radio and Plasma Wave instrument
Authors: Hadid, L. Z.; Edberg, N. J. T.; Chust, T.; Píša, D.;
Dimmock, A. P.; Morooka, M. W.; Maksimovic, M.; Khotyaintsev, Yu. V.;
Souček, J.; Kretzschmar, M.; Vecchio, A.; Le Contel, O.; Retino, A.;
Allen, R. C.; Volwerk, M.; Fowler, C. M.; Sorriso-Valvo, L.; Karlsson,
T.; Santolík, O.; Kolmašová, I.; Sahraoui, F.; Stergiopoulou, K.;
Moussas, X.; Issautier, K.; Dewey, R. M.; Klein Wolt, M.; Malandraki,
O. E.; Kontar, E. P.; Howes, G. G.; Bale, S. D.; Horbury, T. S.;
Martinović, M.; Vaivads, A.; Krasnoselskikh, V.; Lorfèvre, E.;
Plettemeier, D.; Steller, M.; Štverák, Š.; Trávníček, P.;
O'Brien, H.; Evans, V.; Angelini, V.; Velli, M. C.; Zouganelis, I.
Bibcode: 2021A&A...656A..18H
Altcode:
Context. On December 27, 2020, Solar Orbiter completed its first
gravity assist manoeuvre of Venus (VGAM1). While this flyby was
performed to provide the spacecraft with sufficient velocity to get
closer to the Sun and observe its poles from progressively higher
inclinations, the Radio and Plasma Wave (RPW) consortium, along
with other operational in situ instruments, had the opportunity to
perform high cadence measurements and study the plasma properties in
the induced magnetosphere of Venus.
Aims: In this paper, we
review the main observations of the RPW instrument during VGAM1. They
include the identification of a number of magnetospheric plasma wave
modes, measurements of the electron number densities computed using
the quasi-thermal noise spectroscopy technique and inferred from
the probe-to-spacecraft potential, the observation of dust impact
signatures, kinetic solitary structures, and localized structures at the
bow shock, in addition to the validation of the wave normal analysis
on-board from the Low Frequency Receiver.
Methods: We used the
data products provided by the different subsystems of RPW to study
Venus' induced magnetosphere.
Results: The results include the
observations of various electromagnetic and electrostatic wave modes
in the induced magnetosphere of Venus: strong emissions of ∼100 Hz
whistler waves are observed in addition to electrostatic ion acoustic
waves, solitary structures and Langmuir waves in the magnetosheath of
Venus. Moreover, based on the different levels of the wave amplitudes
and the large-scale variations of the electron number densities, we
could identify different regions and boundary layers at Venus.
Conclusions: The RPW instrument provided unprecedented AC magnetic
and electric field measurements in Venus' induced magnetosphere for
continuous frequency ranges and with high time resolution. These
data allow for the conclusive identification of various plasma
waves at higher frequencies than previously observed and a detailed
investigation regarding the structure of the induced magnetosphere
of Venus. Furthermore, noting that prior studies were mainly focused
on the magnetosheath region and could only reach 10-12 Venus radii
(RV) down the tail, the particular orbit geometry of Solar
Orbiter's VGAM1, allowed the first investigation of the nature of the
plasma waves continuously from the bow shock to the magnetosheath,
extending to ∼70RV in the far distant tail region.
Title: The first coronal mass ejection observed in both visible-light
and UV H I Ly-α channels of the Metis coronagraph on board Solar
Orbiter
Authors: Andretta, V.; Bemporad, A.; De Leo, Y.; Jerse, G.; Landini,
F.; Mierla, M.; Naletto, G.; Romoli, M.; Sasso, C.; Slemer, A.;
Spadaro, D.; Susino, R.; Talpeanu, D. -C.; Telloni, D.; Teriaca, L.;
Uslenghi, M.; Antonucci, E.; Auchère, F.; Berghmans, D.; Berlicki,
A.; Capobianco, G.; Capuano, G. E.; Casini, C.; Casti, M.; Chioetto,
P.; Da Deppo, V.; Fabi, M.; Fineschi, S.; Frassati, F.; Frassetto,
F.; Giordano, S.; Grimani, C.; Heinzel, P.; Liberatore, A.; Magli, E.;
Massone, G.; Messerotti, M.; Moses, D.; Nicolini, G.; Pancrazzi, M.;
Pelizzo, M. -G.; Romano, P.; Schühle, U.; Stangalini, M.; Straus,
Th.; Volpicelli, C. A.; Zangrilli, L.; Zuppella, P.; Abbo, L.; Aznar
Cuadrado, R.; Bruno, R.; Ciaravella, A.; D'Amicis, R.; Lamy, P.;
Lanzafame, A.; Malvezzi, A. M.; Nicolosi, P.; Nisticò, G.; Peter,
H.; Plainaki, C.; Poletto, L.; Reale, F.; Solanki, S. K.; Strachan,
L.; Tondello, G.; Tsinganos, K.; Velli, M.; Ventura, R.; Vial, J. -C.;
Woch, J.; Zimbardo, G.
Bibcode: 2021A&A...656L..14A
Altcode:
Context. The Metis coronagraph on board Solar Orbiter offers a new
view of coronal mass ejections (CMEs), observing them for the first
time with simultaneous images acquired with a broad-band filter in
the visible-light interval and with a narrow-band filter around the
H I Ly-α line at 121.567 nm, the so-called Metis UV channel.
Aims: We show the first Metis observations of a CME, obtained on 16
and 17 January 2021. The event was also observed by the EUI/FSI imager
on board Solar Orbiter, as well as by other space-based coronagraphs,
such as STEREO-A/COR2 and SOHO/LASCO/C2, whose images are combined here
with Metis data.
Methods: Different images are analysed here
to reconstruct the 3D orientation of the expanding CME flux rope using
the graduated cylindrical shell model. This also allows us to identify
the possible location of the source region. Measurements of the CME
kinematics allow us to quantify the expected Doppler dimming in the
Ly-α channel.
Results: Observations show that most CME features
seen in the visible-light images are also seen in the Ly-α images,
although some features in the latter channel appear more structured
than their visible-light counterparts. We estimated the expansion
velocity of this event to be below 140 km s−1. Hence,
these observations can be understood by assuming that Doppler dimming
effects do not strongly reduce the Ly-α emission from the CME. These
velocities are comparable with or smaller than the radial velocities
inferred from the same data in a similar coronal structure on the
east side of the Sun.
Conclusions: The first observations by
Metis of a CME demonstrate the capability of the instrument to provide
valuable and novel information on the structure and dynamics of these
coronal events. Considering also its diagnostics capabilities regarding
the conditions of the ambient corona, Metis promises to significantly
advance our knowledge of such phenomena. Movies are available at https://www.aanda.org
Title: The angular-momentum flux in the solar wind observed during
Solar Orbiter's first orbit
Authors: Verscharen, Daniel; Stansby, David; Finley, Adam J.; Owen,
Christopher J.; Horbury, Timothy; Maksimovic, Milan; Velli, Marco;
Bale, Stuart D.; Louarn, Philippe; Fedorov, Andrei; Bruno, Roberto;
Livi, Stefano; Khotyaintsev, Yuri V.; Vecchio, Antonio; Lewis, Gethyn
R.; Anekallu, Chandrasekhar; Kelly, Christopher W.; Watson, Gillian;
Kataria, Dhiren O.; O'Brien, Helen; Evans, Vincent; Angelini, Virginia;
Solar Orbiter SWA, MAG and RPW Teams
Bibcode: 2021A&A...656A..28V
Altcode: 2021arXiv210601780V
Aims: We present the first measurements of the angular-momentum
flux in the solar wind recorded by the Solar Orbiter spacecraft. Our
aim is to validate these measurements to support future studies of
the Sun's angular-momentum loss.
Methods: We combined 60-min
averages of the proton bulk moments and the magnetic field measured by
the Solar Wind Analyser and the magnetometer onboard Solar Orbiter. We
calculated the angular-momentum flux per solid-angle element using
data from the first orbit of the mission's cruise phase in 2020. We
separated the contributions from protons and from magnetic stresses to
the total angular-momentum flux.
Results: The angular-momentum
flux varies significantly over time. The particle contribution
typically dominates over the magnetic-field contribution during our
measurement interval. The total angular-momentum flux shows the largest
variation and is typically anti-correlated with the radial solar-wind
speed. We identify a compression region, potentially associated with a
co-rotating interaction region or a coronal mass ejection, which leads
to a significant localised increase in the angular-momentum flux,
albeit without a significant increase in the angular momentum per
unit mass. We repeated our analysis using the density estimate from
the Radio and Plasma Waves instrument. Using this independent method,
we find a decrease in the peaks of positive angular-momentum flux,
but otherwise, our results remain consistent.
Conclusions:
Our results largely agree with previous measurements of the solar
wind's angular-momentum flux in terms of amplitude, variability, and
dependence on radial solar-wind bulk speed. Our analysis highlights
the potential for more detailed future studies of the solar wind's
angular momentum and its other large-scale properties with data from
Solar Orbiter. We emphasise the need for studying the radial evolution
and latitudinal dependence of the angular-momentum flux in combination
with data from Parker Solar Probe and other assets at heliocentric
distances of 1 au and beyond.
Title: Proton energization by phase steepening of parallel-propagating
Alfvenic fluctuations
Authors: Gonzalez, Carlos; Tenerani, Anna; Matteini, Lorenzo;
Hellinger, Petr; Velli, Marco
Bibcode: 2021AGUFMSH34B..06G
Altcode:
We present recent work on the proton energization process resulting
from the phase-steepening of parallel-propagating and large-amplitude
Alfvénic fluctuation. We have made use of hybrid simulations
complemented by test-particles simulations to understand the complex
behavior of protons during the collapse of an initially circularly
polarized, non-monochromatic Alfvénic wave packet. The wave collapsing
is followed by the formation of rotational discontinuities embedded
in compressional structures that propagate at nearly the Alfven
speed. Proton scattering at the steepened edges causes non-adiabatic
proton perpendicular heating while the parallel electric field at
the propagating fronts mediates the acceleration of protons along the
mean-field, which allows field-aligned proton beams on the velocity
distribution function accompanying the damping of compressible
fluctuations. We discuss the implications of this results on the
context of Alfvénic solar wind.
Title: First light observations of the solar wind in the outer corona
with the Metis coronagraph
Authors: Romoli, M.; Antonucci, E.; Andretta, V.; Capuano, G. E.; Da
Deppo, V.; De Leo, Y.; Downs, C.; Fineschi, S.; Heinzel, P.; Landini,
F.; Liberatore, A.; Naletto, G.; Nicolini, G.; Pancrazzi, M.; Sasso,
C.; Spadaro, D.; Susino, R.; Telloni, D.; Teriaca, L.; Uslenghi,
M.; Wang, Y. -M.; Bemporad, A.; Capobianco, G.; Casti, M.; Fabi, M.;
Frassati, F.; Frassetto, F.; Giordano, S.; Grimani, C.; Jerse, G.;
Magli, E.; Massone, G.; Messerotti, M.; Moses, D.; Pelizzo, M. -G.;
Romano, P.; Schühle, U.; Slemer, A.; Stangalini, M.; Straus, T.;
Volpicelli, C. A.; Zangrilli, L.; Zuppella, P.; Abbo, L.; Auchère,
F.; Aznar Cuadrado, R.; Berlicki, A.; Bruno, R.; Ciaravella, A.;
D'Amicis, R.; Lamy, P.; Lanzafame, A.; Malvezzi, A. M.; Nicolosi,
P.; Nisticò, G.; Peter, H.; Plainaki, C.; Poletto, L.; Reale, F.;
Solanki, S. K.; Strachan, L.; Tondello, G.; Tsinganos, K.; Velli,
M.; Ventura, R.; Vial, J. -C.; Woch, J.; Zimbardo, G.
Bibcode: 2021A&A...656A..32R
Altcode: 2021arXiv210613344R
In this work, we present an investigation of the wind in the solar
corona that has been initiated by observations of the resonantly
scattered ultraviolet emission of the coronal plasma obtained with
UVCS-SOHO, designed to measure the wind outflow speed by applying
Doppler dimming diagnostics. Metis on Solar Orbiter complements the
UVCS spectroscopic observations that were performed during solar
activity cycle 23 by simultaneously imaging the polarized visible
light and the H I Lyman-α corona in order to obtain high spatial and
temporal resolution maps of the outward velocity of the continuously
expanding solar atmosphere. The Metis observations, taken on May 15,
2020, provide the first H I Lyman-α images of the extended corona
and the first instantaneous map of the speed of the coronal plasma
outflows during the minimum of solar activity and allow us to identify
the layer where the slow wind flow is observed. The polarized visible
light (580-640 nm) and the ultraviolet H I Lyα (121.6 nm) coronal
emissions, obtained with the two Metis channels, were combined in
order to measure the dimming of the UV emission relative to a static
corona. This effect is caused by the outward motion of the coronal
plasma along the direction of incidence of the chromospheric photons
on the coronal neutral hydrogen. The plasma outflow velocity was then
derived as a function of the measured Doppler dimming. The static
corona UV emission was simulated on the basis of the plasma electron
density inferred from the polarized visible light. This study leads
to the identification, in the velocity maps of the solar corona, of
the high-density layer about ±10° wide, centered on the extension
of a quiet equatorial streamer present at the east limb - the coronal
origin of the heliospheric current sheet - where the slowest wind
flows at about 160 ± 18 km s−1 from 4 R⊙
to 6 R⊙. Beyond the boundaries of the high-density layer,
the wind velocity rapidly increases, marking the transition between
slow and fast wind in the corona.
Title: Comparative Study of Electric Currents and Energetic Particle
Fluxes in a Solar Flare and Earth Magnetospheric Substorm
Authors: Artemyev, Anton; Zimovets, Ivan; Sharykin, Ivan; Nishimura,
Yukitoshi; Downs, Cooper; Weygand, James; Fiori, Robyn; Zhang,
Xiao-Jia; Runov, Andrei; Velli, Marco; Angelopoulos, Vassilis;
Panasenco, Olga; Russell, Christopher T.; Miyoshi, Yoshizumi; Kasahara,
Satoshi; Matsuoka, Ayako; Yokota, Shoichiro; Keika, Kunihiro; Hori,
Tomoaki; Kazama, Yoichi; Wang, Shiang-Yu; Shinohara, Iku; Ogawa,
Yasunobu
Bibcode: 2021ApJ...923..151A
Altcode: 2021arXiv210503772A
Magnetic field line reconnection is a universal plasma process
responsible for the conversion of magnetic field energy to plasma
heating and charged particle acceleration. Solar flares and Earth's
magnetospheric substorms are two of the most investigated dynamical
systems where global magnetic field reconfiguration is accompanied by
energization of plasma populations. Such a reconfiguration includes
formation of a long-living current system connecting the primary
energy release region and cold dense conductive plasma of the
photosphere/ionosphere. In both flares and substorms the evolution
of this current system correlates with the formation and dynamics of
energetic particle fluxes (although energy ranges can be different
for these systems). Our study is focused on the similarity between
flares and substorms. Using a wide range of data sets available for
flare and substorm investigations, we qualitatively compare the
dynamics of currents and energetic particle fluxes for one flare
and one substorm. We show that there is a clear correlation between
energetic particle precipitations (associated with energy release due to
magnetic reconnection seen from riometer and hard X-ray measurements)
and magnetic field reconfiguration/formation of the current system,
whereas the long-term current system evolution correlates better with
hot plasma fluxes (seen from in situ and soft X-ray measurements). We
then discuss how data sets of in situ measurements of magnetospheric
substorms can help interpret solar flare data.
Title: Structure and dynamics of flux ropes in the solar wind with
implications for switchbacks
Authors: Drake, James; Agapitov, Oleksiy; Swisdak, Marc; Phan, Tai;
Bale, Stuart; Horbury, Timothy; Kasper, Justin; MacDowall, Robert;
Mozer, Forrest; Pulupa, Marc; Raouafi, Nour; Velli, Marco
Bibcode: 2021AGUFMSH33B..07D
Altcode:
The dynamics and structure of flux ropes generated by
interchangereconnection in the corona as they propagate outward in
the solar windis explored with analytic analysis and simulations,
benchmarked withswitchback observations from the Parker Solar
Probe. Keycharacteristics such as size scaling, aspect ratio
andAlfvenicity are explored. Flux rope merging is found to
beenergetically favorable and plays a key role in producing
essentiallyall of the characteristics of switchbacks. Merging
reduces themagnetic field that wraps the flux rope in comparison
with the axialmagnetic field. The weak wrapping field allows the flux
ropes to besquashed by the ambient solar wind magnetic field into the
highlyelongated shapes seen in the data and produces the sharp rotations
ofmagnetic field direction measured at switchbackboundaries. Simulations
of flux rope merger with Alfvenic flowsproduce surprises: merger
amplifies Alfvenic flows and flux ropemergers with high initial
Alfvenicity saturate before merger iscomplete. The amplification of
flows during merger continues untilflows are fully Alfvenic where
merger is no longer energeticallyfavorable. The simulations suggest
that the Alfvenicity of switchbacksshould increase with distance
R from the sun, which is confirmed withPSP data. The saturation of
partially merged flux ropes yields finalstates with remnant, current
layers and density enhancements. Theobservations reveal that many large
switchbacks contain embeddedcurrent sheets, suggesting that they are
made up of several partiallymerged flux ropes. Observations of merging
flux ropes have not yetbeen identified in the data but should be more
likely closer to thesun where the Alfvenicity of flux ropes is predicted
to beweaker. Thus, the flux rope model of switchbacks produces all
of theknown characteristics of switchbacks measured in the solar wind
andsuggests that interchange reconnection in the corona is fundamental
tounderstanding the structure and evolution of the solar wind.
Title: Parker Solar Probe Enters the Magnetically Dominated Solar
Corona
Authors: Kasper, J. C.; Klein, K. G.; Lichko, E.; Huang, Jia; Chen,
C. H. K.; Badman, S. T.; Bonnell, J.; Whittlesey, P. L.; Livi, R.;
Larson, D.; Pulupa, M.; Rahmati, A.; Stansby, D.; Korreck, K. E.;
Stevens, M.; Case, A. W.; Bale, S. D.; Maksimovic, M.; Moncuquet, M.;
Goetz, K.; Halekas, J. S.; Malaspina, D.; Raouafi, Nour E.; Szabo,
A.; MacDowall, R.; Velli, Marco; Dudok de Wit, Thierry; Zank, G. P.
Bibcode: 2021PhRvL.127y5101K
Altcode:
The high temperatures and strong magnetic fields of the solar corona
form streams of solar wind that expand through the Solar System into
interstellar space. At 09:33 UT on 28 April 2021 Parker Solar Probe
entered the magnetized atmosphere of the Sun 13 million km above the
photosphere, crossing below the Alfvén critical surface for five
hours into plasma in casual contact with the Sun with an Alfvén Mach
number of 0.79 and magnetic pressure dominating both ion and electron
pressure. The spectrum of turbulence below the Alfvén critical surface
is reported. Magnetic mapping suggests the region was a steady flow
emerging on rapidly expanding coronal magnetic field lines lying above
a pseudostreamer. The sub-Alfvénic nature of the flow may be due to
suppressed magnetic reconnection at the base of the pseudostreamer,
as evidenced by unusually low densities in this region and the magnetic
mapping.
Title: Small-Scale Solar Activity and its effect on the coronal
environment
Authors: Raouafi, Nour; Stenborg, Guillermo; Seaton, Daniel; DeForest,
Craig; Bale, Stuart; Horbury, Timothy; Kasper, Justin; Velli, Marco;
Karpen, Judith; Kumar, Pankaj; DeVore, C. Richard; Uritsky, Vadim
Bibcode: 2021AGUFMSH25F2144R
Altcode:
Careful analysis of solar observations reveals a myriad of small-scale
jetting activity (i.e., jetlets; Raouafi & Stenborg 2014). Jetlets
are miniature manifestations of the typical coronal jets observed
in both X-rays and extreme-ultraviolet (EUV) solar images. They are
the product of near-ubiquitous magnetic reconnection. Their role in
energy and mass transport to the solar corona and wind has not been
yet well established. Here we provide an overview of this phenomenon
and explore its role at the base of the corona and the young solar
wind. We conjecture that these small dynamic features might be the
source or at least one of the sources of the magnetic switchbacks
observed by the Parker Solar Probe.
Title: A Solar Source of Alfvénic Magnetic Field Switchbacks:
In Situ Remnants of Magnetic Funnels on Supergranulation Scales
Authors: Bale, S. D.; Horbury, T. S.; Velli, M.; Desai, M. I.; Halekas,
J. S.; McManus, M. D.; Panasenco, O.; Badman, S. T.; Bowen, T. A.;
Chandran, B. D. G.; Drake, J. F.; Kasper, J. C.; Laker, R.; Mallet,
A.; Matteini, L.; Phan, T. D.; Raouafi, N. E.; Squire, J.; Woodham,
L. D.; Woolley, T.
Bibcode: 2021ApJ...923..174B
Altcode: 2021arXiv210901069B
One of the striking observations from the Parker Solar Probe (PSP)
spacecraft is the prevalence in the inner heliosphere of large
amplitude, Alfvénic magnetic field reversals termed switchbacks. These
$\delta {B}_{R}/B\sim { \mathcal O }(1$ ) fluctuations occur over a
range of timescales and in patches separated by intervals of quiet,
radial magnetic field. We use measurements from PSP to demonstrate that
patches of switchbacks are localized within the extensions of plasma
structures originating at the base of the corona. These structures
are characterized by an increase in alpha particle abundance, Mach
number, plasma β and pressure, and by depletions in the magnetic
field magnitude and electron temperature. These intervals are in
pressure balance, implying stationary spatial structure, and the field
depressions are consistent with overexpanded flux tubes. The structures
are asymmetric in Carrington longitude with a steeper leading edge
and a small (~1°) edge of hotter plasma and enhanced magnetic field
fluctuations. Some structures contain suprathermal ions to ~85 keV that
we argue are the energetic tail of the solar wind alpha population. The
structures are separated in longitude by angular scales associated with
supergranulation. This suggests that these switchbacks originate near
the leading edge of the diverging magnetic field funnels associated
with the network magnetic field-the primary wind sources. We propose an
origin of the magnetic field switchbacks, hot plasma and suprathermals,
alpha particles in interchange reconnection events just above the
solar transition region and our measurements represent the extended
regions of a turbulent outflow exhaust.
Title: Statistical Observations of Solar Wind Fluctuations in the
de Hoffmann-Teller Frame
Authors: Bowen, Trevor; Mallet, Alfred; McManus, Michael; Squire,
Jonathan; Matteini, Lorenzo; Stawarz, Julia; Woodham, Lloyd; Klein,
Kristopher; Velli, Marco; Badman, Samuel; Bale, Stuart; Larson, Davin;
Chen, Christopher
Bibcode: 2021AGUFMSH41A..04B
Altcode:
PSP observations of turbulence in the inner-heliosphere often
reveal high-cross helicity states that are close to "pure" Alfvén
waves. The wavelike character of high cross-helicity states
enables fluctuations to be studied in a stationary reference
frame with a minimal convected electric field, commonly termed
the de Hoffmann-Teller (dHT) frame. Surprisingly, a dHT frame is
also often found even when fluctuations deviate from high-cross
helicity states, which is a signature of significant alignment of
outer-scale fluctuations. We explore the statistical properties of
the de Hoffmann-Teller frame in the inner heliosphere, with specific
focus on the residual electric field that remains in the stationary
dHT frame. We show that the measured residual electric field is often
associated with the existence of residual energy, and is the result
of counter-propagating Alfvén waves and/or compressive modes: both
of which can contribute to nonlinear turbulent energy transfer in the
heliosphere. Using 3D measurements of the proton and alpha core from
PSP/SWEAP/SPAN we constrain effects of kinetic normalization to Alfvén
velocity, demonstrating that the residual energy and electric fields are
associated with propagating fluctuations and not mis-characterizations
of the Alfvén speed.
Title: Stability of the Magnetotail Current Sheet With Normal Magnetic
Field and Field-Aligned Plasma Flows
Authors: Shi, Chen; Artemyev, Anton; Velli, Marco; Tenerani, Anna
Bibcode: 2021JGRA..12629711S
Altcode: 2021arXiv211008478S
One of the most important problems of magnetotail dynamics is the
substorm onset and the related instability of the magneotail current
sheet. Since the simplest 2D current sheet configuration with monotonic
Bz was proven to be stable to the tearing mode, the focus of
the instability investigation moved to more specific configurations, for
example, kinetic current sheets with strong transient ion currents and
current sheets with non-monotonic Bz (local Bz
minima or/and peaks). The stability of the latter current sheet
configuration has been studied both within kinetic and fluid approaches,
whereas the investigation of the transient ion effects was limited to
kinetic models only. This paper aims to provide a detailed analysis
of the stability of a multi-fluid current sheet configuration that
mimics current sheets with transient ions. Using the system with two
field-aligned ion flows that mimic the effect of pressure non-gyrotropy,
we construct a 1D current sheet with a finite Bz. This
model describes well recent findings of very thin intense magnetotail
current sheets. The stability analysis of this two-ion model confirms
the stabilizing effect of finite Bz and shows that the most
stable current sheet is the one with exactly counter-streaming ion
flows and zero net flow. Such field-aligned flows may substitute the
contribution of the pressure tensor nongyrotropy to the stress balance
but cannot overtake the stabilizing effect of Bz. Obtained
results are discussed in the context of magnetotail dynamical models
and spacecraft observations.
Title: Flux conservation, radial scalings, Mach numbers, and
critical distances in the solar wind: magnetohydrodynamics and
Ulysses observations
Authors: Verscharen, Daniel; Bale, Stuart D.; Velli, Marco
Bibcode: 2021MNRAS.506.4993V
Altcode: 2021MNRAS.tmp.1827V; 2021arXiv210706540V
One of the key challenges in solar and heliospheric physics is to
understand the acceleration of the solar wind. As a super-sonic,
super-Alfvénic plasma flow, the solar wind carries mass, momentum,
energy, and angular momentum from the Sun into interplanetary space. We
present a framework based on two-fluid magnetohydrodynamics to estimate
the flux of these quantities based on spacecraft data independent of
the heliocentric distance of the location of measurement. Applying this
method to the Ulysses data set allows us to study the dependence of
these fluxes on heliolatitude and solar cycle. The use of scaling laws
provides us with the heliolatitudinal dependence and the solar-cycle
dependence of the scaled Alfvénic and sonic Mach numbers as well as the
Alfvén and sonic critical radii. Moreover, we estimate the distance
at which the local thermal pressure and the local energy density
in the magnetic field balance. These results serve as predictions
for observations with Parker Solar Probe, which currently explores
the very inner heliosphere, and Solar Orbiter, which will measure the
solar wind outside the plane of the ecliptic in the inner heliosphere
during the course of the mission.
Title: Evolution of Switchbacks in the Inner Heliosphere
Authors: Tenerani, Anna; Sioulas, Nikos; Matteini, Lorenzo; Panasenco,
Olga; Shi, Chen; Velli, Marco
Bibcode: 2021ApJ...919L..31T
Altcode: 2021arXiv210906341T
We analyze magnetic field data from the first six encounters of Parker
Solar Probe, three Helios fast streams and two Ulysses south polar
passes covering heliocentric distances 0.1 ≲ R ≲ 3 au. We use
this data set to statistically determine the evolution of switchbacks
of different periods and amplitudes with distance from the Sun. We
compare the radial evolution of magnetic field variances with that of
the mean square amplitudes of switchbacks, and quantify the radial
evolution of the cumulative counts of switchbacks per kilometer. We
find that the amplitudes of switchbacks decrease faster than the
overall turbulent fluctuations, in a way consistent with the radial
decrease of the mean magnetic field. This could be the result of a
saturation of amplitudes and may be a signature of decay processes
of large amplitude Alfvénic fluctuations in the solar wind. We find
that the evolution of switchback occurrence in the solar wind is scale
dependent: the fraction of longer-duration switchbacks increases with
radial distance, whereas it decreases for shorter switchbacks. This
implies that switchback dynamics is a complex process involving both
decay and in situ generation in the inner heliosphere. We confirm that
switchbacks can be generated by the expansion, although other types
of switchbacks generated closer to the Sun cannot be ruled out.
Title: Exploring the Solar Wind from Its Source on the Corona into
the Inner Heliosphere during the First Solar Orbiter-Parker Solar
Probe Quadrature
Authors: Telloni, Daniele; Andretta, Vincenzo; Antonucci, Ester;
Bemporad, Alessandro; Capuano, Giuseppe E.; Fineschi, Silvano;
Giordano, Silvio; Habbal, Shadia; Perrone, Denise; Pinto, Rui F.;
Sorriso-Valvo, Luca; Spadaro, Daniele; Susino, Roberto; Woodham, Lloyd
D.; Zank, Gary P.; Romoli, Marco; Bale, Stuart D.; Kasper, Justin C.;
Auchère, Frédéric; Bruno, Roberto; Capobianco, Gerardo; Case,
Anthony W.; Casini, Chiara; Casti, Marta; Chioetto, Paolo; Corso,
Alain J.; Da Deppo, Vania; De Leo, Yara; Dudok de Wit, Thierry;
Frassati, Federica; Frassetto, Fabio; Goetz, Keith; Guglielmino,
Salvo L.; Harvey, Peter R.; Heinzel, Petr; Jerse, Giovanna; Korreck,
Kelly E.; Landini, Federico; Larson, Davin; Liberatore, Alessandro;
Livi, Roberto; MacDowall, Robert J.; Magli, Enrico; Malaspina, David
M.; Massone, Giuseppe; Messerotti, Mauro; Moses, John D.; Naletto,
Giampiero; Nicolini, Gianalfredo; Nisticò, Giuseppe; Panasenco,
Olga; Pancrazzi, Maurizio; Pelizzo, Maria G.; Pulupa, Marc; Reale,
Fabio; Romano, Paolo; Sasso, Clementina; Schühle, Udo; Stangalini,
Marco; Stevens, Michael L.; Strachan, Leonard; Straus, Thomas; Teriaca,
Luca; Uslenghi, Michela; Velli, Marco; Verscharen, Daniel; Volpicelli,
Cosimo A.; Whittlesey, Phyllis; Zangrilli, Luca; Zimbardo, Gaetano;
Zuppella, Paola
Bibcode: 2021ApJ...920L..14T
Altcode: 2021arXiv211011031T
This Letter addresses the first Solar Orbiter (SO)-Parker Solar
Probe (PSP) quadrature, occurring on 2021 January 18 to investigate
the evolution of solar wind from the extended corona to the inner
heliosphere. Assuming ballistic propagation, the same plasma volume
observed remotely in the corona at altitudes between 3.5 and 6.3
solar radii above the solar limb with the Metis coronagraph on SO
can be tracked to PSP, orbiting at 0.1 au, thus allowing the local
properties of the solar wind to be linked to the coronal source region
from where it originated. Thanks to the close approach of PSP to the
Sun and the simultaneous Metis observation of the solar corona, the
flow-aligned magnetic field and the bulk kinetic energy flux density
can be empirically inferred along the coronal current sheet with an
unprecedented accuracy, allowing in particular estimation of the Alfvén
radius at 8.7 solar radii during the time of this event. This is thus
the very first study of the same solar wind plasma as it expands from
the sub-Alfvénic solar corona to just above the Alfvén surface.
Title: On the Role of Solar Wind Expansion as a Source of Whistler
Waves: Scattering of Suprathermal Electrons and Heat Flux Regulation
in the Inner Heliosphere
Authors: Micera, A.; Zhukov, A. N.; López, R. A.; Boella, E.;
Tenerani, A.; Velli, M.; Lapenta, G.; Innocenti, M. E.
Bibcode: 2021ApJ...919...42M
Altcode: 2021arXiv210615975M
The role of solar wind expansion in generating whistler waves is
investigated using the EB-iPic3D code, which models solar wind expansion
self-consistently within a fully kinetic semi-implicit approach. The
simulation is initialized with an electron velocity distribution
function modeled after observations of the Parker Solar Probe during
its first perihelion at 0.166 au, consisting of a dense core and an
antisunward strahl. This distribution function is initially stable
with respect to kinetic instabilities. Expansion drives the solar
wind into successive regimes where whistler heat flux instabilities
are triggered. These instabilities produce sunward whistler waves
initially characterized by predominantly oblique propagation with
respect to the interplanetary magnetic field. The excited waves
interact with the electrons via resonant scattering processes. As
a consequence, the strahl pitch angle distribution broadens and its
drift velocity reduces. The strahl electrons are scattered in the
direction perpendicular to the magnetic field, and an electron halo
is formed. At a later stage, resonant electron firehose instability
is triggered and further affects the electron temperature anisotropy
as the solar wind expands. Wave-particle interaction processes are
accompanied by a substantial reduction of the solar wind heat flux. The
simulated whistler waves are in qualitative agreement with observations
in terms of wave frequencies, amplitudes, and propagation angles. Our
work proposes an explanation for the observations of oblique and
parallel whistler waves in the solar wind. We conclude that solar
wind expansion has to be factored in when trying to explain kinetic
processes at different heliocentric distances.
Title: Direct evidence for magnetic reconnection at the boundaries
of magnetic switchbacks with Parker Solar Probe
Authors: Froment, C.; Krasnoselskikh, V.; Dudok de Wit, T.;
Agapitov, O.; Fargette, N.; Lavraud, B.; Larosa, A.; Kretzschmar,
M.; Jagarlamudi, V. K.; Velli, M.; Malaspina, D.; Whittlesey, P. L.;
Bale, S. D.; Case, A. W.; Goetz, K.; Kasper, J. C.; Korreck, K. E.;
Larson, D. E.; MacDowall, R. J.; Mozer, F. S.; Pulupa, M.; Revillet,
C.; Stevens, M. L.
Bibcode: 2021A&A...650A...5F
Altcode: 2021arXiv210106279F
Context. The first encounters of Parker Solar Probe (PSP) with the Sun
revealed the presence of ubiquitous localised magnetic deflections in
the inner heliosphere; these structures, often called switchbacks, are
particularly striking in solar wind streams originating from coronal
holes.
Aims: We report the direct piece of evidence for magnetic
reconnection occurring at the boundaries of three switchbacks crossed
by PSP at a distance of 45 to 48 solar radii to the Sun during its
first encounter.
Methods: We analyse the magnetic field and
plasma parameters from the FIELDS and Solar Wind Electrons Alphas and
Protons instruments.
Results: The three structures analysed all
show typical signatures of magnetic reconnection. The ion velocity
and magnetic field are first correlated and then anti-correlated at
the inbound and outbound edges of the bifurcated current sheets with
a central ion flow jet. Most of the reconnection events have a strong
guide field and moderate magnetic shear, but one current sheet shows
indications of quasi anti-parallel reconnection in conjunction with
a magnetic field magnitude decrease by 90%.
Conclusions: Given
the wealth of intense current sheets observed by PSP, reconnection at
switchback boundaries appears to be rare. However, as the switchback
boundaries accomodate currents, one can conjecture that the geometry of
these boundaries offers favourable conditions for magnetic reconnection
to occur. Such a mechanism would thus contribute in reconfiguring the
magnetic field of the switchbacks, affecting the dynamics of the solar
wind and eventually contributing to the blending of the structures
with the regular wind as they propagate away from the Sun.
Title: Proton Energization by Phase Steepening of Parallel-propagating
Alfvénic Fluctuations
Authors: González, C. A.; Tenerani, A.; Matteini, L.; Hellinger,
P.; Velli, M.
Bibcode: 2021ApJ...914L..36G
Altcode:
Proton energization at magnetic discontinuities generated by
phase-steepened fronts of parallel-propagating, large-amplitude
Alfvénic fluctuation is studied using hybrid simulations. We find
that dispersive effects lead to the collapse of the wave via phase
steepening and the subsequent generation of compressible fluctuations
that mediate an efficient local energy transfer from the wave to the
protons. Proton scattering at the steepened edges causes nonadiabatic
proton perpendicular heating. Furthermore, the parallel electric
field at the propagating fronts mediates the acceleration of protons
along the mean field. A steady-state is achieved where the proton
distribution function displays a field-aligned beam at the Alfvén
speed, and compressible fluctuations are largely damped. We discuss
the implications of our results in the context of Alfvénic solar wind.
Title: Switchbacks as signatures of magnetic flux ropes generated
by interchange reconnection in the corona
Authors: Drake, J. F.; Agapitov, O.; Swisdak, M.; Badman, S. T.; Bale,
S. D.; Horbury, T. S.; Kasper, J. C.; MacDowall, R. J.; Mozer, F. S.;
Phan, T. D.; Pulupa, M.; Szabo, A.; Velli, M.
Bibcode: 2021A&A...650A...2D
Altcode: 2020arXiv200905645D
The structure of magnetic flux ropes injected into the solar wind
during reconnection in the coronal atmosphere is explored with
particle-in-cell simulations and compared with in situ measurements
of magnetic "switchbacks" from the Parker Solar Probe. We suggest
that multi-x-line reconnection between open and closed flux in the
corona injects flux ropes into the solar wind and that these flux
ropes convect outward over long distances before eroding due to
reconnection. Simulations that explore the magnetic structure of flux
ropes in the solar wind reproduce the following key features of the
switchback observations: a rapid rotation of the radial magnetic field
into the transverse direction, which is a consequence of reconnection
with a strong guide field; and the potential to reverse the radial
field component. The potential implication of the injection of large
numbers of flux ropes in the coronal atmosphere for understanding the
generation of the solar wind is discussed.
Title: Alfvénic versus non-Alfvénic turbulence in the inner
heliosphere as observed by Parker Solar Probe
Authors: Shi, C.; Velli, M.; Panasenco, O.; Tenerani, A.; Réville, V.;
Bale, S. D.; Kasper, J.; Korreck, K.; Bonnell, J. W.; Dudok de Wit, T.;
Malaspina, D. M.; Goetz, K.; Harvey, P. R.; MacDowall, R. J.; Pulupa,
M.; Case, A. W.; Larson, D.; Verniero, J. L.; Livi, R.; Stevens, M.;
Whittlesey, P.; Maksimovic, M.; Moncuquet, M.
Bibcode: 2021A&A...650A..21S
Altcode: 2021arXiv210100830S
Context. Parker Solar Probe (PSP) measures the magnetic field
and plasma parameters of the solar wind at unprecedentedly close
distances to the Sun. These data provide great opportunities to study
the early-stage evolution of magnetohydrodynamic (MHD) turbulence
in the solar wind.
Aims: In this study, we make use of the
PSP data to explore the nature of solar wind turbulence focusing
on the Alfvénic character and power spectra of the fluctuations
and their dependence on the distance and context (i.e., large-scale
solar wind properties), aiming to understand the role that different
effects such as source properties, solar wind expansion, and stream
interaction might play in determining the turbulent state.
Methods: We carried out a statistical survey of the data from the first
five orbits of PSP with a focus on how the fluctuation properties at
the large MHD scales vary with different solar wind streams and the
distance from the Sun. A more in-depth analysis from several selected
periods is also presented.
Results: Our results show that as
fluctuations are transported outward by the solar wind, the magnetic
field spectrum steepens while the shape of the velocity spectrum
remains unchanged. The steepening process is controlled by the "age"
of the turbulence, which is determined by the wind speed together
with the radial distance. Statistically, faster solar wind has higher
"Alfvénicity," with a more dominant outward propagating wave component
and more balanced magnetic and kinetic energies. The outward wave
dominance gradually weakens with radial distance, while the excess of
magnetic energy is found to be stronger as we move closer toward the
Sun. We show that the turbulence properties can significantly vary
from stream to stream even if these streams are of a similar speed,
indicating very different origins of these streams. Especially, the
slow wind that originates near the polar coronal holes has much lower
Alfvénicity compared with the slow wind that originates from the
active regions and pseudostreamers. We show that structures such as
heliospheric current sheets and velocity shears can play an important
role in modifying the properties of the turbulence.
Title: Switchbacks: statistical properties and deviations from
Alfvénicity
Authors: Larosa, A.; Krasnoselskikh, V.; Dudok de Wit, T.; Agapitov,
O.; Froment, C.; Jagarlamudi, V. K.; Velli, M.; Bale, S. D.; Case,
A. W.; Goetz, K.; Harvey, P.; Kasper, J. C.; Korreck, K. E.; Larson,
D. E.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Revillet, C.;
Stevens, M. L.
Bibcode: 2021A&A...650A...3L
Altcode: 2020arXiv201210420L
Context. Parker Solar Probe's first solar encounter has revealed the
presence of sudden magnetic field deflections in the slow Alfvénic
solar wind. These structures, which are often called switchbacks,
are associated with proton velocity enhancements.
Aims: We
study their statistical properties with a special focus on their
boundaries.
Methods: Using data from SWEAP and FIELDS,
we investigate particle and wavefield properties. The magnetic
boundaries are analyzed with the minimum variance technique.
Results: Switchbacks are found to be Alfvénic in 73% of cases
and compressible in 27%. The correlations between magnetic field
magnitude and density fluctuations reveal the existence of both
positive and negative correlations, and the absence of perturbations
in the magnetic field magnitude. Switchbacks do not lead to a magnetic
shear in the ambient field. Their boundaries can be interpreted in
terms of rotational or tangential discontinuities. The former are more
frequent.
Conclusions: Our findings provide constraints on the
possible generation mechanisms of switchbacks, which have to be able
to also account for structures that are not purely Alfvénic. One
of the possible candidates, among others, manifesting the described
characteristics is the firehose instability.
Title: Evolution of Solar Wind Turbulence from 0.1 to 1 au during
the First Parker Solar Probe-Solar Orbiter Radial Alignment
Authors: Telloni, Daniele; Sorriso-Valvo, Luca; Woodham, Lloyd D.;
Panasenco, Olga; Velli, Marco; Carbone, Francesco; Zank, Gary P.;
Bruno, Roberto; Perrone, Denise; Nakanotani, Masaru; Shi, Chen;
D'Amicis, Raffaella; De Marco, Rossana; Jagarlamudi, Vamsee K.;
Steinvall, Konrad; Marino, Raffaele; Adhikari, Laxman; Zhao, Lingling;
Liang, Haoming; Tenerani, Anna; Laker, Ronan; Horbury, Timothy S.;
Bale, Stuart D.; Pulupa, Marc; Malaspina, David M.; MacDowall,
Robert J.; Goetz, Keith; de Wit, Thierry Dudok; Harvey, Peter R.;
Kasper, Justin C.; Korreck, Kelly E.; Larson, Davin; Case, Anthony
W.; Stevens, Michael L.; Whittlesey, Phyllis; Livi, Roberto; Owen,
Christopher J.; Livi, Stefano; Louarn, Philippe; Antonucci, Ester;
Romoli, Marco; O'Brien, Helen; Evans, Vincent; Angelini, Virginia
Bibcode: 2021ApJ...912L..21T
Altcode:
The first radial alignment between Parker Solar Probe and Solar Orbiter
spacecraft is used to investigate the evolution of solar wind turbulence
in the inner heliosphere. Assuming ballistic propagation, two 1.5 hr
intervals are tentatively identified as providing measurements of the
same plasma parcels traveling from 0.1 to 1 au. Using magnetic field
measurements from both spacecraft, the properties of turbulence
in the two intervals are assessed. Magnetic spectral density,
flatness, and high-order moment scaling laws are calculated. The
Hilbert-Huang transform is additionally used to mitigate short sample
and poor stationarity effects. Results show that the plasma evolves
from a highly Alfvénic, less-developed turbulence state near the
Sun, to fully developed and intermittent turbulence at 1 au. These
observations provide strong evidence for the radial evolution of solar
wind turbulence.
Title: Tearing instability inside a 2D current sheet with a normal
magnetic field
Authors: Shi, Chen; Artemyev, Anton; Velli, Marco; Tenerani, Anna
Bibcode: 2021EGUGA..2313282S
Altcode:
Magnetic reconnection converts the magnetic field energy into thermal
and kinetic energies of the plasma. This process usually happens at
extremely fast speed and is therefore believed to be a fundamental
mechanism to explain various explosive phenomena such as coronal mass
ejections and planetary magnetospheric storms. How magnetic reconnection
is triggered from the large magnetohydrodynamic (MHD) scales remains
an open question, with some theoretical and numerical studies showing
the tearing instability to be involved. Observations in the Earth"s
magnetotail and near the magnetopause show that a finite normal magnetic
field is usually present inside the reconnecting current sheet. Besides,
such a normal field may also exist in the solar corona. However, how
this normal magnetic field modifies the tearing instability is not
thoroughly studied. Here we discuss the linear tearing instability
inside a two-dimensional current sheet with a normal component of
magnetic field where the magnetic tension force is balanced by ion
flows parallel and anti-parallel to the magnetic field. We solve the
dispersion relation of the tearing mode with wave vector parallel to
the reconnecting magnetic field. Our results confirm that the finite
normal magnetic field stabilizes the tearing mode and makes the mode
oscillatory instead of purely growing.
Title: Alfvénic versus non-Alfvénic turbulence in the inner
heliosphere as observed by Parker Solar Probe
Authors: Velli, Marco; Shi, Chen; Panasenco, Olga; Tenerani, Anna;
Reville, Victor; the PSP* Team
Bibcode: 2021EGUGA..2312876V
Altcode:
Parker Solar Probe (PSP) measures the magnetic field and plasma
parameters of the solar wind at unprecedentedly close distances to the
Sun, providing a great opportunity to study the early-stage evolution
of magnetohydrodynamic (MHD) turbulence in the solar wind. Here we use
PSP data to explore the nature of solar wind turbulence focusing on the
Alfvénic character and power spectra of the fluctuations and their
dependence on heliocentric distance and context (i.e., large-scale
solar wind properties), aiming to understand the role that different
effects such as source properties, solar wind expansion, and stream
interaction might play in determining the turbulent state. We carried
out a statistical survey of the data from the first five orbits of
PSP with a focus on how the fluctuation properties at the large MHD
scales vary with different solar wind streams and the distance from
the Sun. A more in-depth analysis from several selected periods is
also presented. Our results show that as fluctuations are transported
outward by the solar wind, the magnetic field spectrum steepens while
the shape of the velocity spectrum remains unchanged. The steepening
process is controlled by the age of the turbulence, which is determined
by the wind speed together with the radial distance. Statistically,
faster solar wind has higher Alfvénicity with a more dominant
outward propagating wave component and more balanced magnetic and
kinetic energies. The outward wave dominance gradually weakens with
radial distance, while the excess of magnetic energy is found to be
stronger as we move closer toward the Sun. We show that the turbulence
properties can significantly vary from stream to stream even if these
streams are of a similar speed, indicating very different origins of
these streams. Especially, the slow wind that originates near the polar
coronal holes has much lower Alfvénicity compared with the slow wind
that originates from the active regions and pseudostreamers. We show
that structures such as the heliospheric current sheet and wind stream
velocity shears can play an important role in modifying the properties
of the turbulence.*The PSP Team: Stuart D.Bale, Justin Kasper, Kelly
Korreck, J. W. Bonnell, Thierry Dudok de Wit, Keith Goetz, Peter
R. Harvey, Robert J. MacDowall, David Malaspina, Marc Pulupa, Anthony
W.Case, Davin Larson, Jenny Verniero, Roberto Livi, Michael Stevens,
PhyllisWhittlesey, Milan Maksimovic, and Michel Moncuquet
Title: Theory and observations of switchbacks' evolution in the
solar wind
Authors: Tenerani, Anna; Velli, Marco; Matteini, Lorenzo
Bibcode: 2021EGUGA..2313400T
Altcode:
Alfvénic fluctuations represent the dominant contributions to turbulent
fluctuations in the solar wind, especially, but not limited to, the
fastest streams with velocity of the order of 600-700 km/s. Alfvénic
fluctuations can contribute to solar wind heating and acceleration
via wave pressure and turbulent heating. Observations show that such
fluctuations are characterized by a nearly constant magnetic field
amplitude, a condition which remains largely to be understood and
that may be an indication of how fluctuations evolve and relax in
the expanding solar wind. Interestingly, measurements from Parker
Solar Probe have shown the ubiquitous and persistent presence of
the so-called switchbacks. These are magnetic field lines which are
strongly perturbed to the point that they produce local inversions of
the radial magnetic field. The corresponding signature of switchbacks in
the velocity field is that of local enhancements in the radial speed (or
jets) that display the typical velocity-magnetic field correlation that
characterizes Alfvén waves propagating away from the Sun. While there
is not yet a general consensus on what is the origin of switchbacks
and their connection to coronal activity, a first necessary step to
answer these important questions is to understand how they evolve and
how long they can persist in the solar wind. Here we investigate the
evolution of switchbacks. We address how their evolution is affected
by parametric instabilities and the possible role of expansion, by
comparing models with the observed radial evolution of the fluctuations"
amplitude. We finally discuss what are the implications of our results
for models of switchback generation and related open questions.
Title: Magnetic Reconnection in the Corona as a Source of Switchbacks
in the Solar Wind
Authors: Drake, James; Agapitov, Oleksiy; Swisdak, Marc; Badman, Sam;
Bale, Stuart; Horbury, Timothy; Kasper, Justin; MacDowal, Robert;
Mozer, Forrest; Phan, Tai; Pulupa, Marc; Szabo, Adam; Velli, Marco
Bibcode: 2021EGUGA..23.2865D
Altcode:
The observations from the Parker Solar Probe during the firstperihelion
revealed large numbers of local reversals in the radialcomponent of the
magnetic field with associated velocity spikes. Sincethe spacecraft was
magnetically connected to a coronal hole during theclosest approach to
the sun, one possible source of these spikes ismagnetic reconnection
between the open field lines in the coronal holeand an adjacent region
of closed flux. Reconnection in a low betaenvironment characteristic of
the corona is expected to be burstyrather than steady and is therefore
capable of producing large numbersof magnetic flux ropes with local
reversals of the radial magneticfield that can propagate outward
large radial distances from thesun. Flux ropes with a strong guide
field produce signaturesconsistent with the PSP observations. We have
carried out simulationsof "interchange" reconnection in the corona
and have explored thelocal structure of flux ropes embedded within
the expanding solarwind. We have first established that traditional
interchangereconnection cannot produce the switchbacks since bent field
linesgenerated in the corona quickly straighten. The simulations have
beenextended to the regime dominated by the production of multiple
fluxropes and we have established that flux ropes are injected into
thelocal solar wind. Local simulations of reconnection are also
beingcarried out to explore the structure of flux ropes embedded
in thesolar wind for comparison with observations. Evidence is
presentedthat flux rope merging may be ongoing and might lead to
the highaspect ratio of the switchback structures measured in the
solar wind.
Title: Energetics and 3D Structure of Elementary Events in Solar
Coronal Heating
Authors: Einaudi, G.; Dahlburg, R. B.; Ugarte-Urra, I.; Reep, J. W.;
Rappazzo, A. F.; Velli, M.
Bibcode: 2021ApJ...910...84E
Altcode: 2021arXiv210313499E
Parker first proposed (1972) that coronal heating was the necessary
outcome of an energy flux caused by the tangling of coronal magnetic
field lines by photospheric flows. In this paper we discuss how
this model has been modified by subsequent numerical simulations
outlining in particular the substantial differences between the
"nanoflares" introduced by Parker and "elementary events," defined
here as small-scale spatially and temporally isolated heating
events resulting from the continuous formation and dissipation
of field-aligned current sheets within a coronal loop. We present
numerical simulations of the compressible 3D MHD equations using the
HYPERION code. We use two clustering algorithms to investigate the
properties of the simulated elementary events: an IDL implementation of
a density-based spatial clustering of applications with noise technique,
and our own physical distance clustering algorithm. We identify and
track elementary heating events in time, both in temperature and in
Joule heating space. For every event we characterize properties such
as density, temperature, volume, aspect ratio, length, thickness,
duration, and energy. The energies of the events are in the range
of 1018-1021 erg, with durations shorter
than 100 s. A few events last up to 200 s and release energies
up to 1023 erg. While high temperatures are typically
located at the flux tube apex, the currents extend all the way to
the footpoints. Hence, a single elementary event cannot at present
be detected. The observed emission is due to the superposition of
many elementary events distributed randomly in space and time within
the loop.
Title: The solar wind angular-momentum flux observed during Solar
Orbiter's first orbit
Authors: Verscharen, Daniel; Stansby, David; Finley, Adam; Owen,
Christopher; Horbury, Timothy; Velli, Marco; Bale, Stuart; Louarn,
Philippe; Fedorov, Andrei; Bruno, Roberto; Livi, Stefano; Lewis,
Gethyn; Anekallu, Chandrasekhar; Kelly, Christopher; Watson, Gillian;
Kataria, Dhiren; O'Brien, Helen; Evans, Vincent; Angelini, Virginia
Bibcode: 2021EGUGA..23.6306V
Altcode:
The Solar Orbiter mission is currently in its cruise phase, during which
the spacecraft's in-situ instrumentation measures the solar wind and
the electromagnetic fields at different heliocentric distances. We
evaluate the solar wind angular-momentum flux by combining proton
data from the Solar Wind Analyser (SWA) Proton-Alpha Sensor (PAS)
and magnetic-field data from the Magnetometer (MAG) instruments on
board Solar Orbiter during its first orbit. This allows us to evaluate
the angular momentum in the protons in addition to that stored in
magnetic-field stresses, and compare these to previous observations
from other spacecraft. We discuss the statistical properties of the
angular-momentum flux and its dependence on solar-wind properties. Our
results largely agree with previous measurements of the solar wind"s
angular-momentum flux in the inner heliosphere and demonstrate the
potential for future detailed studies of large-scale properties of
the solar wind with the data from Solar Orbiter.
Title: On Alfvénic Slow Wind: A Journey From the Earth Back to
the Sun
Authors: D'Amicis, R.; Perrone, D.; Bruno, R.; Velli, M.
Bibcode: 2021JGRA..12628996D
Altcode:
Comparative studies of fast and slow solar wind streams performed
over the past decades have illustrated several differences between the
plasma regimes for these different flows, examples including features
such as temperatures, particle distribution function anisotropies, and
the nature of the embedded turbulence, specifically the Alfvénicity
of the fluctuations. Though this two state classification of the
solar wind primarily based on flow speed has been widely adopted,
more in depth studies have found that slow solar wind should be further
categorized, flow speed not being a sufficient descriptor of the plasma
state. Within this framework, slow solar wind streams with a strong
Alfvénic character have been identified and characterized, showing
that in many ways they resemble fast solar wind. The similarities
between fast and slow Alfvénic wind regimes have been explained
in terms of a similar solar origin, with the latter corresponding
to slow winds emanating from rapidly diverging low latitude small
coronal holes. The aim of this review is to describe the state of
art of our understanding of Alfvénic slow solar wind streams. The
results presented cover observations performed at different heliocentric
distances spanning from Wind at L1 to Helios and Parker Solar Probe in
the inner heliosphere, as well as a discussion of their source regions.
Title: A two-step role for plasma expansion in solar wind heat
flux regulation
Authors: Innocenti, Maria Elena; Boella, Elisabetta; Tenerani, Anna;
Velli, Marco
Bibcode: 2021EGUGA..23.6439I
Altcode:
Already several decades ago, it was suggested that kinetic instabilities
play a fundamental role in heat flux regulation at relatively large
distances from the Sun, R> 1 AU [Scime et al, 1994]. Now, Parker
Solar Probe observations have established that this is the case also
closer to it [Halekas et al, 2020].Electron scale instabilities in
the solar wind are driven and affected in their evolution by the
slow, large scale process of solar wind expansion, as demonstrated
observationally [Stverak et al, 2008; Bercic et al, 2020], and via
fully kinetic Expanding Box Model simulations [Innocenti et al,
2019b].Now, connecting the dots, we examine an indirect role of
plasma expansion in heat flux regulation in the solar wind. We show,
as a proof of principle, that plasma expansion can modify heat flux
evolution as a function of heliocentric distance, with respect to what
is expected within an adiabatic framework, due to the onset of kinetic
instabilities, in this case, an oblique firehose instability developing
self consistently in the presence of a core and suprathermal electron
population [Innocenti et al, 2020].This result highlights, once again,
the deeply multi scale nature of the heliospheric environment, that
calls for advanced simulation techniques. In this work, the simulations
are done with the fully kinetic, semi-implicit [Markidis et al, 2010],
Expanding Box Model [Velli et al, 1992] code EB-iPic3D [Innocenti et
al, 2019a].
Title: Proton energization by phase-steepening of parallel propagating
Alfvénic fluctuations
Authors: González, C. A.; Tenerani, A.; Matteini, L.; Hellinger,
P.; Velli, M.
Bibcode: 2021arXiv210402540G
Altcode:
Proton energization at magnetic discontinuities generated by
phase-steepened fronts of parallel propagating, large-amplitude
Alfvénic fluctuation is studied using hybrid simulations. We
find that dispersive effects yield to the collapse of the wave
via phase steepening and the subsequent generation of compressible
fluctuations that mediate an efficient local energy transfer from the
wave to the protons. Proton scattering at the steepened edges causes
non-adiabatic proton perpendicular heating. Furthermore, the parallel
electric field at the propagating fronts mediates the acceleration of
protons along the mean field. A steady-state is achieved where proton
distribution function displays a field-aligned beam at the Alfvén
speed, and compressible fluctuations are largely damped. We discuss
the implications of our results in the context of Alfvénic solar wind.
Title: Investigating the origin of the FIP effect with a shell
turbulence model
Authors: Réville, Victor; Rouillard, Alexis P.; Velli, Marco; Verdini,
Andrea; Buchlin, Éric; Lavarra, Michael; Poirier, Nicolas
Bibcode: 2021FrASS...8....2R
Altcode: 2021arXiv210101440R
The enrichment of coronal loops and the slow solar wind with elements
that have low First Ionization Potential, known as the FIP effect,
has often been interpreted as the tracer of a common origin. A current
explanation for this FIP fractionation rests on the influence of
ponderomotive forces and turbulent mixing acting at the top of the
chromosphere. The implied wave transport and turbulence mechanisms are
also key to wave-driven coronal heating and solar wind acceleration
models. This work makes use of a shell turbulence model run on open
and closed magnetic field lines of the solar corona to investigate
with a unified approach the influence of magnetic topology, turbulence
amplitude and dissipation on the FIP fractionation. We try in particular
to assess whether there is a clear distinction between the FIP effect
on closed and open field regions.
Title: Radial evolution of switchbacks in the inner heliosphere:
observations from PSP to Ulysses
Authors: Tenerani, Anna; Sioulas, Nikos; Matteini, Lorenzo; Panasenco,
Olga; Shi, Chen; Velli, Marco
Bibcode: 2021APS..DPPTO6002T
Altcode:
Measurements from Parker Solar Probe have shown the ubiquitous presence
of the so-called switchbacks. These are magnetic field lines which are
strongly perturbed to the point that they lead to local inversions
of the radial magnetic field. The corresponding signature in the
velocity field is that of a local radial speed jet displaying the
well-known velocity/magnetic field correlation that characterizes
Alfvén waves propagating away from the Sun. While there is not yet a
general consensus on the origins of switchbacks and their connection
to coronal activity, a first necessary step is to understand how
they evolve and how long they can propagate undisturbed in the solar
wind. Characterizing the dynamical evolution of switchbacks in the
solar wind can help us determine whether they are generated in-situ or
not, and whether they contribute to the turbulent cascade by evolving
nonlinearly. In this work, we have analyzed magnetic field data from the
first six encounters of Parker Solar Probe, three fast streams observed
by Helios 1 and 2, and two Ulysses south polar passes, covering the
range of heliocentric distances 0.1 < R < 3 au. We have compared
the radial evolution of the magnetic energy density of switchbacks with
that of the overall turbulent fluctuations, and we have characterized
the radial evolution of the occurrence rate of switchbacks as a function
of their duration. Our results show that switchbacks both decay and
reform in-situ in the inner heliosphere, in-situ generation being more
efficient at the larger scales. Our results confirm that switchbacks
can be generated in the inner heliosphere by the expansion, although
other types of switchbacks, generated closer to the sun, cannot be ruled
out. This research was supported by NASA Grant #80NSS-C18K1211.
Title: Tearing instability and periodic density perturbations in
the slow solar wind
Authors: Reville, Victor; Lavraud, Benoit; Rouillard, Alexis; Velli,
Marco; Tenerani, Anna; Shi, Chen
Bibcode: 2021cosp...43E1745R
Altcode:
In sharp contrast with the fast solar wind, which is thought to
be coming from coronal holes, the origin of the slow wind is still
intensely debated. Intermittent by nature and enriched with low FIP
elements -akin what is observed in closed coronal loops- the slow
wind is thought by many to be born in bursty events at the open/close
boundary of coronal streamers. The slow wind also shows large density
perturbations, which have been shown to be periodic. These density
perturbations could be associated with flux ropes ejected from the
tip of helmet streamers, as shown recently by the WISPR white light
imager onboard Parker Solar Probe. Helmet streamers are indeed likely
unstable and very dynamic. In this work, we study the possibility
that the main process controlling the periodic release of flux ropes
from streamers is a tearing mode. We use MHD simulations of the solar
wind and corona to reproduce realistic configurations and outflows
surrounding the heliospheric current sheet. The reconnection process,
and in particular the fastest growing tearing mode is characterized at
low Lundquist number and we use linear theory to extrapolate to the
so-called ideal regime, and compare with observations. If confirmed,
this process could then explain both the origin of the periodic density
perturbations and the composition of (part of?) the slow solar wind.
Title: The solar wind observed over the first orbits by Parker Solar
Probe : new insights into the origin of the heliosphere
Authors: Velli, Marco; Panasenco, Olga; Tenerani, Anna; Shi, Chen
Bibcode: 2021cosp...43E.932V
Altcode:
Since the launch of Parker Solar Probe (PSP) in 2018, a new window
has opened into understanding the inner heliosphere.The first
Probe encounters, with a perihelion at 35.6 Solar Radii (Rs) from
Sun-center illustrated the complexity of the mapping of the magnetic
field at the Sun even into the inner heliosphere. In Encounter (E)
1, Probe connected to a small, overexpanding coronal hole, and the
resulting slow solar wind flow was dominated by highly Alfvénic
fluctuations, including local radial magnetic field inversions
called switchbacks. Recent Encounters E4 and E5, with perihelia at
a distance of 27.8 Rs, show the importance of the mixing of spatial
and intrinsically time-dependent behavior. Here we describe the
general features of the solar wind seen by PSP in orbits 4 and 5,
with specific emphasis on the polarity of the field, the properties of
the fluctuations observed, and their association with the regions of
origin of the wind and with intrinsically time-dependent processes
at the source. We use the Potential Field Source-Surface (PFSS)
model of De Rosa and Schrijver, based on SDO/HMI magnetogram data in
conjunction photospheric transport, to extrapolate the field from the
solar surface out to an appropriate source surface, and then images
from STEREO, LASCO and SDO/AIA to compare the results with the magnetic
field and plasma seen by Probe. In situ measurements are then used
to compute plasma and turbulence properties, such as Alfvénicity,
and determine the nature of the discontinuities separating different
types of solar wind flows in situ. Probe in E4 and E5 remained very
close to the heliospheric current sheet, and traversed structures
such as pseudostreamer stalks as well as the heliospheric current
sheet itself. It observed both strongly Alfvénic wind and wind with
less clear Alfvénic character. When compared to the first encounter,
the solar wind conditions seen by Probe at the most recent E4 and E5 is
more typical of the wind seen in the ecliptic in periods of increasing
solar activity. Switchbacks are confirmed to be an intrinsic feature
of the nascent solar wind everywhere except above helmet streamers. To
conclude we will discuss how new PSP measurements change our views of
heliospheric magnetic field expansion and solar wind acceleration.
Title: Kinetic physics in the solar wind: local processes and global
consequences
Authors: Innocenti, Maria Elena; Boella, Elisabetta; Tenerani, Anna;
Micera, Alfredo; Velli, Marco
Bibcode: 2021APS..DPPGI1005I
Altcode:
Parker Solar Probe and Solar Orbiter observations have confirmed
that kinetic scale processes are ubiquitous in the solar wind. The
spatial and temporal scales of kinetic instabilities are smaller and
shorter than system scales by several orders of magnitudes. However,
they contribute to shape large-scale solar wind dynamics. Recent PSP
observations [e.g., Cattell et al, 2021; Jagarlamudi et al, 2021] have
focused on whistler waves generated by whistler-type instabilities,
and on their role in scattering electrons from the strahl to the halo
and in heat flux regulation. The contribution of collisionless kinetic
instabilities in heat flux regulation is supported by simulations
[e.g., only in the last two years, Kuzichev et al, 2019; Lopez et al,
2019 & 2020; Vasko et al, 2019, Verscharen et al, 2019; Innocenti
et al, 2020; Micera et al, 2020] and observations, even quite close to
the Sun [Halekas et al, 2020]. Given the role of heat flux in the solar
wind energy balance, one could argue that, through heat flux regulation,
kinetic processes significantly affect global heliospheric dynamics. It
is well known from simulations and observations that solar wind plasma
expansion influences the onset and evolution of a number of kinetic
instabilities, at the ion [Hellinger et al, 2003, 2008, 2013; Matteini
et al, 2006] and electron [Innocenti et al, 2019b] scale. In this talk,
we will review the role of kinetic physics in large scale heliospheric
dynamics. We will focus in particular on the modeling of small-scale,
fast kinetic processes against the backdrop of (slow, large scale)
solar wind plasma expansion. With the support of simulations performed
with the fully kinetic, Expanding Box Model code EB-iPic3D [Innocenti et
al, 2019a], we will then show how solar wind expansion can indirectly
contribute to heat flux regulation by affecting the evolution of heat
flux regulating instabilities [Innocenti at al, 2020; Micera et al,
accepted]. The simulations were performed on the supercomputer
Marconi- Broadwell (Cineca, Italy) under a PRACE allocation. This
research was supported in part by the NASA DRIVE HERMES project,
Grant No. 80NSSC20K0604.
Title: Heliocentric Distance Variation of Interplanetary Field
Enhancements
Authors: Russell, C. T.; Wei, H.; Lai, H.; Horbury, T. S.; Velli,
M. C. M.; O'Brien, H.; Evans, V.; Angelini, V.
Bibcode: 2020AGUFMSH0440027R
Altcode:
Interplanetary Field Enhancement is the name given to a randomly
occurring strengthening and rotation of the interplanetary magnetic
field. The characteristic signature of the IFE is a rapid change
in the strength of the magnetic field to a sharp maximum at which
point there is a sudden change in direction of the field. The source
of these enhancements has been hypothesized to be the mass-loading
of the solar wind associated with collisions of meteors, producing
a dust cloud that charges in the solar UV. This charged dust cloud
is accelerated to the solar wind speed, at which point the magnetic
pressure signature disappears, leaving a twisted magnetic field as the
only remaining signature of the interaction. In this paper, we examine
how the signature changes with decreasing heliocentric distance and the
collisional speed of the impacting meteors increases with proximity
to the Sun. Our largest databases are from 1 AU spacecraft such as
IMP8, Wind, and STEREO, and Pioneer Venus at 0.72 AU. We compare these
records with the more sparse records of missions closer to the Sun,
such as Helios and Messenger, as well as Solar Orbiter and Parker
Solar Probe, as those data become available.
Title: Hybrid simulations of large-amplitude Alfvénic fluctuations:
the role of parametric instabilities in proton heating and
acceleration
Authors: Gonzalez, C.; Tenerani, A.; Velli, M. C. M.; Hellinger, P.
Bibcode: 2020AGUFMSH0290025G
Altcode:
The solar wind is observed to display many non-thermal features such as
a preferential perpendicular heating and a field-aligned proton beam
population that coexist with a spectrum of large-amplitude Alfvénic
fluctuations. While it has become clear that turbulent fluctuations can
contribute to the thermodynamic evolution of the solar wind, it remains
to understand what is the origin of the persistent field-aligned beam
and what mechanisms allow for the observed non-adiabatic expansion of
the solar wind. It is known that large-amplitude Alfvénic fluctuations
tend to be unstable to parametric instabilities, which result in
a decay process of the initial wave into different daughter waves
depending upon the amplitude of the pump wave and the plasma beta. Here
we revisit this problem by means of multidimensional hybrid simulations
and investigate the stability of Alfvénic fluctuations, the saturation
mechanisms of the decay process(es), and the final nonlinear state
reached for different pump wave amplitudes and plasma beta values. We find that the decay process in multi-dimensions persists at
large values of the plasma beta via the filamentation/magnetosonic
decay instabilities that lead to a nonlinear state characterized by a
turbulent, heated plasma displaying a field-aligned beam at the Alfvén
speed. By adopting a test-particle approach, we discuss the resulting
anisotropic proton heating and particle acceleration with an emphasis
on the importance of discontinuities produced by wave steepening in
accelerating particles at the Alfvén speed.
Title: Italian Solar Orbiter-SWA Working Group on "Multiscale Physics"
Authors: D'Amicis, R.; Alberti, T.; Bruno, R.; Califano, F.; Carnevale,
G.; Catapano, F.; Cerri, S. S.; Coco, I.; Del Zanna, L.; De Marco, R.;
Di Matteo, S.; Franci, L.; Greco, A.; Jagarlamudi, V. K.; Landi, S.;
Lepreti, F.; Malara, F.; Marcucci, M. F.; Marino, R.; Matteini, L.;
Nieves-Chinchilla, T.; Nigro, G.; Nisticò, G.; Papini, E.; Pecora, F.;
Perri, S.; Pezzi, O.; Perrone, D.; Primavera, L.; Qamili, E.; Retino,
A.; Servidio, S.; Sorriso-Valvo, L.; Innocenti, M. E.; Telloni, D.;
Tenerani, A.; Trenchi, L.; Valentini, F.; Velli, M. C. M.; Veltri,
P.; Verdini, A.; Villante, U.; Zimbardo, G.
Bibcode: 2020AGUFMSH0360016D
Altcode:
Despite more than a half-century of study, the basic physical processes
responsible for heating and accelerating the solar wind are still not
fully understood. These phenomena are at the center of a hot debate
that is of great interest for the Solar Orbiter mission (as discussed
in details in the Science Activity Plan, SAP) and are strictly linked
to the turbulent nature of solar wind fluctuations which cover an
extended range of spatial and temporal scales. So the identification
of these physical processes is of primary importance for understanding
the origins and evolution of the solar wind and its impact on the
different bodies of the solar system. Moreover, in a broader context,
it would allow also to achieve significant progress in our understanding
of stellar astrophysics. Within this context, the Italian Solar
Orbiter-SWA Working Group (WGs) on `Multiscale Physics' was created in
response to the interest manifested by scientists from several Italian
and international institutions on some important topics such as radial
evolution of turbulence and Alfvénicity and link between fluid and
kinetic scales; solar wind origin; reconnection, intermittency and
particle acceleration in the turbulent solar wind (just to cite some
of them), with a particular attention to the synergies with other
in-situ and remote sensing instruments on board Solar Orbiter and also
with other ESA and NASA missions (e.g. L1 observatories and Parker
Solar Probe). The `Multiscale Physics' WG involves scientists with an
extensive experience in solar wind turbulence and reconnection processes
including expertise in data analysis, simulations and modeling. In this contribution, we present the activity developed so far with
a particular focus on the scientific cases identified.
Title: A new role for solar wind plasma expansion in heat flux
regulation
Authors: Innocenti, M. E.; Boella, E.; Tenerani, A.; Velli, M. C. M.
Bibcode: 2020AGUFMSH033..06I
Altcode:
Ulysses observations showed that neither collisional processes nor solar
wind expansion, alone, can explain the observed heat flux evolution
with heliocentric distance [Scime et al, 1994]: collisionless processes,
namely heat-flux instabilities, must contribute to heat flux regulation
in the solar wind [Gary et al, 1975]. In previous work, we have
introduced the fully kinetic, semi-implicit code EB-iPic3D [Innocenti et
al, 2019a]. EB-iPic3D introduced the Expanding Box Model, EBM [Velli et
al, 1992], into the semi-implicit, fully kinetic code iPic3D [Markidis
et al, 2010], for self-consistent simulations of fully kinetic dynamics
at scales intermediate between the electron and the ion characteristic
scales. Then, we have shown that solar wind expansion can trigger
the onset of kinetic instabilities [Innocenti et al, 2019b]. Now,
we examine the role of plasma expansion in regulating heat flux. We
demonstrate through fully kinetic EBM simulations that this role is
indirect: expansion affects heat flux by triggering, or modifying the
evolution, of kinetic instabilities, which in turn regulate the heat
flux [Innocenti et al, 2020, in press]. Heat flux instabilities
affect the electron Velocity Distribution Function (eVDF) in several
ways: they reduce the drift velocity between the electron species,
alter the skewness of the eVDF, scatter one population into another. We
show that, at least in our simulations, the first process chiefly
determines heat flux evolution. Our work intends to provide an
interpretation framework for coordinated Parker Solar Probe/ Solar
Orbiter/ Earth observations of magnetically connected plasma parcels
at large heliocentric distances.
Title: FIELDS Closer to the Sun: New Insights on the Origins and
Evolution of the Solar Wind.
Authors: Velli, M. C. M.; Bale, S. D.; Goetz, K.; Harvey, P.; Pulupa,
M.; Bowen, T.; Dudok de Wit, T.; Krasnoselskikh, V.; MacDowall, R. J.;
Badman, S. T.; Phan, T.; Moncuquet, M.; Maksimovic, M.; Horbury,
T. S.; Malaspina, D.
Bibcode: 2020AGUFMSH052..03V
Altcode:
We present and discuss the observations carried out by the FIELDS
instrument suite on Parker Solar Probe in the newly explored regions
extending inside 30 Solar Radii from Sun center at the dawn of the
new solar cycle. We will illustrate the main features of the magnetic
and electric fields observed, their source regions on the Sun, and
the nature and characteristics of the turbulent fluctuations as a
function of the plasma parameters. We will also describe the radio
bursts observed and assess how the FIELDS measurements so close to our
star alter our picture of coronal heating and solar wind acceleration,
solar wind origins, and the generation and acceleration of energetic
particles from the Sun.
Title: Observed Turbulent Properties of Spherically Polarized
Alfvénic States
Authors: Bowen, T.; Badman, S. T.; Bale, S. D.; Chandran, B. D. G.;
Chen, C. H. K.; Dudok de Wit, T.; Horbury, T. S.; Kasper, J. C.;
Klein, K. G.; Larson, D.; Mallet, A.; Matteini, L.; McManus, M.;
Tenerani, A.; Velli, M. C. M.; Verniero, J.
Bibcode: 2020AGUFMSH0490012B
Altcode:
Observation of large scale Alfvénic "switchbacks" of the
inner-heliospheric magnetic field, made by Parker Solar Probe, leads
to renewed interest in the nonlinear dynamics of constant magnitude
and spherically polarized states in MHD. Analysis of high cross
helicity streams studied by PSP shows that the turbulent fluctuations
are largely consistent with outward propagating spherically polarized
Alfvén waves. It is well known that such waves are an exact solution
to the MHD equations. In this work, we explore the turbulent scaling
properties of outward propagating spherically polarized Alfvén waves
using an increment analysis of rotation angles of the dominant Elsasser
mode. We show the connection between spherical symmetry and the outer
scale (1/f) turbulent spectrum. The effects of spherical polarization
on the sub-dominant (inward propagating) Alfvén waves, compressive
fluctuations are discussed, as well as implications for the generation
of solar wind turbulence and magnetic switchbacks.
Title: MHD Turbulence in the Solar Wind: Observations from First
Five Encounters of Parker Solar Probe
Authors: Shi, C.; Velli, M. C. M.; Panasenco, O.; Tenerani, A.;
Halekas, J. S.; Stevens, M. L.; Whittlesey, P. L.; Livi, R.; Bowen,
T. A.; Bale, S. D.
Bibcode: 2020AGUFMSH033..05S
Altcode:
Parker Solar Probe (PSP) has finished its first five orbits, reaching
~28 solar radii to the Sun, much lower than any previous spacecraft. The
magnetic field and plasma data collected by PSP provide us with great
opportunities to study the properties and evolution of turbulence in
the young solar wind. Here, we present a statistical analysis of the
PSP data from its first five orbits. We focus on the question that how
the MHD turbulence properties vary with different solar wind streams,
i.e. fast and slow streams. Our results show that, although the plasma
properties, e.g. ion temperature and compressibility, vary significantly
with the solar wind speed, the turbulence properties do not have a
strong wind-speed dependence. The observed faster radial steepening
of magnetic field power spectrum in the slow wind indicates that the
"age" of the turbulence, determined by the wind speed together with the
radial distance, controls the turbulence properties. We observe that as
we get closer to the Sun, the spectral slopes of the magnetic field and
velocity tend to converge to a value ~1.5 and the residual energy rises
from negative values toward 0. This result confirms that the observed
asymmetry between kinetic and magnetic energies and power spectra
beyond 0.3 AU is a result of dynamic evolution of the turbulence.
Title: Sources and Evolution of the Solar Wind Seen by Parker
Solar Probe
Authors: Panasenco, O.; Velli, M. C. M.; Shi, C.; Tenerani, A.;
Réville, V.; Badman, S. T.; Bale, S. D.; D'Amicis, R.; Goetz, K.;
Harvey, P.; Korreck, K. E.; Larson, D. E.; MacDowall, R. J.; Pulupa,
M.; Halekas, J. S.; Stevens, M. B.; Livi, R.; Whittlesey, P. L.
Bibcode: 2020AGUFMSH0290026P
Altcode:
Parker Solar Probe (PSP) has made a number of important discoveries in
its exploration of the inner heliosphere/outer corona inside 36 Rs. Its
observation of ubiquitous large amplitude Alfvénic fluctuations,
regardless of solar wind speed, in all wind streams except for narrow
areas surrounding the heliospheric current sheet, together with large
s-shaped inversions of the magnetic field, called switchbacks, begin to
call into questions standard ideas of solar wind acceleration. In this
presentation we use a wealth of remote sensing and in-situ measurements
to pinpoint the sources of the wind observed by PSP. We then discuss
the origin and evolution of so-called slow Alfvénic wind, the origin
of switchbacks, and the role of magnetic reconnection in the formation
of the solar wind.
Title: Solar wind Alfvénic turbulence: overcoming an old paradigm
Authors: D'Amicis, R.; Bruno, R.; Matteini, L.; Perrone, D.; Velli,
M. C. M.; Telloni, D.; Panasenco, O.
Bibcode: 2020AGUFMSH033..01D
Altcode:
Despite many decades of studies, solar wind turbulence remains an open,
unsolved problem in space plasma physics. The solar wind turbulent
behavior is in many instances dominated by the nonlinear interaction
between inward and outward propagating Alfvén waves, especially
so-called Alfvénic turbulence, that displays a high degree of v-b
correlations (and almost constant number density and magnetic field
magnitude). Also, Alfvénic turbulence is especially prominent in fast
solar wind streams. Such characteristics have strong implications
for spectral features and has motivated the turbulent community to
take a particular care of data selection, separating the streams
according to their speed. However, recent results have shown that the
slow-fast dichotomy should be overcome. Indeed, it has been found that
even slow wind can be sometimes characterized by highly Alfvénic and
high-amplitude fluctuations similar to that of the fast wind. Although
the first observation of this kind of wind dates back to Helios data at
the perihelion passage, L1 measurements during solar maximum revealed,
quite unexpectedly, a statistically significant occurrence of this
kind of wind. On the other hand, recent observations by Parker
Solar Probe (PSP) show the occurrence of Alfvénic slow wind at all
the perihelion passages, putting this topic in the spotlight. In this
talk, we review the main characteristics of the Alfvénic slow wind
from L1 back to PSP closest approach, with a particular focus on the
comparison with the fast wind and the standard slow wind. The several
similarities between the two Alfvénic winds (fast and slow) suggest
a similar origin, with the slow one coming from a low latitude small
coronal hole, with a major role attributed to the super-radial expansion
responsible for the lower velocity of the slow wind. This interpretation
was confirmed by PSP observations. The upcoming Solar Orbiter data will
be of further support in characterizing this kind of slow wind and in
following the radial evolution of Alfvénicity in the inner heliosphere.
Title: Constraining Global Coronal Models with Multiple Independent
Observables
Authors: Badman, S. T.; Brooks, D.; Petrie, G. J. D.; Poirier, N.;
Warren, H.; Bale, S. D.; de Pablos, D.; Harra, L.; Rouillard, A. P.;
Panasenco, O.; Velli, M. C. M.
Bibcode: 2020AGUFMSH032..08B
Altcode:
Global coronal models seek to produce an accurate physical
representation of the Sun's atmosphere which can be used to probe
the dominant plasma physics processes, to connect remote and in situ
observations and operationally to predict space weather events which
can impact the Earth. Assessing their accuracy and usefulness is a
complex task and there are multiple observational pathways to provide
constraints on such models and tune their input parameters. In this
work, we aim to combine several such independent constraints in
a systematic fashion on coronal models. We study the intervals of
Parker Solar Probe's early solar encounters to leverage the unique in
situ observations taken close to the Sun, and the wealth of supporting
observations and prior work analyzing these time intervals. We require
our coronal models to predict the distribution of coronal holes on
the solar surface, and the neutral line topology. We compare these
predictions to (1) direct Extreme Ultraviolet (EUV) observations
of coronal hole locations, (2) white light Carrington maps of the
probable neutral line location at a few solar radii, (3) the magnetic
sector structure measured in situ by Parker Solar Probe as well as
1AU assets. For each of these constraints we compute a simple metric
to evaluate model agreement and compare and contrast these metrics
to evaluate and rank the overall accuracy of the models over a range
of input parameters. Initial results using the coronal hole metric
to analyze Potential Field Source Surface (PFSS) models indicate the
optimum source surface height (Rss) parameter varied from encounter to
encounter. Rss = 1.5 - 2.0 R_sun is shown to work best for Encounters
1 and 3, but higher (2.0-2.5 R_sun) for encounter 2, in agreement with
the magnetic sector structure metric and previous work (e.g. Panasenco
et al. 2020). We discuss the extension of these results to all three
metrics, assess differences in model accuracy among input photospheric
boundary conditions and investigate models with more physics than PFSS.
Title: Onset of fast magnetic reconnection and particle energization
in laboratory and space plasmas
Authors: Pucci, F.; Velli, M.; Shi, C.; Singh, K. A. P.; Tenerani,
A.; Alladio, F.; Ambrosino, F.; Buratti, P.; Fox, W.; Jara-Almonte,
J.; Ji, H.; Yamada, M.; Yoo, J.; Okamura, S.; Ergun, R.; Hoilijoki,
S.; Schwartz, S.
Bibcode: 2020JPlPh..86f5301P
Altcode:
The onset of magnetic reconnection in space, astrophysical and
laboratory plasmas is reviewed discussing results from theory,
numerical simulations and observations. After a brief introduction
on magnetic reconnection and approach to the question of onset, we
first discuss recent theoretical models and numerical simulations,
followed by observations of reconnection and its effects in space and
astrophysical plasmas from satellites and ground-based detectors,
as well as measurements of reconnection in laboratory plasma
experiments. Mechanisms allowing reconnection spanning from collisional
resistivity to kinetic effects as well as partial ionization are
described, providing a description valid over a wide range of plasma
parameters, and therefore applicable in principle to many different
astrophysical and laboratory environments. Finally, we summarize
the implications of reconnection onset physics for plasma dynamics
throughout the Universe and illustrate how capturing the dynamics
correctly is important to understanding particle acceleration. The goal
of this review is to give a view on the present status of this topic
and future interesting investigations, offering a unified approach.
Title: Shear-Driven Transition to Isotropically Turbulent Solar Wind
Outside the Alfvén Critical Zone
Authors: Ruffolo, D. J.; Matthaeus, W. H.; Chhiber, R.; Usmanov, A. V.;
Yang, Y.; Bandyopadhyay, R.; Parashar, T.; Goldstein, M. L.; DeForest,
C.; Wan, M.; Chasapis, A.; Maruca, B.; Velli, M. C. M.; Kasper, J. C.
Bibcode: 2020AGUFMSH0290010R
Altcode:
Motivated by prior remote observations of a transition from striated
solar coronal structures to more isotropic "flocculated" fluctuations,
we propose that the dynamics of the inner solar wind just outside the
Alfvén critical zone, and in the vicinity of the first β=1 surface,
is powered by the relative velocities of adjacent coronal magnetic flux
tubes. We suggest that large amplitude flow contrasts are magnetically
constrained at lower altitude but shear-driven dynamics are triggered
as such constraints are released above the Alfvén critical zone, as
suggested by global magnetohydrodynamic (MHD) simulations that include
self-consistent turbulence transport. We argue that this dynamical
evolution accounts for features observed by Parker Solar Probe ( PSP)
near initial perihelia, including magnetic "switchbacks", and large
transverse velocities that are partially corotational and saturate
near the local Alfvén speed. Large-scale magnetic increments are more
longitudinal than latitudinal, a state unlikely to originate in or
below the lower corona. We attribute this to preferentially longitudinal
velocity shear from varying degrees of corotation. Supporting evidence
includes comparison with a high Mach number three-dimensional
compressible MHD simulation of nonlinear shear-driven turbulence,
reproducing several observed diagnostics, including characteristic
distributions of fluctuations that are qualitatively similar to PSP
observations near the first perihelion. The concurrence of evidence from
remote sensing observations, in situ measurements, and both global and
local simulations supports the idea that the dynamics just above the
Alfvén critical zone boost low-frequency plasma turbulence to the level
routinely observed throughout the explored solar system. This research
has been supported in part by grant RTA6280002 from Thailand Science
Research and Innovation and the Parker Solar Probe mission under the
ISOIS project (contract NNN06AA01C) and a subcontract to University of
Delaware from Princeton University (SUB0000165). M.L.G. acknowledges
support from the Parker Solar Probe FIELDS MAG team. Y.Y. is supported
in part by NSFC grant 11902138. Additional support is acknowledged from
the NASA LWS program (NNX17AB79G) and the HSR program (80NSSC18K1210
& 80NSSC18K1648).
Title: On the stability and evolution of switchbacks in the solar wind
Authors: Tenerani, A.; Velli, M. C. M.; Matteini, L.
Bibcode: 2020AGUFMSH055..04T
Altcode:
Large amplitude, turbulent Alfvénic fluctuations have been commonly
observed in the solar wind since the first in-situ measurements,
and they are thought to provide a possible mechanism to heat the solar
corona and accelerate the solar wind. An important property that remains
to be explained is that these fluctuations display a high degree of
coherence that manifests itself not just via the velocity-magnetic
field correlation that characterizes Alfvé n waves propagating
away from the sun, but also via the observed high degree of plasma
incompressibility. In this context, the recent measurements from Parker
Solar Probe (PSP) have revealed the ubiquitous and persistent presence
of such Alfvénic fluctuations where the magnetic field lines are so
strongly perturbed to the point that they produce local inversions
of the radial magnetic field, known as switchbacks. While there is
not yet a general consensus on what is the origin of switchbacks
and their connection to coronal activity, a first necessary step to
answer these important questions is to understand how they evolve and
how long they can persist in the solar wind. Here w e investigate the
evolution of switchbacks via numerical MHD simulations by including,
in agreement with observations, both a radial magnetic field inversion
and an initially constant total magnetic field pressure. We address in
particular how the evolution of switchbacks is affected by parametric
instabilities and expansion effects, and we finally discuss what are
the implications of our results for models of switchback generation.
Title: Parker Solar Probe In-Situ Observations of Magnetic
Reconnection in the Near-Sun Solar Wind
Authors: Phan, T.; Bale, S. D.; Lavraud, B.; Eastwood, J. P.; Larson,
D. E.; Livi, R.; Halekas, J. S.; Whittlesey, P. L.; Rahmati, A.;
Pulupa, M.; MacDowall, R. J.; Stevens, M. L.; Case, A. W.; Kasper,
J. C.; Korreck, K. E.; Bonnell, J. W.; Drake, J. F.; Shay, M. A.;
Oieroset, M.; Horbury, T. S.; Velli, M. C. M.; Raouafi, N. E.; Szabo,
A.; Koval, A.; Goetz, K.; Harvey, P.
Bibcode: 2020AGUFMSH055..01P
Altcode:
During the first five orbits, PSP has encountered a large number of
current sheets in the near-Sun solar wind. Magnetic reconnection
exhausts have been detected in current sheets associated with the
heliospheric current sheet (HCS), coronal mass ejections, magnetic
flux ropes, and the regular solar wind. PSP crossings of the HCS have
revealed beautiful examples of both sunward and anti-sunward-directed
exhausts. In the sunward exhausts, PSP detected counterstreaming
strahl electrons, indicating that HCS reconnection resulted in the
formation of closed magnetic field lines with both feet on the Sun. In
the anti-sunward exhausts, PSP observed dropouts of strahl electrons,
consistent with the reconnected HCS field lines being disconnected from
the Sun. An early surprise was that the majority of solar wind
current sheets encountered by PSP near perihelia did not appear to
be undergoing local reconnection. Furthermore, reconnection seems to
be occurring more commonly in large-scale heliospheric current sheets
than in small-scale, thin current sheets, raising questions about what
conditions control the onset of reconnection. PSP observations so far
have demonstrated that the near-Sun solar wind is a rich laboratory
ideal for the investigations of the reconnection onset conditions,
as well as the large-scale consequences of reconnection.
Title: A joint study of Solar Orbiter first data and PSP E5 through
3D MHD modeling
Authors: Réville, V.; Strugarek, A.; Brun, S.; Rouillard, A. P.;
Velli, M. C. M.; Poirier, N.; Parenti, S.; Hazra, S.; Perri, B.;
Pinto, R.; Lavraud, B.; Louarn, P.; Fedorov, A.; Owen, C. J.; Bruno,
R.; Livi, R.; Horbury, T. S.; O'Brien, H.; Evans, V.; Angelini, V.;
Bale, S. D.; Kasper, J. C.
Bibcode: 2020AGUFMSH039..09R
Altcode:
The first remote sensing window of Solar Orbiter started mid-June
2020. After a successful commissioning, Solar Orbiter in situ
instruments were then continuously monitoring. This window
is coincidentally close to the fifth perihelion of Parker Solar
Probe. This offers an opportunity for a joint study between the two
probes' data. We use a 3D MHD model of a turbulence driven solar wind,
and compare the structure of the inner heliosphere obtained by the model
and the available in situ and remote sensing data of the spacecraft. We
discuss the key features of the model and tricky points that require
care, such as the choice of the input magnetogram. In particular,
we notice the strong influence of two active regions on the magnetic
sectors and solar wind properties. These regions will be increasingly
important features in future encounters and joint studies as we go
towards solar maximum.
Title: Tearing Modes in Partially Ionized Astrophysical Plasma
Authors: Pucci, Fulvia; Singh, K. Alkendra P.; Tenerani, Anna;
Velli, Marco
Bibcode: 2020ApJ...903L..19P
Altcode: 2020arXiv200603957P
In many astrophysical environments the plasma is only partially ionized,
and therefore the interaction of charged and neutral particles may
alter both the triggering of reconnection and its subsequent dynamical
evolution. We derive the tearing mode maximum growth rate for partially
ionized plasmas in the cases of weak and strong coupling between the
plasma and the neutrals. In addition, critical scalings for current
sheet aspect ratios are presented in terms of Lundquist number and
ion-neutral collision frequencies for which the tearing mode becomes
fast, or ideal. In the decoupled regime the standard tearing mode
is recovered with a small correction that depends on the ion-neutral
collision frequency; in the intermediate regime collisions with neutrals
are shown to stabilize current sheets, resulting in larger critical
aspect ratios for ideal tearing to occur. In the coupled regime, the
growth rate depends on the density ratio between ions and neutrals
through the collision frequency between these two species.
Title: The Role of Parametric Instabilities in Turbulence Generation
and Proton Heating: Hybrid Simulations of Parallel-propagating
Alfvén Waves
Authors: González, C. A.; Tenerani, A.; Velli, M.; Hellinger, P.
Bibcode: 2020ApJ...904...81G
Altcode:
Large-amplitude Alfvén waves tend to be unstable to parametric
instabilities that result in a decay process of the initial wave
into different daughter waves depending upon the amplitude of the
fluctuations and the plasma beta. The propagation angle with respect
to the mean magnetic field of the daughter waves plays an important
role in determining the type of decay. In this paper, we revisit
this problem by means of multidimensional hybrid simulations. In
particular, we study the decay and the subsequent nonlinear evolution
of large-amplitude Alfvén waves by investigating the saturation
mechanism of the instability and its final nonlinear state reached for
different wave amplitudes and plasma beta conditions. As opposed to
one-dimensional simulations where the Decay instability is suppressed
for increasing plasma beta values, we find that the decay process
in multidimensions persists at large values of the plasma beta via
the filamentation/magnetosonic decay instabilities. In general,
the decay process acts as a trigger both to develop a perpendicular
turbulent cascade and to enhance mean field-aligned wave-particle
interactions. We find indeed that the saturated state is characterized
by a turbulent plasma displaying a field-aligned beam at the Alfvén
speed and increased temperatures that we ascribe to the Landau resonance
and pitch-angle scattering in phase space.
Title: The interpretation of data from the Parker Solar Probe mission:
shear-driven transition to an isotropically turbulent solar wind
Authors: Goldstein, Melvyn; Ruffolo, D.; Matthaeus, W. H.; Chhiber, R.;
Usmanov, A. V.; Yang, Y.; Bandyopadhyay, R.; Parashar, T. N.; DeForest,
E.; Wan, M.; Chasapis, A.; Maruca, B. A.; Velli, M.; Kasper, J. C.
Bibcode: 2020REDS..175.1002G
Altcode:
No abstract at ADS
Title: Shear-driven Transition to Isotropically Turbulent Solar Wind
Outside the Alfvén Critical Zone
Authors: Ruffolo, D.; Matthaeus, W. H.; Chhiber, R.; Usmanov, A. V.;
Yang, Y.; Bandyopadhyay, R.; Parashar, T. N.; Goldstein, M. L.;
DeForest, C. E.; Wan, M.; Chasapis, A.; Maruca, B. A.; Velli, M.;
Kasper, J. C.
Bibcode: 2020ApJ...902...94R
Altcode: 2020arXiv200906537R
Motivated by prior remote observations of a transition from striated
solar coronal structures to more isotropic "flocculated" fluctuations,
we propose that the dynamics of the inner solar wind just outside the
Alfvén critical zone, and in the vicinity of the first $\beta =1$
surface, is powered by the relative velocities of adjacent coronal
magnetic flux tubes. We suggest that large-amplitude flow contrasts are
magnetically constrained at lower altitude but shear-driven dynamics are
triggered as such constraints are released above the Alfvén critical
zone, as suggested by global magnetohydrodynamic (MHD) simulations
that include self-consistent turbulence transport. We argue that this
dynamical evolution accounts for features observed by Parker Solar Probe
(PSP) near initial perihelia, including magnetic "switchbacks," and
large transverse velocities that are partially corotational and saturate
near the local Alfvén speed. Large-scale magnetic increments are more
longitudinal than latitudinal, a state unlikely to originate in or
below the lower corona. We attribute this to preferentially longitudinal
velocity shear from varying degrees of corotation. Supporting evidence
includes comparison with a high Mach number three-dimensional
compressible MHD simulation of nonlinear shear-driven turbulence,
reproducing several observed diagnostics, including characteristic
distributions of fluctuations that are qualitatively similar to PSP
observations near the first perihelion. The concurrence of evidence
from remote sensing observations, in situ measurements, and both global
and local simulations supports the idea that the dynamics just above
the Alfvén critical zone boost low-frequency plasma turbulence to
the level routinely observed throughout the explored solar system.
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
Bibcode: 2020A&A...642A...3Z
Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
Title: The Solar Orbiter magnetometer
Authors: Horbury, T. S.; O'Brien, H.; Carrasco Blazquez, I.; Bendyk,
M.; Brown, P.; Hudson, R.; Evans, V.; Oddy, T. M.; Carr, C. M.; Beek,
T. J.; Cupido, E.; Bhattacharya, S.; Dominguez, J. -A.; Matthews, L.;
Myklebust, V. R.; Whiteside, B.; Bale, S. D.; Baumjohann, W.; Burgess,
D.; Carbone, V.; Cargill, P.; Eastwood, J.; Erdös, G.; Fletcher,
L.; Forsyth, R.; Giacalone, J.; Glassmeier, K. -H.; Goldstein, M. L.;
Hoeksema, T.; Lockwood, M.; Magnes, W.; Maksimovic, M.; Marsch, E.;
Matthaeus, W. H.; Murphy, N.; Nakariakov, V. M.; Owen, C. J.; Owens,
M.; Rodriguez-Pacheco, J.; Richter, I.; Riley, P.; Russell, C. T.;
Schwartz, S.; Vainio, R.; Velli, M.; Vennerstrom, S.; Walsh, R.;
Wimmer-Schweingruber, R. F.; Zank, G.; Müller, D.; Zouganelis, I.;
Walsh, A. P.
Bibcode: 2020A&A...642A...9H
Altcode:
The magnetometer instrument on the Solar Orbiter mission is designed
to measure the magnetic field local to the spacecraft continuously
for the entire mission duration. The need to characterise not only
the background magnetic field but also its variations on scales from
far above to well below the proton gyroscale result in challenging
requirements on stability, precision, and noise, as well as magnetic
and operational limitations on both the spacecraft and other
instruments. The challenging vibration and thermal environment has
led to significant development of the mechanical sensor design. The
overall instrument design, performance, data products, and operational
strategy are described.
Title: Oblique Tearing Mode Instability: Guide Field and Hall Effect
Authors: Shi, Chen; Velli, Marco; Pucci, Fulvia; Tenerani, Anna;
Innocenti, Maria Elena
Bibcode: 2020ApJ...902..142S
Altcode: 2020arXiv200700607S
The tearing mode instability is one important mechanism that may explain
the triggering of fast magnetic reconnection in astrophysical plasmas,
such as the solar corona and the Earth's magnetosphere. In this paper,
the linear stability analysis of the tearing mode is carried out for
a current sheet in the presence of a guide field, including the Hall
effect. We show that the presence of a strong guide field does not
modify the most unstable mode in the 2D wavevector space orthogonal
to the current gradient direction, which remains the fastest-growing
parallel mode. With the Hall effect, the inclusion of a guide field
turns the nondispersive propagation along the guide field direction to
a dispersive one. The oblique modes have a wavelike structure along
the normal direction of the current sheet and a strong guide field
suppresses this structure while making the eigenfunctions asymmetric.
Title: Understanding the origins of the heliosphere: integrating
observations and measurements from Parker Solar Probe, Solar Orbiter,
and other space- and ground-based observatories
Authors: Velli, M.; Harra, L. K.; Vourlidas, A.; Schwadron,
N.; Panasenco, O.; Liewer, P. C.; Müller, D.; Zouganelis, I.;
St Cyr, O. C.; Gilbert, H.; Nieves-Chinchilla, T.; Auchère, F.;
Berghmans, D.; Fludra, A.; Horbury, T. S.; Howard, R. A.; Krucker,
S.; Maksimovic, M.; Owen, C. J.; Rodríguez-Pacheco, J.; Romoli,
M.; Solanki, S. K.; Wimmer-Schweingruber, R. F.; Bale, S.; Kasper,
J.; McComas, D. J.; Raouafi, N.; Martinez-Pillet, V.; Walsh, A. P.;
De Groof, A.; Williams, D.
Bibcode: 2020A&A...642A...4V
Altcode:
Context. The launch of Parker Solar Probe (PSP) in 2018, followed
by Solar Orbiter (SO) in February 2020, has opened a new window in
the exploration of solar magnetic activity and the origin of the
heliosphere. These missions, together with other space observatories
dedicated to solar observations, such as the Solar Dynamics Observatory,
Hinode, IRIS, STEREO, and SOHO, with complementary in situ observations
from WIND and ACE, and ground based multi-wavelength observations
including the DKIST observatory that has just seen first light,
promise to revolutionize our understanding of the solar atmosphere
and of solar activity, from the generation and emergence of the Sun's
magnetic field to the creation of the solar wind and the acceleration of
solar energetic particles.
Aims: Here we describe the scientific
objectives of the PSP and SO missions, and highlight the potential for
discovery arising from synergistic observations. Here we put particular
emphasis on how the combined remote sensing and in situ observations of
SO, that bracket the outer coronal and inner heliospheric observations
by PSP, may provide a reconstruction of the solar wind and magnetic
field expansion from the Sun out to beyond the orbit of Mercury in the
first phases of the mission. In the later, out-of-ecliptic portions of
the SO mission, the solar surface magnetic field measurements from SO
and the multi-point white-light observations from both PSP and SO will
shed light on the dynamic, intermittent solar wind escaping from helmet
streamers, pseudo-streamers, and the confined coronal plasma, and on
solar energetic particle transport.
Methods: Joint measurements
during PSP-SO alignments, and magnetic connections along the same
flux tube complemented by alignments with Earth, dual PSP-Earth,
and SO-Earth, as well as with STEREO-A, SOHO, and BepiColumbo will
allow a better understanding of the in situ evolution of solar-wind
plasma flows and the full three-dimensional distribution of the
solar wind from a purely observational point of view. Spectroscopic
observations of the corona, and optical and radio observations,
combined with direct in situ observations of the accelerating solar
wind will provide a new foundation for understanding the fundamental
physical processes leading to the energy transformations from solar
photospheric flows and magnetic fields into the hot coronal plasma
and magnetic fields and finally into the bulk kinetic energy of the
solar wind and solar energetic particles.
Results: We discuss
the initial PSP observations, which already provide a compelling
rationale for new measurement campaigns by SO, along with ground-
and space-based assets within the synergistic context described above.
Title: The Solar Orbiter Heliospheric Imager (SoloHI)
Authors: Howard, R. A.; Vourlidas, A.; Colaninno, R. C.; Korendyke,
C. M.; Plunkett, S. P.; Carter, M. T.; Wang, D.; Rich, N.; Lynch,
S.; Thurn, A.; Socker, D. G.; Thernisien, A. F.; Chua, D.; Linton,
M. G.; Koss, S.; Tun-Beltran, S.; Dennison, H.; Stenborg, G.; McMullin,
D. R.; Hunt, T.; Baugh, R.; Clifford, G.; Keller, D.; Janesick, J. R.;
Tower, J.; Grygon, M.; Farkas, R.; Hagood, R.; Eisenhauer, K.; Uhl,
A.; Yerushalmi, S.; Smith, L.; Liewer, P. C.; Velli, M. C.; Linker,
J.; Bothmer, V.; Rochus, P.; Halain, J. -P.; Lamy, P. L.; Auchère,
F.; Harrison, R. A.; Rouillard, A.; Patsourakos, S.; St. Cyr, O. C.;
Gilbert, H.; Maldonado, H.; Mariano, C.; Cerullo, J.
Bibcode: 2020A&A...642A..13H
Altcode:
Aims: We present the design and pre-launch performance of
the Solar Orbiter Heliospheric Imager (SoloHI) which is an instrument
prepared for inclusion in the ESA/NASA Solar Orbiter mission, currently
scheduled for launch in 2020.
Methods: The goal of this paper
is to provide details of the SoloHI instrument concept, design, and
pre-flight performance to give the potential user of the data a better
understanding of how the observations are collected and the sources
that contribute to the signal.
Results: The paper discusses
the science objectives, including the SoloHI-specific aspects, before
presenting the design concepts, which include the optics, mechanical,
thermal, electrical, and ground processing. Finally, a list of planned
data products is also presented.
Conclusions: The performance
measurements of the various instrument parameters meet or exceed the
requirements derived from the mission science objectives. SoloHI is
poised to take its place as a vital contributor to the science success
of the Solar Orbiter mission.
Title: The Solar Orbiter mission. Science overview
Authors: Müller, D.; St. Cyr, O. C.; Zouganelis, I.; Gilbert, H. R.;
Marsden, R.; Nieves-Chinchilla, T.; Antonucci, E.; Auchère, F.;
Berghmans, D.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic,
M.; Owen, C. J.; Rochus, P.; Rodriguez-Pacheco, J.; Romoli, M.;
Solanki, S. K.; Bruno, R.; Carlsson, M.; Fludra, A.; Harra, L.;
Hassler, D. M.; Livi, S.; Louarn, P.; Peter, H.; Schühle, U.;
Teriaca, L.; del Toro Iniesta, J. C.; Wimmer-Schweingruber, R. F.;
Marsch, E.; Velli, M.; De Groof, A.; Walsh, A.; Williams, D.
Bibcode: 2020A&A...642A...1M
Altcode: 2020arXiv200900861M
Aims: Solar Orbiter, the first mission of ESA's Cosmic Vision
2015-2025 programme and a mission of international collaboration between
ESA and NASA, will explore the Sun and heliosphere from close up and
out of the ecliptic plane. It was launched on 10 February 2020 04:03
UTC from Cape Canaveral and aims to address key questions of solar and
heliospheric physics pertaining to how the Sun creates and controls
the Heliosphere, and why solar activity changes with time. To answer
these, the mission carries six remote-sensing instruments to observe
the Sun and the solar corona, and four in-situ instruments to measure
the solar wind, energetic particles, and electromagnetic fields. In
this paper, we describe the science objectives of the mission, and how
these will be addressed by the joint observations of the instruments
onboard.
Methods: The paper first summarises the mission-level
science objectives, followed by an overview of the spacecraft and
payload. We report the observables and performance figures of each
instrument, as well as the trajectory design. This is followed by a
summary of the science operations concept. The paper concludes with a
more detailed description of the science objectives.
Results:
Solar Orbiter will combine in-situ measurements in the heliosphere
with high-resolution remote-sensing observations of the Sun to address
fundamental questions of solar and heliospheric physics. The performance
of the Solar Orbiter payload meets the requirements derived from the
mission's science objectives. Its science return will be augmented
further by coordinated observations with other space missions and
ground-based observatories. ARRAY(0x207ce98)
Title: Metis: the Solar Orbiter visible light and ultraviolet
coronal imager
Authors: Antonucci, Ester; Romoli, Marco; Andretta, Vincenzo; Fineschi,
Silvano; Heinzel, Petr; Moses, J. Daniel; Naletto, Giampiero; Nicolini,
Gianalfredo; Spadaro, Daniele; Teriaca, Luca; Berlicki, Arkadiusz;
Capobianco, Gerardo; Crescenzio, Giuseppe; Da Deppo, Vania; Focardi,
Mauro; Frassetto, Fabio; Heerlein, Klaus; Landini, Federico; Magli,
Enrico; Marco Malvezzi, Andrea; Massone, Giuseppe; Melich, Radek;
Nicolosi, Piergiorgio; Noci, Giancarlo; Pancrazzi, Maurizio; Pelizzo,
Maria G.; Poletto, Luca; Sasso, Clementina; Schühle, Udo; Solanki,
Sami K.; Strachan, Leonard; Susino, Roberto; Tondello, Giuseppe;
Uslenghi, Michela; Woch, Joachim; Abbo, Lucia; Bemporad, Alessandro;
Casti, Marta; Dolei, Sergio; Grimani, Catia; Messerotti, Mauro;
Ricci, Marco; Straus, Thomas; Telloni, Daniele; Zuppella, Paola;
Auchère, Frederic; Bruno, Roberto; Ciaravella, Angela; Corso,
Alain J.; Alvarez Copano, Miguel; Aznar Cuadrado, Regina; D'Amicis,
Raffaella; Enge, Reiner; Gravina, Alessio; Jejčič, Sonja; Lamy,
Philippe; Lanzafame, Alessandro; Meierdierks, Thimo; Papagiannaki,
Ioanna; Peter, Hardi; Fernandez Rico, German; Giday Sertsu, Mewael;
Staub, Jan; Tsinganos, Kanaris; Velli, Marco; Ventura, Rita; Verroi,
Enrico; Vial, Jean-Claude; Vives, Sebastien; Volpicelli, Antonio;
Werner, Stephan; Zerr, Andreas; Negri, Barbara; Castronuovo, Marco;
Gabrielli, Alessandro; Bertacin, Roberto; Carpentiero, Rita; Natalucci,
Silvia; Marliani, Filippo; Cesa, Marco; Laget, Philippe; Morea, Danilo;
Pieraccini, Stefano; Radaelli, Paolo; Sandri, Paolo; Sarra, Paolo;
Cesare, Stefano; Del Forno, Felice; Massa, Ernesto; Montabone, Mauro;
Mottini, Sergio; Quattropani, Daniele; Schillaci, Tiziano; Boccardo,
Roberto; Brando, Rosario; Pandi, Arianna; Baietto, Cristian; Bertone,
Riccardo; Alvarez-Herrero, Alberto; García Parejo, Pilar; Cebollero,
María; Amoruso, Mauro; Centonze, Vito
Bibcode: 2020A&A...642A..10A
Altcode: 2019arXiv191108462A
Aims: Metis is the first solar coronagraph designed for a
space mission and is capable of performing simultaneous imaging of the
off-limb solar corona in both visible and UV light. The observations
obtained with Metis aboard the Solar Orbiter ESA-NASA observatory
will enable us to diagnose, with unprecedented temporal coverage and
spatial resolution, the structures and dynamics of the full corona
in a square field of view (FoV) of ±2.9° in width, with an inner
circular FoV at 1.6°, thus spanning the solar atmosphere from 1.7
R⊙ to about 9 R⊙, owing to the eccentricity
of the spacecraft orbit. Due to the uniqueness of the Solar Orbiter
mission profile, Metis will be able to observe the solar corona
from a close (0.28 AU, at the closest perihelion) vantage point,
achieving increasing out-of-ecliptic views with the increase of the
orbit inclination over time. Moreover, observations near perihelion,
during the phase of lower rotational velocity of the solar surface
relative to the spacecraft, allow longer-term studies of the off-limb
coronal features, thus finally disentangling their intrinsic evolution
from effects due to solar rotation.
Methods: Thanks to a novel
occultation design and a combination of a UV interference coating of
the mirrors and a spectral bandpass filter, Metis images the solar
corona simultaneously in the visible light band, between 580 and 640
nm, and in the UV H I Lyman-α line at 121.6 nm. The visible light
channel also includes a broadband polarimeter able to observe the
linearly polarised component of the K corona. The coronal images in
both the UV H I Lyman-α and polarised visible light are obtained at
high spatial resolution with a spatial scale down to about 2000 km
and 15 000 km at perihelion, in the cases of the visible and UV light,
respectively. A temporal resolution down to 1 s can be achieved when
observing coronal fluctuations in visible light.
Results: The
Metis measurements, obtained from different latitudes, will allow for
complete characterisation of the main physical parameters and dynamics
of the electron and neutral hydrogen/proton plasma components of the
corona in the region where the solar wind undergoes the acceleration
process and where the onset and initial propagation of coronal mass
ejections (CMEs) take place. The near-Sun multi-wavelength coronal
imaging performed with Metis, combined with the unique opportunities
offered by the Solar Orbiter mission, can effectively address crucial
issues of solar physics such as: the origin and heating/acceleration
of the fast and slow solar wind streams; the origin, acceleration,
and transport of the solar energetic particles; and the transient
ejection of coronal mass and its evolution in the inner heliosphere,
thus significantly improving our understanding of the region connecting
the Sun to the heliosphere and of the processes generating and driving
the solar wind and coronal mass ejections.
Conclusions: This
paper presents the scientific objectives and requirements, the overall
optical design of the Metis instrument, the thermo-mechanical design,
and the processing and power unit; reports on the results of the
campaigns dedicated to integration, alignment, and tests, and to
the characterisation of the instrument performance; describes the
operation concept, data handling, and software tools; and, finally,
the diagnostic techniques to be applied to the data, as well as a brief
description of the expected scientific products. The performance of the
instrument measured during calibrations ensures that the scientific
objectives of Metis can be pursued with success. Metis website:
http://metis.oato.inaf.it
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena in Solar and
Heliospheric Plasmas
Authors: Ji, H.; Karpen, J.; Alt, A.; Antiochos, S.; Baalrud, S.;
Bale, S.; Bellan, P. M.; Begelman, M.; Beresnyak, A.; Bhattacharjee,
A.; Blackman, E. G.; Brennan, D.; Brown, M.; Buechner, J.; Burch, J.;
Cassak, P.; Chen, B.; Chen, L. -J.; Chen, Y.; Chien, A.; Comisso,
L.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.; Dong, C. F.;
Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun, R.; Eyink,
G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.; Fujimoto,
K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo, F.; Hare,
J.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.; Le,
A.; Lebedev, S.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.;
Liu, W.; Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sironi, L.; Sitnov, M.; Stanier, A.; Swisdak, M.; TenBarge,
J.; Tharp, T.; Uzdensky, D.; Vaivads, A.; Velli, M.; Vishniac, E.;
Wang, H.; Werner, G.; Xiao, C.; Yamada, M.; Yokoyama, T.; Yoo, J.;
Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200908779J
Altcode:
Magnetic reconnection underlies many explosive phenomena in the
heliosphere and in laboratory plasmas. The new research capabilities in
theory/simulations, observations, and laboratory experiments provide the
opportunity to solve the grand scientific challenges summarized in this
whitepaper. Success will require enhanced and sustained investments
from relevant funding agencies, increased interagency/international
partnerships, and close collaborations of the solar, heliospheric,
and laboratory plasma communities. These investments will deliver
transformative progress in understanding magnetic reconnection and
related explosive phenomena including space weather events.
Title: Collisionless Heat Flux Regulation via the Electron Firehose
Instability in the Presence of a Core and Suprathermal Population
in the Expanding Solar Wind
Authors: Innocenti, Maria Elena; Boella, Elisabetta; Tenerani, Anna;
Velli, Marco
Bibcode: 2020ApJ...898L..41I
Altcode: 2020arXiv200707143I
The evolution of the electron heat flux in the solar wind is regulated
by the interplay between several effects: solar wind expansion, which
can potentially drive velocity-space instabilities, turbulence,
wave-particle interactions, and, possibly, collisions. Here we
address the respective role played by the solar wind expansion and
the electron firehose instability (EFI), developing in the presence of
multiple electron populations, in regulating the heat flux. We carry
out fully kinetic, expanding box model simulations and separately
analyze the enthalpy, bulk, and velocity distribution function skewness
contributions for each of the electron species. We observe that the
key factor determining electron energy flux evolution is the reduction
of the drift velocity of the electron populations in the rest frame
of the solar wind. In our simulations, redistribution of the electron
thermal energy from the parallel to the perpendicular direction after
the onset of the EFI is observed. However, this process seems to impact
energy flux evolution only minimally. Hence, reduction of the electron
species drift velocity in the solar wind frame appears to directly
correlate with efficiency for heat flux instabilities.
Title: Tearing Instability and Periodic Density Perturbations in
the Slow Solar Wind
Authors: Réville, Victor; Velli, Marco; Rouillard, Alexis P.; Lavraud,
Benoit; Tenerani, Anna; Shi, Chen; Strugarek, Antoine
Bibcode: 2020ApJ...895L..20R
Altcode: 2020arXiv200502679R
In contrast with the fast solar wind, which originates in coronal holes,
the source of the slow solar wind is still debated. Often intermittent
and enriched with low first ionization potential elements—akin to
what is observed in closed coronal loops—the slow wind could form
in bursty events nearby helmet streamers. Slow winds also exhibit
density perturbations that have been shown to be periodic and could be
associated with flux ropes ejected from the tip of helmet streamers,
as shown recently by the WISPR white-light imager on board Parker
Solar Probe (PSP). In this work, we propose that the main mechanism
controlling the release of flux ropes is a flow-modified tearing mode
at the heliospheric current sheet (HCS). We use magnetohydrodynamic
simulations of the solar wind and corona to reproduce realistic
configurations and outflows surrounding the HCS. We find that this
process is able to explain long (∼10-20 hr) and short (∼1-2 hr)
timescales of density structures observed in the slow solar wind. This
study also sheds new light on the structure, topology, and composition
of the slow solar wind, and could be, in the near future, compared
with white light and in situ PSP observations.
Title: Collisionless electron dynamics in the expanding solar wind
Authors: Innocenti, Maria Elena; Boella, Elisabetta; Tenerani, Anna;
Velli, Marco
Bibcode: 2020EGUGA..2212596I
Altcode:
Observations of solar wind electron properties, as displayed in
the Tperp/Tpar vs βpar plane, appear to be constrained both in the
Tperp/Tpar <1 and in the Tperp/Tpar >1 regimes by the electron
firehose instability (EFI) and by the whistler instability respectively
[Štverák 2008]. The onset mechanism of the EFI is established: solar
wind expansion results in an electron thermal anisotropy, which in turns
promotes the development of the instability that contributes to limit
that same anisotropy [Innocenti 2019a]. However, if this were the only
mechanism at work in the expanding solar wind, electron observations
would pool at the EFI marginal instability line. Instead, they populate
the "stable" interval bound by EFI and whistler marginal instability
lines. It is not fully clear which role fully kinetic processes have in
lifting the observed data points above the EFI marginal stability line
and into the "stable" area. Other competing processes redistributing
excess parallel energy into the perpendicular direction, such as
collisions, may be at work as well [Yoon 2019].We investigate this issue
with Particle In Cell, Expanding Box Model simulations [Innocenti 2019b]
of EFI developing self consistently in the expanding solar wind. Our
results show that after the EFI marginal stability line is reached,
further collisionless evolution brings our simulated data points in
the "stable" area. We thus demonstrate that, at least under certain
circumstances, purely collisionless processes may explain observed
solar wind observations, without the need of invoking collisions
as a way to channel excess parallel energy into the perpendicular
direction. Štverák, Štěpán, et al. "Electron temperature anisotropy
constraints in the solar wind." Journal of Geophysical Research:
Space Physics 113.A3 (2008).Innocenti, Maria Elena, et al. "Onset and
Evolution of the Oblique, Resonant Electron Firehose Instability in the
Expanding Solar Wind Plasma." The Astrophysical Journal 883.2 (2019):
146.Yoon, P. H., et al. "Solar Wind Temperature Isotropy." Physical
review letters 123.14 (2019): 145101.Innocenti, Maria Elena, Anna
Tenerani, and Marco Velli. "A Semi-implicit Particle-in-cell Expanding
Box Model Code for Fully Kinetic Simulations of the Expanding Solar
Wind Plasma." The Astrophysical Journal 870.2 (2019): 66.
Title: Alfvénic Slow Solar Wind Observed in the Inner Heliosphere
by Parker Solar Probe
Authors: Huang, Jia; Kasper, J. C.; Stevens, M.; Vech, D.; Klein,
K. G.; Martinović, Mihailo M.; Alterman, B. L.; Jian, Lan K.; Hu,
Qiang; Velli, Marco; Horbury, Timothy S.; Lavraud, B.; Parashar,
T. N.; Ďurovcová, Tereza; Niembro, Tatiana; Paulson, Kristoff;
Hegedus, A.; Bert, C. M.; Holmes, J.; Case, A. W.; Korreck, K. E.;
Bale, Stuart D.; Larson, Davin E.; Livi, Roberto; Whittlesey, P.;
Pulupa, Marc; Dudok de Wit, Thierry; Malaspina, David M.; MacDowall,
Robert J.; Bonnell, John W.; Harvey, Peter R.; Goetz, Keith
Bibcode: 2020arXiv200512372H
Altcode:
The slow solar wind is typically characterized as having
low Alfvénicity. However, Parker Solar Probe (PSP) observed
predominately Alfvénic slow solar wind during several of its initial
encounters. From its first encounter observations, about 55.3\% of
the slow solar wind inside 0.25 au is highly Alfvénic ($|\sigma_C|
> 0.7$) at current solar minimum, which is much higher than
the fraction of quiet-Sun-associated highly Alfvénic slow wind
observed at solar maximum at 1 au. Intervals of slow solar wind with
different Alfvénicities seem to show similar plasma characteristics
and temperature anisotropy distributions. Some low Alfvénicity
slow wind intervals even show high temperature anisotropies,
because the slow wind may experience perpendicular heating as
fast wind does when close to the Sun. This signature is confirmed
by Wind spacecraft measurements as we track PSP observations to 1
au. Further, with nearly 15 years of Wind measurements, we find that
the distributions of plasma characteristics, temperature anisotropy
and helium abundance ratio ($N_\alpha/N_p$) are similar in slow winds
with different Alfvénicities, but the distributions are different
from those in the fast solar wind. Highly Alfvénic slow solar wind
contains both helium-rich ($N_\alpha/N_p\sim0.045$) and helium-poor
($N_\alpha/N_p\sim0.015$) populations, implying it may originate from
multiple source regions. These results suggest that highly Alfvénic
slow solar wind shares similar temperature anisotropy and helium
abundance properties with regular slow solar winds, and they thus
should have multiple origins.
Title: Localized Magnetic-field Structures and Their Boundaries in
the Near-Sun Solar Wind from Parker Solar Probe Measurements
Authors: Krasnoselskikh, V.; Larosa, A.; Agapitov, O.; de Wit,
T. Dudok; Moncuquet, M.; Mozer, F. S.; Stevens, M.; Bale, S. D.;
Bonnell, J.; Froment, C.; Goetz, K.; Goodrich, K.; Harvey, P.; Kasper,
J.; MacDowall, R.; Malaspina, D.; Pulupa, M.; Raouafi, N.; Revillet,
C.; Velli, M.; Wygant, J.
Bibcode: 2020ApJ...893...93K
Altcode: 2020arXiv200305409K
One of the discoveries of the Parker Solar Probe during its first
encounters with the Sun is ubiquitous presence of relatively
small-scale structures standing out as sudden deflections of the
magnetic field. They were named "switchbacks" since some of them show
a full reversal of the radial component of the magnetic field and
then return to "regular" conditions. We carried out an analysis of
three typical switchback structures having different characteristics:
I. Alfvénic structure, where the variations of the magnetic field
components take place while conserving the magnitude of the magnetic
field; II. Compressional structure, where the magnitude of the field
varies together with changes of its components; and III. Structure
manifesting full reversal of the magnetic field, presumably Alfvén,
which is an extremal example of a switchback. We analyzed the
properties of the magnetic fields of these structures and of their
boundaries. Observations and analyses lead to the conclusion that
they represent localized twisted magnetic tubes moving with respect
to surrounding plasma. An important feature is the existence of a
relatively narrow boundary layer at the surface of the tube that
accommodates flowing currents. These currents are closed on the
surface of the structure and typically have comparable azimuthal and
tube-axis-aligned components. They are supported by the presence of an
effective electric field due to strong gradients of the density and ion
plasma pressure. The ion beta is typically larger inside the structure
than outside. The surface of the structure may also accommodate
electromagnetic waves that assist particles in carrying currents.
Title: Large Amplitude Fluctuations in the Alfvénic Solar Wind
Authors: D'Amicis, R.; Matteini, L.; Bruno, R.; Velli, M.
Bibcode: 2020SoPh..295...46D
Altcode:
Large amplitude fluctuations, often with characteristics reminiscent
of large amplitude Alfvén waves propagating away from the Sun, are
ubiquitous in the solar wind. Such features are most frequently found
within fast solar wind streams and most often at solar minimum. The
fluctuations found in slow solar wind streams usually have a smaller
relative amplitude, are less Alfvénic in character and present more
variability. However, intervals of slow wind displaying Alfvénic
correlations have been recently identified in different solar cycle
phases. In the present paper we report Alfvénic slow solar wind streams
seen during the maximum of solar activity that are characterized not
only by a very high correlation between velocity and magnetic field
fluctuations (as required by outwardly propagating Alfvén modes) -
comparable to that seen in fast wind streams - but also by higher
amplitude relative fluctuations comparable to those seen in fast
wind. Our results suggest that the Alfvénic slow wind has a different
origin from the slow wind found near the boundary of coronal holes,
where the amplitude of the Alfvénic fluctuations decreases together
with decreasing the wind speed.
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, H.; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.; Guo,
F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte, J.;
Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian, A.;
Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu, W.;
Longcope, D.; Loureiro, N.; Lu, Q. -M.; Ma, Z-W.; Matthaeus, W. H.;
Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson, P.;
Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan, T.;
Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2020arXiv200400079J
Altcode:
This white paper summarizes major scientific challenges and
opportunities in understanding magnetic reconnection and related
explosive phenomena as a fundamental plasma process.
Title: Proton Temperature Anisotropy Variations in Inner Heliosphere
Estimated with the First Parker Solar Probe Observations
Authors: Huang, Jia; Kasper, J. C.; Vech, D.; Klein, K. G.; Stevens,
M.; Martinović, Mihailo M.; Alterman, B. L.; Ďurovcová, Tereza;
Paulson, Kristoff; Maruca, Bennett A.; Qudsi, Ramiz A.; Case, A. W.;
Korreck, K. E.; Jian, Lan K.; Velli, Marco; Lavraud, B.; Hegedus,
A.; Bert, C. M.; Holmes, J.; Bale, Stuart D.; Larson, Davin E.;
Livi, Roberto; Whittlesey, P.; Pulupa, Marc; MacDowall, Robert J.;
Malaspina, David M.; Bonnell, John W.; Harvey, Peter; Goetz, Keith;
Dudok de Wit, Thierry
Bibcode: 2020ApJS..246...70H
Altcode: 2019arXiv191203871H
We present a technique for deriving the temperature anisotropy of solar
wind protons observed by the Parker Solar Probe (PSP) mission in the
near-Sun solar wind. The radial proton temperature measured by the
Solar Wind Electrons, Alphas, and Protons (SWEAP) Solar Probe Cup is
compared with the orientation of local magnetic field measured by the
FIELDS fluxgate magnetometer, and the proton temperatures parallel and
perpendicular to the magnetic field are extracted. This procedure is
applied to different data products, and the results are compared and
optimum timescales for data selection and trends in the uncertainty
in the method are identified. We find that the moment-based proton
temperature anisotropy is more physically consistent with the expected
limits of the mirror and firehose instabilities, possibly because the
nonlinear fits do not capture a significant non-Maxwellian shape to
the proton velocity distribution function near the Sun. The proton beam
has a small effect on total proton temperature anisotropy owing to its
much smaller density relative to the core compared to what was seen
by previous spacecraft farther from the Sun. Several radial trends
in the temperature components and the variation of the anisotropy
with parallel plasma beta are presented. Our results suggest that we
may see stronger anisotropic heating as PSP moves closer to the Sun,
and that a careful treatment of the shape of the proton distribution
may be needed to correctly describe the temperature.
Title: Clustering of Intermittent Magnetic and Flow
Structures near Parker Solar Probe's First Perihelion—A
Partial-variance-of-increments Analysis
Authors: Chhiber, Rohit; Goldstein, M. L.; Maruca, B. A.; Chasapis,
A.; Matthaeus, W. H.; Ruffolo, D.; Bandyopadhyay, R.; Parashar, T. N.;
Qudsi, R.; de Wit, T. Dudok; Bale, S. D.; Bonnell, J. W.; Goetz, K.;
Harvey, P. R.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Kasper,
J. C.; Korreck, K. E.; Case, A. W.; Stevens, M.; Whittlesey, P.;
Larson, D.; Livi, R.; Velli, M.; Raouafi, N.
Bibcode: 2020ApJS..246...31C
Altcode: 2019arXiv191203608C
During the Parker Solar Probe's (PSP) first perihelion pass,
the spacecraft reached within a heliocentric distance of ∼37
R⊙ and observed numerous magnetic and flow structures
characterized by sharp gradients. To better understand these
intermittent structures in the young solar wind, an important property
to examine is their degree of correlation in time and space. To
this end, we use the well-tested partial variance of increments
(PVI) technique to identify intermittent events in FIELDS and SWEAP
observations of magnetic and proton-velocity fields (respectively)
during PSP's first solar encounter, when the spacecraft was within 0.25
au from the Sun. We then examine distributions of waiting times (WT)
between events with varying separation and PVI thresholds. We find
power-law distributions for WT shorter than a characteristic scale
comparable to the correlation time of the fluctuations, suggesting
a high degree of correlation that may originate in a clustering
process. WT longer than this characteristic time are better described
by an exponential, suggesting a random memory-less Poisson process at
play. These findings are consistent with near-Earth observations of
solar wind turbulence. The present study complements the one by Dudok
de Wit et al., which focuses on WT between observed "switchbacks"
in the radial magnetic field.
Title: Parker Solar Probe In Situ Observations of Magnetic
Reconnection Exhausts during Encounter 1
Authors: Phan, T. D.; Bale, S. D.; Eastwood, J. P.; Lavraud, B.;
Drake, J. F.; Oieroset, M.; Shay, M. A.; Pulupa, M.; Stevens, M.;
MacDowall, R. J.; Case, A. W.; Larson, D.; Kasper, J.; Whittlesey,
P.; Szabo, A.; Korreck, K. E.; Bonnell, J. W.; de Wit, T. Dudok;
Goetz, K.; Harvey, P. R.; Horbury, T. S.; Livi, R.; Malaspina, D.;
Paulson, K.; Raouafi, N. E.; Velli, M.
Bibcode: 2020ApJS..246...34P
Altcode: 2020arXiv200106048P
Magnetic reconnection in current sheets converts magnetic energy
into particle energy. The process may play an important role
in the acceleration and heating of the solar wind close to the
Sun. Observations from Parker Solar Probe (PSP) provide a new
opportunity to study this problem, as it measures the solar wind at
unprecedented close distances to the Sun. During the first orbit,
PSP encountered a large number of current sheets in the solar wind
through perihelion at 35.7 solar radii. We performed a comprehensive
survey of these current sheets and found evidence for 21 reconnection
exhausts. These exhausts were observed in heliospheric current sheets,
coronal mass ejections, and regular solar wind. However, we find
that the majority of current sheets encountered around perihelion,
where the magnetic field was strongest and plasma β was lowest, were
Alfvénic structures associated with bursty radial jets, and these
current sheets did not appear to be undergoing local reconnection. We
examined conditions around current sheets to address why some current
sheets reconnected while others did not. A key difference appears to
be the degree of plasma velocity shear across the current sheets: the
median velocity shear for the 21 reconnection exhausts was 24% of the
Alfvén velocity shear, whereas the median shear across 43 Alfvénic
current sheets examined was 71% of the Alfvén velocity shear. This
finding could suggest that large, albeit sub-Alfvénic, velocity
shears suppress reconnection. An alternative interpretation is that
the Alfvénic current sheets are isolated rotational discontinuities
that do not undergo local reconnection.
Title: Measures of Scale-dependent Alfvénicity in the First PSP
Solar Encounter
Authors: Parashar, T. N.; Goldstein, M. L.; Maruca, B. A.; Matthaeus,
W. H.; Ruffolo, D.; Bandyopadhyay, R.; Chhiber, R.; Chasapis, A.;
Qudsi, R.; Vech, D.; Roberts, D. A.; Bale, S. D.; Bonnell, J. W.; de
Wit, T. Dudok; Goetz, K.; Harvey, P. R.; MacDowall, R. J.; Malaspina,
D.; Pulupa, M.; Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens,
M.; Whittlesey, P.; Larson, D.; Livi, R.; Velli, M.; Raouafi, N.
Bibcode: 2020ApJS..246...58P
Altcode:
The solar wind shows periods of highly Alfvénic activity, where
velocity fluctuations and magnetic fluctuations are aligned or
antialigned with each other. It is generally agreed that solar
wind plasma velocity and magnetic field fluctuations observed by
the Parker Solar Probe (PSP) during the first encounter are mostly
highly Alfvénic. However, quantitative measures of Alfvénicity are
needed to understand how the characterization of these fluctuations
compares with standard measures from prior missions in the inner and
outer heliosphere, in fast wind and slow wind, and at high and low
latitudes. To investigate this issue, we employ several measures to
quantify the extent of Alfvénicity—the Alfvén ratio rA,
the normalized cross helicity σc, the normalized residual
energy σr, and the cosine of angle between velocity and
magnetic fluctuations $\cos {\theta }_{{vb}}$ . We show that despite
the overall impression that the Alfvénicity is large in the solar wind
sampled by PSP during the first encounter, during some intervals the
cross helicity starts decreasing at very large scales. These length
scales (often >1000dI) are well inside inertial range,
and therefore, the suppression of cross helicity at these scales
cannot be attributed to kinetic physics. This drop at large scales
could potentially be explained by large scale shears present in the
inner heliosphere sampled by PSP. In some cases, despite the cross
helicity being constant down to the noise floor, the residual energy
decreases with scale in the inertial range. These results suggest that
it is important to consider all these measures to quantify Alfvénicity.
Title: Magnetic Field Kinks and Folds in the Solar Wind
Authors: Tenerani, Anna; Velli, Marco; Matteini, Lorenzo; Réville,
Victor; Shi, Chen; Bale, Stuart D.; Kasper, Justin C.; Bonnell, John
W.; Case, Anthony W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey,
Peter R.; Klein, Kristopher G.; Korreck, Kelly; Larson, Davin; Livi,
Roberto; MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc;
Stevens, Michael; Whittlesey, Phyllis
Bibcode: 2020ApJS..246...32T
Altcode: 2019arXiv191203240T
Parker Solar Probe (PSP) observations during its first encounter at
35.7 R⊙ have shown the presence of magnetic field lines
that are strongly perturbed to the point that they produce local
inversions of the radial magnetic field, known as switchbacks. Their
counterparts in the solar wind velocity field are local enhancements
in the radial speed, or jets, displaying (in all components) the
velocity-magnetic field correlation typical of large amplitude
Alfvén waves propagating away from the Sun. Switchbacks and radial
jets have previously been observed over a wide range of heliocentric
distances by Helios, Wind, and Ulysses, although they were prevalent
in significantly faster streams than seen at PSP. Here we study
via numerical magnetohydrodynamics simulations the evolution of such
large amplitude Alfvénic fluctuations by including, in agreement with
observations, both a radial magnetic field inversion and an initially
constant total magnetic pressure. Despite the extremely large excursion
of magnetic and velocity fields, switchbacks are seen to persist for
up to hundreds of Alfvén crossing times before eventually decaying
due to the parametric decay instability. Our results suggest that such
switchback/jet configurations might indeed originate in the lower corona
and survive out to PSP distances, provided the background solar wind is
sufficiently calm, in the sense of not being pervaded by strong density
fluctuations or other gradients, such as stream or magnetic field
shears, that might destabilize or destroy them over shorter timescales.
Title: Anticorrelation between the Bulk Speed and the Electron
Temperature in the Pristine Solar Wind: First Results from the Parker
Solar Probe and Comparison with Helios
Authors: Maksimovic, M.; Bale, S. D.; Berčič, L.; Bonnell, J. W.;
Case, A. W.; Dudok de Wit, T.; Goetz, K.; Halekas, J. S.; Harvey,
P. R.; Issautier, K.; Kasper, J. C.; Korreck, K. E.; Jagarlamudi,
V. Krishna; Lahmiti, N.; Larson, D. E.; Lecacheux, A.; Livi, R.;
MacDowall, R. J.; Malaspina, D. M.; Martinović, M. M.; Meyer-Vernet,
N.; Moncuquet, M.; Pulupa, M.; Salem, C.; Stevens, M. L.; Štverák,
Š.; Velli, M.; Whittlesey, P. L.
Bibcode: 2020ApJS..246...62M
Altcode:
We discuss the solar wind electron temperatures Te as
measured in the nascent solar wind by Parker Solar Probe during its
first perihelion pass. The measurements have been obtained by fitting
the high-frequency part of quasi-thermal noise spectra recorded by the
Radio Frequency Spectrometer. In addition we compare these measurements
with those obtained by the electrostatic analyzer discussed in Halekas
et al. These first electron observations show an anticorrelation between
Te and the wind bulk speed V: this anticorrelation is most
likely the remnant of the well-known mapping observed at 1 au and beyond
between the fast wind and its coronal hole sources, where electrons
are observed to be cooler than in the quiet corona. We also revisit
Helios electron temperature measurements and show, for the first time,
that an in situ ( ${T}_{e},V$ ) anticorrelation is well observed at
0.3 au but disappears as the wind expands, evolves, and mixes with
different electron temperature gradients for different wind speeds.
Title: Enhanced Energy Transfer Rate in Solar Wind Turbulence Observed
near the Sun from Parker Solar Probe
Authors: Bandyopadhyay, Riddhi; Goldstein, M. L.; Maruca, B. A.;
Matthaeus, W. H.; Parashar, T. N.; Ruffolo, D.; Chhiber, R.; Usmanov,
A.; Chasapis, A.; Qudsi, R.; Bale, Stuart D.; Bonnell, J. W.; Dudok
de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; MacDowall, Robert
J.; Malaspina, David M.; Pulupa, Marc; Kasper, J. C.; Korreck, K. E.;
Case, A. W.; Stevens, M.; Whittlesey, P.; Larson, D.; Livi, R.; Klein,
K. G.; Velli, M.; Raouafi, N.
Bibcode: 2020ApJS..246...48B
Altcode: 2019arXiv191202959B
Direct evidence of an inertial-range turbulent energy cascade has been
provided by spacecraft observations in heliospheric plasmas. In the
solar wind, the average value of the derived heating rate near 1 au
is $\sim {10}^{3}\,{\rm{J}}\,{\mathrm{kg}}^{-1}\,{{\rm{s}}}^{-1}$ ,
an amount sufficient to account for observed departures from adiabatic
expansion. Parker Solar Probe, even during its first solar encounter,
offers the first opportunity to compute, in a similar fashion, a
fluid-scale energy decay rate, much closer to the solar corona than any
prior in situ observations. Using the Politano-Pouquet third-order law
and the von Kármán decay law, we estimate the fluid-range energy
transfer rate in the inner heliosphere, at heliocentric distance
R ranging from 54 R⊙ (0.25 au) to 36 R⊙
(0.17 au). The energy transfer rate obtained near the first perihelion
is about 100 times higher than the average value at 1 au, which is in
agreement with estimates based on a heliospheric turbulence transport
model. This dramatic increase in the heating rate is unprecedented in
previous solar wind observations, including those from Helios, and
the values are close to those obtained in the shocked plasma inside
the terrestrial magnetosheath.
Title: Observations of Energetic-particle Population Enhancements
along Intermittent Structures near the Sun from the Parker Solar Probe
Authors: Bandyopadhyay, Riddhi; Matthaeus, W. H.; Parashar, T. N.;
Chhiber, R.; Ruffolo, D.; Goldstein, M. L.; Maruca, B. A.; Chasapis,
A.; Qudsi, R.; McComas, D. J.; Christian, E. R.; Szalay, J. R.; Joyce,
C. J.; Giacalone, J.; Schwadron, N. A.; Mitchell, D. G.; Hill, M. E.;
Wiedenbeck, M. E.; McNutt, R. L., Jr.; Desai, M. I.; Bale, Stuart D.;
Bonnell, J. W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey, Peter R.;
MacDowall, Robert J.; Malaspina, David M.; Pulupa, Marc; Velli, M.;
Kasper, J. C.; Korreck, K. E.; Stevens, M.; Case, A. W.; Raouafi, N.
Bibcode: 2020ApJS..246...61B
Altcode: 2019arXiv191203424B
Observations at 1 au have confirmed that enhancements in measured
energetic-particle (EP) fluxes are statistically associated with "rough"
magnetic fields, I.e., fields with atypically large spatial derivatives
or increments, as measured by the Partial Variance of Increments (PVI)
method. One way to interpret this observation is as an association
of the EPs with trapping or channeling within magnetic flux tubes,
possibly near their boundaries. However, it remains unclear whether
this association is a transport or local effect; I.e., the particles
might have been energized at a distant location, perhaps by shocks
or reconnection, or they might experience local energization or
re-acceleration. The Parker Solar Probe (PSP), even in its first
two orbits, offers a unique opportunity to study this statistical
correlation closer to the corona. As a first step, we analyze the
separate correlation properties of the EPs measured by the Integrated
Science Investigation of the Sun (IS⊙IS) instruments during the
first solar encounter. The distribution of time intervals between
a specific type of event, I.e., the waiting time, can indicate the
nature of the underlying process. We find that the IS⊙IS observations
show a power-law distribution of waiting times, indicating a correlated
(non-Poisson) distribution. Analysis of low-energy (∼15 - 200 keV/nuc)
IS⊙IS data suggests that the results are consistent with the 1
au studies, although we find hints of some unexpected behavior. A
more complete understanding of these statistical distributions will
provide valuable insights into the origin and propagation of solar EPs,
a picture that should become clear with future PSP orbits.
Title: Exploring Solar Wind Origins and Connecting Plasma Flows
from the Parker Solar Probe to 1 au: Nonspherical Source Surface
and Alfvénic Fluctuations
Authors: Panasenco, Olga; Velli, Marco; D'Amicis, Raffaella; Shi,
Chen; Réville, Victor; Bale, Stuart D.; Badman, Samuel T.; Kasper,
Justin; Korreck, Kelly; Bonnell, J. W.; Wit, Dudok de Thierry; Goetz,
Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina, David M.;
Pulupa, Marc; Case, Anthony W.; Larson, Davin; Livi, Roberto; Stevens,
Michael; Whittlesey, Phyllis
Bibcode: 2020ApJS..246...54P
Altcode:
The magnetic field measurements of the FIELDS instrument on the
Parker Solar Probe (PSP) have shown intensities, throughout its first
solar encounter, that require a very low source surface (SS) height (
${R}_{\mathrm{SS}}\leqslant 1.8\,{R}_{\odot }$ ) to be reconciled with
magnetic field measurements at the Sun via potential field extrapolation
(PFSS). However, during PSP's second encounter, the situation
went back to a more classic SS height ( ${R}_{\mathrm{SS}}\leqslant
2.5\,{R}_{\odot }$ ). Here we use high-resolution observations of the
photospheric magnetic field (Solar Dynamics Observatory/Helioseismic
and Magnetic Imager) to calculate neutral lines and boundaries of the
open field regions for SS heights from 1.2 to 2.5 R⊙ using
an evolving PFSS model and the measured solar wind speed to trace the
source of the wind observed by PSP to the low corona and photosphere. We
adjust RSS to get the best match for the field polarity
over the period 2018 October-November and 2019 March-April, finding
that the best fit for the observed magnetic field polarity inversions
requires a nonspherical SS. The geometry of the coronal hole boundaries
for different RSS is tested using the PSP perihelion passes,
3D PFSS models, and LASCO/C2 observations. We investigate the sources
of stronger-than-average magnetic fields and times of Alfvénic fast
and slow wind. Only some of the strongly Alfvénic slow wind streams
seen by PSP survive and are observed at 1 au: the origins and peculiar
topology of the background in which they propagate is discussed.
Title: Magnetic Connectivity of the Ecliptic Plane within 0.5 au:
Potential Field Source Surface Modeling of the First Parker Solar
Probe Encounter
Authors: Badman, Samuel T.; Bale, Stuart D.; Martínez Oliveros, Juan
C.; Panasenco, Olga; Velli, Marco; Stansby, David; Buitrago-Casas,
Juan C.; Réville, Victor; Bonnell, John W.; Case, Anthony W.; Dudok
de Wit, Thierry; Goetz, Keith; Harvey, Peter R.; Kasper, Justin
C.; Korreck, Kelly E.; Larson, Davin E.; Livi, Roberto; MacDowall,
Robert J.; Malaspina, David M.; Pulupa, Marc; Stevens, Michael L.;
Whittlesey, Phyllis L.
Bibcode: 2020ApJS..246...23B
Altcode: 2019arXiv191202244B
We compare magnetic field measurements taken by the FIELDS instrument
on board Parker Solar Probe (PSP) during its first solar encounter
to predictions obtained by potential field source surface (PFSS)
modeling. Ballistic propagation is used to connect the spacecraft to the
source surface. Despite the simplicity of the model, our results show
striking agreement with PSP's first observations of the heliospheric
magnetic field from ∼0.5 au (107.5 R⊙) down to 0.16
au (35.7 R⊙). Further, we show the robustness of the
agreement is improved both by allowing the photospheric input to the
model to vary in time, and by advecting the field from PSP down to
the PFSS model domain using in situ PSP/Solar Wind Electrons Alphas
and Protons measurements of the solar wind speed instead of assuming
it to be constant with longitude and latitude. We also explore
the source surface height parameter (RSS) to the PFSS
model, finding that an extraordinarily low source surface height
(1.3-1.5 R⊙) predicts observed small-scale polarity
inversions, which are otherwise washed out with regular modeling
parameters. Finally, we extract field line traces from these models. By
overlaying these on extreme ultraviolet images we observe magnetic
connectivity to various equatorial and mid-latitude coronal holes,
indicating plausible magnetic footpoints and offering context for
future discussions of sources of the solar wind measured by PSP.
Title: The Role of Alfvén Wave Dynamics on the Large-scale Properties
of the Solar Wind: Comparing an MHD Simulation with Parker Solar
Probe E1 Data
Authors: Réville, Victor; Velli, Marco; Panasenco, Olga; Tenerani,
Anna; Shi, Chen; Badman, Samuel T.; Bale, Stuart D.; Kasper, J. C.;
Stevens, Michael L.; Korreck, Kelly E.; Bonnell, J. W.; Case, Anthony
W.; de Wit, Thierry Dudok; Goetz, Keith; Harvey, Peter R.; Larson,
Davin E.; Livi, Roberto; Malaspina, David M.; MacDowall, Robert J.;
Pulupa, Marc; Whittlesey, Phyllis L.
Bibcode: 2020ApJS..246...24R
Altcode: 2019arXiv191203777R
During Parker Solar Probe's first orbit, the solar wind plasma
was observed in situ closer than ever before, the perihelion on
2018 November 6 revealing a flow that is constantly permeated by
large-amplitude Alfvénic fluctuations. These include radial magnetic
field reversals, or switchbacks, that seem to be a persistent feature
of the young solar wind. The measurements also reveal a very strong,
unexpected, azimuthal velocity component. In this work, we numerically
model the solar corona during this first encounter, solving the MHD
equations and accounting for Alfvén wave transport and dissipation. We
find that the large-scale plasma parameters are well reproduced,
allowing the computation of the solar wind sources at Probe with
confidence. We try to understand the dynamical nature of the solar
wind to explain both the amplitude of the observed radial magnetic
field and of the azimuthal velocities.
Title: Switchbacks in the Near-Sun Magnetic Field: Long Memory and
Impact on the Turbulence Cascade
Authors: Dudok de Wit, Thierry; Krasnoselskikh, Vladimir V.; Bale,
Stuart D.; Bonnell, John W.; Bowen, Trevor A.; Chen, Christopher
H. K.; Froment, Clara; Goetz, Keith; Harvey, Peter R.; Jagarlamudi,
Vamsee Krishna; Larosa, Andrea; MacDowall, Robert J.; Malaspina, David
M.; Matthaeus, William H.; Pulupa, Marc; Velli, Marco; Whittlesey,
Phyllis L.
Bibcode: 2020ApJS..246...39D
Altcode: 2019arXiv191202856D
One of the most striking observations made by Parker Solar Probe
during its first solar encounter is the omnipresence of rapid polarity
reversals in a magnetic field that is otherwise mostly radial. These
so-called switchbacks strongly affect the dynamics of the magnetic
field. We concentrate here on their macroscopic properties. First,
we find that these structures are self-similar, and have neither a
characteristic magnitude, nor a characteristic duration. Their waiting
time statistics show evidence of aggregation. The associated long memory
resides in their occurrence rate, and is not inherent to the background
fluctuations. Interestingly, the spectral properties of inertial range
turbulence differ inside and outside of switchback structures; in the
latter the 1/f range extends to higher frequencies. These results
suggest that outside of these structures we are in the presence
of lower-amplitude fluctuations with a shorter turbulent inertial
range. We conjecture that these correspond to a pristine solar wind.
Title: Observations of Heating along Intermittent Structures in the
Inner Heliosphere from PSP Data
Authors: Qudsi, R. A.; Maruca, B. A.; Matthaeus, W. H.; Parashar,
T. N.; Bandyopadhyay, Riddhi; Chhiber, R.; Chasapis, A.; Goldstein,
Melvyn L.; Bale, S. D.; Bonnell, J. W.; Dudok de Wit, T.; Goetz, K.;
Harvey, P. R.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Kasper,
J. C.; Korreck, K. E.; Case, A. W.; Stevens, M.; Whittlesey, P.;
Larson, D.; Livi, R.; Velli, M.; Raouafi, N.
Bibcode: 2020ApJS..246...46Q
Altcode: 2019arXiv191205483Q
The solar wind proton temperature at 1 au has been found to be
correlated with small-scale intermittent magnetic structures,
I.e., regions with enhanced temperature are associated with coherent
structures, such as current sheets. Using Parker Solar Probe data from
the first encounter, we study this association using measurements of the
radial proton temperature, employing the partial variance of increments
(PVI) technique to identify intermittent magnetic structures. We observe
that the probability density functions of high PVI events have higher
median temperatures than those with lower PVI. The regions in space
where PVI peaks were also locations that had enhanced temperatures
when compared with similar regions, suggesting a heating mechanism in
the young solar wind that is associated with intermittency developed
by a nonlinear turbulent cascade in the immediate vicinity.
Title: Sharp Alfvénic Impulses in the Near-Sun Solar Wind
Authors: Horbury, Timothy S.; Woolley, Thomas; Laker, Ronan; Matteini,
Lorenzo; Eastwood, Jonathan; Bale, Stuart D.; Velli, Marco; Chandran,
Benjamin D. G.; Phan, Tai; Raouafi, Nour E.; Goetz, Keith; Harvey,
Peter R.; Pulupa, Marc; Klein, K. G.; Dudok de Wit, Thierry; Kasper,
Justin C.; Korreck, Kelly E.; Case, A. W.; Stevens, Michael L.;
Whittlesey, Phyllis; Larson, Davin; MacDowall, Robert J.; Malaspina,
David M.; Livi, Roberto
Bibcode: 2020ApJS..246...45H
Altcode:
Measurements of the near-Sun solar wind by the Parker Solar Probe
have revealed the presence of large numbers of discrete Alfvénic
impulses with an anti-sunward sense of propagation. These are similar
to those previously observed near 1 au, in high speed streams over the
Sun's poles and at 60 solar radii. At 35 solar radii, however, they
are typically shorter and sharper than seen elsewhere. In addition,
these spikes occur in "patches" and there are also clear periods within
the same stream when they do not occur; the timescale of these patches
might be related to the rate at which the spacecraft magnetic footpoint
tracks across the coronal hole from which the plasma originated. While
the velocity fluctuations associated with these spikes are typically
under 100 km s-1, due to the rather low Alfvén speeds
in the streams observed by the spacecraft to date, these are still
associated with large angular deflections of the magnetic field—and
these deflections are not isotropic. These deflections do not appear
to be related to the recently reported large-scale, pro-rotation
solar wind flow. Estimates of the size and shape of the spikes reveal
high aspect ratio flow-aligned structures with a transverse scale of
≈104 km. These events might be signatures of near-Sun
impulsive reconnection events.
Title: Solar Probe Cup — First Results
Authors: Case, A. W.; Kasper, J.; Stevens, M.; Korreck, K.; Mello,
T.; Lamirato, T.; Larson, D.; Whittlesey, P.; Livi, R.; Horbury, T.;
Klein, K.; Velli, M.; Bale, S.; Pulupa, M.; Malaspina, D.; Bonnell,
J.; Harvey, P.; Goetz, K.; Dudok de Wit, T.; MacDowall, R.
Bibcode: 2020AAS...23514909C
Altcode:
The Solar Probe Cup (SPC) is a scientific instrument on the Parker Solar
Probe (PSP) mission designed to measure the thermal plasma present in
the solar corona and solar wind. PSP has now completed three orbits of
the Sun, approaching as close as 35 solar radii and sampling a region
of space that has been previously unexplored. Despite the extremely
harsh operating environment, SPC has successfully made measurements
throughout those orbits that reveal a new view of the physical
processes taking place in the near-Sun environment. Throughout these
solar encounters, many transient features have been seen in the solar
wind data, consisting of temporally short (seconds to 10s of minutes)
spikes in the solar wind speed that then return to a baseline speed,
occurring along with switchbacks in the magnetic field, in which the
radial component of the magnetic briefly changes sign. Additionally,
the first two orbits have provided us measurements that hint at PSP's
location relative to the Alfven point. This presentation will discuss
SPC's successful operation over the first three orbits, characterize
the statistical properties of the switchback features (e.g., sizes,
durations, and flow directions), and provide an initial calculation
of the Alfven point location relative to PSP.
Title: Alfvénic fluctuations in the solar wind: nonlinearities and
pressure anisotropy effects
Authors: Tenerani, Anna; Velli, Marco
Bibcode: 2020PPCF...62a4001T
Altcode:
Large amplitude, turbulent Alfvénic fluctuations have been commonly
observed in the solar wind since the first in situ measurements. An
important but still unexplained property of such nonlinear fluctuations
seen typically in the fastest streams is that, despite the large
excursion of the magnetic field fluctuations, the magnitude of
the total magnetic field remains nearly constant, a condition that
corresponds to spherical polarization. How is this Alfvénic turbulent
state achieved in the solar wind remains a fundamental open question
in space physics. Although nonlinear Alfvénic fluctuations have been
studied for several decades, most of previous work has considered a
plasma in thermodynamic equilibrium. The solar wind however displays
many non-thermal features and here we discuss how non-thermal effects,
in particular pressure anisotropy, and nonlinearities affect the
stability and nonlinear evolution of Alfvénic fluctuations with
constant total magnetic field magnitude in different plasma-β regimes.
Title: Large amplitude Alfvénic turbulence, Switchbacks and the
Acceleration of the Solar Wind.
Authors: Velli, Marco
Bibcode: 2020APS..DPPB01002V
Altcode:
Perhaps the most striking observation made by Parker Solar Probe
during its first few orbits is that of the prevalence of extremely
large amplitude oscillations in the radial magnetic field, leading
to reversals in sign not connected to crossings of the heliospheric
current sheet but rather to kinks of the field line themselves, as
demonstrated by the permanence of the electron pitch angle. Such rapid
folds in the field, also called switchbacks, are seen with periods
going from seconds to more than an hour, while an analysis of the
corresponding velocity field shows that the fluctuations in radial
velocity, δVr are well correlated to those of the radial
magnetic field, with a correlation with δBr corresponding to
Alfvén waves propagating away from the Sun. Switchbacks however belong
to a well - developed power spectrum, so the appropriate description is
that of Alfvénic turbulence. In addition, the magnitude of the total
magnetic field often remains almost constant, i.e., the compressibility
of the fluctuations is very small. The present talk will discuss
these intriguing Probe observations, including the prevalence of high
velocity magnetic field correlation even in extremely low speed wind,
to suggest scenarios for the origin and evolution of such fluctuations
in the solar wind, and their potential role in coronal heating and
solar wind acceleration. This research was funded by the FIELDS
experiment on the Parker Solar Probe spacecraft, designed and developed
under NASA contract NNN06AA01C.
Title: Spectral signatures of recursive magnetic field reconnection
Authors: Tenerani, A.; Velli, M.
Bibcode: 2020MNRAS.491.4267T
Altcode: 2019MNRAS.tmp.2963T; 2019arXiv190705243T; 2019MNRAS.tmp.2898T
We use 2.5D magnetohydrodynamic simulations to investigate the
spectral signatures of the non-linear disruption of a tearing unstable
current sheet via the generation of multiple secondary current
sheets and magnetic islands. During the non-linear phase of tearing
mode evolution, there develops a regime in which the magnetic energy
density shows a spectrum with a power law close to B(k)2 ∼
k-0.8. Such an energy spectrum is found in correspondence
of the neutral line, within the diffusion region of the primary
current sheet, where energy is conveyed towards smaller scales via a
'recursive' process of fast tearing-type instabilities. Far from the
neutral line, we find that magnetic energy spectra evolve towards
slopes compatible with the 'standard' Kolmogorov spectrum. Starting
from a self-similar description of the non-linear stage at the neutral
line, we provide a model that predicts a reconnecting magnetic field
energy spectrum scaling as k-4/5, in good agreement with
numerical results. An extension of the predicted power law to generic
current sheet profiles is also given and possible implications for
turbulence phenomenology are discussed. These results provide a step
forward to understand the 'recursive' generation of magnetic islands
(plasmoids), which has been proposed as a possible explanation for the
energy release during flares, but which, more in general, can have an
impact on the subsequent turbulent evolution of unstable sheets that
naturally form in the high Lundquist number and collisionless plasmas
found in most of the astrophysical environments.
Title: A New Model for Self-Consistent Simulations of Kinetic Dynamics
in the Expanding Solar Wind
Authors: Innocenti, Maria Elena; Boella, Elisabetta; Tenerani, Anna;
Velli, Marco
Bibcode: 2020APS..DPPTO16004
Altcode:
With the launch of Solar Orbiter, it is now possible to probe
magnetically connected solar wind plasma across significantly separated
heliocentric distances (at Parker Solar Probe, Solar Orbiter, Earth),
and have a direct insight into the evolution of solar wind kinetic
process with heliocentric distance. Kinetic features are ubiquitous
in the young solar wind and rarer (but still non negligible) at
1 AU. During propagation, kinetic processes constrain solar wind
parameters and regulate heat flux. We simulate this evolution with
the fully kinetic semi-implicit Expanding Box Model code EB-iPic3D,
which models kinetically a solar wind plasma parcel moving away from
the Sun while expanding in the transverse direction. We investigate
how plasma expansion triggers the onset and modifies the evolution of
kinetic instabilities (eg, electron firehose and whistler instability)
that constrain solar wind parameters and impact heat flux evolution with
heliocentric distance. We then study the competition of expansion and
turbulence in determining the solar wind temperature radial dependence.
Title: Propagation of Alfvén Waves in the Expanding Solar Wind with
the Fast-Slow Stream Interaction
Authors: Shi, Chen; Velli, Marco; Tenerani, Anna; Rappazzo, Franco;
Réville, Victor
Bibcode: 2020ApJ...888...68S
Altcode: 2019arXiv191002108S
We carry out two-dimensional magnetohydrodynamic simulations of an
ensemble of Alfvénic fluctuations propagating in a structured,
expanding solar wind including the presence of fast and slow
solar wind streams. Using an appropriate expanding box model, the
simulations incorporate the effects of fast-slow stream shear and
compression and rarefaction self-consistently. We investigate the
radial and longitudinal evolution of the cross helicity, the total
and residual energies and the power spectra of outward and inward
Alfvénic fluctuations. The stream interaction is found to strongly
affect the radial evolution of Alfvénic turbulence. The total energy
in the Alfvén waves is depleted within the velocity shear regions,
accompanied by the decrease of the normalized cross helicity. The
presence of stream compression facilitates this process. Residual energy
fluctuates around zero due to the correlation and de-correlation between
the inward/outward waves but no net growth or decrease of the residual
energy is observed. The radial power spectra of the inward/outward
Alfvén waves show significant longitudinal variations. Kolmogorov-like
spectra are developed only inside the fast and slow streams and when
both the compression and shear are present. On the other hand, the
spectra along the longitudinal direction show clear Kolmogorov-like
inertial ranges in all cases.
Title: The role of Alfvén wave dynamics in the large scale properties
of the solar wind: comparing 3D MHD simulation and PSP data
Authors: Réville, V.; Velli, M.; Panasenco, O.; Tenerani, A.; Shi,
C.; Rouillard, A. P.; Bale, S. D.; Kasper, J. C.; Badman, S. T.;
Korreck, K. E.; Pulupa, M.; Bonnell, J. W.; Case, A. W.; Larson,
D. E.; Livi, R.; Stevens, M. L.; Whittlesey, P. L.; Malaspina, D.;
Harvey, P.; Goetz, K.; Dudok de Wit, T.; MacDowall, R. J.
Bibcode: 2019AGUFMSH51A..03R
Altcode:
The first two encounters of Parker Solar Probe have shown features
that already challenge our understanding of the solar wind. During
E1, PSP went through a slow Alfvénic solar wind, likely coming from
equatorial regions. Large amplitude Alfvén waves are present over
many frequencies and show a spherical polarization consistent with
non-linear solutions of the MHD equations. To study this phenomenon we
use 3D MHD simulations of the solar corona, including the propagation
and the dissipation of Alfvén waves to power the solar wind. We first
check the agreement of the simulations with coronal images obtained
from EUV instruments as well as white light images obtained with WISPR
onboard PSP. We then can find the sources of the observed solar wind and
compare with simpler potential field models (PFSS). Finally, we propose
a way to interpret the differences in the properties of the simulations
and the observed data, by accounting for the wave dynamics in the large
scale (or average) solar wind properties. This could lead to important
progress regarding the open flux problem and the computation of the
solar wind angular momentum. This research was supported by NASA
Parker Solar Probe Observatory Scientist grant NNX 15AF34G and by the
European Research Council (ERC) project SLOW_SOURCE - DLV-819189 .
Title: Observations of Magnetic Island Formation by the Wide Field
Imager on Parker Solar Probe (WISPR/PSP)
Authors: Linton, M.; Stenborg, G.; Howard, R. A.; Ko, Y. K.; Vourlidas,
A.; Higginson, A. K.; Tenerani, A.; Velli, M.; Liewer, P. C.
Bibcode: 2019AGUFMSH33D3397L
Altcode:
We report on observations by the Wide Field Imager on Parker Solar
Probe of possible magnetic island formation in a solar coronal streamer
current sheet. Remote sensing observations during the second perihelion
of Parker Solar Probe show the formation and ejection of an elliptical
structure in the center of the streamer current sheet. The morphology
and evolution of this candidate magnetic island is consistent with
magnetohydrodynamical simulations of island formation and ejection
in solar wind current sheets. We will review the relevant theory and
simulations as they relate to this observation, and will compare
and contrast these observations with corresponding remote sensing
observation of candidate island formation events made from 1 AU by
the SOHO and STEREO spacecraft. This work was supported by the
NASA Parker Solar Probe Program Office.
Title: Linear and nonlinear evolution of jets and microstreams in
the solar wind
Authors: Derr, J.; Tenerani, A.; Velli, M.
Bibcode: 2019AGUFMSH53B3397D
Altcode:
Fast solar wind streams are known to be dominated by Alfvénic
turbulence, i.e. large amplitude magnetic field and quasi-incompressible
velocity fluctuations with a correlation corresponding to waves
propagating away from the Sun. At the same time the Ulysses spacecraft
showed that microstreams, persistent long period (1/2-2 days)
fluctuations in the radial velocity field are ubiquitous in the fast
wind. This contribution explores the possible causal relation between
microstreams and Alfvénic turbulence. We carry out a parametric
study of the stability of the microstream jets to Kelvin-Helmholtz
(KH) instabilities: starting from the profiles of density, radial
speed and magnetic field observed in the solar wind, we investigate
both at what distance from the Sun KH instabilities may be triggered
and the ensuing nonlinear dynamics.
Title: Coronal Origins of the Alfvénic Slow Solar Wind
Authors: Panasenco, O.; Velli, M.; D'Amicis, R.
Bibcode: 2019AGUFMSH44A..04P
Altcode:
As demonstrated by the Ulysses mission the filling factor of the
slow wind in the heliosphere is too large to arise only from the
helmet streamer cusps, so magnetic field and plasma transport and
instabilities involving processes at coronal hole boundaries and quiet
sun must be at work. Outwardly propagating Alfvénic fluctuations are
usually hosted by fast solar wind streams, however a number of slow
solar wind periods have been identified where the turbulence is also
dominated by outward Alfvénic modes (Marsch et al. 1981, D'Amicis and
Bruno 2015 and initial Parker Solar Probe results Bale et al. 2019). 80%
of the wind at Helios was shown to be Alfvénic (Stansby et al. 2019)
and ~ 37% Alfvenic slow. Is the difference between Alfvénic slow wind
and standard slow wind associated with a different dynamics, or is
the coronal topology at the source completely different, as initial
indications seem to show? Here we discuss magnetic topology
and properties of the coronal sources for the peculiar Alfvénic
slow solar wind. We illustrate the specific role played by different
coronal hole types (polar CHs, equatorial extensions of polar CHs,
isolated CHs both at high latitude and close to the equator), as well
as by solar filaments and active regions at coronal hole boundaries,
that strongly influence the magnetic topology of the lower corona and
solar wind properties. Pseudostreamers (PSs) are multipolar features,
which develop into open fields that are unipolar at greater heights
requiring the presence of two or more nearby coronal holes of the same
polarity. MHD solar wind models along magnetic field lines show that
the properties of the solar wind emanating from CHs with pseudostreamers
are different from regular CHs (Panasenco et al. 2019). Here we explain
the coronal conditions required for the development of Alfvénic slow
solar wind.
Title: Sharp Alfvenic Impulses in the Near-Sun Solar Wind: Properties
and Possible Origins
Authors: Horbury, T. S.; Matteini, L.; Woolley, T.; Laker, R.; Perrone,
D.; Stansby, D.; Velli, M.; Chandran, B. D. G.; Bale, S. D.; Kasper,
J. C.; Stevens, M. L.; Pulupa, M.; Korreck, K. E.; Larson, D. E.;
Livi, R.; Whittlesey, P. L.; Malaspina, D.; Bonnell, J. W.; Harvey,
P.; Goetz, K.; Dudok de Wit, T.; MacDowall, R. J.
Bibcode: 2019AGUFMSH51A..01H
Altcode:
Parker Solar Probe has revealed the presence of large numbers
of discrete Alfvenic impulses in the near-Sun solar wind with an
anti-Sunward sense of propagation. These are similar to those previously
observed near 1 AU and in high speed streams over the Sun's poles and at
60 solar radii. At 35 solar radii, however, they are typically shorter
and sharper than seen elsewhere. In addition, these spikes occur in
"patches" and there are also clear periods within the same stream
when they do not occur. While the velocity fluctuations associated
with these spikes are typically under 100 km/s due to the rather low
Alfven speeds in the streams observed by Probe to date, these are still
associated with large angular deflections of the magnetic field - and
these deflections are not isotropic. We discuss the scales, amplitudes
and orientations of these structures and their links to other properties
measured by Probe, such as the bulk plasma flow. We also discuss how
these new observations, combined with those from earlier missions,
provide evidence for the possible origins of these events and in
particular whether they are the long-sought interplanetary signature
of discrete reconnection jets in the solar corona.
Title: Parker Solar Probe Observations of Magnetic Reconnection
Exhausts during Encounter 1
Authors: Phan, T.; Bale, S. D.; Eastwood, J. P.; Lavraud, B.; Pulupa,
M.; Stevens, M. L.; MacDowall, R. J.; Case, A. W.; Larson, D. E.;
Kasper, J. C.; Whittlesey, P. L.; Szabo, A.; Korreck, K. E.; Bonnell,
J. W.; Drake, J. F.; Dudok de Wit, T.; Goetz, K.; Harvey, P.; Horbury,
T. S.; Lehman, E.; Livi, R.; Malaspina, D.; Oieroset, M.; Paulson,
K.; Phan, J.; Shay, M. A.; Velli, M.; Weschler, B.
Bibcode: 2019AGUFMSH23A..05P
Altcode:
Magnetic reconnection in current sheets converts magnetic energy into
particle energy. It has been suggested that reconnection may therefore
play an important role in the heating of the solar wind close to
the Sun. Observations from Parker Solar Probe (PSP) provide a new
opportunity to study this problem, as it measures the solar wind in
situ at unprecedented distances to the Sun. During the 1st
orbit, PSP encountered a large number of current sheets in the solar
wind through perihelion at 35.7 solar radii. We have performed a
comprehensive survey of these current sheets and found clear evidence
for reconnection exhausts in a variety of phenomena including (1)
Heliospheric current sheets, (2) coronal mass ejections, (3) small
flux ropes, and (4) regular solar wind. However, we find that the
majority of the current sheets encountered around the closest approach
to the Sun were mostly Alfvenic structures associated with bursty radial
jets. Although it has been suggested that these Alfvenic structures may
be driven by reconnection lower in the corona, the majority of these
current sheets do not appear to be undergoing local reconnection. We
will show sample examples of reconnection in the aforementioned
phenomena, and discuss why some current sheets reconnect, while others
do not. The PSP findings could help reveal the key conditions that
control the presence or absence of reconnection in current sheets.
Title: Combining Remote and in situ Parker Solar Probe and STEREO
Data to Understand Solar Wind Density Structures
Authors: Viall, N. M.; Howard, R. A.; Vourlidas, A.; DeForest, C.;
Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens, M. L.; Whittlesey,
P. L.; Larson, D. E.; Livi, R.; Szabo, A.; Kepko, L.; Lavraud, B.;
Rouillard, A. P.; Velli, M.
Bibcode: 2019AGUFMSH13C3432V
Altcode:
The instrument suite on Parker Solar Probe offers an unprecedented
viewpoint of the ambient solar wind and structure therein, shortly after
its formation and release from the solar corona. We take advantage of
the synergistic observations of the first Parker Solar Probe encounters
and the STEREO COR2 deep field campaigns covering the same time periods
to study mesoscale solar wind density structures. They often occur
in a quasi-periodic train, especially near the heliospheric current
sheet. Some may be a consequence of the development of dynamics en
route; many are remnants of the formation and release of the solar
wind, and provide important constraints on solar wind models. The
opportunity to combine the different observing angles and fields of
view of the white light WISPR observations and white light STEREO COR2
observations with in situ density and plasma measurements from SWEAP
allows better understanding of the characteristics and properties of
mesoscale density structures. The in situ data measure precise size
scales, plasma boundaries, and relationships between density and
other parameters. They help in the interpretation of the structures
seen in white light images and in unraveling projection effects. The
white light images enhance the in situ data by providing global
heliospheric context, as well as the occurrence rate and 2-D size
scales of structures as a function of latitude and distance from the
Sun. Together, these observations provide crucial constraints on the
formation of structures in the solar wind.
Title: An introductory guide to fluid models with anisotropic
temperatures. Part 1. CGL description and collisionless fluid
hierarchy
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Khomenko, E.;
Goldstein, M. L.; Webb, G. M.; Cally, P. S.; Collados, M.; Velli,
M.; Adhikari, L.
Bibcode: 2019JPlPh..85f2002H
Altcode: 2019arXiv190109354H
We present a detailed guide to advanced collisionless fluid models
that incorporate kinetic effects into the fluid framework, and that are
much closer to the collisionless kinetic description than traditional
magnetohydrodynamics. Such fluid models are directly applicable to
modelling the turbulent evolution of a vast array of astrophysical
plasmas, such as the solar corona and the solar wind, the interstellar
medium, as well as accretion disks and galaxy clusters. The text
can be viewed as a detailed guide to Landau fluid models and it is
divided into two parts. Part 1 is dedicated to fluid models that
are obtained by closing the fluid hierarchy with simple (non-Landau
fluid) closures. Part 2 is dedicated to Landau fluid closures. Here
in Part 1, we discuss the fluid model of Chew-Goldberger-Low (CGL)
in great detail, together with fluid models that contain dispersive
effects introduced by the Hall term and by the finite Larmor radius
corrections to the pressure tensor. We consider dispersive effects
introduced by the non-gyrotropic heat flux vectors. We investigate
the parallel and oblique firehose instability, and show that the
non-gyrotropic heat flux strongly influences the maximum growth rate of
these instabilities. Furthermore, we discuss fluid models that contain
evolution equations for the gyrotropic heat flux fluctuations and that
are closed at the fourth-moment level by prescribing a specific form
for the distribution function. For the bi-Maxwellian distribution,
such a closure is known as the `normal' closure. We also discuss a
fluid closure for the bi-kappa distribution. Finally, by considering
one-dimensional Maxwellian fluid closures at higher-order moments,
we show that such fluid models are always unstable. The last possible
non Landau fluid closure is therefore the `normal' closure, and beyond
the fourth-order moment, Landau fluid closures are required.
Title: Flocculation, switchbacks, and loss of Alfvenicity: Indicators
of shear-driven turbulence in the young solar wind?
Authors: Matthaeus, W. H.; Ruffolo, D. J.; DeForest, C.; Parashar,
T.; Goldstein, M. L.; Roberts, D. A.; Chhiber, R.; Usmanov, A. V.;
Dudok de Wit, T.; Bandyopadhyay, R.; Chasapis, A.; Maruca, B.; Velli,
M. C. M.; Kasper, J. C.
Bibcode: 2019AGUFMSH53B3374M
Altcode:
Since the first preliminary announcements of Parker Solar Probe
results [1], there has been increased discussion of "switchbacks"
and speed enhancements such as those observed in Helios data [2]. A
familiar explanation relies on outward propagation of large amplitude
remnants of magnetic reconnection at lower altitudes. Such a mechanism
is plausible and difficult to rule out. However, another possibility
exists, namely that the onset of strong shear-driven turbulence,
beginning just outside the Alfvén critical region, may induce the
switchbacks through large-scale perturbation of the flow. This scenario
is consistent with a suite of observable effects already apparent in
imaging [3] and in situ datasets [2]. DeForest et al. interpreted the
transition from elongated striae to relatively isotropic flocculae
as a signature of the onset of shear-driven turbulent activity some
20-80 Rs from the photosphere, where the magnetic field ceases to be
a dominant constraint on transverse motions; this interpretation has
received support from turbulence-driven global simulations of the solar
wind [4]. The presence of velocity shears is also strongly suggested by
coronal imaging at lower altitudes [5]. Somewhere above the conventional
Alfvén point such shears can begin supplying turbulence energy [6]
while also destroying Alfvénicity by injection of kinetic energy but
not cross helicity [7]. If indeed the flocculation signifies large
fluctuations or even turnover associated with vortices, then specific
features of shear driven turbulence may be anticipated in imaging data
from the upcoming PUNCH mission and in ongoing analysis of in situ
Parker Solar Probe observations. Details of these signatures will be
given here. Research supported in part by grant RTA5980003 from the
Thailand Research Fund, by NASA under NNX17AB79G, 80NSSC18K1210,
80NSSC18K1648, and by the PSP ISOIS project as subcontract under
NNN06AA01C. [1] S. Bale, invited talk, 2018 Fall AGU Meeting
[2] T. Horbury, L. Matteini & D. Stansby, MNRAS 478, 1980
(2018) [3] C. DeForest et al., Astrophys. J. 828, 66 (2016) [4] R. Chhiber et al., Astophys. J. Lett. 856, L39 (2018) [5]
C. DeForest et al., Astrophys. J. 862, 18 (2018) [6] G. Zank et
al., JGR 101, 17093 (1996); B. Breech et al., JGR 113, A08105 (2008)
[7] D. A. Roberts et al., JGR 97, 17115 (1992); see also Fig. 3
of D. A. Roberts, Astrophys. J. 711, 1044 (2010)
Title: Temperature-anisotropy-driven instabilities and electron
and ion energy budget in the expanding solar wind: fully-kinetic
Expanding Box Model simulations with EB-iPic3D
Authors: Innocenti, M. E.; Tenerani, A.; Boella, E.; Velli, M.
Bibcode: 2019AGUFMSH52A..04I
Altcode:
Observations (Matteini et al, 2013; Stverak et al, 2008 ),
analytical work (Yoon 2017), simulations (Matteini et al 2006,
Hellinger et al 2008, Innocenti et al, sub) point to the role of
kinetic instabilities in constraining the bulk parameters of the ion
and electron populations in the solar wind. In particular, ion and
electron firehose instabilities constrain solar wind observations in
the Tperp< Tpar regime, towards which the
solar wind is pushed by spherical plasma expansion. Analytical
work (Yoon et al 2017) and simulations (Micera et al, sub) point
to temperature-anisotropy-driven instabilities as a mean for energy
exchange between ions and electrons. This would be consistent with
the electron and ion temperature evolution as a function of the
heliocentric distance, which highlights the need for a positive
and negative energy balance for ions and electrons to explain
observed trends (Stverak et al, 2015). We investigate these
dynamics through fully kinetic simulations where ion and electron
temperature-anisotropy-induced kinetic instabilities develop self
consistently as a result of solar wind expansion. The simulations are
carried out with EB-iPic3D (Innocenti et al, 2019), a semi-implicit,
Expanding Box, fully kinetic code. Our simulations allow us to
investigate in details the electron/ion energy exchange and the
trajectory of a solar wind plasma parcel in the ßpar
vs Tperp/Tpar plane, as a function of the
heliocentric distance. They also show that, under certain solar wind
conditions, the thermal anisotropy triggering the electron firehose
instability is drastically reduced by the instability itself through
the generation of short-scale, short-lived, complex electron velocity
distribution patterns. Hellinger,P.,& Travnıcek,P.M.2008,
JGR: SP, 113 Innocenti, M. E., Tenerani, A., & Velli,
M. 2019, APJ, 870, 66 Innocenti, M.E., Tenerani, A., Boella,
E.,& Velli, M. Submitted Matteini, L., Landi, S., Hellinger,
P., & Velli, M. 2006, JGR: SP, 111 Matteini, L., Hellinger,
P., Goldstein, B. E., et al. 2013,JGR: SP, 118, 2771 Micera A.,
Boella E., Zhukov, A. N., Shaaban, S. M., Lazar M., & Lapenta,
G., arXiv:1907.08502
Stverak, S., Travnıcek, P., Maksimovic, M., et al. 2008,JGR: SP,
113 Stverak, S., Travnıcek, P. M., & Hellinger, P. 2015,JGR:
SP, 120, 8177 Yoon, P. H., & Sarfraz, M. 2017, ApJ, 835, 246
Title: Statistics of Energetic Particles in the first Parker Solar
Probe Orbit: Correlations and Association with Magnetic Structures
Authors: Matthaeus, W. H.; Bandyopadhyay, R.; Parashar, T.; Chasapis,
A.; Chhiber, R.; Ruffolo, D. J.; Qudsi, R. A.; McComas, D. J.;
Christian, E. R.; Szalay, J. R.; Joyce, C.; Goldstein, M. L.; Maruca,
B.; Giacalone, J.; Schwadron, N.; Mitchell, D. G.; Wiedenbeck, M. E.;
McNutt, R. L., Jr.; Bale, S.; Bonnell, J. W.; Dudok de Wit, T.; Goetz,
K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Kasper,
J. C.; Korreck, K. E.; Case, A. W.; Stevens, M.; Whittlesey, P. L.;
Larson, D. E.; Livi, R.; Velli, M.; Raouafi, N.
Bibcode: 2019AGUFMSH52A..07M
Altcode:
Observations at 1 au have confirmed that measured energetic particle
fluxes are statistically associated with "rough" magnetic fields, as
measured by the Partial Variance of Increments (PVI) method [1]. This
has been interpreted as association due to trapping within magnetic flux
tubes, or near their boundaries[2,3]. However it has so far remained
unclear if this is a transport effect, with particles energized at
a distant location, perhaps by shocks or reconnection, or, on the
other hand, if the particles might experience local energization, or
re-acceleration. Parker Solar Probe (PSP), even in its first orbit,
offers a unique opportunity to examine this phenomenon closer to
the corona. There, if due to transport, the effect may be recorded
closer to the sources, or earlier in the acceleration process. If
the effect is due to local acceleration, the parameters in which it
is observed will likely be much different than at 1 au. As a first
step, we analyze the separate correlation properties of the magnetic
signal recorded by the MAG instrument, and the energetic particles
recorded by the ISOIS instruments. We find that FGM observations show
a power-law distribution of waiting times in the magnetic-field PVI,
while an analysis of ISOIS counts also shows power laws, indicating
a correlated (non-Poisson) distribution. Preliminary analysis of low
energy ISOIS data suggests results consistent with the Tessein et al
studies [2,3], while results of higher energy count distributions
conditioned on PVI are less clear. A more complete understanding
of these statistical distributions will provide valuable insights
into the origin and propagation of solar energetic particles, a
picture that should become clear with future PSP orbits. Research
partially supported by a subcontract to NASA NNN06AA01C. [1]
A. Greco, W. H. Matthaeus, S. Perri, K. T. Osman, S. Servidio,
M. Wan and P. Dmitruk, Space Sci Rev., 214, 1 (2018) [2]
J. A. Tessein, D. Ruffolo, W. H. Matthaeus et al., ApJ, 812, 68 (2015),
https://doi.org/10.1088/0004-637X/812/1/68 [3] J. A. Tessein,
D. Ruffolo, W. H. Matthaeus, and M. Wan. Geophys. Res. Lett., 43:3620,
(2016).
Title: Young Solar Wind in the Grip of the Sun's Corona
Authors: Kasper, J. C.; Bale, S. D.; Belcher, J. W.; Berthomier, M.;
Case, A. W.; Chandran, B. D. G.; Curtis, D. W.; Gallagher, D. L.;
Gary, S. P.; Golub, L.; Halekas, J. S.; Ho, G. C.; Horbury, T. S.;
Hu, Q.; Huang, J.; Klein, K. G.; Korreck, K. E.; Larson, D. E.; Livi,
R.; Maruca, B.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Martinović,
M.; McGinnis, D.; Pogorelov, N. V.; Richardson, J. D.; Skoug, R. M.;
Steinberg, J. T.; Stevens, M. L.; Szabo, A.; Velli, M.; Whittlesey,
P. L.; Wright, K. H., Jr.; Zank, G. P.; MacDowall, R. J.; McComas,
D. J.; McNutt, R. L., Jr.; Pulupa, M.; Raouafi, N. E.; Schwadron, N.
Bibcode: 2019AGUFMSH11A..02K
Altcode:
The birthplace of the solar wind is the corona of our Sun, where
unidentified mechanisms heat the plasma to millions of Kelvin and
magnetic fields shape the flow of particles and waves. The plasma is
unstable, accelerating as it expands away from the Sun, exceeding
the speed of sound at a heliocentric distance of a few solar radii
(Rs) and the Alfvén speed at 36 Rs, beyond which the wind decouples
from the corona. Here, we show the first observations of the young,
low-Alfvén-mach-number wind obtained by the Solar Wind Electrons Alphas
and Protons (SWEAP) plasma instruments on Parker Solar Probe (PSP)
during its first two encounters with the Sun. Once PSP dropped below a
quarter of the distance from the Sun to the Earth, SWEAP began to detect
(for the first time) a persistent and growing rotational circulation
of the plasma around the Sun peaking at 40-50 km/s at perihelion as the
Alfvén mach number fell to 1.5. This finding may support theories for
enhanced stellar angular momentum loss due to rigid coronal rotation,
but the circulation is large, and angular momentum does not appear
to be conserved, suggesting that torques still act on the young wind
at these distances. PSP also measured numerous intense and organized
Alfvénic velocity spikes with strong propagating field reversals
and large jumps in speed. These field reversals and jets call for an
overhaul in our understanding of the turbulent fluctuations that may,
by energizing the solar wind, hold the key to its origin.
Title: Large scale 1/f magnetic field spectrum in the solar wind
close to the Sun: comparison between 0.15 and 0.3AU
Authors: Matteini, L.; Chen, C. H. K.; Stansby, D.; Horbury, T. S.;
Perrone, D.; Tenerani, A.; Velli, M.; Bale, S.; Pulupa, M.; Malaspina,
D.; Bonnell, J. W.; Harvey, P.; Goetz, K.; Dudok de Wit, T.; MacDowall,
R. J.
Bibcode: 2019AGUFMSH21C3329M
Altcode:
We investigate properties of the spectrum of magnetic field fluctuations
observed by PSP in the solar wind inside 0.3 AU. We focus on large
scales (low frequencies in the spacecraft frame) above the MHD-inertial
range with typical spectral index -5/3, where the spectrum displays
a shallower slope, close to -1. The radial evolution of the break
scale separating the inertial and 1/f ranges is investigated for
different wind regimes and compared with analogous conditions observed
by Helios at 0.3 AU and beyond. We analyze data taking into account
different physical parameters that can play a role in the evolution
of the fluctuations, such as their absolute and relative amplitude,
the flow expansion rate, and the estimated non-linear time of turbulent
interactions. Observations are discussed in the framework of existing
models for the solar wind 1/f spectrum and results are compared with
theoretical predictions.
Title: Waiting time (distance) distributions of magnetic field and
velocity PVI events during the first Parker Solar Probe encounter
Authors: Chhiber, R.; Goldstein, M. L.; Matthaeus, W. H.;
Bandyopadhyay, R.; Maruca, B.; Parashar, T.; Ruffolo, D. J.; Qudsi,
R. A.; Bale, S. D.; Chasapis, A.; Bonnell, J. W.; Dudok de Wit, T.;
Goetz, K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.;
Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens, M. L.; Whittlesey,
P. L.; Larson, D. E.; Livi, R.; Velli, M.; Raouafi, N. E.
Bibcode: 2019AGUFMSH13C3451C
Altcode:
During the Parker Solar Probe's (PSP) first perihelion pass, the
spacecraft reached to within a heliocentric distance of ~37 solar
radii and observed magnetic and flow structures characterized by
sharp gradients. As we try and understand these intermittent coronal
structures better, an important property to examine is their degree
of correlation. To this end, we use the well-tested Partial Variance
of Increments (PVI) technique [1] to identify intermittent events
in FIELDS and SWEAP observations of magnetic and velocity fields. We
then examine the distributions of waiting times between events with
varying separation and PVI levels. We find power-law distributions,
suggesting a high degree of correlation that may originate in a
clustering process, as opposed to a random distribution produced by
a memory-less Poisson process [2]. We also find that waiting times
between events with separations larger than inertial-range scales
follow a power-law close to -1, hinting at a possible connection with
observations of "1/f noise" associated with signals originating near
the source solar surface [3]. The present study complements the one by
Dudok de Wit et al., which focuses on the waiting times between the
observed "switchbacks" in the radial magnetic field. [1] Greco
et al. Space Sci. Rev. (2018) 214:1 [2] Greco et al. Phys. Rev. E
(2009) 80, 046401 [3] Matthaeus & Goldstein PRL (1986) 57, 4
Title: MHD-Scale Energy Transfer in the Inner Heliosphere from
PSP observations
Authors: Bandyopadhyay, R.; Goldstein, M. L.; Maruca, B.; Matthaeus,
W. H.; Parashar, T.; Ruffolo, D. J.; Chhiber, R.; Usmanov, A. V.;
Chasapis, A.; Qudsi, R. A.; Bale, S.; Bonnell, J. W.; Dudok de Wit,
T.; Goetz, K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.;
Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens, M. L.; Whittlesey,
P. L.; Larson, D. E.; Livi, R.; Velli, M. C. M.; Raouafi, N. E.
Bibcode: 2019AGUFMSH21C3318B
Altcode:
Observations at 1 AU have reported direct evidence of an inertial-range
energy cascade [1]. The average value of energy cascade rate in 1 AU
solar wind plasma is around 1000 J/kg/s, which is shown to be sufficient
to account for the heating of the solar-wind [2]. Parker Solar Probe
(PSP) offers the first opportunity to estimate a similar, fluid-scale
energy decay rate closer to the solar corona. Using a Politano-Pouquet
[3] third-order law, we provide estimates of fluid-range energy
cascade rate at 50 to 36 solar radii, during the first perihelion first
encounter of PSP. Despite the cross-helicity being high in these regions
of the heliosphere, there is an inertial-range cascade occurring. The
energy transfer rate is at least 100 times higher than the average
value at 1AU. Further, we estimate the global energy decay rate at
the energy-containing scales using a Taylor-Karman decay phenomenology
[4]. The von Karman energy decay estimates agree reasonably well with
the third-order-law estimates. We also compare the two estimates with
the heating rate obtained from a turbulence-based, global solar wind
simulation [5]. [1] Sorriso-Valvo et al., Phys. Rev. Lett., 99,
115001 (2007) [2] MacBride et al., ApJ, 679, 1644 (2008) [3]
Politano & Pouquet, GRL, 25, 273 (1998) [4] Wan et al., JFM,
697, 296315 (2012) [5] Usmanov et al., ApJ, 865, 25 (2018)
Title: The Solar Orbiter Heliospheric Imager (SoloHI) for the Solar
Orbiter Mission: Science and Instrument Status
Authors: Vourlidas, A.; Howard, R. A.; Colaninno, R. C.; Korendyke,
C.; Thernisien, A.; Linton, M.; Tun Beltran, S.; Liewer, P. C.; Velli,
M.; Linker, J.; Bothmer, V.; Rochus, P. L.; Lamy, P. L.
Bibcode: 2019AGUFMSH24A..08V
Altcode:
The SoloHI instrument has completed its development effort and has been
integrated onto the Solar Orbiter spacecraft. The mission, scheduled
for launch in February 2020, will undergo gravity assist maneuvers
around Venus to change both the perihelion distance as well as the
plane of the orbit to ultimately achieve a minimum perihelion of 0.28
AU and an orbital inclination of about 35° relative to the ecliptic
plane. The remote sensing instruments will operate for three 10-day
periods out of the nominal 6-month orbit. SoloHI detects sunlight
scattered by free electrons in the corona and solar wind from 5° to
45° elongation in visible wavelengths, providing linkage between solar
and solar wind observations. The science investigation focuses mainly on
the solar wind, including streamers, small-scale intensity and density
fluctuations, jets, and Coronal Mass Ejections (CMEs). SoloHI is very
similar to the HI-1 instrument on STEREO/SECCHI but with double the FOV
of HI-1. In this paper, we present our preparations for the mission
including the instrument status, our science planning strategy, our
observing plans for cruise phase, calibrations, early science and our
low-latency and science data products This work has been supported
by NASA.
Title: An introductory guide to fluid models with anisotropic
temperatures. Part 2. Kinetic theory, Padé approximants and Landau
fluid closures
Authors: Hunana, P.; Tenerani, A.; Zank, G. P.; Goldstein, M. L.;
Webb, G. M.; Khomenko, E.; Collados, M.; Cally, P. S.; Adhikari, L.;
Velli, M.
Bibcode: 2019JPlPh..85f2003H
Altcode: 2019arXiv190109360H
In Part 2 of our guide to collisionless fluid models, we concentrate
on Landau fluid closures. These closures were pioneered by Hammett
and Perkins and allow for the rigorous incorporation of collisionless
Landau damping into a fluid framework. It is Landau damping that sharply
separates traditional fluid models and collisionless kinetic theory,
and is the main reason why the usual fluid models do not converge to the
kinetic description, even in the long-wavelength low-frequency limit. We
start with a brief introduction to kinetic theory, where we discuss in
detail the plasma dispersion function Z(ζ), and the associated plasma
response function R(ζ)=1+ζZ(ζ)=-Z^' }(ζ)/2. We then consider a
one-dimensional (1-D) (electrostatic) geometry and make a significant
effort to map all possible Landau fluid closures that can be constructed
at the fourth-order moment level. These closures for parallel moments
have general validity from the largest astrophysical scales down to
the Debye length, and we verify their validity by considering examples
of the (proton and electron) Landau damping of the ion-acoustic mode,
and the electron Landau damping of the Langmuir mode. We proceed by
considering 1-D closures at higher-order moments than the fourth order,
and as was concluded in Part 1, this is not possible without Landau
fluid closures. We show that it is possible to reproduce linear
Landau damping in the fluid framework to any desired precision,
thus showing the convergence of the fluid and collisionless kinetic
descriptions. We then consider a 3-D (electromagnetic) geometry in the
gyrotropic (long-wavelength low-frequency) limit and map all closures
that are available at the fourth-order moment level. In appendix Ae
provide comprehensive tables with Padé approximants of R(ζ) up to
the eighth-pole order, with many given in an analytic form.
Title: Alfvénic velocity spikes and rotational flows in the near-Sun
solar wind
Authors: Kasper, J. C.; Bale, S. D.; Belcher, J. W.; Berthomier,
M.; Case, A. W.; Chandran, B. D. G.; Curtis, D. W.; Gallagher, D.;
Gary, S. P.; Golub, L.; Halekas, J. S.; Ho, G. C.; Horbury, T. S.;
Hu, Q.; Huang, J.; Klein, K. G.; Korreck, K. E.; Larson, D. E.; Livi,
R.; Maruca, B.; Lavraud, B.; Louarn, P.; Maksimovic, M.; Martinovic,
M.; McGinnis, D.; Pogorelov, N. V.; Richardson, J. D.; Skoug, R. M.;
Steinberg, J. T.; Stevens, M. L.; Szabo, A.; Velli, M.; Whittlesey,
P. L.; Wright, K. H.; Zank, G. P.; MacDowall, R. J.; McComas, D. J.;
McNutt, R. L.; Pulupa, M.; Raouafi, N. E.; Schwadron, N. A.
Bibcode: 2019Natur.576..228K
Altcode:
The prediction of a supersonic solar wind1 was first
confirmed by spacecraft near Earth2,3 and later
by spacecraft at heliocentric distances as small as 62 solar
radii4. These missions showed that plasma accelerates
as it emerges from the corona, aided by unidentified processes that
transport energy outwards from the Sun before depositing it in the
wind. Alfvénic fluctuations are a promising candidate for such a
process because they are seen in the corona and solar wind and contain
considerable energy5-7. Magnetic tension forces the corona
to co-rotate with the Sun, but any residual rotation far from the Sun
reported until now has been much smaller than the amplitude of waves
and deflections from interacting wind streams8. Here we
report observations of solar-wind plasma at heliocentric distances
of about 35 solar radii9-11, well within the distance
at which stream interactions become important. We find that Alfvén
waves organize into structured velocity spikes with duration of up to
minutes, which are associated with propagating S-like bends in the
magnetic-field lines. We detect an increasing rotational component
to the flow velocity of the solar wind around the Sun, peaking at
35 to 50 kilometres per second—considerably above the amplitude of
the waves. These flows exceed classical velocity predictions of a few
kilometres per second, challenging models of circulation in the corona
and calling into question our understanding of how stars lose angular
momentum and spin down as they age12-14.
Title: Measures of Scale Dependent Alfvénicity in the First PSP
Solar Encounter
Authors: Parashar, T. N.; Goldstein, M. L.; Maruca, B. A.; Matthaeus,
W. H.; Ruffolo, D.; Bandyopadhyay, R.; Chhiber, R.; Chasapis, A.;
Qudsi, R.; Vech, D.; Roberts, D. A.; Bale, S. D.; Bonnell, J. W.; Dudok
de Wit, T.; Goetz, K.; Harvey, P. R.; MacDowall, R. J.; Malaspina,
D.; Pulupa, M.; Kasper, J. C.; Korreck, K. E.; Case, A. W.; Stevens,
M.; Whittlesey, P.; Larson, D.; Livi, R.; Velli, M.; Raouafi, N.
Bibcode: 2019arXiv191207181P
Altcode:
The solar wind shows periods of highly Alfvénic activity, where
velocity fluctuations and magnetic fluctuations are aligned or
anti-aligned with each other. It is generally agreed that solar
wind plasma velocity and magnetic field fluctuations observed
by Parker Solar Probe (PSP) during the first encounter are mostly
highly Alfvénic. However, quantitative measures of Alfvénicity are
needed to understand how the characterization of these fluctuations
compares with standard measures from prior missions in the inner
and outer heliosphere, in fast wind and slow wind, and at high and
low latitudes. To investigate this issue, we employ several measures
to quantify the extent of Alfvénicity -- the Alfvén ratio $r_A$,
{normalized} cross helicity $\sigma_c$, {normalized} residual energy
$\sigma_r$, and the cosine of angle between velocity and magnetic
fluctuations $\cos\theta_{vb}$. We show that despite the overall
impression that the Alfvénicity is large in the solar wind sampled
by PSP during the first encounter, during some intervals the cross
helicity starts decreasing at very large scales. These length-scales
(often $> 1000 d_i$) are well inside inertial range, and therefore,
the suppression of cross helicity at these scales cannot be attributed
to kinetic physics. This drop at large scales could potentially be
explained by large-scale shears present in the inner heliosphere sampled
by PSP. In some cases, despite the cross helicity being constant down
to the noise floor, the residual energy decreases with scale in the
inertial range. These results suggest that it is important to consider
all these measures to quantify Alfvénicity.
Title: Alfvenic slow solar wind and proton temperature anisotropy
in inner heliosphere by PSP observations
Authors: Huang, J.; Kasper, J. C.; Vech, D.; Klein, K. G.; Velli,
M.; Stevens, M. L.; Paulson, K.; Maruca, B.; Qudsi, R. A.; Alterman,
B. L.; Lavraud, B.; Case, A. W.; Korreck, K. E.; Bale, S.; Larson,
D. E.; Livi, R.; Whittlesey, P. L.; Pulupa, M.; MacDowall, R. J.;
Malaspina, D.; Bonnell, J. W.; Harvey, P.; Goetz, K.; Dudok de Wit, T.
Bibcode: 2019AGUFMSH13C3452H
Altcode:
We use Parker Solar Probe (PSP) observations to investigate two aspects
of the inner heliosphere solar wind: (1) Alfvenic slow wind and (2)
proton temperature anisotropy . (1) Slow solar wind is generally
found to have low Alfvenicity, but PSP observed mainly slow solar
wind with high Alfvenicity during its first two encounters. Based on
PSP measurements, we study the characteristics of Alfvenic slow solar
wind near the Sun, in particular the variation of helium abundance
and proton temperature anisotropy. (2) We fit the proton temperature
anisotropy using SWEAP and magnetic field observations from the FIELDS
instrument suite. Accordingly, we study the temperature anisotropy
variations as a function of plasma beta, heliocentric distance,
and different solar wind stream. These results may lead to some new
understandings of solar wind origin and evolution.
Title: Magnetic connectivity of the ecliptic plane within 0.5 AU :
PFSS modelling of the early PSP encounters
Authors: Badman, S. T.; Bale, S. D.; Martinez Oliveros, J. C.;
Panasenco, O.; Velli, M.; Stansby, D.; Buitrago-Casas, J. C.; Réville,
V.; Pulupa, M.; Malaspina, D.; Bonnell, J. W.; Harvey, P.; Goetz,
K.; Dudok de Wit, T.; MacDowall, R. J.; Kasper, J. C.; Case, A. W.;
Korreck, K. E.; Larson, D. E.; Livi, R.; Stevens, M. L.; Whittlesey,
P. L.
Bibcode: 2019AGUFMSH13C3453B
Altcode:
We compare Parker Solar Probe (PSP) FIELDS early magnetic field
measurements to predictions obtained by Potential Field Source Surface
modeling (PFSS). Ballistic propagation (Parker spiral assumption)
is used to connect the spacecraft to the source surface. Despite the
simplicity of the model, our results show striking agreement with PSP's
first observations of the heliospheric magnetic field from 0.5 AU down
to 0.16 AU. Further, we show the robustness of the agreement is improved
both by allowing the photospheric input to the model to vary in time,
and by advecting the field from PSP down to the PFSS model domain
using in situ PSP/SWEAP measurements of the solar wind speed instead
of assuming it to be constant with longitude and latitude. We also
explore the source surface height parameter to the PFSS model: Overall,
we find evidence that a lower source surface height (< 2 solar radii)
provides improvements to the prediction. We find for PSP Encounter 1
(Nov. 2018) that an extraordinarily low source surface height (1.3-1.5
solar radii) predicts observed small scale polarity inversions which
are otherwise washed out with regular modeling parameters. Finally,
we extract field line traces from these models. By overlaying these
on EUV images we observe magnetic connectivity to various equatorial
and mid-latitude coronal holes, providing a sanity check and offering
context for future discussions of sources of the solar wind measured
by PSP.
Title: Intermittent heating in the inner Heliosphere: PSP observations
Authors: Qudsi, R. A.; Maruca, B.; Matthaeus, W. H.; Parashar, T.;
Bandyopadhyay, R.; Chhiber, R.; Chasapis, A.; Kasper, J. C.; Korreck,
K. E.; Case, A. W.; Stevens, M. L.; Whittlesey, P. L.; Larson, D. E.;
Livi, R.; Goldstein, M. L.; Bale, S.; Bonnell, J. W.; Dudok de Wit,
T.; Goetz, K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa,
M.; Velli, M.; Raouafi, N. E.
Bibcode: 2019AGUFMSH21C3317Q
Altcode:
Solar Wind temperature at 1 AU exhibits statistical correlation with
the magnetic structure, wherein regions with high temperature are found
to be associated with coherent structures [1]. Using Parker Solar Probe
(PSP) data from the first encounter, we studied this correlation between
the magnetic field structure, measured using the Partial Variance of
Increments (PVI) [2], and the radial temperature of the ionized hydrogen
atoms (protons). For the magnetic field, we used the low cadence data
from FIELDS instrument and for proton temperature we used the moments
data from SWEAP. We observed that the probability distribution function
(PDF) of events with high PVIs have a higher median temperature than
those with lower PVI, implying the presence of heating mechanism in
the solar wind, associated with turbulence driven structures. [1]
Osman, K. T., Matthaeus, W. H., Greco, A., & Servidio, S.2011, ApJ,
727, L11 [2] A. Greco, W. H. Matthaeus, S,. Perri, K. T. Osman,
S. Servidio, M. Wan and P. Dmitruk, Space Sci Rev., 214, 1 (2018)
Title: Proton-Alpha Temperature Relaxation and Preferential Heating
in the Corona: a Ulysses Case Study.
Authors: Sun, W.; Shi, C.; Velli, M.; Tenerani, A.
Bibcode: 2019AGUFMSH53B3384S
Altcode:
The solar wind in the inner heliosphere displays many non-equilibrium
features, including unequal temperatures of different ion species,
temperature anisotropies and beams. Previous work (Kasper et al.,
2017) has shown that in the ecliptic plane and for solar wind whose
speed is under 450 km/s, a zone of preferential ion heating extends
about 20-40 solar radius from the sun. Beyond this zone, the solar
wind helium-to-hydrogen temperature ratio declines with increasing
Coulomb collisional age. Here we attempt the same determination by
using the Ulysses data set. The Ulysses spacecraft carried out an
extensive investigation of the heliosphere outside of the ecliptic
plane over about a full solar cycle. Its location varied over distances
from 1.3 to 5 AU and latitudes up to 80o. We report a
similar preferential heating zone exists whose boundary height is
calculated. Also, we analyze the boundary's oscillation with the
solar cycle.
Title: The magnetic structure and electrodynamics of the emerging
solar wind
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; Dudok de Wit, T.;
Eastwood, J. P.; Webster, J.; Farrell, W. M.; Fong, C.; Goetz, K.;
Goldstein, M. L.; Goodrich, K.; Harvey, P.; Horbury, T. S.; Howes,
G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Martinez Oliveros,
J. C.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F.;
Phan, T.; Pulupa, M.; Raouafi, N. E.; Salem, C. S.; Stansby, D.;
Stevens, M. L.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
Bibcode: 2019AGUFMSH11A..05B
Altcode:
Convection and rotation drive the solar dynamo and, ultimately,
provide the mechanical energy flux required to heat the solar corona
and accelerate the solar wind. However, the way in which energy is then
dissipated to heat the corona and wind are not well understood. Some
energization models invoke non-thermal energy flux imparted by plasma
Alfvén waves, while others rely on a carpet of small nano-flares as
energy input, however these models have been unconstrained by direct
measurements of the solar wind near its origin. Here we use in situ
measurements from the FIELDS instrument suite during the first solar
encounter (E1) at 35.7 solar radii (Rs) of the NASA Parker Solar
Probe (PSP) mission to reveal the magnetic structure and kinetics of
slow Alfvénic solar wind emerging from a small, equatorial coronal
hole. Our measurements show that, at solar minimum, the slow wind can
escape from above the low-lying, complex magnetic structures of the
equatorial streamer belt, carrying a magnetic field that is highly
dynamic, exhibiting polarity reversals on timescales from seconds
to hours. These rapidly oscillating field structures are associated
with clustered radial jets of plasma in which the energy flux is
dramatically enhanced and turbulence levels are higher. Time intervals
between groups of jets indicate a solar wind that is steady with a
mostly radial magnetic field and relatively low levels of Alfvénic
turbulent fluctuations. This 'quiet' wind however shows clear signatures
of plasma micro-instabilities associated with ion and electron beams
and velocity-space structure.
Title: Solar wind heating by Alfvén waves: compressible effects
Authors: Réville, V.; Velli, M.; Tenerani, A.; Shi, C.
Bibcode: 2019sf2a.conf..365R
Altcode:
We study the heating produced by a compressible cascade in
unidimensional solutions of the solar wind using the numerical setup
described in \citet{Reville2018}. Alfvén waves are injected from the
photosphere and may be, depending on their frequency and amplitude,
unstable to parametric decay, in which case they create a compressible
cascade of forward and inward Elsässer variables. Dissipation at small
scales then create an extended heat deposition in the corona, which
accelerates the wind in addition to the wave pressure. This process
can provide enough heating to fully sustain a solar wind solution.
Title: Photospheric Vortices and Coronal Energy Storage And Release
Authors: Rappazzo, F.; Velli, M. C. M.; Einaudi, G.; Dahlburg, R. B.
Bibcode: 2019AGUFMSH53B3376R
Altcode:
The dynamics of a closed corona where photospheric vortices twist the
coronal magnetic field lines is investigated by means of a Cartesian
reduced magnetohydrodynamic (MHD) model. We consider first two
corotating or counter-rotating vortices localized at the center of
a photospheric plate, and additionally more corotating vortices that
fill the plate entirely. The other plate is line-tied. After an initial
linear transient stage, during which the vortices create laminar and
smoothly twisting flux tubes, the system relaxes to a fully non-linear
state that approaches a form of statistical stationarity: the main goal
of this investigation is to understand this state, i.e. a vortex-forced
coronal magnetic configuration permeated by finite amplitude broadband
fluctuations. We find that depending on the system parameters and the
arrangement and handedness of the photospheric vortices an inverse
cascade storing a significant amount of magnetic energy may or may
not occur. In the first case a reservoir of magnetic energy available
to large events such as destabilization of a pre-CME configuration
develops, while in the second case the outcome is a turbulent heated
corona. Although our geometry is simplified our simulations are shown
to have relevant implications for coronal dynamics and CME initiation.
Title: Parker Solar Probe: Mission Status and Outlook After One Year
of Operation
Authors: Raouafi, N. E.; Bale, S.; Kasper, J. C.; Howard, R. A.;
McComas, D. J.; Velli, M.; Posner, A.; Szabo, A.
Bibcode: 2019AGUFMSH11A..01R
Altcode:
NASA's Parker Solar Probe, which launched on August 12, 2018, flew
closer to our star than any spacecraft has come before. Parker Solar
Probe completed two solar orbits and started the third one, all with
a perihelion of 35.6 Solar Radii. The second Venus gravity assist will
take place on December 26, 2019, after which the orbit perihelion will
decrease to 27.8 Solar Radii. Parker will potentially revolutionize
our understanding of this mysterious region by answering questions
that puzzled scientists for decades: how the solar wind is heated
and accelerated and how solar energetic particles are accelerated and
transported throughout the heliosphere.Data from the first two orbits
show plasma properties that have not been observed before in the solar
wind.The initial results of the mission and the data from the first
two orbits will be published in the fall. We provide an overview on
the status and outlook of the mission after the first year of operation.
Title: Propagation of Alfvén waves and evolution of turbulence in
the expanding solar wind with the presence of stream interaction
Authors: Shi, C.; Velli, M.; Tenerani, A.; Réville, V.
Bibcode: 2019AGUFMSH51A..05S
Altcode:
We carry out two-dimensional magnetohydrodynamic (MHD) simulations
of Alfvénic fluctuations propagating in a structured solar wind
including the effects of spherical expansion. In the simulations,
fluctuations propagate in a wind that includes the effects of
fast-slow stream interactions with shear, compression and rarefaction
self-consistently. We investigate the radial evolution and the
longitudinal variation of quantities that are frequently used in
turbulence studies, e.g. the cross helicity, the residual energy,
the spectra of Elsässer variables, etc. We show that the stream
interaction strongly affects the radial evolution of Alfvénic
turbulence. The total energy in the Alfvén waves is depleted within
the velocity shear regions, accompanied by a decrease of the normalized
cross helicity, with and without stream compression. The presence
of compression facilitates this process. Residual energy fluctuates
around zero due to the correlation and de-correlation between the
inward/outward waves but no systematic growth of the residual energy
is observed. The parallel power spectra of the outward/inward Alfvén
waves show significant longitudinal variations. Kolmogorov-like spectra
are developed only inside the fast and slow streams and when both
the compression and shear are present. However, the spectra along the
longitudinal direction show clear Kolmogorov-like inertial ranges in
all cases. Dominance of magnetic energy and decrease of cross helicity
are observed near the current sheet embedded inside the slow wind. We
propose to use the Parker Solar Probe data to study the radial evolution
of the Alfvénic turbulence in the future.
Title: Observed Properties of Solar Wind Jets inside 0.25 AU
Authors: Case, A. W.; Kasper, J. C.; Lamirato, T. R.; Mello, T.;
Stevens, M. L.; Korreck, K. E.; Larson, D. E.; Livi, R.; Whittlesey,
P. L.; Horbury, T. S.; Klein, K. G.; Velli, M.; Bale, S. D.; Pulupa,
M.; Malaspina, D.; Bonnell, J. W.; Harvey, P.; Goetz, K.; Dudok de Wit,
T.; MacDowall, R. J.
Bibcode: 2019AGUFMSH12A..06C
Altcode:
Over the course of the first two orbits of Parker Solar Probe (PSP),
the spacecraft has made solar wind and interplanetary magnetic field
measurements inside 0.25 AU for the first time. Being this close to
the Sun allows a view of the solar wind much closer to the source of
its initial acceleration than has been previously observed. At this
distance, smaller features have not yet interacted with the surrounding
plasma, and are still clearly visible with in situ data. Observations
from these first two orbits show an extraordinary number of transient
features that appear as "jets" in the plasma data. These jets consist of
temporally short (seconds to 10s of minutes) spikes in the solar wind
speed that promptly return to the baseline wind speed. The velocity
spikes are accompanied by "switchbacks" in the magnetic field, where
the radial component of the magnetic field briefly changes sign and
the electron strahl direction follows the field reversal. This study
characterizes the statistical properties of these jets (e.g., sizes,
durations, and flow directions) to develop a picture of the overall
structure of the jets and to assess whether they may have a significant
influence over the dynamics of solar wind in the inner heliosphere.
Title: Fast Magnetic Reconnection in the Presence of a Normal
Component: Macroscopic Fluid Prediction and Microscopic Physics
Through Kinetic Simulations with Pictor.
Authors: Pucci, F.; Velli, M.; Tenerani, A.; Shi, C.; Kumar, R.;
Ergun, R.
Bibcode: 2019AGUFMSM13D3344P
Altcode:
In many configurations, from magnetospheric tails to line-tied
magnetic fields in the solar corona, current sheets contain a small
non vanishing normal component, i.e. the field does not present null
lines in any projection. Here we present an analysis of the linear
stability of the tearing instability for non-neutral current sheets
in terms of the intensity of the normal component Bn and the aspect
ratio L/a. In particular, we derive the critical aspect ratio scalings
at which the growth rates become independent of the Lundquist number
S. We also address the transition from macroscopic fluid scales to
kinetic scales as the thickness of the sheet decreases, performing
a kinetic simulation for a Harris sheet configuration altered by a
normal component. This preliminary simulation with the PIC code PICTOR
will then allow us to address how the criterion for fast reconnection
in thicker current sheets evolves into fast kinetic reconnection with
increasing Lundquist number.
Title: Alfvenicity in PSP observations: comparing different measures
Authors: Bandyopadhyay, R.; Parashar, T. N.; Goldstein, M. L.; Maruca,
B.; Matthaeus, W. H.; Chasapis, A.; Chhiber, R.; Ruffolo, D. J.;
Qudsi, R. A.; Bale, S.; Bonnell, J. W.; Dudok de Wit, T.; Goetz,
K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.; Pulupa, M.; Kasper,
J. C.; Korreck, K. E.; Case, A. W.; Stevens, M. L.; Whittlesey, P. L.;
Larson, D. E.; Livi, R.; Velli, M.; Raouafi, N. E.
Bibcode: 2019AGUFMSH13C3436B
Altcode:
Alfv'enicity is an important concept in plasma dynamics, but its
meaning is sometimes ambiguous, as it is used to imply different
(related) constructs by different authors. There are three measures:
Alfvén ratio r_A
Title: Numerical simulations of the evolution of magnetic field
kinks in the solar wind
Authors: Tenerani, A.; Velli, M.; Réville, V.; Shi, C.; Bale,
S. D.; Kasper, J. C.; Stevens, M. L.; Case, A. W.; Korreck, K. E.;
Larson, D. E.; Livi, R.; Klein, K. G.; Whittlesey, P. L.; Pulupa, M.;
Malaspina, D.; Bonnell, J. W.; Harvey, P.; Goetz, K.; Dudok de Wit,
T.; MacDowall, R. J.
Bibcode: 2019AGUFMSH51A..02T
Altcode:
We investigate, via Magnetohydrodynamic (MHD) numerical simulations,
the evolution of large amplitude Alfvénic fluctuations that include a
component parallel to the background magnetic field leading to kinked
magnetic field lines. When the amplitude of those kinks is sufficiently
large, the total magnetic field folds back on itself leading to a local
inversion of its polarity. Local magnetic field polarity inversions,
also known as "switchbacks", are common features of the Alfvénic solar
wind and have been observed in a wide range of heliocentric distances
(from 0.3 AU all the way out to a few AU), and most recently by Parker
Solar Probe during its first two encounters. The stability and evolution
of switchbacks in the expanding solar wind has not yet been studied
in detail. In this work we compare the dynamical evolution of exact
nonlinear Alfvénic solutions to the MHD system, characterized by
constant total magnetic pressure, with the evolution of unbalanced
structures, and we determine under which conditions "switchbacks"
can propagate in the solar wind, their stability and lifetime.
Title: Highly structured slow solar wind emerging from an equatorial
coronal hole
Authors: Bale, S. D.; Badman, S. T.; Bonnell, J. W.; Bowen, T. A.;
Burgess, D.; Case, A. W.; Cattell, C. A.; Chandran, B. D. G.;
Chaston, C. C.; Chen, C. H. K.; Drake, J. F.; de Wit, T. Dudok;
Eastwood, J. P.; Ergun, R. E.; Farrell, W. M.; Fong, C.; Goetz,
K.; Goldstein, M.; Goodrich, K. A.; Harvey, P. R.; Horbury, T. S.;
Howes, G. G.; Kasper, J. C.; Kellogg, P. J.; Klimchuk, J. A.; Korreck,
K. E.; Krasnoselskikh, V. V.; Krucker, S.; Laker, R.; Larson, D. E.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D. M.; Martinez-Oliveros,
J.; McComas, D. J.; Meyer-Vernet, N.; Moncuquet, M.; Mozer, F. S.;
Phan, T. D.; Pulupa, M.; Raouafi, N. E.; Salem, C.; Stansby, D.;
Stevens, M.; Szabo, A.; Velli, M.; Woolley, T.; Wygant, J. R.
Bibcode: 2019Natur.576..237B
Altcode:
During the solar minimum, when the Sun is at its least active, the solar
wind1,2 is observed at high latitudes as a predominantly fast
(more than 500 kilometres per second), highly Alfvénic rarefied stream
of plasma originating from deep within coronal holes. Closer to the
ecliptic plane, the solar wind is interspersed with a more variable slow
wind3 of less than 500 kilometres per second. The precise
origins of the slow wind streams are less certain4; theories
and observations suggest that they may originate at the tips of helmet
streamers5,6, from interchange reconnection near coronal hole
boundaries7,8, or within coronal holes with highly diverging
magnetic fields9,10. The heating mechanism required to
drive the solar wind is also unresolved, although candidate mechanisms
include Alfvén-wave turbulence11,12, heating by reconnection
in nanoflares13, ion cyclotron wave heating14
and acceleration by thermal gradients1. At a distance of
one astronomical unit, the wind is mixed and evolved, and therefore
much of the diagnostic structure of these sources and processes
has been lost. Here we present observations from the Parker Solar
Probe15 at 36 to 54 solar radii that show evidence of
slow Alfvénic solar wind emerging from a small equatorial coronal
hole. The measured magnetic field exhibits patches of large,
intermittent reversals that are associated with jets of plasma and
enhanced Poynting flux and that are interspersed in a smoother and
less turbulent flow with a near-radial magnetic field. Furthermore,
plasma-wave measurements suggest the existence of electron and ion
velocity-space micro-instabilities10,16 that are associated
with plasma heating and thermalization processes. Our measurements
suggest that there is an impulsive mechanism associated with solar-wind
energization and that micro-instabilities play a part in heating, and
we provide evidence that low-latitude coronal holes are a key source
of the slow solar wind.
Title: Magnetic Field Line Twisting by Photospheric Vortices: Energy
Storage and Release
Authors: Rappazzo, A. F.; Velli, M.; Dahlburg, R. B.; Einaudi, G.
Bibcode: 2019ApJ...883..148R
Altcode: 2019arXiv190504420R
We investigate the dynamics of a closed-corona Cartesian reduced
magnetohydrodynamic model where photospheric vortices twist the coronal
magnetic field lines. We consider two corotating or counterrotating
vortices localized at the center of the photospheric plate and,
additionally, more corotating vortices that fill the plate entirely. Our
investigation is specifically devoted to studying the fully nonlinear
stage, after the linear stage during which the vortices create laminar
and smoothly twisting flux tubes. Our main goal is to understand the
dynamics of the photospheric vortices twisting the field lines of a
coronal magnetic configuration permeated by finite-amplitude broadband
fluctuations. We find that, depending on the system parameters and the
arrangement and handedness of the photospheric vortices, an inverse
cascade storing a significant amount of magnetic energy may or may not
occur. In the first case, a reservoir of magnetic energy available to
large events, such as destabilization of a pre-coronal mass ejection
(CME) configuration, develops, while in the second case, the outcome
is a turbulent heated corona. Although our geometry is simplified,
our simulations are shown to have relevant implications for coronal
dynamics and CME initiation.
Title: Fast Recursive Reconnection and the Hall Effect: Hall-MHD
Simulations
Authors: Shi, Chen; Tenerani, Anna; Velli, Marco; Lu, San
Bibcode: 2019ApJ...883..172S
Altcode: 2019arXiv190605961S
Magnetohydrodynamic (MHD) theory and simulations have shown that
reconnection is triggered via a fast “ideal” tearing instability
in current sheets whose inverse aspect ratio decreases to a/L∼
{S}-1/3, with S as the Lundquist number defined by the
half-length L of the current sheet (of a thickness of 2a). Ideal
tearing, in 2D sheets, triggers a hierarchical collapse via stretching
of X-points and recursive instability. At each step, the local
Lundquist number decreases, until the subsequent sheet thickness
either approaches kinetic scales or the Lundquist number becomes
sufficiently small. Here we carry out a series of Hall-MHD simulations
to show how the Hall effect modifies recursive reconnection once the
ion inertial scale is approached. We show that as the ion inertial
length becomes of the order of the inner, singular layer thickness at
some step of the recursive collapse, reconnection transits from the
plasmoid-dominant regime to an intermediate plasmoid+Hall regime and
then to the Hall-dominant regime. The structure around the X-point,
the reconnection rate, the dissipation property, and the power spectra
are also modified significantly by the Hall effect.
Title: Onset and Evolution of the Oblique, Resonant Electron Firehose
Instability in the Expanding Solar Wind Plasma
Authors: Innocenti, Maria Elena; Tenerani, Anna; Boella, Elisabetta;
Velli, Marco
Bibcode: 2019ApJ...883..146I
Altcode:
A double adiabatically expanding solar wind would quickly develop large
parallel to perpendicular temperature anisotropies in electrons and
ions that are not observed. One reason is that firehose instabilities
would be triggered, leading to an ongoing driving/saturation
evolution mechanism. We verify this assumption here for the
first time for the electron distribution function and the electron
firehose instability (EFI), using fully kinetic simulations with the
Expanding Box Model. This allows the self-consistent study of onset and
evolution of the oblique, resonant EFI in an expanding solar wind. We
characterize how the competition between EFI and adiabatic expansion
plays out in high- and low-beta cases, in high- and low-speed solar
wind streams. We observe that, even when competing against expansion,
the EFI results in perpendicular heating and parallel cooling. These two
concurrent processes effectively limit the expansion-induced increase
in temperature anisotropy and parallel electron beta. We show that
the EFI goes through cycles of stabilization and destabilization:
when higher wave number EFI modes saturate, lower wave number modes
are destabilized by the effects of the expansion. We show how resonant
wave/ particle interaction modifies the electron velocity distribution
function after the onset of the EFI. The simulations are performed
with the fully kinetic, semi-implicit expanding box code EB-iPic3D.
Title: Dynamic Evolution of Current Sheets, Ideal Tearing, Plasmoid
Formation and Generalized Fractal Reconnection Scaling Relations
Authors: Singh, K. A. P.; Pucci, Fulvia; Tenerani, Anna; Shibata,
Kazunari; Hillier, Andrew; Velli, Marco
Bibcode: 2019ApJ...881...52S
Altcode: 2019arXiv190400755S
Magnetic reconnection may be the fundamental process allowing energy
stored in magnetic fields to be released abruptly, with solar flares and
coronal mass ejection being archetypal natural plasma examples. Magnetic
reconnection is much too slow of a process to be efficient on the
large scales, but accelerates once small enough scales are formed in
the system. For this reason, the fractal reconnection scenario was
introduced to explain explosive events in the solar atmosphere; it was
based on the recursive triggering and collapse via tearing instability
of a current sheet originally thinned during the rise of a filament in
the solar corona. Here we compare the different fractal reconnection
scenarios that have been proposed, and derive generalized scaling
relations for the recursive triggering of fast, “ideal” —i.e.,
Lundquist number independent—tearing in collapsing current sheet
configurations with arbitrary current profile shapes. An important
result is that the Sweet-Parker scaling with Lundquist number, if
interpreted as the aspect ratio of the singular layer in an ideally
unstable sheet, is universal and does not depend on the details of
the current profile in the sheet. Such a scaling, however, must not
be interpreted in terms of stationary reconnection, rather it defines
a step in the accelerating sequence of events of the ideal tearing
mediated fractal cascade. We calculate scalings for the expected number
of plasmoids for such generic profiles and realistic Lundquist numbers,
showing that in ideal tearing scenarios a smaller number of plasmoids,
by orders of magnitude, is generated compared to the original fractal
model.
Title: Turbulence and Particle Acceleration in Collisionless
Magnetic Reconnection: Effects of Temperature Inhomogeneity across
Pre-reconnection Current Sheet
Authors: Lu, San; Angelopoulos, V.; Artemyev, A. V.; Pritchett, P. L.;
Liu, J.; Runov, A.; Tenerani, A.; Shi, C.; Velli, M.
Bibcode: 2019ApJ...878..109L
Altcode:
Magnetic reconnection is an important process in various collisionless
plasma environments because it reconfigures the magnetic field
and releases magnetic energy to accelerate charged particles. Its
dynamics depend critically on the properties of the pre-reconnection
current sheet. One property in particular, cross-sheet temperature
inhomogeneity, which is ubiquitous throughout the heliosphere, has
been shown to increase reconnection outflow speed, energy conversion
efficiency, and secondary island formation rate using two-dimensional
particle-in-cell simulations. Here we expand upon these findings,
considering two cases with a long, thin current sheet, one with
homogeneous temperature and one with inhomogeneous temperature
across the current sheet. In the inhomogeneous temperature case,
numerous secondary islands form continuously, which increases current
sheet turbulence (well-developed cascade power spectra) at large
wavenumbers. Current density, energy conversion, dissipation, and
acceleration of high-energy particles are also enhanced relative to the
homogenous temperature case. Our results suggest that inhomogeneous
temperature profiles, which are realistic, need to be incorporated
into studies of turbulence and particle acceleration in collisionless
magnetic reconnection.
Title: Explosive Magnetotail Activity
Authors: Sitnov, Mikhail; Birn, Joachim; Ferdousi, Banafsheh; Gordeev,
Evgeny; Khotyaintsev, Yuri; Merkin, Viacheslav; Motoba, Tetsuo; Otto,
Antonius; Panov, Evgeny; Pritchett, Philip; Pucci, Fulvia; Raeder,
Joachim; Runov, Andrei; Sergeev, Victor; Velli, Marco; Zhou, Xuzhi
Bibcode: 2019SSRv..215...31S
Altcode:
Modes and manifestations of the explosive activity in the Earth's
magnetotail, as well as its onset mechanisms and key pre-onset
conditions are reviewed. Two mechanisms for the generation of
the pre-onset current sheet are discussed, namely magnetic flux
addition to the tail lobes, or other high-latitude perturbations,
and magnetic flux evacuation from the near-Earth tail associated
with dayside reconnection. Reconnection onset may require stretching
and thinning of the sheet down to electron scales. It may also
start in thicker sheets in regions with a tailward gradient of the
equatorial magnetic field Bz; in this case it begins
as an ideal-MHD instability followed by the generation of bursty
bulk flows and dipolarization fronts. Indeed, remote sensing and
global MHD modeling show the formation of tail regions with increased
Bz, prone to magnetic reconnection, ballooning/interchange
and flapping instabilities. While interchange instability may also
develop in such thicker sheets, it may grow more slowly compared
to tearing and cause secondary reconnection locally in the dawn-dusk
direction. Post-onset transients include bursty flows and dipolarization
fronts, micro-instabilities of lower-hybrid-drift and whistler waves,
as well as damped global flux tube oscillations in the near-Earth
region. They convert the stretched tail magnetic field energy into
bulk plasma acceleration and collisionless heating, excitation
of a broad spectrum of plasma waves, and collisional dissipation
in the ionosphere. Collisionless heating involves ion reflection
from fronts, Fermi, betatron as well as other, non-adiabatic,
mechanisms. Ionospheric manifestations of some of these magnetotail
phenomena are discussed. Explosive plasma phenomena observed in the
laboratory, the solar corona and solar wind are also discussed.
Title: Propagation of Alfven waves and evolution of turbulence in
the expanding solar wind with the presence of stream interaction
Authors: Shi, Chen; Tenerani, Anna; Velli, Marco; Reville, Victor
Bibcode: 2019shin.confE..65S
Altcode:
We carry out two-dimensional magnetohydrodynamics (MHD) simulations
of the propagation of Alfven waves in the solar wind with the
expansion effect taken into consideration. Especially, the fast-slow
stream interaction region with shear, compression and rarefaction is
constructed in the simulations. We investigate the radial evolution and
the longitudinal variation of various quantities that are frequently
used in turbulence studies, e.g. the cross helicity, the residual
energy, the power spectra of Elsasser variables, etc. We show that
the stream interaction strongly affects the radial evolution of the
alfvenic turbulence. The total energy in the alfven waves is depleted
within the velocity shear regions, accompanied by decrease of the
normalized cross helicity no matter compression is present or not. The
presence of compression facilitates the energy depletion and decrease
of cross helicity inside the compression region. Residual energy is
produced in the radial evolution but is fluctuating around zero. The
power spectra of the outward/inward alfven waves show significant
longitudinal variations. Inside the fast and slow streams, Kolmogorov
spectra are well developed while in the shear regions the energy decays
rapidly with the wave number.
Title: Major Scientific Challenges and Opportunities in Understanding
Magnetic Reconnection and Related Explosive Phenomena throughout
the Universe
Authors: Ji, Hantao; Alt, A.; Antiochos, S.; Baalrud, S.; Bale, S.;
Bellan, P. M.; Begelman, M.; Beresnyak, A.; Blackman, E. G.; Brennan,
D.; Brown, M.; Buechner, J.; Burch, J.; Cassak, P.; Chen, L. -J.;
Chen, Y.; Chien, A.; Craig, D.; Dahlin, J.; Daughton, W.; DeLuca, E.;
Dong, C. F.; Dorfman, S.; Drake, J.; Ebrahimi, F.; Egedal, J.; Ergun,
R.; Eyink, G.; Fan, Y.; Fiksel, G.; Forest, C.; Fox, W.; Froula, D.;
Fujimoto, K.; Gao, L.; Genestreti, K.; Gibson, S.; Goldstein, M.;
Guo, F.; Hesse, M.; Hoshino, M.; Hu, Q.; Huang, Y. -M.; Jara-Almonte,
J.; Karimabadi, H.; Klimchuk, J.; Kunz, M.; Kusano, K.; Lazarian,
A.; Le, A.; Li, H.; Li, X.; Lin, Y.; Linton, M.; Liu, Y. -H.; Liu,
W.; Longcope, D.; Louriero, N.; Lu, Q. -M.; Ma, Z. -W.; Matthaeus,
W. H.; Meyerhofer, D.; Mozer, F.; Munsat, T.; Murphy, N. A.; Nilson,
P.; Ono, Y.; Opher, M.; Park, H.; Parker, S.; Petropoulou, M.; Phan,
T.; Prager, S.; Rempel, M.; Ren, C.; Ren, Y.; Rosner, R.; Roytershteyn,
V.; Sarff, J.; Savcheva, A.; Schaffner, D.; Schoeffier, K.; Scime, E.;
Shay, M.; Sitnov, M.; Stanier, A.; TenBarge, J.; Tharp, T.; Uzdensky,
D.; Vaivads, A.; Velli, M.; Vishniac, E.; Wang, H.; Werner, G.; Xiao,
C.; Yamada, M.; Yokoyama, T.; Yoo, J.; Zenitani, S.; Zweibel, E.
Bibcode: 2019BAAS...51c...5J
Altcode: 2019astro2020T...5J
This is a group white paper of 100 authors (each with explicit
permission via email) from 51 institutions on the topic of magnetic
reconnection which is relevant to 6 thematic areas. Grand challenges
and research opportunities are described in observations, numerical
modeling and laboratory experiments in the upcoming decade.
Title: Alfvénicity in the solar wind: high- and low-speed streams
Authors: D'Amicis, Raffaella; Matteini, Lorenzo; Bruno, Roberto;
Velli, Marco; De Marco, Rossana
Bibcode: 2019EGUGA..2114625D
Altcode:
Recent studies support evidence that the classification of solar wind
according to its speed in fast and slow solar wind must be revised. For
example one of the main features characterizing the fast solar wind,
i.e. Alfvénicity, is shared also by a kind of slow wind which is a
peculiarity found especially during maximum of solar cycle 23. The
amplitude of the fluctuations of these two kinds of solar wind
are comparable as well as their charge state but this result still
lacks a clear explanation. It has been suggested that the two kinds
of Alfvénic solar wind have a similar coronal origin with a major
role in the origin of the (Alfvénic) slow solar wind played by the
super-radial expansion. However, further study is needed for a thorough
understanding of the mechanism of origin. The results of this study
are relevant for Parker Solar Probe and for the upcoming Solar Orbiter,
and more generally for solar wind measurements close to the Sun, since
one of the main objectives of these two missions is to investigate the
solar sources of the slow solar wind. We show further insight drawing
attention on both protons and alpha particles.
Title: [Plasma 2020 Decadal] The essential role of multi-point
measurements in turbulence investigations: the solar wind beyond
single scale and beyond the Taylor Hypothesis
Authors: Matthaeus, W. H.; Bandyopadhyay, R.; Brown, M. R.; Borovsky,
J.; Carbone, V.; Caprioli, D.; Chasapis, A.; Chhiber, R.; Dasso,
S.; Dmitruk, P.; Del Zanna, L.; Dmitruk, P. A.; Franci, Luca; Gary,
S. P.; Goldstein, M. L.; Gomez, D.; Greco, A.; Horbury, T. S.; Ji,
Hantao; Kasper, J. C.; Klein, K. G.; Landi, S.; Li, Hui; Malara, F.;
Maruca, B. A.; Mininni, P.; Oughton, Sean; Papini, E.; Parashar, T. N.;
Petrosyan, Arakel; Pouquet, Annick; Retino, A.; Roberts, Owen; Ruffolo,
David; Servidio, Sergio; Spence, Harlan; Smith, C. W.; Stawarz, J. E.;
TenBarge, Jason; Vasquez1, B. J.; Vaivads, Andris; Valentini, F.;
Velli, Marco; Verdini, A.; Verscharen, Daniel; Whittlesey, Phyllis;
Wicks, Robert; Bruno, R.; Zimbardo, G.
Bibcode: 2019arXiv190306890M
Altcode:
This paper briefly reviews a number of fundamental measurements that
need to be made in order to characterize turbulence in space plasmas
such as the solar wind. It has long been known that many of these
quantities require simultaneous multipoint measurements to attain a
proper characterization that would reveal the fundamental physics
of plasma turbulence. The solar wind is an ideal plasma for such
an investigation, and it now appears to be technologically feasible
to carry out such an investigation, following the pioneering Cluster
and MMS missions. Quantities that need to be measured using multipoint
measurements include the two-point, two-time second correlation function
of velocity, magnetic field and density, and higher order statistical
objects such as third and fourth order structure functions. Some
details of these requirements are given here, with a eye towards
achieving closure on fundamental questions regarding the cascade rate,
spectral anisotropy, characteristic coherent structures, intermittency,
and dissipation mechanisms that describe plasma turbuelence, as well as
its variability with plasma parameters in the solar wind. The motivation
for this discussion is the current planning for a proposed Helioswarm
mission that would be designed to make these measurements,leading to
breakthrough understanding of the physics of space and astrophysical
turbulence.
Title: Large-scale Magnetic Funnels in the Solar Corona
Authors: Panasenco, Olga; Velli, Marco; Panasenco, Aram
Bibcode: 2019ApJ...873...25P
Altcode:
We describe open coronal magnetic fields with a specific
geometry—large-scale coronal magnetic funnels—that are found to play
an important role in coronal dynamics. Coronal magnetic funnels can be
attributed to three main factors: (i) the presence of pseudostreamer(s),
(ii) the presence of filament channels, and (iii) the presence of
active regions in the close vicinity of a pseudostreamer. The geometry
of magnetic funnels displays a strongly nonmonotonic expansion below 2
R ⊙. We present a detailed study of a funnel arising from
a double pseudostreamer near the equator, formed between a triplet of
coronal holes of the same polarity. By following the evolution of these
coronal holes we find that the pseudostreamer and, therefore, funnel
topology, changes when two coronal holes have merged together. The
funnel geometry of the open magnetic field becomes smoother, with
a monotonic expansion factor, after this merging. The presence of
magnetic funnels is indirectly confirmed by the appearance of coronal
cloud prominences in the solar corona, typically in the 304 Å passband,
as a result of colder plasma debris falling back toward the Sun in the
wake of eruptions in the surrounding atmosphere. The coronal clouds
appear suspended at heights of 1.2-1.3 R ⊙, coinciding
with the region of strongest gradients in the magnetic field. By
studying the evolution of funnel open magnetic fields over several
solar rotations we find a direct relation between the presence of
coronal clouds high in the solar corona and the coincident existence
of funnel magnetic fields below them.
Title: Ion Charge States in a Time-Dependent Wave-Turbulence-Driven
Model of the Solar Wind
Authors: Lionello, Roberto; Downs, Cooper; Linker, Jon A.; Mikić,
Zoran; Raymond, John; Shen, Chengcai; Velli, Marco
Bibcode: 2019SoPh..294...13L
Altcode: 2019arXiv190103748L; 2018SoPh..294...13L
Ion fractional charge states, measured in situ in the heliosphere,
depend on the properties of the plasma in the inner corona. As the ions
travel outward in the solar wind and the electron density drops, the
charge states remain essentially unaltered or "frozen in". Thus they
can provide a powerful constraint on heating models of the corona and
acceleration of the solar wind. We have implemented non-equilibrium
ionization calculations into a 1D wave-turbulence-driven (WTD)
hydrodynamic solar wind model and compared modeled charge states
with the Ulysses 1994 - 1995 in situ measurements. We have found
that modeled charge-state ratios of C6+/C5+ and
O7+/O6+, among others, were too low compared with
Ulysses measurements. However, a heuristic reduction of the plasma
flow speed has been able to bring the modeled results in line with
observations, though other ideas have been proposed to address this
discrepancy. We discuss implications of our results and the prospect
of including ion charge-state calculations into our 3D MHD model of
the inner heliosphere.
Title: A Semi-implicit Particle-in-cell Expanding Box Model Code
for Fully Kinetic Simulations of the Expanding Solar Wind Plasma
Authors: Innocenti, Maria Elena; Tenerani, Anna; Velli, Marco
Bibcode: 2019ApJ...870...66I
Altcode:
We address the challenges that come with fully kinetic Particle-In-Cell
(PIC) simulations of the expanding solar wind by introducing a
semi-implicit, Expanding Box Model (EBM) approach to the study of solar
wind kinetic physics. Plasma propagation and expansion are dealt with
via the variable change of the EBM. In this way the large separation
between scales of interest and domain size is addressed by including
solar wind propagation and expansion as time-dependent coupling
terms and coordinate stretching. The semi-implicit discretization,
in the widely used Implicit Moment Method (IMM) flavor, promises
to increase the simulated domain size and duration with respect to
explicit discretization. The EBM IMM equations are derived and tested
against expected behavior of expanding plasma.
Title: Dependence of Coronal Loop Temperature on Loop Length and
Magnetic Field Strength
Authors: Dahlburg, R. B.; Einaudi, G.; Ugarte-Urra, I.; Rappazzo,
A. F.; Velli, M.
Bibcode: 2018ApJ...868..116D
Altcode:
The temperature characteristics of solar coronal loops over a wide
range of lengths and magnetic field strengths are investigated by
means of numerical simulations. A very high correlation between
magnetic field strength (B 0) and maximum temperature
(T max) is found. Shorter loops rooted at stronger fields
are those that reach higher maximum temperatures. High temperatures
constitute a small part of the loop volume. For loops of equal length,
those with stronger magnetic fields have broader emission measure
distributions. The conditions underlying the variety of loops observed
in the solar corona are discussed, an explanation of why both cold
and hot loops exist is provided, and suggestions are given as to
what observations need to be made to confirm the results. Data in
the analysis are provided by numerical simulations using HYPERION,
an explicit massively parallel Fourier collocation-finite-difference
code. In the simulations footpoints are convected with a randomized
large-scale flow. This produces a Poynting flux which leads to the
buildup of magnetic energy in the loop. The magnetic energy is then
transformed into thermal energy by a magnetic reconnection process
occurring within current sheets formed locally by an energy cascade
toward small scales.
Title: Nonlinear Firehose Relaxation and Constant-B Field Fluctuations
Authors: Tenerani, Anna; Velli, Marco
Bibcode: 2018ApJ...867L..26T
Altcode: 2018arXiv180804453T
The nonlinear evolution of Alfvénic fluctuations in the firehose
unstable regime is investigated numerically and theoretically for
an anisotropic plasma described by the one-fluid double adiabatic
equations. We revisit the traditional theory of the instability
and examine the nonlinear saturation mechanism, showing that it
corresponds to evolution toward states that minimize an appropriate
energy functional. We demonstrate that such states correspond to
broadband magnetic and velocity field fluctuations with an overall
constant magnitude of the magnetic field. These nonlinear states
provide a basin of attraction for the long-term nonlinear evolution
of the instability, a self-organization process that may play a role
in maintaining the constant-B Alfvénic states seen in the solar wind
in the high-β regime.
Title: Parametric Decay and the Origin of the Low-frequency Alfvénic
Spectrum of the Solar Wind
Authors: Réville, Victor; Tenerani, Anna; Velli, Marco
Bibcode: 2018ApJ...866...38R
Altcode:
The fast solar wind shows a wide spectrum of transverse magnetic
and velocity field perturbations. These perturbations are strongly
correlated in the sense of Alfvén waves propagating mostly outward,
from the Sun to the interplanetary medium. They are likely to
be fundamental to the acceleration and the heating of the solar
wind. However, the precise origin of the broadband spectrum is
unknown to date. Typical periods of chromospheric Alfvén waves
are limited to a few minutes, and any longer period perturbations
should be strongly reflected at the transition region. In this work,
we show that minute long Alfvénic fluctuations are unstable to the
parametric instability. Parametric instability enables an inverse energy
cascade by exciting several-hour-long periods of Alfvénic fluctuations
together with strong density fluctuations (typically between 1 and 20
R ⊙). These results may improve our understanding of the
origin of the solar wind turbulent spectrum and will be tested by the
Parker Solar Probe.
Title: Roadmap for Reliable Ensemble Forecasting of the Sun-Earth
System
Authors: Nita, Gelu; Angryk, Rafal; Aydin, Berkay; Banda, Juan;
Bastian, Tim; Berger, Tom; Bindi, Veronica; Boucheron, Laura; Cao,
Wenda; Christian, Eric; de Nolfo, Georgia; DeLuca, Edward; DeRosa,
Marc; Downs, Cooper; Fleishman, Gregory; Fuentes, Olac; Gary, Dale;
Hill, Frank; Hoeksema, Todd; Hu, Qiang; Ilie, Raluca; Ireland,
Jack; Kamalabadi, Farzad; Korreck, Kelly; Kosovichev, Alexander;
Lin, Jessica; Lugaz, Noe; Mannucci, Anthony; Mansour, Nagi; Martens,
Petrus; Mays, Leila; McAteer, James; McIntosh, Scott W.; Oria, Vincent;
Pan, David; Panesi, Marco; Pesnell, W. Dean; Pevtsov, Alexei; Pillet,
Valentin; Rachmeler, Laurel; Ridley, Aaron; Scherliess, Ludger; Toth,
Gabor; Velli, Marco; White, Stephen; Zhang, Jie; Zou, Shasha
Bibcode: 2018arXiv181008728N
Altcode:
The authors of this report met on 28-30 March 2018 at the New Jersey
Institute of Technology, Newark, New Jersey, for a 3-day workshop
that brought together a group of data providers, expert modelers, and
computer and data scientists, in the solar discipline. Their objective
was to identify challenges in the path towards building an effective
framework to achieve transformative advances in the understanding
and forecasting of the Sun-Earth system from the upper convection
zone of the Sun to the Earth's magnetosphere. The workshop aimed to
develop a research roadmap that targets the scientific challenge
of coupling observations and modeling with emerging data-science
research to extract knowledge from the large volumes of data (observed
and simulated) while stimulating computer science with new research
applications. The desire among the attendees was to promote future
trans-disciplinary collaborations and identify areas of convergence
across disciplines. The workshop combined a set of plenary sessions
featuring invited introductory talks and workshop progress reports,
interleaved with a set of breakout sessions focused on specific topics
of interest. Each breakout group generated short documents, listing
the challenges identified during their discussions in addition to
possible ways of attacking them collectively. These documents were
combined into this report-wherein a list of prioritized activities
have been collated, shared and endorsed.
Title: Solar Physics from Unconventional Viewpoints
Authors: Gibson, Sarah E.; Vourlidas, Angelos; Hassler, Donald M.;
Rachmeler, Laurel A.; Thompson, Michael J.; Newmark, Jeffrey; Velli,
Marco; Title, Alan; McIntosh, Scott W.
Bibcode: 2018FrASS...5...32G
Altcode: 2018arXiv180509452G
We explore new opportunities for solar physics that could be realized
by future missions providing sustained observations from vantage
points away from the Sun-Earth line. These include observations from
the far side of the Sun, at high latitudes including over the solar
poles, or from near-quadrature angles relative to the Earth (e.g.,
the Sun-Earth L4 and L5 Lagrangian points). Such observations fill
known holes in our scientific understanding of the three-dimensional,
time-evolving Sun and heliosphere, and have the potential to open new
frontiers through discoveries enabled by novel viewpoints.
Title: Subresolution activity in solar and stellar coronae from
magnetic field line tangling
Authors: Rappazzo, A. F.; Dahlburg, R. B.; Einaudi, G.; Velli, M.
Bibcode: 2018MNRAS.478.2257R
Altcode: 2018MNRAS.tmp.1146R; 2018arXiv180500480R
The heating of coronal loops is investigated to understand the
observational consequences in terms of the thermodynamics and radiative
losses from the Sun and the magnetized coronae of stars with an outer
convective envelope. The dynamics of the Parker coronal heating model
are studied for different ratios of the photospheric forcing velocity
time-scale tp to the Alfvén crossing time along a loop
tA. It is shown that for tp/tA
≳ 10-24 the heating rate and maximum temperature are largest and
approximately independent of tp/tA, leading to
a strong emission in X-rays and extreme ultraviolet. On the opposite
decreasing tp/tA to smaller values leads to
lower heating rates and plasma temperatures, and consequently fading
high-energy radiative emission once tp/tA
≲ 1-3. The average volumetric loop heating rate is shown to scale
as ℓ_p u_p B_0^2/4π L^2, where ℓp and up
are, respectively, the convective granule length-scale and velocity,
B0 is the intensity of the strong magnetic field threading
the loop, and L the loop length. These findings support a recent
hypothesis explaining ultracool dwarf observations of stars with similar
magnetic field strength but radically different topologies displaying
different radiative emission.
Title: Fully kinetic, semi-implicit expanding box method:
implementations and first results
Authors: Innocenti, Maria Elena; Tenerani, Anna; Velli, Marco
Bibcode: 2018shin.confE.174I
Altcode:
The solar wind is far from thermodynamical equilibrium. Both protons
and electrons display highly anisotropic distribution functions
that evolve with radial distance possibly due to a combination of
expansion effects and kinetic instabilities [Maksimovic 2005, Matteini
2013]. While models that include solar wind expansion within the
hybrid description of the plasma have already been employed to study
the effects of the the expansion on proton dynamics [Liewer 2001], a
self-consistent investigation of the dynamics of protons and electrons
in the expanding solar wind is still lacking
Title: Pseudostreamers and widely distributed SEP events
Authors: Panasenco, Olga; Panasenco, Aram; Velli, Marco
Bibcode: 2018cosp...42E2565P
Altcode:
Our analysis of the pseudostreamer magnetic topology reveals new
interesting implications for understanding SEP acceleration in
CMEs. The possible reasons for the wide distribution of some SEP
events can be the presence of pseudostreamers in the vicinity of the
SEP source region which creates conditions for the existence of strong
longitudinal spread of energetic particles as well as an anomalous
longitudinal solar wind magnetic field component. We reconstructed
the 3D magnetic configurations of pseudostreamers with a potential
field source surface (PFSS) model, which uses as a lower boundary
condition the magnetic field derived from an evolving surface-flux
transport model. In order to estimate the possible magnetic connections
between the spacecraft and the SEP source region, we used the Parker
spiral, ENLIL and PFSS models. We found that in cases of the wide SEP
distributions a specific configuration of magnetic field appears to
exist at low solar latitudes all the way around the sun, we named this
phenomenon a pseudostreamers belt. It appears that the presence of the
well developed pseudostreamer or, rather multiple pseudostreamers,
organized into the pseudostreamer belt can be considered as a very
favorable condition for wide SEP events.
Title: Onset and nonlinear evolution of fast reconnection: Lundquist
number and Hall effects
Authors: Shi, Chen; Velli, Marco; Tenerani, Anna
Bibcode: 2018shin.confE.240S
Altcode:
Magnetic reconnection is a process that leads to global changes of
magnetic field line connectivity by releasing, often explosively, part
of the magnetic energy to the surrounding plasma in the form of kinetic
energy and heat. Although our knowledge has been greatly advanced in
the last few decades, the problem of how and under which conditions
magnetic reconnection can be triggered explosively still remains
open. Theory and resistive MHD simulations of a collapsing current
sheet have confirmed that, at very large values of the macroscopic
Lundquist number S (S>>10^4) an 'ideal' tearing is triggered
once a critical aspect ratio, scaling as S^(-1/3), is approached
from above, pointing to the fundamental difficulty to naturally form
Sweet-Parker type current sheets at those values of S. It was shown
that the same reasoning, when applied recursively, can describe the
complete nonlinear disruption of the original current sheet (in 2D)
until small scale, marginally stable current sheets are formed via what
is called 'fractal' reconnection. In space plasmas, where resistivity
is very small, however, the thickness of the reconnection diffusion
region, or even of the current sheet, may be comparable to the ion
inertial length: recent linear analysis has shown that the Hall term
modifies the critical current sheet aspect ratio for the onset of
'ideal' tearing. Here we start by discussing which are the conditions
for stability of current sheets with respect to the tearing mode
instability at low values of S within the MHD framework, and estimate
the critical Lundquist number S_c that separates stable from unstable
current sheets. We next investigate numerically the dynamics of a
collapsing current sheet at large S (S>>S_c) by including the
Hall effect, and study how the latter affects the reconnection onset
process as well as the subsequent nonlinear evolution.
Title: Parker Solar Probe: Exploring the Plasma Physics of the Solar
Corona and Inner Heliosphere
Authors: Velli, Marco; Bale, S.; Fox, N.; Howard, R.
Bibcode: 2018shin.confE.269V
Altcode:
The magnetic field is fundamental to solar activity and shapes the
inter-planetary environment, as demonstrated by many past and present
interplanetary and remote sensing spacecraft. Magnetic fields are also
the source for coronal heating and the very existence of the solar
wind; produced by the sun's dynamo and emerging into the corona,
magnetic fields become a conduit for waves, act to store energy,
and then propel plasma into the Heliosphere in the form of Coronal
Mass Ejections (CMEs). Magnetic fields are also at the heart of the
generation and acceleration of Solar Energetic Particle (SEPs) that
modify the space weather environment of the Earth and other planets.
Title: The Highly Structured Outer Solar Corona
Authors: DeForest, C. E.; Howard, R. A.; Velli, M.; Viall, N.;
Vourlidas, A.
Bibcode: 2018ApJ...862...18D
Altcode:
We report on the observation of fine-scale structure in the outer
corona at solar maximum, using deep-exposure campaign data from the
Solar Terrestrial Relations Observatory-A (STEREO-A)/COR2 coronagraph
coupled with postprocessing to further reduce noise and thereby improve
effective spatial resolution. The processed images reveal radial
structure with high density contrast at all observable scales down to
the optical limit of the instrument, giving the corona a “woodgrain”
appearance. Inferred density varies by an order of magnitude on spatial
scales of 50 Mm and follows an f -1 spatial spectrum. The
variations belie the notion of a smooth outer corona. They are
inconsistent with a well-defined “Alfvén surface,” indicating
instead a more nuanced “Alfvén zone”—a broad trans-Alfvénic
region rather than a simple boundary. Intermittent compact structures
are also present at all observable scales, forming a size spectrum
with the familiar “Sheeley blobs” at the large-scale end. We use
these structures to track overall flow and acceleration, finding that
it is highly inhomogeneous and accelerates gradually out to the limit
of the COR2 field of view. Lagged autocorrelation of the corona has
an enigmatic dip around 10 R ⊙, perhaps pointing to new
phenomena near this altitude. These results point toward a highly
complex outer corona with far more structure and local dynamics than
has been apparent. We discuss the impact of these results on solar
and solar-wind physics and what future studies and measurements are
necessary to build upon them.
Title: In situ categorization and coronal origins of different slow
solar wind types
Authors: Panasenco, Olga; Tenerani, Anna; Velli, Marco; Panasenco, Aram
Bibcode: 2018shin.confE.236P
Altcode:
The slow solar wind is not as regular as the fast wind, and a number
of periods have been identified where the turbulence is essentially
Alfvénic (Marsch et al. 1981, D'Amicis and Bruno, 2015). What creates
the difference between “standard" and Alfvénic slow wind? Is the
Title: Large-Scale Magnetic Funnels in the Solar Corona
Authors: Panasenco, Olga; Panasenco, Aram; Velli, Marco
Bibcode: 2018cosp...42E2566P
Altcode:
The existence of open coronal magnetic fields with peculiar geometry -
large-scale coronal magnetic funnels - can be attributed to two main
factors: (i) the presence of two or more coronal holes of the same
polarity forming coronal pseudostreamers, (ii) specific configurations
of closed magnetic field in the low corona - filament channels. The
important property of magnetic funnels is their strongly non-monotonic
expansion factor below 2 solar radii. In the case study presented here
we consider a double pseudostreamer near the equator, formed between a
triplet of isolated coronal holes of the same polarity, and harboring
two pairs of twin filaments in its base. Following the evolution of
these coronal holes we find that the pseudostreamer and, therefore,
magnetic funnel topology, changes when two coronal holes have merged
together. Using a potential field source-surface (PFSS) extrapolation
to compute the coronal field from photospheric maps (SDO/HMI), we show
that the funnel - like geometry of the open magnetic field changes
to a regular one with monotonic expansion factor after the merging
of coronal holes. The presence of coronal magnetic funnels becomes
directly visible when sufficient plasma accumulates inside them:
when the plasma density grows to become observable, coronal cloud
prominences appear in the corona, mostly in 304 A spectral line. We
study the evolution of the funnel - like open magnetic fields during
several solar rotations and find a direct relation between magnetic
funnels and the presence of coronal clouds at great heights in the
solar corona. 1D numerical analysis of pseudostreamers with funnel
topology shows that the properties of the solar wind from coronal
magnetic funnels depend on the presence/absence of filament channels,
number of channels and chirality at the pseudostreamer base low in
the solar corona.
Title: The problem of constant-B field Alfvénic fluctuations
Authors: Tenerani, Anna; Velli, Marco
Bibcode: 2018shin.confE...8T
Altcode:
One of the outstanding problems in astrophysics is the origin of stellar
coronae, winds, and, more generally, the ubiquitous existence in the
universe of hot million degree (or more) plasmas. The solar corona and
wind provide an accessible environment to understand plasma heating and
acceleration, and this is one of the main goals of the upcoming NASA
mission Parker Solar Probe, which will arrive closer to the Sun (10 Rs)
than any previous spacecraft. Here we focus on the problem of the
existence and dynamical accessibility of constant-B nonlinear states
in collisionless plasmas. We investigate the stability properties
of Alfvénic fluctuations to both parametric decay and firehose
instability, and we show that broadband, constant-B nonlinear states
are a basin of attraction of the firehose instability. We discuss
possible implications for Parker Solar Probe.
Title: Parametric instability of high frequencies Alfvén waves,
inverse cascade and the generation of the solar wind turbulence
spectrum
Authors: Réville, Victor; Tenerani, Anna; Velli, Marco
Bibcode: 2018shin.confE..37R
Altcode:
Alfvén waves are thought to play an important dynamical role in solar
wind heating and acceleration. Observations of such waves covering a
wide range of frequencies- from hours through minutes- have been made
in-situ for almost 50 years. Remote sensing measurements, however,
have shown the dominance of few minute period Alfvén waves in the
chromosphere and theoreticals studies suggest that any longer period
fluctuations should be strongly reflected in crossing the transition
region. We study in a realistic solar wind profile the propagation
of high frequency (few minutes) Alfvén waves in the corona solving
the fully non-linear compressible MHD equations. We demonstrate that
parametric decay may affect these waves at low heights, mostly below or
around the Alfvén point. This process then triggers an inverse cascade
that is able to transfer significant power to hour long periods.These
results imply that the generation of the solar wind spectrum might
need to involve compressible processes. In-situ measurements by Parker
Solar Probe should be able to either confirm or deny these results.
Title: Marginal Stability of Sweet-Parker Type Current Sheets at
Low Lundquist Numbers
Authors: Shi, Chen; Velli, Marco; Tenerani, Anna
Bibcode: 2018ApJ...859...83S
Altcode: 2018arXiv180207162S
Magnetohydrodynamic simulations have shown that a nonunique critical
Lundquist number S c exists, hovering around S
c ∼ 104, above which threshold Sweet-Parker type
stationary reconnecting configurations become unstable to a fast tearing
mode dominated by plasmoid generation. It is known that the flow along
the sheet plays a stabilizing role, though a satisfactory explanation
of the nonuniversality and variable critical Lundquist numbers observed
is still lacking. Here we discuss this question using 2D linear MHD
simulations and linear stability analyses of Sweet-Parker type current
sheets in the presence of background stationary inflows and outflows at
low Lundquist numbers (S ≤ 104). Simulations show that the
inhomogeneous outflow stabilizes the current sheet by stretching the
growing magnetic islands and at the same time evacuating the magnetic
islands out of the current sheet. This limits the time during which
fluctuations that begin at any given wavelength can remain unstable,
rendering the instability nonexponential. We find that the linear theory
based on the expanding-wavelength assumption works well for S larger
than ∼1000. However, we also find that the inflow and location of
the initial perturbation also affect the stability threshold.
Title: Solar Observations Away from the Sun-Earth Line
Authors: Gibson, Sarah E.; McIntosh, Scott William; Rachmeler,
Laurel; Thompson, Michael J.; Title, Alan M.; Velli, Marco C. M.;
Vourlidas, Angelos
Bibcode: 2018tess.conf40340G
Altcode:
Observations from satellite missions have transformed the field of solar
physics. High-resolution observations with near-continuous temporal
coverage have greatly extended our capability for studying long-term
and transient phenomena, and the opening of new regions of the solar
spectrum has made detailed investigation of the solar atmosphere
possible. However, to date most solar space-based missions
have been restricted to an observational vantage in the vicinity of
the Sun-Earth line, either in orbit around the Earth or from the L1
Lagrangian point. As a result, observations from these satellites
represent the same geometrical view of the Sun that is accessible
from the Earth. Understanding the deep interior structure of the
Sun and the full development of solar activity would really benefit
from fully three-dimensional monitoring of the solar atmosphere and
heliosphere. On the one hand, simultaneous spacecraft observations
from multiple vantage points would allow studies of the deep interior
structure of the sun via stereoscopic helioseismology; on the other,
distributed observations would allow the understanding of the complete
evolution of activity complexes and enhance space weather predictions
dramatically. Presently, observations of the Sun away from Earth
are obtained by the STEREO pair of satellites, which have provided
an unprecedented global view by orbiting around to the far side of
the Sun, and the Ulysses mission, which achieved a high-inclination
(80˚) near-polar orbit (but which, however, did not include any solar
imaging instruments). The forthcoming Solar Orbiter mission, which
will orbit the sun and reach a maximum inclination of 34˚ out of the
ecliptic should provide the first detailed mapping of the sun's polar
fields. In addition, Solar Probe Plus will explore the outer corona
and inner Heliosphere with very rapid solar encounters at a minimum
perihelion 9.86 solar radii from the center of the Sun. We explore
some of the new opportunities for solar physics that can be realized
by future missions that provide sustained observations from vantage
points away from the Sun-Earth line (and in some cases the ecliptic
plane): observations from the far side of the Sun, over its poles,
or from the L5 Lagrangian point.
Title: Waves, turbulence and reconnection in the accelerating
solar wind
Authors: Tenerani, Anna; Velli, Marco C. M.
Bibcode: 2018tess.conf31201T
Altcode:
The dynamics of fluctuations and structures in the acceleration region
Title: Scientific Drivers for a Solar Polar Mission
Authors: Hassler, Don; Velli, Marco C.; Murphy, Neil; Creyke Liewer,
Paulett
Bibcode: 2018tess.conf11102H
Altcode:
Solar and Heliospheric physics has experienced a golden age of
discovery over the past 20+ years, and the launches of Parker Solar
Probe and Solar Orbiter promise to add exciting new observations and
insights into our understanding of the Sun-Heliosphere system. So
what is next? Although these missions have, and promise to continue
to, revolutionize our understanding of the Sun, the one region
that is still unexplored is the solar pole…the solar pole is one
of the final frontiers of solar physics. Although Solar Orbiter
will achieve a latitude of ~32 degrees at the end of it's extended
mission, providing a first glimpse of this unexplored polar region,
its observations will still be significantly foreshortened over much
of the pole, and it will only collect imaging observations for 30 days
per 168 day orbit. A dedicated, extended solar polar mission offers
the opportunity to revolutionize our understanding of the relationship
between the magnetic field and dynamics of the Sun's polar region, it's
internal structure and dynamics, and the solar cycle. This talk
will discuss the various scientific drivers for a Solar Polar Mission,
what scientific objectives can ONLY be addressed by a dedicated Solar
Polar Mission, and what requirements do these scientific objectives
place on the observational and orbital requirements of such a mission.
Title: Solar Polar Diamond Explorer (SPDEx): Understanding the
Origins of Solar Activity Using a New Perspective
Authors: Vourlidas, A.; Liewer, P. C.; Velli, M.; Webb, D.
Bibcode: 2018arXiv180504172V
Altcode:
Our knowledge of the Sun, its atmosphere, long term activity and
space weather potential is severely limited by the lack of good
observations of the polar and far-side regions. Observations from
a polar vantage point would revolutionize our understanding of the
mechanism of solar activity cycles, polar magnetic field reversals,
the internal structure and dynamics of the Sun and its atmosphere. Only
with extended (many day) observations of the polar regions can the
polar flows be determined down to the tachocline where the dynamo is
thought to originate. Rapid short period polar orbits, using in situ
and remote sensing instrumentation, distributed over a small number of
spacecraft, will provide continuous 360o coverage of the solar surface
and atmosphere in both longitude and latitude for years on end. This
unprecedented full coverage will enable breakthrough studies of the
physical connection between the solar interior, the solar atmosphere,
the solar wind, solar energetic particles and the inner heliosphere at
large. A potential implementation, the Solar Polar Diamond Explorer
(SPDEx) built upon the Solar Polar Imager mission design, involves
up to four small spacecraft in a 0.48-AU orbit with an inclination
of 75o. The orbit is achieved using solar sails or ion engines, both
technologies already demonstrated in space.
Title: Turtles All The Way Down: The finely structured outer corona,
and its implications for PSP
Authors: DeForest, Craig E.; Howard, Russell A.; Velli, Marco C. M.;
Viall, Nicholeen M.; Vourlidas, Angelos
Bibcode: 2018tess.conf30928D
Altcode:
Based on optical resolution of the starfield with SOHO/LASCO,
STEREO/COR, and other coronagraphs, there is widespread intuition that
the solar corona becomes more smooth with altitude. This is an optical
illusion, caused by the interplay between signal-to-noise ratio (SNR)
and feature size in typical coronal images. Processed, low-noise,
deep-field COR2 images of the outer corona reveal rich structure at
all observable scales, with surprising time variability and very short
spatial correlation scales under 50 Mm, at altitudes near 10 Rs. This
has deep implications not only for the solar wind and outer coronal
physics, but also for the types of structure that Parker Solar Probe
will encounter. We will present and discuss the fundamental result,
and explore its implications for in-situ science and required context
imaging from PSP. We will also make specific predictions about the
environment PSP will encounter at solar altitudes of 10-15 Rs.
Title: SAFARI: Solar Activity Far Side Investigation
Authors: Velli, Marco C. M.; Hassler, Don; Jefferies, Stuart; Murphy,
Neil; Panasenco, Olga
Bibcode: 2018tess.conf40341V
Altcode:
The Solar Activity Far Side Investigation, or SAFARI, is a small mission
concept, or an element of a larger mission, devoted to exploring the
origins of solar magnetic activity by carrying out observations of the
velocity and magnetic fields at the solar surface from a vantage point
widely separated from Earth in longitude and latitude. SAFARI images
the Sun from orbits trailing/leading the Earth at 1 AU (SAFARI-S),
with important excursion in latitude, while at the same time the
Sun is imaged from the Earth (SAFARI-E). SAFARI carries out these
observations using a compact Doppler magnetograph based on a simple,
robust design with magneto-optical filters. SAFARI's ground based
component, SAFARI-E, uses a similar observational technique, allowing
precise inter-calibration of magnetograms and providing an opportunity
to implement the novel technique of stereoscopic helioseismology,
probing flows and structural heterogeneities deep in the convection
zone, reaching below the tachocline and opening a new observational
window into the Sun. The combined measurements of solar magnetic fields
from Earth and spacecraft viewpoints extends the longitudinal and
latitudinal coverage of the solar disk allowing extended simultaneous
observations permitting the full study of active region development and
decay that cannot be observed in its entirety from a single point due to
solar rotation. In addition, the structure and depths of sunspots can
be addressed with stereoscopic local helioseismology. Combined scalar
magnetic field measurements from multiple vantage points provide the
vector magnetic field; combined LOS velocity field measurements frm
different vantage points provide the vector velocity field: fundamental
measurements to understand solar activity.
Title: The Solar Corona and Accelerating Solar Wind: Parker Solar
Probe
Authors: Velli, Marco
Bibcode: 2018EGUGA..20.9352V
Altcode:
The magnetic field is fundamental to solar activity and shapes the
interplanetary environment. Magnetic fields are also the source for
coronal heating and the very existence of the solar wind; produced
by the sun's dynamo and emerging into the corona, magnetic fields
become a conduit for waves, act to store energy, and then propel
plasma into the heliosphere in the form of Coronal Mass Ejections
(CMEs). In 2018 the Parker Solar Probe (PSP) mission will launch to
carry out the first in situ exploration of the outer solar corona
and inner heliosphere. Direct measurements of the plasma in the
closest atmosphere of our star should lead to a new understanding of
the questions of coronal heating and solar wind acceleration. I will
describe the PSP scientific objectives, instrument suites, and models of
solar magnetic activity, coronal heating, and solar wind acceleration
that PSP may confirm or falsify. The latter involve our most recent
understanding of MHD turbulence, magnetic reconnection, and electron
and ion heating and acceleration in complex magnetic fields.
Title: The slow solar wind that resembles the fast wind: new insights
Authors: D'Amicis, Raffaella; Matteini, Lorenzo; Velli, Marco;
Bruno, Roberto
Bibcode: 2018EGUGA..2014114D
Altcode:
Over the past few years it has become increasingly clear that the
paradigm according to which the solar wind comes in two distinct flavors
(fast and slow solar wind) must be revised. Especially around solar
maximum, a slow wind with many characteristics similar to the fast
wind has been found. One such feature is the high Alfvenic content of
velocity and magnetic field fluctuations, whose correlation can be as
great as that found in the fast wind, and with a similar dominance of
outward propagating waves, a peculiarity which is still under study. It
has been suggested that the similarity of this and other characteristics
is due to a common origin for the two types of solar wind, with a
major role attributed to the super-radial expansion responsible for
the lower velocity of the slow wind. These new findings have relevant
implications for the upcoming Solar Orbiter and Parker Solar Probe
missions and more generally for solar wind measurements close to the
Sun. One of the main objectives of these two missions is to investigate
the solar sources of the slow solar wind. Composition analysis can
provide a better understanding in this regard. A particular focus will
be placed on helium (alpha particles) whose abundance in the solar wind
oscillates but is typically a few percent and is thus well resolved in
plasma analyzers. Alpha particles moments will be compared with the
protons ones and some derived parameters will be studied including
for instance temperature ratio, density ratio, drift velocity and
anisotropy. This study will be performed for different solar wind
regimes to better characterized their differences and similarities.
Title: Onset of fast "ideal" tearing in thin current sheets:
Dependence on the equilibrium current profile
Authors: Pucci, F.; Velli, M.; Tenerani, A.; Del Sarto, D.
Bibcode: 2018PhPl...25c2113P
Altcode: 2018arXiv180108412P
In this paper, we study the scaling relations for the triggering of
the fast, or "ideal," tearing instability starting from equilibrium
configurations relevant to astrophysical as well as laboratory plasmas
that differ from the simple Harris current sheet configuration. We
present the linear tearing instability analysis for equilibrium
magnetic fields which (a) go to zero at the boundary of the domain
and (b) contain a double current sheet system (the latter previously
studied as a Cartesian proxy for the m = 1 kink mode in cylindrical
plasmas). More generally, we discuss the critical aspect ratio scalings
at which the growth rates become independent of the Lundquist number
S, in terms of the dependence of the Δ' parameter on the wavenumber
k of unstable modes. The scaling Δ'(k) with k at small k is found to
categorize different equilibria broadly: the critical aspect ratios may
be even smaller than L/a ∼ Sα with α = 1/3 originally
found for the Harris current sheet, but there exists a general lower
bound α ≥ 1/4.
Title: Modeling Solar Wind Expansion with Wave-Particle Interactions
and Coulomb Collisions
Authors: Matteini, L.; Hellinger, P.; Landi, S.; Pantellini, F. G. E.;
Velli, M.; Franci, L.; Verdini, A.
Bibcode: 2017AGUFMSH32A..06M
Altcode:
The evolution of the solar wind plasma is strongly influenced by
its spherical expansion in interplanetary space. Due to the weak
- but not fully negligible - collisionality of the plasma, the
behaviour of the system can be hardly modelled through standard
approaches, either fluid or fully collisionless. Moreover, solar
wind microphysics depends on many different processes, including
the interaction of particles with background waves and turbulence,
and plasma instabilities. Disentangling the effect of these processes
from the role of intra- and inter-species particle collisions in the
framework of the overall secular evolution imposed by the expansion
is particularly challenging.In this presentation we will review some
basics of the solar wind expansion as well as some of the recent results
obtained by means of kinetic numerical models which take into account
the radial expansion on the plasma, with emphasis on the comparison
with in situ observations and the role of the forthcoming Solar Orbiter
and Parker Solar Probe missions.
Title: ICARUS Mission, Next Step of Coronal Exploration after Solar
Orbiter and Solar Probe Plus
Authors: Krasnoselskikh, V.; Tsurutani, B.; Velli, M.; Maksimovic,
M.; Balikhin, M. A.; Dudok de Wit, T.; Kretzschmar, M.
Bibcode: 2017AGUFMSH14B..08K
Altcode:
The primary scientific goal of ICARUS, a mother-daughter satellite
mission, will be to determine how the magnetic field and plasma dynamics
in the outer solar atmosphere give rise to the corona, the solar wind
and the heliosphere. Reaching this goal will be a Rosetta-stone step,
with results broadly applicable in the fields of space plasma and
astrophysics. Within ESA's Cosmic Vision roadmap, these goals address
Theme 2: How does the solar system work ?" Investigating basic processes
occurring from the Sun to the edge of the Solar System". ICARUS will
not only advance our understanding of the plasma environment around
the Sun, but also of the numerous magnetically active stars with hot
plasma coronae. ICARUS I will perform the firstever direct in situ
measurements of electromagnetic fields, particle acceleration, wave
activity, energy distribution and flows directly in the regions where
the solar wind emerges from the coronal plasma. ICARUS I will have a
perihelion at 1 Solar radius from its surface, it will cross the region
where the major energy deposition occurs. The polar orbit of ICARUS I
will enable crossing the regions where both the fast and slow wind are
generated. It will probe local characteristics of the plasma and provide
unique information about the processes involved in the creation of the
solar wind. ICARUS II will observe this region using remote-sensing
instruments, providing simultaneous information about regions crossed
by ICARUS I and the solar atmosphere below as observed by solar
telescopes. It will provide bridges for understanding the magnetic
links between heliosphere and solar atmosphere. Such information is
crucial to understanding of the physics and electrodynamics of the
solar atmosphere. ICARUS II will also play an important relay role,
enabling the radio-link with ICARUS I. It will receive, collect and
store information transmitted from ICARUS I during its closest approach
to the Sun. It will perform preliminary data processing and transmit it
to the Earth. Performing such unique in situ measurements in the region
where deadly solar energetic particles are energized, ICARUS will make
fundamental contributions to our ability to monitor and forecast the
space radiation environment. Such knowledge is extremely important
for space explorations, especially for long-term manned space missions.
Title: Widely distributed SEP events and pseudostreamers
Authors: Panasenco, O.; Panasenco, A.; Velli, M.
Bibcode: 2017AGUFMSH33C..07P
Altcode:
Our analysis of the pseudostreamer magnetic topology reveals new
interesting implications for understanding SEP acceleration in
CMEs. The possible reasons for the wide distribution of some SEP
events can be the presence of pseudostreamers in the vicinity of the
SEP source region which creates conditions for the existence of strong
longitudinal spread of energetic particles as well as an anomalous
longitudinal solar wind magnetic field component. We reconstructed
the 3D magnetic configurations of pseudostreamers with a potential
field source surface (PFSS) model, which uses as a lower boundary
condition the magnetic field derived from an evolving surface-flux
transport model. In order to estimate the possible magnetic connections
between the spacecraft and the SEP source region, we used the Parker
spiral, ENLIL and PFSS models. We found that in cases of the wide SEP
distributions a specific configuration of magnetic field appears to
exist at low solar latitudes all the way around the sun, we named this
phenomenon a pseudostreamers belt. It appears that the presence of the
well developed pseudostreamer or, rather multiple pseudostreamers,
organized into the pseudostreamer belt can be considered as a very
favorable condition for wide SEP events.
Title: The Solar Orbiter Heliospheric Imager (SoloHI) for the Solar
Orbiter Mission
Authors: Howard, R.; Colaninno, R. C.; Plunkett, S. P.; Thernisien,
A. F.; Wang, D.; Rich, N.; Korendyke, C.; Socker, D. G.; Linton, M.;
McMullin, D. R.; Vourlidas, A.; Liewer, P. C.; De Jong, E.; Velli,
M.; Mikic, Z.; Bothmer, V.; Philippe, L.; Carter, M. T.
Bibcode: 2017AGUFMSH23D2681H
Altcode:
The SoloHI instrument has completed its development effort and has been
integrated onto the Solar Orbiter (SolO) spacecraft. The SolO mission,
scheduled for launch in February 2019, will undergo gravity assist
maneuvers around Venus to change both the perihelion distance as well
as the plane of the orbit to ultimately achieve a minimum perihelion
of 0.28 AU and an orbital inclination of about 35° relative to the
ecliptic plane. The remote sensing instruments will operate for three
10-day periods out of the nominal 6-month orbit. SoloHI will observe
sunlight scattered by free electrons in the corona/solar wind from 5°
to 45° elongation in visible wavelengths and will provide a coupling
between remote sensing and in situ observations. It is very similar
to the HI-1 instrument on STEREO/SECCHI except that the FOV is twice
the size at 40o. We present our efforts to prepare for the mission
including our observing plans, quick-look plans and some results of
the calibration activities. We gratefully acknowledge the support of
the NASA Solar Orbiter Collaboration project.
Title: Global solar magetic field organization in the extended corona:
influence on the solar wind speed and density over the cycle.
Authors: Réville, V.; Velli, M.; Brun, S.
Bibcode: 2017AGUFMSH11B2453R
Altcode:
The dynamics of the solar wind depends intrinsically on the structure
of the global solar magnetic field, which undergoes fundamental
changes over the 11yr solar cycle. For instance, the wind terminal
velocity is thought to be anti-correlated with the expansion factor,
a measure of how the magnetic field varies with height in the solar
corona, usually computed at a fixed height (≈ 2.5 R⊙,
the source surface radius which approximates the distance at which
all magnetic field lines become open). However, the magnetic field
expansion affects the solar wind in a more detailed way, its influence
on the solar wind properties remaining significant well beyond the
source surface: we demonstrate this using 3D global MHD simulations
of the solar corona, constrained by surface magnetograms over half
a solar cycle (1989-2001). For models to comply with the constraints
provided by observed characteristics of the solar wind, namely, that
the radial magnetic field intensity becomes latitude independent at some
distance from the Sun (Ulysses observations beyond 1 AU), and that the
terminal wind speed is anti-correlated with the mass flux, they must
accurately describe expansion beyond the solar wind critical point
(even up to 10R⊙ and higher in our model). We also show
that near activity minimum, expansion in the higher corona beyond 2.5
R⊙ is actually the dominant process affecting the wind
speed. We discuss the consequences of this result on the necessary
acceleration profile of the solar wind, the location of the sonic
point and of the energy deposition by Alfvén waves.
Title: Wave Propagation Around Coronal Structures: Stratification,
Buoyancy, Small Scale Formation
Authors: Tomlinson, S. M.; Rappazzo, F.; Velli, M.
Bibcode: 2017AGUFMSH33B2783T
Altcode:
We study the propagation of waves in a coronal medium characterized
by stratification and structure in density. temperature and magnetic
field. It is well known that average gradients affect the propagation
of Alfvén and other MHD waves via reflection, phase mixing, resonant
absorption and other coupling phenomena. Here we discuss how the
interplay of propagation on inhomogeneous, stratified structures
with nonlinear interactions may lead to interesting effects including
preferential heating, buoyancy, and plasma acceleration.
Title: Plasmoids everywhere: ideal tearing, the transition to fast
reconnection, and solar activity.
Authors: Velli, M. C. M.; Pucci, F.; Tenerani, A.; Shi, C.; Del Sarto,
D.; Rappazzo, A. F.
Bibcode: 2017AGUFMSH11B2452V
Altcode:
We discuss the role of generalized ``ideal" tearing (IT) as a possible
trigger mechanism for magnetic reconnection to understand energetic
phenomena in the solar atmosphere. We begin with a pedagogical
introduction to the IT concept, how it stems from the classical
analysis of the tearing instability, what is meant by plasmoids, and
the connections of IT to the plasmoid instability and Sweet Parker
current sheets. We then proceed to analyze how the IT concept extends
to equilibria with flows, small scale kinetic effects, different current
structures and different magnetic field topology configurations. Finally
we discuss the relationship of reconnection triggering to nonlinear
cascades and turbulent evolution, and how different situations may
arise depending on scale, boundary conditions, and time-history,
from coronal heating via nanoflares, to solar flares and coronal mass
ejections. Issues of local topology, dimensionality, anisotropy will
also be discussed.
Title: The Parametric Instability of Alfvén Waves: Effects of
Temperature Anisotropy
Authors: Tenerani, Anna; Velli, Marco; Hellinger, Petr
Bibcode: 2017ApJ...851...99T
Altcode: 2017arXiv171106371T
We study the stability of large-amplitude, circularly polarized Alfvén
waves in an anisotropic plasma described by the double-adiabatic/CGL
closure, and in particular the effect of a background thermal pressure
anisotropy on the well-known properties of Alfvén wave parametric decay
in magnetohydrodynamics (MHD). Anisotropy allows instability over a
much wider range of values of parallel plasma beta (β ∥)
when ξ = p 0⊥/p 0∥ > 1. When the pressure
anisotropy exceeds a critical value, ξ ≥ ξ* with ξ* ≃ 2.7,
there is a new regime in which the parametric instability is no longer
quenched at high β ∥, and in the limit β ∥
≫ 1, the growth rate becomes independent of β ∥. In the
opposite case of ξ < ξ*, the instability is strongly suppressed
with increasing parallel plasma beta, similarly to the MHD case. We
analyze marginal stability conditions for parametric decay in the (ξ,
β ∥) parameter space and discuss possible implications
for Alfvénic turbulence in the solar wind.
Title: Marginal Stability of Current Sheets at Low Lundquist Numbers
and the Hall Effect
Authors: Shi, C.; Velli, M.; Tenerani, A.
Bibcode: 2017AGUFMSH11B2451S
Altcode:
Magnetohydrodynamic simulations suggest that there exists a non-unique
critical Lundquist number S, around S 104, above which current sheets
transition from a stationary Sweet-Parker (SP) like reconnecting
configuration to a highly tearing-unstable (turbulent) state dominated
by plasmoid generation. It is known that the flow along the sheet plays
a stabilizing role, as one would expect that the plasmoid evacuation
time-scale must be longer than the typical growth time for islands in
order for the sheet to be tearing unstable. However, a satisfactory
explanation of the existence of such a critical threshold for the
tearing instability, and why it is not universal, is still lacking. A
detailed understanding of this effect is important even for very large
Lundquist number plasmas, because it has been shown that in such cases
fast reconnection occurs, at least in 2D, starting from sheets that are
much thicker than SP (so-called ideally tearing sheets) and evolving
in a self-similar way over smaller and smaller scales (and therefore
smalle Lundquist numbers) until a sufficiently low Reynolds number
is reached for which sheets are stable and dissipate rapidly. So the
ending of this so-called fractal reconnection regime is determined
by the low-Lundquist number stabilization of SP-like sheets. Here we
present the linear stability analysis of two-dimensional SP current
sheets at relatively low-S with flows across and along the sheet, and
carry out 2D MHD simulations to validate the linear results, and extend
those results by including the Hall effect in order to inspect in which
way it affects both the marginal stability of SP-like current sheets at
low-S and the disruption of forming current sheets above the critical S.
Title: The Solar Wind from Pseudostreamers and their Environs:
Opportunities for Observations with Parker Solar Probe and Solar
Orbiter
Authors: Panasenco, O.; Velli, M.; Panasenco, A.; Lionello, R.
Bibcode: 2017AGUFMSH23D2703P
Altcode:
The solar dynamo and photospheric convection lead to three main types of
structures extending from the solar surface into the corona - active
regions, solar filaments (prominences when observed at the limb) and
coronal holes. These structures exist over a wide range of scales,
and are interlinked with each other in evolution and dynamics. Active
regions can form clusters of magnetic activity and the strongest
overlie sunspots. In the decay of active regions, the boundaries
separating opposite magnetic polarities (neutral lines) develop specific
structures called filament channels above which filaments form. In the
presence of flux imbalance decaying active regions can also give birth
to lower latitude coronal holes. The accumulation of magnetic flux at
coronal hole boundaries also creates conditions for filament formation:
polar crown filaments are permanently present at the boundaries of
the polar coronal holes. Mid-latitude and equatorial coronal holes -
the result of active region evolution - can create pseudostreamers
if other coronal holes of the same polarity are present. While
helmet streamers form between open fields of opposite polarities,
the pseudostreamer, characterized by a smaller coronal imprint,
typically shows a more prominent straight ray or stalk extending
from the corona. The pseudostreamer base at photospheric heights
is multipolar; often one observes tripolar magnetic configurations
with two neutral lines - where filaments can form - separating the
coronal holes. Here we discuss the specific role of filament channels
on pseudostreamer topology and on solar wind properties. 1D numerical
analysis of pseudostreamers shows that the properties of the solar wind
from around PSs depend on the presence/absence of filament channels,
number of channels and chirality at thepseudostreamer base low in the
corona. We review and model possible coronal magnetic configurations
and solar wind plasma properties at different distances from the solar
surface that may be observed by Parker Solar Probe and Solar Orbiter.
Title: Signatures Of Coronal Heating Driven By Footpoint Shuffling:
Closed and Open Structures.
Authors: Velli, M. C. M.; Rappazzo, A. F.; Dahlburg, R. B.; Einaudi,
G.; Ugarte-Urra, I.
Bibcode: 2017AGUFMSH41D..01V
Altcode:
We have previously described the characteristic state of the confined
coronal magnetic field as a special case of magnetically dominated
magnetohydrodynamic (MHD) turbulence, where the free energy in
the transverse magnetic field is continuously cascaded to small
scales, even though the overall kinetic energy is small. This coronal
turbulence problem is defined by the photospheric boundary conditions:
here we discuss recent numerical simulations of the fully compressible
3D MHD equations using the HYPERION code. Loops are forced at their
footpoints by random photospheric motions, energizing the field to
a state with continuous formation and dissipation of field-aligned
current sheets: energy is deposited at small scales where heating
occurs. Only a fraction of the coronal mass and volume gets heated
at any time. Temperature and density are highly structured at scales
that, in the solar corona, remain observationally unresolved: the
plasma of simulated loops is multithermal, where highly dynamical
hotter and cooler plasma strands are scattered throughout the loop at
sub-observational scales. We will also compare Reduced MHD simulations
with fully compressible simulations and photospheric forcings with
different time-scales compared to the Alfv'en transit time. Finally,
we will discuss the differences between the closed field and open field
(solar wind) turbulence heating problem, leading to observational
consequences that may be amenable to Parker Solar Probe and Solar
Orbiter.
Title: Parametric instability of nonlinear Alfvén waves in
anisotropic plasmas
Authors: Tenerani, A.; Velli, M.; Hellinger, P.
Bibcode: 2017AGUFMSH14B..05T
Altcode:
We study the stability of large amplitude, circularly polarized Alfvén
waves in an anisotropic plasma described by the double adiabatic
two-temperature CGL equations, and in particular the effect of a
background pressure anisotropy on the well-known properties of Alfvén
wave parametric decay in Magnetohydrodynamics (MHD). A temperature
anisotropy allows instability over a much wider range of values of
parallel plasma beta (β||) when the pressure anisotropy p0perp/p0||
> 1. For p0perp/p0|| > 2.7 there is a new regime in which the
growth rates of the parametric instability become finite and independent
of β|| in the limit β||>>1. In the opposite case of p0perp/p0||
< 2.7, the instability is strongly suppressed for increasing plasma
beta, similar to the MHD case. We analyze marginal stability conditions
for parametric and firehose instabilities in parameter space defined
by p0perp/p0|| vs. β|| and the wave amplitude B vs. β||, and discuss
possible implications for Alfvén waves in the solar wind. We validate
our results by means of hybrid simulations and we show under which
conditions the parametric decay is unaffected by the onset of kinetic
instabilities at smaller scales.
Title: Fast Magnetic Reconnection: “Ideal” Tearing and the
Hall Effect
Authors: Pucci, Fulvia; Velli, Marco; Tenerani, Anna
Bibcode: 2017ApJ...845...25P
Altcode: 2017arXiv170408793P
One of the main questions in magnetic reconnection is the origin of
triggering behavior with on/off properties that, once it is activated,
accounts for the fast magnetic energy conversion to kinetic and
thermal energies at the heart of explosive events in astrophysical and
laboratory plasmas. Over the past decade, progress has been made on the
initiation of fast reconnection via the plasmoid instability and what
has been called “ideal” tearing, which sets in once current sheets
thin to a critical inverse aspect ratio {(a/L)}c. As shown
by Pucci & Velli, at {(a/L)}c∼ {S}-1/3,
the timescale for the instability to develop becomes of the order
of the Alfvén time and independent of the Lundquist number (here
defined in terms of current sheet length L). However, given the
large values of S in natural plasmas, this transition might occur for
thicknesses of the inner resistive singular layer that are comparable
to the ion inertial length d I . When this occurs,
Hall currents produce a three-dimensional quadrupole structure of
the magnetic field, and the dispersive waves introduced by the Hall
effect accelerate the instability. Here we present a linear study
showing how the “ideal” tearing mode critical aspect ratio is
modified when Hall effects are taken into account, including more
general scaling laws of the growth rates in terms of sheet inverse
aspect ratio: the critical inverse aspect ratio is amended to a/L≃
{({di}/L)}0.29{(1/S)}0.19, at which point
the instability growth rate becomes Alfvénic and does not depend on
either of the (small) parameters {d}I/L,1/S. We discuss the
implications of this generalized triggering aspect ratio for recently
developed phase diagrams of magnetic reconnection.
Title: Solar Orbiter Status Report
Authors: Gilbert, Holly; St. Cyr, Orville Chris; Mueller, Daniel;
Zouganelis, Yannis; Velli, Marco
Bibcode: 2017SPD....4811004G
Altcode:
With the delivery of the instruments to the spacecraft builder, the
Solar Orbiter mission is in the midst of Integration & Testing
phase at Airbus in Stevenage, U.K. This mission to “Explore the
Sun-Heliosphere Connection” is the first medium-class mission of
ESA’s Cosmic Vision 2015-2025 program and is being jointly implemented
with NASA. The dedicated payload of 10 remote-sensing and in-situ
instruments will orbit the Sun as close as 0.3 A.U. and will provide
measurments from the photosphere into the solar wind. The three-axis
stabilized spacecraft will use Venus gravity assists to increase the
orbital inclination out of the ecliptic to solar latitudes as high as 34
degrees in the extended mission. The science team of Solar Orbiter has
been working closely with the Solar Probe Plus scientists to coordinate
observations between these two highly-complementary missions. This will
be a status report on the mission development; the interested reader
is referred to the recent summary by Müller et al., Solar Physics 285
(2013).
Title: Solar Orbiter Status Report
Authors: Gilbert, Holly; Cyr, Chris S.; Mueller, Daniel; Zouganelis,
Yannis; Velli, Marco
Bibcode: 2017shin.confE.139G
Altcode:
With the delivery of the instruments to the spacecraft builder,
the Solar Orbiter mission is in the midst of Integration &
Testing phase at Airbus in Stevenage, U.K. This mission to "Explore
the Sun-Heliosphere Connection" is the first medium-class mission of
ESA's Cosmic Vision 2015-2025 program and is being jointly implemented
with NASA. The dedicated payload of 10 remote-sensing and in-situ
instruments will orbit the Sun as close as 0.3 A.U. and will provide
measurments from the photosphere into the solar wind. The three-axis
stabilized spacecraft will use Venus gravity assists to increase the
orbital inclination out of the ecliptic to solar latitudes as high as 34
degrees in the extended mission. The science team of Solar Orbiter has
been working closely with the Solar Probe Plus scientists to coordinate
observations between these two highly-complementary missions. This will
be a status report on the mission development; the interested reader
is referred to the recent summary by Müller et al., Solar Physics 285
(2013).
Title: Evolving Waves and Turbulence in the Outer Corona and Inner
Heliosphere: The Accelerating Expanding Box
Authors: Tenerani, Anna; Velli, Marco
Bibcode: 2017ApJ...843...26T
Altcode: 2017arXiv170203014T
Alfvénic fluctuations in the solar wind display many properties
reflecting an ongoing nonlinear cascade, e.g., a well-defined spectrum
in frequency, together with some characteristics more commonly
associated with the linear propagation of waves from the Sun, such
as the variation of fluctuation amplitude with distance, dominated
by solar wind expansion effects. Therefore, both nonlinearities and
expansion must be included simultaneously in any successful model of
solar wind turbulence evolution. Because of the disparate spatial scales
involved, direct numerical simulations of turbulence in the solar wind
represent an arduous task, especially if one wants to go beyond the
incompressible approximation. Indeed, most simulations neglect solar
wind expansion effects entirely. Here we develop a numerical model
to simulate turbulent fluctuations from the outer corona to 1 au and
beyond, including the sub-Alfvénic corona. The accelerating expanding
box (AEB) extends the validity of previous expanding box models by
taking into account both the acceleration of the solar wind and the
inhomogeneity of background density and magnetic field. Our method
incorporates a background accelerating wind within a magnetic field
that naturally follows the Parker spiral evolution using a two-scale
analysis in which the macroscopic spatial effect coupling fluctuations
with background gradients becomes a time-dependent coupling term in
a homogeneous box. In this paper we describe the AEB model in detail
and discuss its main properties, illustrating its validity by studying
Alfvén wave propagation across the Alfvén critical point.
Title: Coronal Heating Topology: The Interplay of Current Sheets
and Magnetic Field Lines
Authors: Rappazzo, A. F.; Matthaeus, W. H.; Ruffolo, D.; Velli, M.;
Servidio, S.
Bibcode: 2017ApJ...844...87R
Altcode: 2017arXiv170608983R
The magnetic topology and field line random walk (FLRW) properties of
a nanoflare-heated and magnetically confined corona are investigated
in the reduced magnetohydrodynamic regime. Field lines originating
from current sheets form coherent structures, called current sheet
connected (CSC) regions, which extend around them. CSC FLRW is strongly
anisotropic, with preferential diffusion along the current sheets’
in-plane length. CSC FLRW properties remain similar to those of
the entire ensemble but exhibit enhanced mean square displacements
and separations due to the stronger magnetic field intensities in
CSC regions. The implications for particle acceleration and heat
transport in the solar corona and wind, and for solar moss formation
are discussed.
Title: ICARUS mission, next step of coronal exploration after Solar
Orbiter and Solar Probe Plus
Authors: Krasnoselskikh, Vladimir; Tsurutani, Bruce T.; Velli,
Marco; Maksimovic, Milan; Balikhin, Mikhael; Dudok de Wit, Thierry;
Kretzschmar, Matthieu
Bibcode: 2017EGUGA..19.3971K
Altcode:
The primary scientific goal of ICARUS (Investigation of Coronal
AcceleRation and heating Up to the Sun), a mother-daughter satellite
mission, will be to determine how the magnetic _field and plasma
dynamics in the outer solar atmosphere give rise to the corona,
the solar wind and the entire heliosphere. Reaching this goal will
be a Rosetta-stone step, with results broadly applicable within
the fields of space plasma physics and astrophysics. Within ESA's
Cosmic Vision roadmap, these science goals address Theme 2: How does
the solar system work ?" by investigating basic processes occurring
From the Sun to the edge of the Solar System". ICARUS will not only
advance our understanding of the plasma environment around our the
Sun, but also of the numerous magnetically active stars with hot
plasma coronae. ICARUS I will perform the first-ever direct in situ
measurements of electromagnetic fields, particle acceleration, wave
activity, energy distribution and flows directly in the regions where
the solar wind emerges from the coronal plasma. ICARUS I will have a
perihelion at 1 Solar radius from its surface, it will cross the region
where the major energy deposition occurs. The polar orbit of ICARUS
I will enable crossing the regions where both the fast and slow wind
are generated. It will probe local characteristics of the plasma and
provide unique information about the physical processes involved in the
creation of the solar wind. ICARUS II will observe this region using
remote-sensing instruments, providing simultaneous information about
regions crossed by ICARUS I and the solar atmosphere below as observed
by solar telescopes. It will thus provide bridges for understanding the
magnetic links between the heliosphere and the solar atmosphere. Such
information is crucial to our understanding of the plasma physics and
electrodynamics of the solar atmosphere. ICARUS II will also play a
very important relay role, enabling the radio-link with ICARUS I. It
will receive, collect and store information transmitted from ICARUS I
during its closest approach to the Sun. It will also perform preliminary
data processing before transmitting it to the Earth. Performing such
unique in situ measurements in the region where presumably deadly
solar energetic particles are energized, ICARUS will make fundamental
contributions to our ability to monitor and forecast the space radiation
environment. Such a knowledge is extremely important for future space
explorations, especially for long-term manned space missions.
Title: Stability Analysis of Two-dimensional Current Sheets at
Arbitrary Aspect Ratio
Authors: Shi, C.; Tenerani, A.; Velli, M.
Bibcode: 2016AGUFMSH51B2596S
Altcode:
Magnetohydrodynamic simulations suggest that there exists a threshold
Lundquist number Sc, around Sc 104, above which current sheets
transition from a laminar, Sweet-Parker like reconnecting configuration,
to a highly tearing-unstable (turbulent) state dominated by plasmoid
generation. In this context, it is known that the flow along the sheet
plays a stabilizing role, as one would expect the evacuation time-scale
to be longer than the typical growth time for islands in order for
the sheet to be tearing unstable. However, a satisfactory detailed
explanation of the critical threshold for the tearing instability
and its dependence on boundary conditions is still lacking. Here we
present results from linear stability analysis of two-dimensional
current sheets with flows across and along the sheet, spanning a wide
range of Lundquist numbers (S) and current sheet aspect ratio. Since
the growth rate of the tearing mode is strongly affected by the aspect
ratio, we inspect how it should scale with S in order to overcome
the stabilizing effect of flows. This work complements a companion
one on one-dimensional sheets embedded in a jet, highlighting the
effects introduced by both inhomogeneity along the sheet and boundary
conditions.
Title: Closed-field Coronal Heating Driven by Wave Turbulence
Authors: Downs, Cooper; Lionello, Roberto; Mikić, Zoran; Linker,
Jon A.; Velli, Marco
Bibcode: 2016ApJ...832..180D
Altcode: 2016arXiv161002113D
To simulate the energy balance of coronal plasmas on macroscopic scales,
we often require the specification of the coronal heating mechanism
in some functional form. To go beyond empirical formulations and to
build a more physically motivated heating function, we investigate
the wave-turbulence-driven (WTD) phenomenology for the heating of
closed coronal loops. Our implementation is designed to capture the
large-scale propagation, reflection, and dissipation of wave turbulence
along a loop. The parameter space of this model is explored by solving
the coupled WTD and hydrodynamic evolution in 1D for an idealized
loop. The relevance to a range of solar conditions is also established
by computing solutions for over one hundred loops extracted from a
realistic 3D coronal field. Due to the implicit dependence of the WTD
heating model on loop geometry and plasma properties along the loop and
at the footpoints, we find that this model can significantly reduce
the number of free parameters when compared to traditional empirical
heating models, and still robustly describe a broad range of quiet-Sun
and active region conditions. The importance of the self-reflection
term in producing relatively short heating scale heights and thermal
nonequilibrium cycles is also discussed.
Title: Observational Signatures of Coronal Heating
Authors: Dahlburg, R. B.; Einaudi, G.; Ugarte-Urra, I.; Warren, H. P.;
Rappazzo, A. F.; Velli, M.; Taylor, B.
Bibcode: 2016AGUFMSH42A..06D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numericalsimulations of the fully compressible
three-dimensionalmagnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection,nonlinear thermal conduction and
optically thin radiation.The footpoints of the loop magnetic field
are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi-thermal, where highly
dynamical hotter and cooler plasma strands arescattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla.To connect these simulations to observations
the computed numberdensities and temperatures are used to synthesize
the intensities expected inemission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer(EIS) on Hinode. These
intensities are then employed to compute differentialemission measure
distributions, which are found to be very similar to those derivedfrom
observations of solar active regions.
Title: Double Current Sheet Instabilities and the Transition to
Turbulence.
Authors: Pucci, F.; Velli, M.; Biferale, L.; Sahoo, G.
Bibcode: 2016AGUFMSH41A2528P
Altcode:
The double tearing instability has often been studied as a proxy for
the m=1 kink mode in cylindrical plasma. In this paper we describe
the results of 3D simulations of an initially periodic double current
sheet described by Harris equilibria with a guide field in two cases: 1)
zero net helicity and an average magnetic field and 2) a well defined
helicity (force free but non constant alpha). We study and contrast
the de-stabilization and transition to turbulence for these two cases:
we describe spectra, cascades, and possible application to heliospheric
phenomena, in particular CME evolution and relaxation. The research
leading to these results has received fund- ing from the European
Union's Seventh Framework Pro- gramme (FP7/2007-2013) under grant
agreement No. 339032
Title: Predicting the Orientation of the Bz Component
of CMEs
Authors: Panasenco, O.; Velli, M.
Bibcode: 2016AGUFMSH14A..02P
Altcode:
We present a step-by-step procedure to forecast the magnetic field
vector B and more specifically the Bz component associated with the
passage of the Coronal Mass Ejections (CMEs) at 1 AU via observational
and modeling efforts. This is a fundamental component in determining
the geo-effectiveness of Interplanetary (I)CMEs. We discuss a detailed
observational and modeling investigation of pre-eruptive filament
channels and filaments (prominences at the limb), the source regions of
CMEs on the sun, to determine the direction of the tangential component
of the magnetic field in the source region before CMEs (chirality), its
evolution during CME deflection and rotation when filaments erupt. We
analyze the local and global magnetic background near the CME source
region to predict the possible changes in the CME orientation during
the essential early stages of the eruption when magnetic pressure and
tension of the surrounding field are still significant compared to that
in the CME. The question we will answer: What is the direction of the
magnetic field vector B in a pre-eruptive system, with emphasis on the
axial field, and how does it evolve in the early stages of eruption
in the low corona?
Title: Filament Structure and Stability in the Solar Corona
Authors: Tomlinson, S. M.; Velli, M. C. M.; Panasenco, O.
Bibcode: 2016AGUFMSH51B2597T
Altcode:
We summarize and critically evaluate the basic theories for solar
filaments and prominences presented up to date, as well as their
stability. In particular we will discuss the role of the various current
systems that may be present in determining equilibrium and stability
properties, including those responsible for the torus instability and
loss of equilibrium. We will then attempt to introduce some additional
elements, such as the possible role of the longitudinal magnetic field
evolution in the stability of prominences, as well as the consequences
of their intrinsically dynamic nature.
Title: Fast Tearing Mode Instability in Thin Current Sheets Embedded
in a Jet
Authors: Tenerani, A.; Velli, M.
Bibcode: 2016AGUFMSH51B2594T
Altcode:
A longstanding problem has been to understand why at relatively low
Lundquist number current sheets are observed to be stable with respect
to the tearing mode. In particular, simulations suggest that instability
sets-in above a minimum aspect ratio A which is around A 100. Assuming
a scaling with the Lundquist number S as A S1/2 (as in Sweet Parker
sheets), this implies the existence of a critical Lundquist number
of around Sc 104 as has been numerically seen.While it is known that
flows along current sheets have a stabilizing effects, the existence
of a threshold for instability in terms of Lundquist number and
aspect ratio of the current sheet has not yet been theoretically
satisfactorily shown. Here we approach this problem starting with
a simple one dimensional equilibrium current sheet embedded in a
sheared flow (Bickley jet) to locally mimic the dynamics of current
sheets. It is known that the growth rate of the tearing mode increases
with the aspect ratio, while the latter should have little effects
on flows. Here we therefore extend and generalize previous linear
studies to a wider range of Lundquist and Alfvén Mach numbers, by
varying the current sheet aspect ratio, assuming a generic scaling A
Sα. This study is complementary to the stability of two-dimensional
current sheet configurations with flows.
Title: Complexity Variations in the Interplanetary Magnetic Field
Between 0.3 and 5.4 AU
Authors: Weygand, J. M.; Kivelson, M.; Khurana, K. K.; Walker, R. J.;
Strangeway, R. J.; Velli, M.; Angelopoulos, V.
Bibcode: 2016AGUFMSH41A2511W
Altcode:
We have investigated how the character of magnetic fluctuations in the
solar wind depends on radial distance from the Sun. We use measurements
of the magnetic field taken at different distances from the Sun by
different spacecraft: Helios between 0.3 and 1 AU and Ulysses between
1.4 and 5.4 AU. We selected data intervals determined to have only
turbulent magnetic field fluctuations and no other structures. With
these data we calculate the Jensen-Shannon complexity as a function of
permutation entropy. Jensen-Shannon complexity maps indicate if the
fluctuations in the magnetic fields are stochastic (low complexity),
chaotic (maximal complexity and lower entropy), or chaotic with a
strong noise component (moderate complexity and high entropy). The
Jensen-Shannon complexity values determined from the Helios and Ulysses
spacecraft measurements for the turbulent magnetic fluctuations indicate
the fluctuations are stochastic. This conclusion is supported by power
spectra. The Jensen-Shannon complexity values calculated for slow
(<450 km/s) and fast solar wind (>550 km/s) turbulent magnetic
field fluctuations evolve from low complexity and high entropy at 1
AU to lower complexity and higher entropy farther from the Sun (to
5.4 AU). No clear dependence on heliospheric latitude is apparent in
the Ulysses data. We interpret these data to mean that the magnetic
field fluctuations become more stochastic at greater distances from
the Sun. We investigate whether with this change in complexity is due
to expansion of the solar wind or the age of the turbulent magnetic
fluctuations. These results can be tested by Solar Probe Plus in 2018.
Title: Solar Wind Origins, Heating and Turbulence Evolution with
Solar Probe Plus: The First Three Perihelia
Authors: Velli, M. C. M.; Panasenco, O.; Rappazzo, A. F.; Tenerani,
A.; Bale, S. D.; Fox, N. J.; Howard, R.; Kasper, J. C.; McComas, D. J.
Bibcode: 2016AGUFMSH54A..07V
Altcode:
In this presentation we will focus on some of the early science return
made possible by the Solar Probe Plus mission, and more specifically
the returns from the first three perihelia at 35.66 solar radii (Rs),
just over half the distance from the Sun of the previous closest
approaching spacecraft, Helios (62.4 Rs). The increased exploration of
the inner heliosphere will allow important new measurements on slow and
fast solar wind turbulent fluctuations, their spectra, and therefore
the origin and dynamics of the so-called Alfvénic turbulence, with
fundamental implications on both the acceleration and heating of the
wind. Will the Alfvénic turbulence cause further bursty jetting in
fast wind streams? How will the anisotropy of the particle distribution
functions eveolve and how will this impact our understanding of the
role plasma instabilities in the wind? During these first encounters,
the Solar Probe Plus spacecraft will already achieve sufficient speeds
to cross the corotation orbit at perihelion: we will therefore also
focus on the questions of the different origins of the slow and fast
solar wind, and specifically the role of the heliospheric current sheet,
the s-web, and coronal streamers and pseudo-streamers in influencing
the different plasma velocities, temperatures and fluctuation properties
in the solar wind inside 40 Rs.
Title: The Wide-Field Imager for Solar Probe Plus (WISPR)
Authors: Vourlidas, Angelos; Howard, Russell A.; Plunkett, Simon P.;
Korendyke, Clarence M.; Thernisien, Arnaud F. R.; Wang, Dennis; Rich,
Nathan; Carter, Michael T.; Chua, Damien H.; Socker, Dennis G.; Linton,
Mark G.; Morrill, Jeff S.; Lynch, Sean; Thurn, Adam; Van Duyne, Peter;
Hagood, Robert; Clifford, Greg; Grey, Phares J.; Velli, Marco; Liewer,
Paulett C.; Hall, Jeffrey R.; DeJong, Eric M.; Mikic, Zoran; Rochus,
Pierre; Mazy, Emanuel; Bothmer, Volker; Rodmann, Jens
Bibcode: 2016SSRv..204...83V
Altcode: 2015SSRv..tmp....8V; 2015SSRv..tmp...66B
The Wide-field Imager for Solar PRobe Plus (WISPR) is the sole imager
aboard the Solar Probe Plus (SPP) mission scheduled for launch in
2018. SPP will be a unique mission designed to orbit as close as
7 million km (9.86 solar radii) from Sun center. WISPR employs a
95∘ radial by 58∘ transverse field of view
to image the fine-scale structure of the solar corona, derive the 3D
structure of the large-scale corona, and determine whether a dust-free
zone exists near the Sun. WISPR is the smallest heliospheric imager to
date yet it comprises two nested wide-field telescopes with large-format
(2 K × 2 K) APS CMOS detectors to optimize the performance for their
respective fields of view and to minimize the risk of dust damage,
which may be considerable close to the Sun. The WISPR electronics are
very flexible allowing the collection of individual images at cadences
up to 1 second at perihelion or the summing of multiple images to
increase the signal-to-noise when the spacecraft is further from the
Sun. The dependency of the Thomson scattering emission of the corona
on the imaging geometry dictates that WISPR will be very sensitive
to the emission from plasma close to the spacecraft in contrast to
the situation for imaging from Earth orbit. WISPR will be the first
`local' imager providing a crucial link between the large-scale corona
and the in-situ measurements.
Title: The FIELDS Instrument Suite for Solar Probe Plus. Measuring
the Coronal Plasma and Magnetic Field, Plasma Waves and Turbulence,
and Radio Signatures of Solar Transients
Authors: Bale, S. D.; Goetz, K.; Harvey, P. R.; Turin, P.; Bonnell,
J. W.; Dudok de Wit, T.; Ergun, R. E.; MacDowall, R. J.; Pulupa,
M.; Andre, M.; Bolton, M.; Bougeret, J. -L.; Bowen, T. A.; Burgess,
D.; Cattell, C. A.; Chandran, B. D. G.; Chaston, C. C.; Chen,
C. H. K.; Choi, M. K.; Connerney, J. E.; Cranmer, S.; Diaz-Aguado, M.;
Donakowski, W.; Drake, J. F.; Farrell, W. M.; Fergeau, P.; Fermin, J.;
Fischer, J.; Fox, N.; Glaser, D.; Goldstein, M.; Gordon, D.; Hanson,
E.; Harris, S. E.; Hayes, L. M.; Hinze, J. J.; Hollweg, J. V.; Horbury,
T. S.; Howard, R. A.; Hoxie, V.; Jannet, G.; Karlsson, M.; Kasper,
J. C.; Kellogg, P. J.; Kien, M.; Klimchuk, J. A.; Krasnoselskikh,
V. V.; Krucker, S.; Lynch, J. J.; Maksimovic, M.; Malaspina, D. M.;
Marker, S.; Martin, P.; Martinez-Oliveros, J.; McCauley, J.; McComas,
D. J.; McDonald, T.; Meyer-Vernet, N.; Moncuquet, M.; Monson, S. J.;
Mozer, F. S.; Murphy, S. D.; Odom, J.; Oliverson, R.; Olson, J.;
Parker, E. N.; Pankow, D.; Phan, T.; Quataert, E.; Quinn, T.; Ruplin,
S. W.; Salem, C.; Seitz, D.; Sheppard, D. A.; Siy, A.; Stevens, K.;
Summers, D.; Szabo, A.; Timofeeva, M.; Vaivads, A.; Velli, M.; Yehle,
A.; Werthimer, D.; Wygant, J. R.
Bibcode: 2016SSRv..204...49B
Altcode: 2016SSRv..tmp...16B
NASA's Solar Probe Plus (SPP) mission will make the first in situ
measurements of the solar corona and the birthplace of the solar
wind. The FIELDS instrument suite on SPP will make direct measurements
of electric and magnetic fields, the properties of in situ plasma waves,
electron density and temperature profiles, and interplanetary radio
emissions, amongst other things. Here, we describe the scientific
objectives targeted by the SPP/FIELDS instrument, the instrument
design itself, and the instrument concept of operations and planned
data products.
Title: The Solar Probe Plus Mission: Humanity's First Visit to
Our Star
Authors: Fox, N. J.; Velli, M. C.; Bale, S. D.; Decker, R.; Driesman,
A.; Howard, R. A.; Kasper, J. C.; Kinnison, J.; Kusterer, M.; Lario,
D.; Lockwood, M. K.; McComas, D. J.; Raouafi, N. E.; Szabo, A.
Bibcode: 2016SSRv..204....7F
Altcode: 2015SSRv..tmp..105F
Solar Probe Plus (SPP) will be the first spacecraft to fly into the low
solar corona. SPP's main science goal is to determine the structure
and dynamics of the Sun's coronal magnetic field, understand how
the solar corona and wind are heated and accelerated, and determine
what processes accelerate energetic particles. Understanding these
fundamental phenomena has been a top-priority science goal for over five
decades, dating back to the 1958 Simpson Committee Report. The scale
and concept of such a mission has been revised at intervals since that
time, yet the core has always been a close encounter with the Sun. The
mission design and the technology and engineering developments enable
SPP to meet its science objectives to: (1) Trace the flow of energy that
heats and accelerates the solar corona and solar wind; (2) Determine
the structure and dynamics of the plasma and magnetic fields at the
sources of the solar wind; and (3) Explore mechanisms that accelerate
and transport energetic particles. The SPP mission was confirmed in
March 2014 and is under development as a part of NASA's Living with
a Star (LWS) Program. SPP is scheduled for launch in mid-2018, and
will perform 24 orbits over a 7-year nominal mission duration. Seven
Venus gravity assists gradually reduce SPP's perihelion from 35 solar
radii (RS) for the first orbit to {<}10 RS
for the final three orbits. In this paper we present the science,
mission concept and the baseline vehicle for SPP, and examine how the
mission will address the key science questions
Title: Solar Wind Electrons Alphas and Protons (SWEAP) Investigation:
Design of the Solar Wind and Coronal Plasma Instrument Suite for
Solar Probe Plus
Authors: Kasper, Justin C.; Abiad, Robert; Austin, Gerry;
Balat-Pichelin, Marianne; Bale, Stuart D.; Belcher, John W.; Berg,
Peter; Bergner, Henry; Berthomier, Matthieu; Bookbinder, Jay; Brodu,
Etienne; Caldwell, David; Case, Anthony W.; Chandran, Benjamin D. G.;
Cheimets, Peter; Cirtain, Jonathan W.; Cranmer, Steven R.; Curtis,
David W.; Daigneau, Peter; Dalton, Greg; Dasgupta, Brahmananda;
DeTomaso, David; Diaz-Aguado, Millan; Djordjevic, Blagoje; Donaskowski,
Bill; Effinger, Michael; Florinski, Vladimir; Fox, Nichola; Freeman,
Mark; Gallagher, Dennis; Gary, S. Peter; Gauron, Tom; Gates, Richard;
Goldstein, Melvin; Golub, Leon; Gordon, Dorothy A.; Gurnee, Reid; Guth,
Giora; Halekas, Jasper; Hatch, Ken; Heerikuisen, Jacob; Ho, George; Hu,
Qiang; Johnson, Greg; Jordan, Steven P.; Korreck, Kelly E.; Larson,
Davin; Lazarus, Alan J.; Li, Gang; Livi, Roberto; Ludlam, Michael;
Maksimovic, Milan; McFadden, James P.; Marchant, William; Maruca,
Bennet A.; McComas, David J.; Messina, Luciana; Mercer, Tony; Park,
Sang; Peddie, Andrew M.; Pogorelov, Nikolai; Reinhart, Matthew J.;
Richardson, John D.; Robinson, Miles; Rosen, Irene; Skoug, Ruth M.;
Slagle, Amanda; Steinberg, John T.; Stevens, Michael L.; Szabo, Adam;
Taylor, Ellen R.; Tiu, Chris; Turin, Paul; Velli, Marco; Webb, Gary;
Whittlesey, Phyllis; Wright, Ken; Wu, S. T.; Zank, Gary
Bibcode: 2016SSRv..204..131K
Altcode: 2015SSRv..tmp..119K
The Solar Wind Electrons Alphas and Protons (SWEAP) Investigation
on Solar Probe Plus is a four sensor instrument suite that provides
complete measurements of the electrons and ionized helium and hydrogen
that constitute the bulk of solar wind and coronal plasma. SWEAP
consists of the Solar Probe Cup (SPC) and the Solar Probe Analyzers
(SPAN). SPC is a Faraday Cup that looks directly at the Sun and measures
ion and electron fluxes and flow angles as a function of energy. SPAN
consists of an ion and electron electrostatic analyzer (ESA) on
the ram side of SPP (SPAN-A) and an electron ESA on the anti-ram
side (SPAN-B). The SPAN-A ion ESA has a time of flight section that
enables it to sort particles by their mass/charge ratio, permitting
differentiation of ion species. SPAN-A and -B are rotated relative to
one another so their broad fields of view combine like the seams on a
baseball to view the entire sky except for the region obscured by the
heat shield and covered by SPC. Observations by SPC and SPAN produce
the combined field of view and measurement capabilities required to
fulfill the science objectives of SWEAP and Solar Probe Plus. SWEAP
measurements, in concert with magnetic and electric fields, energetic
particles, and white light contextual imaging will enable discovery
and understanding of solar wind acceleration and formation, coronal and
solar wind heating, and particle acceleration in the inner heliosphere
of the solar system. SPC and SPAN are managed by the SWEAP Electronics
Module (SWEM), which distributes power, formats onboard data products,
and serves as a single electrical interface to the spacecraft. SWEAP
data products include ion and electron velocity distribution functions
with high energy and angular resolution. Full resolution data are stored
within the SWEM, enabling high resolution observations of structures
such as shocks, reconnection events, and other transient structures
to be selected for download after the fact. This paper describes the
implementation of the SWEAP Investigation, the driving requirements
for the suite, expected performance of the instruments, and planned
data products, as of mission preliminary design review.
Title: Integrated Science Investigation of the Sun (ISIS): Design
of the Energetic Particle Investigation
Authors: McComas, D. J.; Alexander, N.; Angold, N.; Bale, S.; Beebe,
C.; Birdwell, B.; Boyle, M.; Burgum, J. M.; Burnham, J. A.; Christian,
E. R.; Cook, W. R.; Cooper, S. A.; Cummings, A. C.; Davis, A. J.;
Desai, M. I.; Dickinson, J.; Dirks, G.; Do, D. H.; Fox, N.; Giacalone,
J.; Gold, R. E.; Gurnee, R. S.; Hayes, J. R.; Hill, M. E.; Kasper,
J. C.; Kecman, B.; Klemic, J.; Krimigis, S. M.; Labrador, A. W.;
Layman, R. S.; Leske, R. A.; Livi, S.; Matthaeus, W. H.; McNutt,
R. L.; Mewaldt, R. A.; Mitchell, D. G.; Nelson, K. S.; Parker, C.;
Rankin, J. S.; Roelof, E. C.; Schwadron, N. A.; Seifert, H.; Shuman,
S.; Stokes, M. R.; Stone, E. C.; Vandegriff, J. D.; Velli, M.; von
Rosenvinge, T. T.; Weidner, S. E.; Wiedenbeck, M. E.; Wilson, P.
Bibcode: 2016SSRv..204..187M
Altcode: 2014SSRv..tmp...24M
The Integrated Science Investigation of the Sun (ISIS) is a complete
science investigation on the Solar Probe Plus (SPP) mission, which
flies to within nine solar radii of the Sun's surface. ISIS comprises a
two-instrument suite to measure energetic particles over a very broad
energy range, as well as coordinated management, science operations,
data processing, and scientific analysis. Together, ISIS observations
allow us to explore the mechanisms of energetic particles dynamics,
including their: (1) Origins—defining the seed populations and
physical conditions necessary for energetic particle acceleration;
(2) Acceleration—determining the roles of shocks, reconnection,
waves, and turbulence in accelerating energetic particles; and (3)
Transport—revealing how energetic particles propagate from the corona
out into the heliosphere. The two ISIS Energetic Particle Instruments
measure lower (EPI-Lo) and higher (EPI-Hi) energy particles. EPI-Lo
measures ions and ion composition from ∼20 keV/nucleon-15 MeV total
energy and electrons from ∼25-1000 keV. EPI-Hi measures ions from
∼1-200 MeV/nucleon and electrons from ∼0.5-6 MeV. EPI-Lo comprises
80 tiny apertures with fields-of-view (FOVs) that sample over nearly
a complete hemisphere, while EPI-Hi combines three telescopes that
together provide five large-FOV apertures. ISIS observes continuously
inside of 0.25 AU with a high data collection rate and burst data
(EPI-Lo) coordinated with the rest of the SPP payload; outside of
0.25 AU, ISIS runs in low-rate science mode whenever feasible to
capture as complete a record as possible of the solar energetic
particle environment and provide calibration and continuity for
measurements closer in to the Sun. The ISIS Science Operations Center
plans and executes commanding, receives and analyzes all ISIS data,
and coordinates science observations and analyses with the rest of
the SPP science investigations. Together, ISIS' unique observations
on SPP will enable the discovery, untangling, and understanding of
the important physical processes that govern energetic particles in
the innermost regions of our heliosphere, for the first time. This
paper summarizes the ISIS investigation at the time of the SPP mission
Preliminary Design Review in January 2014.
Title: `Ideally' unstable current sheets and the triggering of fast
magnetic reconnection
Authors: Tenerani, A.; Velli, M.; Pucci, F.; Landi, S.; Rappazzo, A. F.
Bibcode: 2016JPlPh..82e5301T
Altcode: 2016arXiv160805066T
Magnetic reconnection is thought to be the dynamical mechanism
underlying many explosive phenomena observed both in space and in the
laboratory, although the question of how fast magnetic reconnection
is triggered in such high Lundquist ( ) number plasmas has remained
elusive. It has been well established that reconnection can develop
over time scales faster than those predicted traditionally once
kinetic scales are reached. It has also been shown that, within the
framework of resistive magnetohydrodynamics (MHD), fast reconnection is
achieved for thin enough sheets via the onset of the so-called plasmoid
instability. The latter was discovered in studies specifically devoted
to the Sweet-Parker current sheet, either as an initial condition
or an apparent transient state developing in nonlinear studies. On
the other hand, a fast tearing instability can grow on an ideal,
i.e. -independent, time scale (dubbed `ideal' tearing) within current
sheets whose aspect ratio scales with the macroscopic Lundquist number
as 1/3$]]> - much smaller than the Sweet-Parker one - suggesting a
new way to approach to the initiation of fast reconnection in collapsing
current configurations. Here we present an overview of what we have
called `ideal' tearing in resistive MHD, and discuss how the same
reasoning can be extended to other plasma models commonly used that
include electron inertia and kinetic effects. We then discuss a scenario
for the onset of `ideal' fast reconnection via collapsing current
sheets and describe a quantitative model for the interpretation of
the nonlinear evolution of `ideally' unstable sheets in two dimensions.
Title: Complexity Variations in the Interplanetary Magnetic Field
between 0.4 and 5.3 AU
Authors: Weygand, James M.; Kivelson, M. G.; Velli, M.; Khurana,
K. K.; Angelopoulos, V.; Walker, R. J.
Bibcode: 2016shin.confE.181W
Altcode:
We have investigated how the character of magnetic fluctuations
in solar wind depends on radial distance from the Sun. We use
measurements of the magnetic field taken at different distances from
the Sun by different spacecraft: Helios between 0.4 and 1 AU, Wind at
about 1 AU, and Ulysses at about 5.4 AU. Data intervals are selected
to contain turbulent magnetic fluctuations, coronal mass ejections
(CMEs), and co-rotating interaction regions (CIRs). With these data we
calculate the Jensen-Shannon complexity as a function of permutation
entropy. Jensen-Shannon complexity maps indicate if the fluctuations
in the magnetic fields are stochastic (low complexity) chaotic (maximal
complexity and lower entropy), or chaotic with a strong noise component
(moderate complexity and high entropy). The Jensen-Shannon complexity
values determined from the spacecraft measurements for the turbulent
magnetic fluctuations indicate the fluctuations are stochastic
in nature. This observation is supported by power spectra. The
Jensen-Shannon complexity values determined for the CMEs and CIRs
indicate the fluctuations are chaotic. The CME Jensen-Shannon complexity
values evolve from high complexity and moderate entropy at 1 AU to
lower complexity and higher entropy farther from the Sun at 5.4 AU. We
interpret these data to mean that as the solar wind plasma expands
outward, the magnetic field fluctuations evolve from chaotic (i.e.,
low dimensionality, deterministic fluctuations) to stocastic (i.e.,
low dimensionality, non-deterministic fluctuations). These results
can be tested by Solar Probe Plus to be launched in 2018.
Title: Inward Motions in the Outer Solar Corona between 7 and 12 R
⊙: Evidence for Waves or Magnetic Reconnection Jets?
Authors: Tenerani, Anna; Velli, Marco; DeForest, Craig
Bibcode: 2016ApJ...825L...3T
Altcode:
DeForest et al. used synoptic visible-light image sequences from the
COR2 coronagraph on board the STEREO-A spacecraft to identify inbound
wave motions in the outer corona beyond 7 solar radii and inferred, from
the observation, that the Alfvén surface separating the magnetically
dominated corona from the flow dominated wind must be located beyond at
least 12 solar radii from the Sun over polar coronal holes and beyond
15 solar radii in the streamer belt. Here, we attempt identification of
the observed inward signal by theoretically reconstructing height-speed
diagrams and comparing them to the observed profiles. Interpretation
in terms of Alfvén waves or Alfvénic turbulence appears to be
ruled out by the fact that the observed signal shows a deceleration
of inward motion when approaching the Sun. Fast magnetoacoustic waves
are not directly ruled out in this way, as it is possible for inward
waves observed in quadrature, but not propagating exactly radially,
to suffer total reflection as the Alfvén speed rises close to the
Sun. However, the reconstructed signal in the height-speed diagram has
the wrong concavity. A final possibility is decelerating reconnection
jets, most probably from component reconnection, in the accelerating
wind: the profile in this case appears to match the observations very
well. This interpretation does not alter the conclusion that the Alfvén
surface must be at least 12 solar radii from the photosphere. Further
observations should help constrain this process, never identified
previously in this way, in the distance range from 7 to 12 solar radii.
Title: Morphology of Pseudostreamers and Solar Wind Properties
Authors: Panasenco, Olga; Velli, Marco
Bibcode: 2016SPD....47.0324P
Altcode:
The solar dynamo and photospheric convection lead to three main types of
structures extending from the solar surface into the corona - active
regions, solar filaments (prominences when observed at the limb) and
coronal holes. These structures exist over a wide range of scales,
and are interlinked with each other in evolution and dynamics. Active
regions can form clusters of magnetic activity and the strongest
overlie sunspots. In the decay of active regions, the boundaries
separating opposite magnetic polarities (neutral lines) develop the
specific structures called filament channels above which filaments
form. In the presence of flux imbalance decaying active regions can
also give birth to lower latitude coronal holes. The accumulation of
magnetic flux at coronal hole boundaries also creates the conditions
for filament formation: polar crown filaments are permanently present
at the boundaries of the polar coronal holes. Middle-latitude and
equatorial coronal holes - the result of active region evolution
- can create pseudostreamers (PSs) if other coronal holes of the
same polarity are present. While helmet streamers form between open
fields of opposite polarities, the pseudostreamer, characterized by
a smaller coronal imprint, typically shows a more prominent straight
ray or stalk extending from the corona. The pseudostreamer base
at photospheric heights is multipolar; often one observes tripolar
magnetic configurations with two neutral lines - where filaments can
form - separating the coronal holes. Here we discuss the specific role
of filament channels on pseudostreamer topology and on solar wind
properties. 1D numerical analysis of PSs shows that the properties
of the solar wind from around PSs depend on the presence/absence of
filament channels, number of channels and chirality at the PS base
low in the corona.
Title: Formation and Evolution of Large-Scale Magnetic Funnels in
the Solar Corona
Authors: Panasenco, Olga; Velli, Marco
Bibcode: 2016SPD....4740204P
Altcode:
The existence of open coronal magnetic fields with peculiar geometry -
large-scale magnetic funnels - can be attributed to three factors:
(i) the presence of two or more corona holes of the same polarity
(or pseudostreamers - PSs), (ii) specific configurations of closed
magnetic field in the low corona up to 1.3 Rs (filament channels) and
(iii) the presence of strong active regions in the vicinity of the
pseudostreamer. The important property of magnetic funnels is their
strongly non-monotonic expansion factor below 2 Rs. The case study
presented here is a pseudostreamer near the equator, formed between
two isolated coronal holes of the same polarity, and harboring a
pair of twin filaments in its base. Following the evolution of these
coronal holes we find that the PS topology changes when two coronal
holes merged together. Using a potential field source-surface (PFSS)
extrapolation to compute the coronal field from photospheric maps
(SDO/HMI), we show that the funnel-like geometry of the open magnetic
field changes to a regular one with monotonic expansion factor after
the merging of coronal holes. The presence of coronal magnetic funnels
becomes directly visible when sufficient plasma accumulates inside
them: when the plasma density grows to become observable coronal cloud
prominences appear in the corona. The plasma suspension at heights
of 0.3 Rs coincides with the largest gradients in the field which
naturally leads to a diamagnetic hypothesis for the force counteracting
gravity. We study the evolution of the funnel-like open fields during
several solar rotations and find a direct relation between funnels
and the presence of coronal clouds at great heights in the solar corona.
Title: Numerical Simulation of DC Coronal Heating
Authors: Dahlburg, Russell B.; Einaudi, G.; Taylor, Brian D.;
Ugarte-Urra, Ignacio; Warren, Harry; Rappazzo, A. F.; Velli, Marco
Bibcode: 2016SPD....47.0305D
Altcode:
Recent research on observational signatures of turbulent heating of
a coronal loop will be discussed. The evolution of the loop is is
studied by means of numerical simulations of the fully compressible
three-dimensional magnetohydrodynamic equations using the HYPERION
code. HYPERION calculates the full energy cycle involving footpoint
convection, magnetic reconnection, nonlinear thermal conduction
and optically thin radiation. The footpoints of the loop magnetic
field are convected by random photospheric motions. As a consequence
the magnetic field in the loop is energized and develops turbulent
nonlinear dynamics characterized by the continuous formation and
dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is non-uniformly
distributed so that only a fraction of thecoronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales which, in the solar corona, remain observationally unresolved:
the plasma of the simulated loop is multi thermal, where highly
dynamical hotter and cooler plasma strands are scattered throughout
the loop at sub-observational scales. Typical simulated coronal loops
are 50000 km length and have axial magnetic field intensities ranging
from 0.01 to 0.04 Tesla. To connect these simulations to observations
the computed number densities and temperatures are used to synthesize
the intensities expected in emission lines typically observed with
the Extreme ultraviolet Imaging Spectrometer (EIS) on Hinode. These
intensities are then employed to compute differential emission measure
distributions, which are found to be very similar to those derived
from observations of solar active regions.
Title: Inward Motions in the Outer Solar Corona Between 6 And 12 R :
Evidence For Waves or Magnetic Reconnection Jets?
Authors: Velli, Marco; Tenerani, Anna; DeForest, Craig
Bibcode: 2016SPD....4740205V
Altcode:
DeForest et al. (2014) used synoptic visible-light image sequences
from the COR2 coronagraph on board the STEREO-A spacecraft to identify
inbound wave motions in the outer corona beyond 6 solar radii and
inferred, from the observation, that the Alfven surface separating
the magnetically dominated corona from the ow dominated wind must be
located at least 12 solar radii from the Sun over polar coronal holes
and 15 solar radii in the streamer belt. Here we will discuss both
this and previous observations of inflows further down and attempt
identification of the observed inward signals. We will theoretically
reconstruct height-speed diagrams and compare them to the observed
profiles. Interpretation in terms of Alfven / magnetoacouatic modes
or Alfvenic turbulence appears to be ruled out by the fact that the
observed signal shows a deceleration of inward motion when approaching
the Sun. Fast magnetoacoustic waves are not directly ruled out in
this way, as it is possible for inward waves observed in quadrature,
but not propagating exactly radially, to suffer total reflection as
the Alfven speed rises close to the Sun. However, the reconstructed
signal in the height speed diagram has the wrong concavity. A final
possibility is decelerating reconnection jets, most probably from
component reconnection, in the accelerating wind: the profile in this
case appears to match the observations very well. This interpretation
does not alter the conclusion that the Alfven surface must be at least
12 solar radii from the photosphere.
Title: The ideal tearing mode: theory and resistive MHD simulations
Authors: Del Zanna, L.; Landi, S.; Papini, E.; Pucci, F.; Velli, M.
Bibcode: 2016JPhCS.719a2016D
Altcode: 2016arXiv160304995D
Classical MHD reconnection theories, both the stationary Sweet-Parker
model and the tearing instability, are known to provide rates which
are too slow to explain the observations. However, a recent analysis
has shown that there exists a critical threshold on current sheet's
thickness, namely a/L ∼ S -1/3, beyond which the tearing
modes evolve on fast macroscopic Alfvénic timescales, provided the
Lunquist number S is high enough, as invariably found in solar and
astrophysical plasmas. Therefore, the classical Sweet-Parker scenario,
for which the diffusive region scales as a/L ∼ S -1/2
and thus can be up to ∼ 100 times thinner than the critical value,
is likely to be never realized in nature, as the current sheet itself
disrupts in the elongation process. We present here two-dimensional,
compressible, resistive MHD simulations, with S ranging from
105 to 107, that fully confirm the linear
analysis. Moreover, we show that a secondary plasmoid instability
always occurs when the same critical scaling is reached on the local,
smaller scale, leading to a cascading explosive process, reminiscent
of the flaring activity.
Title: Reconnection in thin current sheets
Authors: Tenerani, Anna; Velli, Marco; Pucci, Fulvia; Rappazzo, A. F.
Bibcode: 2016SPD....47.1401T
Altcode:
It has been widely believed that reconnection is the underlying
mechanism of many explosive processes observed both in nature and
laboratory, but the question of reconnection speed and initial trigger
have remained mysterious. How is fast magnetic energy release triggered
in high Lundquist (S) and Reynolds (R) number plasmas?It has been
shown that a tearing mode instability can grow on an ideal timescale,
i.e., independent from the the Lundquist number, once the current sheet
thickness becomes thin enough, or rather the inverse aspect ratio a/L
reaches a scale a/L~S-1/3. As such, the latter provides a natural,
critical threshold for current sheets that can be formed in nature
before they disrupt in a few Alfvén time units. Here we discuss
the transition to fast reconnection extended to simple viscous and
kinetic models and we propose a possible scenario for the transition
to explosive reconnection in high-Lundquist number plasmas, that we
support with fully nonlinear numerical MHD simulations of a collapsing
current sheet.
Title: "Ideal" tearing and the transition to fast reconnection in
the weakly collisional MHD and EMHD regimes
Authors: Del Sarto, Daniele; Pucci, Fulvia; Tenerani, Anna; Velli,
Marco
Bibcode: 2016JGRA..121.1857D
Altcode: 2015arXiv151100035D
This paper discusses the transition to fast growth of the tearing
instability in thin current sheets in the collisionless limit where
electron inertia drives the reconnection process. It has been previously
suggested that in resistive MHD there is a natural maximum aspect
ratio (ratio of sheet length and breadth to thickness) which may be
reached for current sheets with a macroscopic length L, the limit being
provided by the fact that the tearing mode growth time becomes of the
same order as the Alfvén time calculated on the macroscopic scale. For
current sheets with a smaller aspect ratio than critical the normalized
growth rate tends to zero with increasing Lundquist number S, while for
current sheets with an aspect ratio greater than critical the growth
rate diverges with S. Here we carry out a similar analysis but with
electron inertia as the term violating magnetic flux conservation:
previously found scalings of critical current sheet aspect ratios
with the Lundquist number are generalized to include the dependence
on the ratio de2/L2, where de is the electron
skin depth, and it is shown that there are limiting scalings which,
as in the resistive case, result in reconnecting modes growing on
ideal time scales. Finite Larmor radius effects are then included,
and the rescaling argument at the basis of "ideal" reconnection is
proposed to explain secondary fast reconnection regimes naturally
appearing in numerical simulations of current sheet evolution.
Title: Observational Signatures of Coronal Loop Heating and Cooling
Driven by Footpoint Shuffling
Authors: Dahlburg, R. B.; Einaudi, G.; Taylor, B. D.; Ugarte-Urra,
I.; Warren, H. P.; Rappazzo, A. F.; Velli, M.
Bibcode: 2016ApJ...817...47D
Altcode: 2015arXiv151203079D
The evolution of a coronal loop is studied by means of
numerical simulations of the fully compressible three-dimensional
magnetohydrodynamic equations using the HYPERION code. The footpoints
of the loop magnetic field are advected by random motions. As a
consequence, the magnetic field in the loop is energized and develops
turbulent nonlinear dynamics characterized by the continuous formation
and dissipation of field-aligned current sheets: energy is deposited
at small scales where heating occurs. Dissipation is nonuniformly
distributed so that only a fraction of the coronal mass and volume gets
heated at any time. Temperature and density are highly structured at
scales that, in the solar corona, remain observationally unresolved:
the plasma of our simulated loop is multithermal, where highly dynamical
hotter and cooler plasma strands are scattered throughout the loop at
sub-observational scales. Numerical simulations of coronal loops of
50,000 km length and axial magnetic field intensities ranging from 0.01
to 0.04 T are presented. To connect these simulations to observations,
we use the computed number densities and temperatures to synthesize
the intensities expected in emission lines typically observed with the
Extreme Ultraviolet Imaging Spectrometer on Hinode. These intensities
are used to compute differential emission measure distributions using
the Monte Carlo Markov Chain code, which are very similar to those
derived from observations of solar active regions. We conclude that
coronal heating is found to be strongly intermittent in space and time,
with only small portions of the coronal loop being heated: in fact,
at any given time, most of the corona is cooling down.
Title: Resistive Magnetohydrodynamic Simulations of Fast Reconnection
in Thin Current Sheets: Analysis of the Linear and Nonlinear Stages
of the "Ideal" Tearing Mode
Authors: Landi, S.; Del Zanna, L.; Papini, E.; Pucci, F.; Velli, M.
Bibcode: 2015AGUFMSH43A2429L
Altcode:
Thin current sheets are known to be unstable to tearing and even
super-tearing modes, leading to explosive reconnection events
as required to explain the rapid release of magnetic energy in
astrophysical plasmas (solar flares, magnetar bursts, dissipation in
pulsar winds). Here we study by means of resistive, compressible MHD
simulations the behavior of current sheets whose inverse aspect ratio
scales with the Lundquist number S as S-1/3, known to give rise to fast,
ideal reconnection, with an evolution and growth that are independent
of S. In the linear phase we retrieve the expected eigenmodes and the
growth rate, which can be as high as γ ≈ 0.6 τA-1, where τA is
the ideal Alfvénic time set by the macroscopic scales. The nonlinear
stages are characterized by the coalescence of magnetic islands and by
secondary reconnection events, obeying the same critical scaling with
the local S, leading to the production and ejection of plasmoids on
increasingly shorter timescales. Preliminary simulations of the ideal
tearing mode are presented also for magnetically dominated plasmas,
in the relativistic MHD regime.
Title: Complexity Variations in the Interplanetary Magnetic Field
between 0.4 and 5.3 AU
Authors: Weygand, J. M.; Kivelson, M.; Velli, M.; Gekelman, W. N.;
Khurana, K. K.; Angelopoulos, V.; Walker, R. J.
Bibcode: 2015AGUFMSH33A2450W
Altcode:
We have investigated how the character of magnetic fluctuations of solar
wind plasma depends on radial distance from the Sun. We use measurements
of the magnetic field taken at different distances from the Sun by
different spacecraft: Helios between 0.4 and 1 AU, ACE and Wind at
about 1 AU, and Ulysses at about 5.3 AU. Data intervals are selected
to contain only what appear to be random fluctuations and to exclude
solar wind structures such as coronal mass ejections, co-rotating
interaction regions, heliospheric current sheets, shocks, etc. With
these data we calculate the Jensen-Shannon complexity as a function
of permutation entropy. Jensen-Shannon complexity maps indicate if the
fluctuations in the magnetic fields are stochastic (low complexity and
high entropy), or if they exhibit minimal or maximal complexity and
lower entropy. The Jensen-Shannon complexity values determined from
the spacecraft measurements evolve from moderate complexity and high
entropy at 0.4 AU to lower complexity and higher entropy farther from
the Sun. We interpret these data to mean that as the solar wind plasma
expands outward, the magnetic field fluctuations evolve from chaotic
(i.e., low dimensionality, deterministic fluctuations) to turbulent
(i.e., low dimensionality, non-deterministic fluctuations). By
separating the magnetic fluctuations into slow solar wind (<450
km/s) and fast solar wind (>550 km/s), we find that the younger
solar wind (transported outward rapidly) has higher complexity than
the older solar wind (transported outward slowly). These results can
be tested by Solar Probe Plus to be launched in 2018.
Title: Ideal Tearing in the Hall Regime
Authors: Pucci, F.; Velli, M.; Tenerani, A.
Bibcode: 2015AGUFMSH43A2430P
Altcode:
Magnetic reconnection is generally believed to be the mechanism
thatexplains explosive events in astrophysical plasmas, such as
flares inthe solar corona, substorms. One of the main questions which
remainsconcerns how magnetic reconnection may account for the fast
magneticenergy conversion to kinetic and thermal energies. Recently
it hasbeen shown by Pucci and Velli (2014) that, assuming that
currentsheets scales as different powers of the magnetic Reynolds
number S,the growth rate of the tearing mode instability in current
sheetsincreases as the sheets thin and, once the thickness reaches a
scalinga/L ∼ S-1/3, the time scale for the instability to develop
becomesof the order of the Alfvén time. In Hall reconnection,
dispersivewaves introduced by the Hall effect make the energy
release ratesfaster. This effect becomes important to the collisional
tearing modeinstability when the thickness of magnetic reversal layer
iscomparable to the ion inertia length, where Hall currents produce
athree-dimensional quadrupole structure of magnetic field. Here
wepresent a linear study aiming to show how an "ideal tearing mode"
isachieved when Hall effects are included, including scaling laws
forsheet aspect ratios and growth rates.
Title: The Slow and Fast Solar Wind: Understanding Heating,
Acceleration and Turbulence from Observations with Solar Probe Plus
and Solar Orbiter
Authors: Velli, M. C. M.
Bibcode: 2015AGUFMSH24A..02V
Altcode:
The Solar Probe Plus and Solar Orbiter missions have as part of their
goals to understand the coronal heating and source regions of the solar
wind and the role of turbulence in the solar wind acceleration and
dynamics. In this presentation a summary of the questions associated
with the distibution of wind speeds and magnetic fields in the inner
heliosphere and their origin on the sun will be summarized. Where and
how does the sharp gradient in speeds develop close to the Sun? Is
the wind source for fast and slow the same, and is there a steady
component or is its origin always intermittent in nature? Where does
the heliospheric current sheet form and how stable is it close to the
Sun? Where does Alfvénic turbulence form and what is its role in
coronal heating? I will describe how the multiple Solar Probe Plus
passes together with Solar Orbiter alignments and quadratures will
help to make fundamental progress on these questions.
Title: Filament Channels: Isolated Laboratories of Plasma Heating
in the Solar Corona
Authors: Panasenco, O.; Velli, M.
Bibcode: 2015AGUFMSH13C2454P
Altcode:
Solar filament channels are complex systems comprising photospheric,
chromospheric and coronal components. These components include
magnetic neutral lines, supergranule cells, fibrils (spicules),
filaments (prominences when observed on the limb), coronal cells,
filament cavities and their overlying coronal arcades. Filaments are
very highly structured and extend in height from the photosphere to
the corona. Filament cores have chromospheric temperatures - 10,000 K
(even at coronal heights ~ 100 Mm), surrounded by hotter plasma with
temperature up to ~50,000 K. The whole filament is isolated from
the rest of the solar corona by an envelope - the filament channel
cavity - with temperatures of about 2,000,000 K. The filament channel
cavity is even hotter than the solar corona outside the filament
channel arcade. The compactness and big temperature variations make
filament channels unique ready-to-go laboratories of coronal plasma
heating and thermodynamics. In this work we discuss possible sources
and mechanisms of heating in the filament channel environment. In
particular, we address the mechanisms of magnetic canceling and current
sheet dissipation.
Title: Trigger of Fast Reconnection via Collapsing Current Sheets
Authors: Tenerani, A.; Velli, M.; Rappazzo, A. F.; Pucci, F.
Bibcode: 2015AGUFMSH43A2433T
Altcode:
It has been widely believed that reconnection is the underlying
mechanism of many explosive processes observed both in astrophysical
and laboratory plasmas. However, both the questions of how magnetic
reconnection is triggered in high Lundquist (S) and Reynolds (R)
number plasmas, and how it can then occur on fast, ideal, time-scales
remain open. Indeed, it has been argued that fast reconnection rates
could be achieved once kinetic scales are reached, or, alternatively,
by the onset of the so-called plasmoid instability within Sweet-Parker
current sheets. However, it has been shown recently that a tearing
mode instability (the "ideal tearing") can grow on an ideal, i.e.,
S-independent, timescale once the width a of a current sheet becomes
thin enough with respect to its macroscopic length L, a/L ~ S-1/3. This
suggests that current sheet thinning down to such a threshold aspect
ratio —much larger, for S>>1, than the Sweet-Parker one that
scales as a/L ~ S-1/2— might provide the trigger for fast reconnection
even within the fluid plasma framework. Here we discuss the transition
to fast reconnection by studying with visco-resistive MHD simulations
the onset and evolution of the tearing instability within a single
collapsing current sheet. We indeed show that the transition to a fast
tearing mode instability takes place when an inverse aspect ratio of the
order of the threshold a/L ~ S-1/3 is reached, and that the secondary
current sheets forming nonlinearly become the source of a succession
of recursive tearing instabilities. The latter is reminiscent of the
fractal reconnection model of flares, which we modify in the light of
the "ideal tearing" scenario.
Title: Large-Eddy Simulations of Magnetohydrodynamic Turbulence in
Heliophysics and Astrophysics
Authors: Miesch, Mark; Matthaeus, William; Brandenburg, Axel;
Petrosyan, Arakel; Pouquet, Annick; Cambon, Claude; Jenko, Frank;
Uzdensky, Dmitri; Stone, James; Tobias, Steve; Toomre, Juri; Velli,
Marco
Bibcode: 2015SSRv..194...97M
Altcode: 2015arXiv150501808M; 2015SSRv..tmp...83M
We live in an age in which high-performance computing is transforming
the way we do science. Previously intractable problems are now becoming
accessible by means of increasingly realistic numerical simulations. One
of the most enduring and most challenging of these problems is
turbulence. Yet, despite these advances, the extreme parameter regimes
encountered in space physics and astrophysics (as in atmospheric and
oceanic physics) still preclude direct numerical simulation. Numerical
models must take a Large Eddy Simulation (LES) approach, explicitly
computing only a fraction of the active dynamical scales. The success
of such an approach hinges on how well the model can represent the
subgrid-scales (SGS) that are not explicitly resolved. In addition
to the parameter regime, heliophysical and astrophysical applications
must also face an equally daunting challenge: magnetism. The presence
of magnetic fields in a turbulent, electrically conducting fluid flow
can dramatically alter the coupling between large and small scales,
with potentially profound implications for LES/SGS modeling. In this
review article, we summarize the state of the art in LES modeling of
turbulent magnetohydrodynamic (MHD) flows. After discussing the nature
of MHD turbulence and the small-scale processes that give rise to energy
dissipation, plasma heating, and magnetic reconnection, we consider how
these processes may best be captured within an LES/SGS framework. We
then consider several specific applications in heliophysics and
astrophysics, assessing triumphs, challenges, and future directions.
Title: Magnetic Reconnection: Recursive Current Sheet Collapse
Triggered by “Ideal” Tearing
Authors: Tenerani, Anna; Velli, Marco; Rappazzo, Antonio Franco;
Pucci, Fulvia
Bibcode: 2015ApJ...813L..32T
Altcode: 2015arXiv150608921T
We study, by means of MHD simulations, the onset and evolution of fast
reconnection via the “ideal” tearing mode within a collapsing
current sheet at high Lundquist numbers (S\gg {10}4). We
first confirm that as the collapse proceeds, fast reconnection is
triggered well before a Sweet-Parker-type configuration can form: during
the linear stage, plasmoids rapidly grow in a few Alfvén times when the
predicted “ideal” tearing threshold S-1/3 is approached
from above; after the linear phase of the initial instability, X-points
collapse and reform nonlinearly. We show that these give rise to a
hierarchy of tearing events repeating faster and faster on current
sheets at ever smaller scales, corresponding to the triggering of
“ideal” tearing at the renormalized Lundquist number. In resistive
MHD, this process should end with the formation of sub-critical (S
≤ 104) Sweet-Parker sheets at microscopic scales. We
present a simple model describing the nonlinear recursive evolution
that explains the timescale of the disruption of the initial sheet.
Title: ADAHELI: exploring the fast, dynamic Sun in the x-ray, optical,
and near-infrared
Authors: Berrilli, Francesco; Soffitta, Paolo; Velli, Marco; Sabatini,
Paolo; Bigazzi, Alberto; Bellazzini, Ronaldo; Bellot Rubio, Luis
Ramon; Brez, Alessandro; Carbone, Vincenzo; Cauzzi, Gianna; Cavallini,
Fabio; Consolini, Giuseppe; Curti, Fabio; Del Moro, Dario; Di Giorgio,
Anna Maria; Ermolli, Ilaria; Fabiani, Sergio; Faurobert, Marianne;
Feller, Alex; Galsgaard, Klaus; Gburek, Szymon; Giannattasio, Fabio;
Giovannelli, Luca; Hirzberger, Johann; Jefferies, Stuart M.; Madjarska,
Maria S.; Manni, Fabio; Mazzoni, Alessandro; Muleri, Fabio; Penza,
Valentina; Peres, Giovanni; Piazzesi, Roberto; Pieralli, Francesca;
Pietropaolo, Ermanno; Martinez Pillet, Valentin; Pinchera, Michele;
Reale, Fabio; Romano, Paolo; Romoli, Andrea; Romoli, Marco; Rubini,
Alda; Rudawy, Pawel; Sandri, Paolo; Scardigli, Stefano; Spandre,
Gloria; Solanki, Sami K.; Stangalini, Marco; Vecchio, Antonio;
Zuccarello, Francesca
Bibcode: 2015JATIS...1d4006B
Altcode:
Advanced Astronomy for Heliophysics Plus (ADAHELI) is a project concept
for a small solar and space weather mission with a budget compatible
with an European Space Agency (ESA) S-class mission, including launch,
and a fast development cycle. ADAHELI was submitted to the European
Space Agency by a European-wide consortium of solar physics research
institutes in response to the "Call for a small mission opportunity
for a launch in 2017," of March 9, 2012. The ADAHELI project builds
on the heritage of the former ADAHELI mission, which had successfully
completed its phase-A study under the Italian Space Agency 2007 Small
Mission Programme, thus proving the soundness and feasibility of
its innovative low-budget design. ADAHELI is a solar space mission
with two main instruments: ISODY: an imager, based on Fabry-Pérot
interferometers, whose design is optimized to the acquisition of
highest cadence, long-duration, multiline spectropolarimetric images
in the visible/near-infrared region of the solar spectrum. XSPO: an
x-ray polarimeter for solar flares in x-rays with energies in the 15
to 35 keV range. ADAHELI is capable of performing observations that
cannot be addressed by other currently planned solar space missions,
due to their limited telemetry, or by ground-based facilities, due to
the problematic effect of the terrestrial atmosphere.
Title: Resistive Magnetohydrodynamics Simulations of the Ideal
Tearing Mode
Authors: Landi, S.; Del Zanna, L.; Papini, E.; Pucci, F.; Velli, M.
Bibcode: 2015ApJ...806..131L
Altcode: 2015arXiv150407036L
We study the linear and nonlinear evolution of the tearing
instability on thin current sheets by means of two-dimensional
numerical simulations, within the framework of compressible,
resistive MHD. In particular we analyze the behavior of current
sheets whose inverse aspect ratio scales with the Lundquist number S as
{{S}-1/3}. This scaling has been recently recognized to yield
the threshold separating fast, ideal reconnection, with an evolution
and growth that are independent of S provided this is high enough,
as it should be natural having the ideal case as a limit for S\to ∞
. Our simulations confirm that the tearing instability growth rate
can be as fast as γ ≈ 0.6 {{τ }A}-1, where
{{τ }A} is the ideal Alfvénic time set by the macroscopic
scales, for our least diffusive case with S={{10}7}. The
expected instability dispersion relation and eigenmodes are
also retrieved in the linear regime, for the values of S explored
here. Moreover, in the nonlinear stage of the simulations we observe
secondary events obeying the same critical scaling with S, here
calculated on the local, much smaller lengths, leading to increasingly
faster reconnection. These findings strongly support the idea that in
a fully dynamic regime, as soon as current sheets develop, thin, and
reach this critical threshold in their aspect ratio, the tearing mode
is able to trigger plasmoid formation and reconnection on the local
(ideal) Alfvénic timescales, as required to explain the explosive
flaring activity often observed in solar and astrophysical plasmas.
Title: Models of coronal heating, turbulence and fast reconnection
Authors: Velli, M.; Pucci, F.; Rappazzo, F.; Tenerani, A.
Bibcode: 2015RSPTA.37340262V
Altcode:
Coronal heating is at the origin of the EUV and X-ray emission
and mass loss from the sun and many other stars. While different
scenarios have been proposed to explain the heating of magnetically
confined and open regions of the corona, they must all rely on the
transfer, storage and dissipation of the abundant energy present in
photospheric motions, which, coupled to magnetic fields, give rise to
the complex phenomenology seen at the chromosphere and transition region
(i.e. spicules, jets, 'tornadoes'). Here we discuss models and numerical
simulations which rely on magnetic fields and electric currents both
for energy transfer and for storage in the corona. We will revisit the
sources and frequency spectrum of kinetic and electromagnetic energies,
the role of boundary conditions, and the routes to small scales required
for effective dissipation. Because reconnection in current sheets has
been, and still is, one of the most important processes for coronal
heating, we will also discuss recent aspects concerning the triggering
of reconnection instabilities and the transition to fast reconnection.
Title: The Tearing Mode Instability of Thin Current Sheets: the
Transition to Fast Reconnection in the Presence of Viscosity
Authors: Tenerani, Anna; Rappazzo, Antonio Franco; Velli, Marco;
Pucci, Fulvia
Bibcode: 2015ApJ...801..145T
Altcode: 2014arXiv1412.0047T
This paper studies the growth rate of reconnection instabilities in thin
current sheets in the presence of both resistivity and viscosity. In a
previous paper, Pucci & Velli, it was argued that at sufficiently
high Lundquist number S it is impossible to form current sheets with
aspect ratios L/a that scale as L/a∼ {{S}α } with α \gt
1/3 because the growth rate of the tearing mode would then diverge in
the ideal limit S\to ∞ . Here we extend their analysis to include the
effects of viscosity, always present in numerical simulations along with
resistivity, and which may play a role in the solar corona and other
astrophysical environments. A finite Prandtl number allows current
sheets to reach larger aspect ratios before becoming rapidly unstable
in pileup-type regimes. Scalings with Lundquist and Prandtl numbers
are discussed, as well as the transition to kinetic reconnection.
Title: Ion Kinetic Energy Conservation and Magnetic Field Strength
Constancy in Multi-fluid Solar Wind Alfvénic Turbulence
Authors: Matteini, L.; Horbury, T. S.; Pantellini, F.; Velli, M.;
Schwartz, S. J.
Bibcode: 2015ApJ...802...11M
Altcode: 2015arXiv150100702M
We investigate the properties of plasma fluid motion in the
large-amplitude, low-frequency fluctuations of highly Alfvénic fast
solar wind. We show that protons locally conserve total kinetic energy
when observed from an effective frame of reference comoving with the
fluctuations. For typical properties of the fast wind, this frame can
be reasonably identified by alpha particles which, due to their drift
with respect to protons at about the Alfvén speed along the magnetic
field, do not partake in the fluid low-frequency fluctuations. Using
their velocity to transform the proton velocity into the frame of
Alfvénic turbulence, we demonstrate that the resulting plasma motion
is characterized by a constant absolute value of the velocity, zero
electric fields, and aligned velocity and magnetic field vectors as
expected for unidirectional Alfvénic fluctuations in equilibrium. We
propose that this constraint, via the correlation between velocity
and magnetic field in Alfvénic turbulence, is the origin of the
observed constancy of the magnetic field; while the constant velocity
corresponding to constant energy can only be observed in the frame of
the fluctuations, the corresponding constant total magnetic field,
invariant for Galilean transformations, remains the observational
signature in the spacecraft frame of the constant total energy in the
Alfvén turbulence frame.
Title: Basics of Plasma Astrophysics
Authors: Chiuderi, C.; Velli, M.
Bibcode: 2015bps..book.....C
Altcode:
No abstract at ADS
Title: Parametric decay of parallel and oblique Alfvén waves in
the expanding solar wind
Authors: Del Zanna, L.; Matteini, L.; Landi, S.; Verdini, A.; Velli, M.
Bibcode: 2015JPlPh..81a3202D
Altcode: 2014arXiv1407.5851D
The long-term evolution of large-amplitude Alfvén waves propagating
in the solar wind is investigated by performing two-dimensional
MHD simulations within the expanding box model. The linear and
nonlinear phases of the parametric decay instability are studied
for both circularly polarized waves in parallel propagation and for
arc-polarized waves in oblique propagation. The non-monochromatic
case is also considered. In the oblique case, the direct excitation of
daughter modes transverse to the local background field is found for
the first time in an expanding environment, and this transverse cascade
seems to be favored for monochromatic mother waves. The expansion
effect reduces the instability growth rate, and it can even suppress
its onset for the lowest frequency modes considered here, possibly
explaining the persistence of these outgoing waves in the solar wind.
Title: Application of a Solar Wind Model Driven by Turbulence
Dissipation to a 2D Magnetic Field Configuration
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikić, Zoran
Bibcode: 2014ApJ...796..111L
Altcode: 2014arXiv1410.1789L
Although it is widely accepted that photospheric motions provide
the energy source and that the magnetic field must play a key role
in the process, the detailed mechanisms responsible for heating
the Sun's corona and accelerating the solar wind are still not
fully understood. Cranmer et al. developed a sophisticated,
one-dimensional (1D), time-steady model of the solar wind with
turbulence dissipation. By varying the coronal magnetic field, they
obtain, for a single choice of wave properties, a realistic range
of slow and fast wind conditions with a sharp latitudinal transition
between the two streams. Using a 1D, time-dependent model of the solar
wind of Lionello et al., which incorporates turbulent dissipation
of Alfvén waves to provide heating and acceleration of the plasma,
we have explored a similar configuration, obtaining qualitatively
equivalent results. However, our calculations suggest that the rapid
transition between slow and fast wind suggested by this 1D model may
be disrupted in multidimensional MHD simulations by the requirement
of transverse force balance.
Title: Visco-resistive tearing in thin current sheets.
Authors: Velli, M. M. C.; Tenerani, A.; Rappazzo, A. F.; Pucci, F.
Bibcode: 2014AGUFMSH31B..06V
Altcode:
How fast magnetic energy release is triggered and occurs in high
Lundquist (S) and high Reynolds number ( R ) plasmas such as that
of the solar corona is a fundamental problem for understanding
phenomena ranging from coronal heating to flares and CMEs. Diffusion or
collisional reconnection driven by macroscopic flows in quasi-steady
Sweet-Parker (SP) current sheets are processes far too slow to fit
observational data. Spontaneous reconnection, driven by the onset of
the tearing instability inside current sheets, provides an alternative
paradigm to SP reconnection. Nevertheless, as long as macroscopic
current layers are considered, the growth of such an instability is also
a slow process. Recently it has been shown that SP current sheets are
rapidly unstable in high S plasmas, indeed have a growth rate diverging
with increasing S. It has been suggested that such instabilities are
triggered during the nonlinear stage of the primary tearing instability
of a macroscopic layer. The formation of plasmoids in this presumed
SP sheet speeds up the reconnection rate to ideal values. Recently,
we have suggested that SP sheets can not be realized in quasi-ideal
plasmas, and that the plasmoid instability is triggered on a much
larger scale (i.e. with current sheets having a much larger ration of
thickness to length than SP). Here we present a linear parametric study
of the tearing instability for a Harris current sheet, while taking into
account both viscosity and current sheets of variable aspect ratios. The
present study shows that an explosive growth of the reconnection rate
may be reached during the linear stage, once a critical width of the
current layer is reached. In the absence of a strong guide field this
depends on viscosity and a range of critical aspect ratios can be
found for different values of S, R, or S and Prandtl number.
Title: Interchange Reconnection and Slow Solar Wind Formation at
the boundaries of open field regions in the Solar Corona
Authors: Rappazzo, A. F.; Matthaeus, W. H.; Ruffolo, D. J.; Servidio,
S.; Velli, M.
Bibcode: 2014AGUFMSH31B..05R
Altcode:
Interchange reconnection, i.e., magnetic reconnection at the interface
between open and closed corona, is thought to contribute to the
formation of the slowsolar wind, since it can inject the hotter and
denserplasma from closed regions into the heliosphere,and account for
the different slow wind composition (thatis similar to the plasma of
closed regions) respectto the fast wind. The interchange process has
mostly been investigatedfor magnetic field lines with opposite polarity
and null points, either for the case of counterdirected loops (e.g.,
Fisk et al. 1999, Fisk and Schwadron 2001), or in correspondence of
null points at the apex of streamers or pseudo-streamers (e.g., Wang
et al. 1998,Edmondson et al. 2010, Del Zanna et al. 2011).Magnetic
reconnection can certainly occur in these configurations,but they occupy
a very small volume of the corona. On the other hand component magnetic
reconnection at the boundarybetween coronal holes and streamers or
pseudo-streamers hasreceived less attention, even though it can occur
aroundthe entire extension of such boundaries. Magnetic reconnection is
at the basis of Parker'snanoflare scenario for the heating of coronal
loops.Modeling such regions in cartesian geometry with a strongguide
field, it has been shown numerically that photosphericmotions
induce a magnetic fieldcomponent orthogonal to the strong axial
field characterizedby the presence of many current sheets, where
the field lines are locally oppositely directed, and can reconnect
(Einaudi et al. 1996; Dmitruk and Gomez 1997).The reconnection of
the orthogonal component of the magneticfield leads to a change
of connectivity of the field linesof the total magnetic field that
connect one photospheric boundaryto the other. We have shown that a
similar interchange mechanismcan operate in and around the boundaries
between open and closedregions inducing a continual stochastic
rearrangement of connectivityeverywhere along the open-closed boundary
(Rappazzo et al. 2012). We examine a reduced MHD model of a simplified
interfaceregion between open and closed corona. We extend previous
results to quantify the flux of mass density,heat and momentum from
the closed to the adjacent open regionthrough their shared boundary,
and model the impact of this fluxon the acceleration of the slow
component of the solar wind.
Title: A Nonlinear Model for Dynamics in the Expanding Accelerating
Solar Wind
Authors: Tenerani, A.; Velli, M.
Bibcode: 2014AGUFMSH33A4120T
Altcode:
One of the outstanding problems in astrophysics is the origin of stellar
coronae, winds, and, more generally, the ubiquitous existence in the
universe of hot million degree (or more) plasmas. The solar corona and
wind provide an accessible environment to understand plasma heating and
acceleration, and this is one of the main goals of the upcoming NASA
mission Solar Probe Plus, which will arrive closer to the Sun (10 Rs),
within the acceleration region than any previous spacecraft. Alfvén
waves, which can easily propagate along magnetic field lines from the
cooler photosphere to the hot corona and above, are thought to provide
a possible mechanism to supply the energy required to heat and boost
the solar wind, through turbulent dissipation and pressure. In-situ
observations show that a nonlinear cascade of Alfvén waves, mainly
propagating outward, is taking place, and that it evolves with
heliocentric distance. In spite of the well defined observational
signatures, the evolution of such Alfvénic turbulence in the solar wind
is still a matter under debate, as neither linear theory nor numerical
simulations can account for the observed properties. In particular,
the effects of the expansion of the underlying solar atmosphere are a
crucial element which must be taken into account, since the observed
decrease in overall rms energies is best accounted for by expansion
effects. Here we present a model to study the dynamics of a plasma
parcel embedded in a radially accelerating solar wind, all the way
from the acceleration region to the inner heliosphere, called the
Accelerating Expanding Box. This model takes describes the radial
evolution of turbulence and structures as they are observed in the
expanding solar wind in a relatively simple way. As a first application,
we show how expansion affects the onset and the radial evolution of
the decay of large amplitude Alfvén waves through interaction with
magnetoacoustic waves, the parametric decay instability.
Title: Characterizing a Model of Coronal Heating and Solar Wind
Acceleration Based on Wave Turbulence.
Authors: Downs, C.; Lionello, R.; Mikic, Z.; Linker, J.; Velli, M.
Bibcode: 2014AGUFMSH31B..04D
Altcode:
Understanding the nature of coronal heating and solar wind acceleration
is a key goal in solar and heliospheric research. While there have
been many theoretical advances in both topics, including suggestions
that they may be intimately related, the inherent scale coupling
and complexity of these phenomena limits our ability to construct
models that test them on a fundamental level for realistic solar
conditions. At the same time, there is an ever increasing impetus to
improve our spaceweather models, and incorporating treatments for
these processes that capture their basic features while remaining
tractable is an important goal. With this in mind, I will give an
overview of our exploration of a wave-turbulence driven (WTD) model for
coronal heating and solar wind acceleration based on low-frequency
Alfvénic turbulence. Here we attempt to bridge the gap between
theory and practical modeling by exploring this model in 1D HD and
multi-dimensional MHD contexts. The key questions that we explore
are: What properties must the model possess to be a viable model for
coronal heating? What is the influence of the magnetic field topology
(open, closed, rapidly expanding)? And can we simultaneously capture
coronal heating and solar wind acceleration with such a quasi-steady
formulation? Our initial results suggest that a WTD based formulation
performs adequately for a variety of solar and heliospheric conditions,
while significantly reducing the number of free parameters when
compared to empirical heating and solar wind models. The challenges,
applications, and future prospects of this type of approach will also
be discussed.
Title: Solar Probe Plus: A NASA Mission to Touch the Sun
Authors: Fox, N. J.; Velli, M. M. C.; Kasper, J. C.; McComas, D. J.;
Howard, R.; Bale, S. D.; Decker, R. B.
Bibcode: 2014AGUFMSH21B4096F
Altcode:
Solar Probe Plus (SPP), currently in Phase C, will be the first
mission to fly into the low solar corona, revealing how the corona is
heated and the solar wind and energetic particles are accelerated,
solving fundamental mysteries that have been top priority science
goals since such a mission was first proposed in 1958. The scale
and concept of such a mission has been revised at intervals since
that time, yet the core has always been a close encounter with the
Sun. The primary science goal of the Solar Probe Plus mission is to
determine the structure and dynamics of the Sun's coronal magnetic
field, understand how the solar corona and wind are heated and
accelerated, and determine what mechanisms accelerate and transport
energetic particles. The SPP mission will achieve this by identifying
and quantifying the basic plasma physical processes at the heart of
the Heliosphere. SPP uses an innovative mission design, significant
technology development and a risk-reducing engineering development
to meet the SPP science objectives: 1) Trace the flow of energy that
heats and accelerates the solar corona and solar wind; 2) Determine
the structure and dynamics of the plasma and magnetic fields at the
sources of the solar wind; and 3) Explore mechanisms that accelerate
and transport energetic particles. In this presentation, we present
Solar Probe Plus and examine how the mission will address the science
questions that have remained unanswered for over 5 decades.
Title: Pseudostreamers: Formation, Magnetic Topology and Plasma
Properties
Authors: Panasenco, O.; Velli, M. M. C.
Bibcode: 2014AGUFMSH33A4121P
Altcode:
A traditional view of the origins of the solar wind states that slow
wind streams arise from coronal hole boundaries due to the larger
expansion factor. It is hard in this explanation to understand why the
slow wind occupies so much space in the heliosphere. Pseudostreamers
are multipolar features which develop into fields that are unipolar
at greater heights. There is debate as to the speed and nature of the
wind from pseudostreamers: it could be fast, slow, or in between. And,
in general, they might form a network of slow wind which may or may
not connect in the heliosphere to slow wind coming from around the
heliospheric current sheet. Here we discuss the relationship between
the expansion factor along PFSS extrapolated magnetic field lines of
pseudostreamers and wind speed and plasma properties calculated with
numeral modeling. We demonstrate how the resulting wind type depends on
the stage of pseudostreamer development in the context of the global
coronal environment: factors in determining wind speed include the
height of the pseudostreamer null point, the presence or absence of
filament channels, and the expansion of coronal magnetic field lines
in the neighborhood of the pseudostreamer spine. This study helps
to better understand the sources of slow and fast solar wind for the
Solar Probe Plus mission.
Title: Characterizing a Closed Field Coronal Heating Model Inspired
by Wave Turbulence
Authors: Downs, Cooper; Lionello, Roberto; Mikić, Zoran; Linker,
Jon A.; Velli, Marco
Bibcode: 2014shin.confE.156D
Altcode:
To simulate the energy balance of coronal plasmas on macroscopic scales,
we often require the specification of the coronal heating mechanism
in some functional form. To go beyond empirical formulations and to
build a more physically motivated heating function, we investigate
the wave-turbulence driven (WTD) phenomenology for the heating of
closed coronal loops. To do so, we employ an implementation of non-WKB
equations designed to capture the large-scale propagation, reflection,
and dissipation of wave turbulence along a loop. The parameter space
of this model is explored by solving the coupled WTD and hydrodynamic
equations in 1D for an idealized loop, and the relevance to a range
of solar conditions is established by computing solutions for several
hundred loops extracted from a realistic 3D coronal field. Due to
the implicit dependence of the WTD heating model on loop geometry and
plasma properties along the loop and at the footpoints, we find that
this model can significantly reduce the number of free parameters when
compared to traditional empirical heating models, and still robustly
describe a broad range of quiet-sun and active region conditions. The
importance of the self-reflection term in producing realistic heating
scale heights and thermal non-equilibrium cycles is discussed, which
has relevance to the heating and cooling signatures often observed in
active region cores.
Title: Application of a Solar Wind Model Driven by Turbulence
Dissipation to a 2D Magnetic Field Configuration
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikic, Zoran
Bibcode: 2014shin.confE.141L
Altcode:
Solar physicists are still actively investigating the mechanisms
responsible for heating the Sun's corona and accelerating the solar
wind, although it is widely accepted that photospheric motions provide
the energy source and that the magnetic field must play a key role in
the process. Cranmer et al 2007 developed a sophisticated, 1D,
time-steady model of the solar wind with turbulence dissipation. By
varying the coronal magnetic field, they obtain, for a single choice
of wave properties, a realistic range of slow and fast wind conditions
with a sharp latitudinal transition between the two streams. Using
the 1D, time-dependent model of the solar wind of Cranmer et al. 2007,
which incorporates turbulent dissipation of Alfvén waves to provide
heating and acceleration of the plasma, we have explored a similar
configuration, obtaining qualitatively equivalent results. However,
we suspect that the bifurcation between slow and fast wind suggested
by this 1D model may not occur in multidimensional MHD simulations.
Title: Validating a Time-dependent Turbulence-driven Model of the
Solar Wind
Authors: Lionello, Roberto; Velli, Marco; Downs, Cooper; Linker,
Jon A.; Mikić, Zoran; Verdini, Andrea
Bibcode: 2014ApJ...784..120L
Altcode: 2014arXiv1402.4188L
Although the mechanisms responsible for heating the Sun's corona and
accelerating the solar wind are still being actively investigated, it
is largely accepted that photospheric motions provide the energy source
and that the magnetic field must play a key role in the process. Verdini
et al. presented a model for heating and accelerating the solar wind
based on the turbulent dissipation of Alfvén waves. We first use a
time-dependent model of the solar wind to reproduce one of Verdini et
al.'s solutions; then, we extend its application to the case where the
energy equation includes thermal conduction and radiation losses, and
the upper chromosphere is part of the computational domain. Using this
model, we explore the parameter space and describe the characteristics
of a fast solar wind solution. We discuss how this formulation may be
applied to a three-dimensional MHD model of the corona and solar wind.
Title: Apparent Solar Tornado-Like Prominences
Authors: Panasenco, Olga; Martin, Sara F.; Velli, Marco
Bibcode: 2014SoPh..289..603P
Altcode: 2013arXiv1307.2303P
Recent high-resolution observations from the Solar Dynamics Observatory
(SDO) have reawakened interest in the old and fascinating phenomenon
of solar tornado-like prominences. This class of prominences was
first introduced by Pettit (Astrophys. J.76, 9, 1932), who studied
them over many years. Observations of tornado prominences similar to
the ones seen by SDO had already been documented by Secchi (Le Soleil,
1877). High-resolution and high-cadence multiwavelength data obtained
by SDO reveal that the tornado-like appearance of these prominences is
mainly an illusion due to projection effects. We discuss two different
cases where prominences on the limb might appear to have a tornado-like
behavior. One case of apparent vortical motions in prominence spines
and barbs arises from the (mostly) 2D counterstreaming plasma motion
along the prominence spine and barbs together with oscillations along
individual threads. The other case of apparent rotational motion is
observed in a prominence cavity and results from the 3D plasma motion
along the writhed magnetic fields inside and along the prominence cavity
as seen projected on the limb. Thus, the "tornado" impression results
either from counterstreaming and oscillations or from the projection
on the plane of the sky of plasma motion along magnetic-field lines,
rather than from a true vortical motion around an (apparent) vertical or
horizontal axis. We discuss the link between tornado-like prominences,
filament barbs, and photospheric vortices at their base.
Title: Reconnection of Quasi-singular Current Sheets: The "Ideal"
Tearing Mode
Authors: Pucci, Fulvia; Velli, Marco
Bibcode: 2014ApJ...780L..19P
Altcode:
A strong indication that fast reconnection regimes exist within
resistive magnetohydrodynamics was given by the proof that the
Sweet-Parker current sheet, maintained by a flow field with
an appropriate inflow-outflow structure, could be unstable to a
reconnecting instability which grows without bound as the Lundquist
number, S, tends to infinity. The requirement of a minimum value
for S in order for the plasmoid instability to kick in does little to
resolve the paradoxical nature of the result. Here we argue against the
realizability of Sweet-Parker current sheets in astrophysical plasmas
with very large S by showing that an "ideal" tearing mode takes over
before current sheets reach such a thickness. While the Sweet-Parker
current sheet thickness scales as ~S -1/2, the tearing mode
becomes effectively ideal when a current sheet collapses to a thickness
of the order of ~S -1/3, up to 100 times thicker than S
-1/2, when (as happens in many astrophysical environments)
S is as large as 1012. Such a sheet, while still diffusing
over a very long time, is unstable to a tearing mode with multiple
x-points: here we detail the characteristics of the instability and
discuss how it may help solve the flare trigger problem and effectively
initiate the turbulent disruption of the sheet.
Title: Coordinated science with the Solar Orbiter, Solar Probe Plus,
Interhelioprobe and SPORT missions
Authors: Maksimovic, Milan; Vourlidas, Angelos; Zimovets, Ivan; Velli,
Marco; Zhukov, Andrei; Kuznetsov, Vladimir; Liu, Ying; Bale, Stuart;
Ming, Xiong
Bibcode: 2014cosp...40E1956M
Altcode:
The concurrent science operations of the ESA Solar Orbiter (SO), NASA
Solar Probe Plus (SPP), Russian Interhelioprobe (IHP) and Chinese SPORT
missions will offer a truly unique epoch in heliospheric science. While
each mission will achieve its own important science objectives, taken
together the four missions will be capable of doing the multi-point
measurements required to address many problems in Heliophysics such
as the coronal origin of the solar wind plasma and magnetic field or
the way the Solar transients drive the heliospheric variability. In
this presentation, we discuss the capabilities of the four missions
and the Science synergy that will be realized by concurrent operations
Title: Solar Probe Plus: A NASA Mission to Touch the Sun
Authors: Fox, N. J.; Bale, S. D.; Decker, R. B.; Howard, R.; Kasper,
J. C.; McComas, D. J.; Szabo, A.; Velli, M. M.
Bibcode: 2013AGUFMSM53A2207F
Altcode:
Solar Probe Plus (SPP), currently in Phase B, will be the first mission
to fly into the low solar corona, revealing how the corona is heated
and the solar wind is accelerated, solving two fundamental mysteries
that have been top priority science goals since such a mission was
first proposed in 1958. The scale and concept of such a mission has
been revised at intervals since that time, yet the core has always been
a close encounter with the Sun. The primary science goal of the Solar
Probe Plus mission is to determine the structure and dynamics of the
Sun's coronal magnetic field, understand how the solar corona and wind
are heated and accelerated, and determine what mechanisms accelerate
and transport energetic particles. The SPP mission will achieve this
by identifying and quantifying the basic plasma physical processes at
the heart of the Heliosphere. SPP uses an innovative mission design,
significant technology development and a risk-reducing engineering
development to meet the SPP science objectives: 1) Trace the flow of
energy that heats and accelerates the solar corona and solar wind;
2) Determine the structure and dynamics of the plasma and magnetic
fields at the sources of the solar wind; and 3) Explore mechanisms
that accelerate and transport energetic particles. In this poster,
we present Solar Probe Plus and examine how the mission will address
the science questions that have remained unanswered for over 5 decades.
Title: Parametric decay of radial Alfvén waves in the expanding
accelerating solar wind
Authors: Tenerani, A.; Velli, M.
Bibcode: 2013JGRA..118.7507T
Altcode:
We study the onset and evolution of the Alfvén wave parametric
decay instability within the Accelerating Expanding Box model in
the framework of a one-fluid description of the plasma. As we are
interested in understanding wave propagation and dissipation in the
inner heliosphere and solar wind, the expansion of the solar wind
itself may not be neglected. In this sense, the Accelerating Expanding
Box provides a useful and simple model to mimic the effects that
the expansion of the underlying atmosphere has on wave propagation
and plasma dynamics. In the simulations, we follow the evolution of
Alfvén waves along a fast solar wind stream, from the sub-Alfvénic
region up to a maximum heliocentric distance of nearly 4 AU. We
consider exact solutions of the compressible MHD system given by
circularly polarized Alfvén waves which propagate in the radial
direction, along the mean magnetic field. Both monochromatic waves
and a nonmonochromatic wave are considered. Monochromatic waves have
periods ranging from a few minutes to a few hours, the latter being
stabilized by the expansion. The nonmonochromatic wave has a central
period of the order of a few minutes, with a broad spectrum containing
frequencies near the threshold of the instability. In this case the
Alfvén wave partly decays into backward daughter Alfvén waves up to
the instability saturation, then giving rise to a nonlinear cascade
of incompressible and compressible modes.
Title: Origins of Rolling, Twisting, and Non-radial Propagation of
Eruptive Solar Events
Authors: Panasenco, Olga; Martin, Sara F.; Velli, Marco; Vourlidas,
Angelos
Bibcode: 2013SoPh..287..391P
Altcode: 2012arXiv1211.1376P; 2012SoPh..tmp..321P
We demonstrate that major asymmetries in erupting filaments and CMEs,
namely major twists and non-radial motions are typically related to
the larger-scale ambient environment around eruptive events. Our
analysis of prominence eruptions observed by the STEREO, SDO, and
SOHO spacecraft shows that prominence spines retain, during the
initial phases, the thin ribbon-like topology they had prior to
the eruption. This topology allows bending, rolling, and twisting
during the early phase of the eruption, but not before. The combined
ascent and initial bending of the filament ribbon is non-radial
in the same general direction as for the enveloping CME. However,
the non-radial motion of the filament is greater than that of the
CME. In considering the global magnetic environment around CMEs,
as approximated by the Potential Field Source Surface (PFSS) model,
we find that the non-radial propagation of both erupting filaments and
associated CMEs is correlated with the presence of nearby coronal holes,
which deflect the erupting plasma and embedded fields. In addition,
CME and filament motions, respectively, are guided towards weaker
field regions, namely null points existing at different heights in
the overlying configuration. Due to the presence of the coronal hole,
the large-scale forces acting on the CME may be asymmetric. We find
that the CME propagates usually non-radially in the direction of least
resistance, which is always away from the coronal hole. We demonstrate
these results using both low- and high-latitude examples.
Title: Field Lines Twisting in a Noisy Corona: Implications for
Energy Storage and Release, and Initiation of Solar Eruptions
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.
Bibcode: 2013ApJ...771...76R
Altcode: 2013arXiv1301.7678R
We present simulations modeling closed regions of the solar corona
threaded by a strong magnetic field where localized photospheric
vortical motions twist the coronal field lines. The linear and nonlinear
dynamics are investigated in the reduced magnetohydrodynamic regime in
Cartesian geometry. Initially the magnetic field lines get twisted and
the system becomes unstable to the internal kink mode, confirming and
extending previous results. As typical in this kind of investigations,
where initial conditions implement smooth fields and flux-tubes,
we have neglected fluctuations and the fields are laminar until the
instability sets in. However, previous investigations indicate that
fluctuations, excited by photospheric motions and coronal dynamics,
are naturally present at all scales in the coronal fields. Thus,
in order to understand the effect of a photospheric vortex on a more
realistic corona, we continue the simulations after kink instability
sets in, when turbulent fluctuations have already developed in the
corona. In the nonlinear stage the system never returns to the simple
initial state with ordered twisted field lines, and kink instability
does not occur again. Nevertheless, field lines get twisted, although
in a disordered way, and energy accumulates at large scales through
an inverse cascade. This energy can subsequently be released in
micro-flares or larger flares, when interaction with neighboring
structures occurs or via other mechanisms. The impact on coronal
dynamics and coronal mass ejections initiation is discussed.
Title: A Time-Dependent Turbulence-Driven Model of the Solar Wind
Authors: Lionello, Roberto; Downs, C.; Linker, J. A.; Mikic, Z.;
Velli, M.; Verdini, A.
Bibcode: 2013SPD....44...22L
Altcode:
Although the mechanisms responsible for heating the Sun's corona and
accelerating the solar wind are still actively investigated, it is
largely accepted that photospheric motions provide the energy source
and that the magnetic field must play a key role in the process. Verdini
et al. (2010) presented a model for heating and accelerating the solar
wind based on the turbulent dissipation of Alfven waves. We first
use our time-dependent model of the solar wind to reproduce Verdini
et al's solution; then we extend its application to the case when the
energy equation includes thermal conduction and radiation losses, and
the upper chromosphere is part of the computational domain.Application
of this formulation to our 3D MHD model of the solar corona and solar
wind will be discussed.
Title: Coronal pseudostreamers: Source of fast or slow solar wind?
Authors: Panasenco, Olga; Velli, Marco
Bibcode: 2013AIPC.1539...50P
Altcode: 2012arXiv1211.6171P
We discuss observations of pseudostreamers and their 3D magnetic
configuration as reconstructed with potential field source surface
(PFSS)models to study their contribution to the solar wind. To
understand the outflow from pseudostreamers the 3D expansion factor must
be correctly estimated. Pseudostreamers may contain filament channels
at their base in which case the open field lines diverge more strongly
and the corresponding greater expansion factors lead to slower wind
outflow, compared with pseudostreamers in which filament channels
are absent. In the neighborhood of pseudostreamers the expansion
factor does not increase monotonically with distance from the sun,
and doesn't simply depend on the height of the pseudostreamer null
point but on the entire magnetic field configuration.
Title: Proton temperature anisotropy and current sheet stability:
2-D hybrid simulations
Authors: Matteini, Lorenzo; Landi, Simone; Velli, Marco; Matthaeus,
William H.
Bibcode: 2013AIPC.1539..247M
Altcode:
The solar wind is a weakly collisional non homogeneous plasma; gradients
associated to density, velocity shears and current sheets are often
observed. In situ observations also show that the solar wind plasma
is far from thermal equilibrium and particle distribution functions
are not isotropic. The presence of a temperature anisotropy can be
the source of free energy for kinetic instabilities and their unstable
fluctuations may grow and propagate in the plasma. However, how these
fluctuations evolve in a non homogeneous medium and how they interact
and influence local coherent structures, is still an open question. We
report preliminary numerical simulations that describe the evolution
of current sheets in a non thermal plasma, focusing on the interaction
between kinetic effects driven by a proton temperature anisotropy and
magnetic reconnection processes.
Title: Signatures of kinetic instabilities in the solar wind
Authors: Matteini, Lorenzo; Hellinger, Petr; Goldstein, Bruce E.;
Landi, Simone; Velli, Marco; Neugebauer, Marcia
Bibcode: 2013JGRA..118.2771M
Altcode:
An analysis of ion non-thermal properties in the fast solar wind based
on Ulysses data is reported. The radial evolution of the main proton
moments (density, temperature, and drift velocities) and their empirical
correlations with other plasma parameters are investigated in detail
and compared with theoretical expectations. The stability of the plasma
is studied against different ion kinetic instabilities driven by ion
temperature anisotropies and differential velocities, focusing on the
identification of possible signatures of relevant instabilities in the
observed core-beam structure of proton distributions. The temperature
anisotropy of the total proton distribution appears to be constrained
by fire hose instabilities, in agreement with previous studies, while
if considered separately, beam and core populations exhibit opposite
anisotropies, with core protons characterized by perpendicular
temperatures larger than the parallel ones, possibly (marginally)
unstable for ion-cyclotron instability. The evolution with distance
of the drift velocity between the secondary population and the main
core is found to be nonadiabatic, leading to the identification of a
marginal stability path of a magnetosonic ion-beam instability. As a
conclusion, we find that a large fraction of the proton distributions
observed by Ulysses display signatures of either a beam or a fire hose
instability, suggesting that such kinetic processes play an important
role in regulating the solar wind thermal energetics during the plasma
expansion.
Title: Building small scales in MHD turbulence
Authors: Verdini, A.; Grappin, R.; Pinto, R.; Velli, M.
Bibcode: 2013AIPC.1539...74V
Altcode:
Magneto-hydrodynamic turbulence (MHD) with a mean large-scale field
is known to produce an anisotropic cascade, with energy mostly in
perpendicular scales. We use a shell-model version of the Reduced MHD
equations to simulate turbulence in homogeneous periodic conditions,
in coronal loops, and in the solar wind. We compare the perpendicular
and parallel spectra and show that different regimes of weak turbulence
develop in loops and in the solar wind. We briefly comment on the way
their characteristic large-scale features influence the weak turbulence
spectra and their transition to strong turbulence.
Title: Integrating physics-based coronal heating and solar wind
acceleration in a global MHD model
Authors: Lionello, Roberto; Velli, Marco; Linker, Jon A.; Mikić, Zoran
Bibcode: 2013AIPC.1539...30L
Altcode:
Although the mechanisms responsible for heating the Sun's corona
and accelerating the solar wind are being actively investigated,
it is largely accepted that photospheric motions provide the energy
source and that the magnetic field must play a key role in the
process. 3D MHD models of the corona and of the solar wind usually
employ some phenomenological artifice to accelerate the wind and heat
the corona. Within the framework of a 3D full MHD numerical code,
we apply the results of Rappazzo et al. [1] on turbulent cascade to
heat the closed-field region, and those of Verdini and Velli [2],
Verdini et al. [3] on Alfvén turbulence dissipation to accelerate
the solar wind. We also compare the properties of the solar wind at
1 A.U. using a formulation based on Verdini and Velli [2], Verdini et
al. [3] and one based on Chandran et al. [4].
Title: Expansion effects on solar wind hybrid simulations
Authors: Parashar, Tulasi N.; Velli, Marco; Goldstein, Bruce E.
Bibcode: 2013AIPC.1539...54P
Altcode:
Ion kinetic simulations of the solar wind using hybrid codes can model
local wave input, heating and instabilities, but generally do not
include long term evolution effects in the expanding solar wind. We
further develop the expanding box model used in earlier studies to
include the mirror force effects and study their role in the evolution
of the proton distribution functions in the outer corona and inner
heliosphere. The mirror force, significant in the acceleration region
of the solar wind, is required for consistency with the conservation
of magnetic moment of particles in the expanding wind. We present
preliminary results from the modified 1D expanding box hybrid (EBHM)
simulations.
Title: Coupling the solar surface and the corona: Coronal rotation,
Alfvén wave-driven polar plumes
Authors: Pinto, R. F.; Grappin, R.; Velli, M.; Verdini, A.
Bibcode: 2013AIPC.1539...58P
Altcode: 2013arXiv1301.0341P
The dynamical response of the solar corona to surface and sub-surface
perturbations depends on the chromospheric stratification, and
specifically on how efficiently these layers reflect or transmit
incoming Alfvén waves. While it would be desirable to include the
chromospheric layers in the numerical simulations used to study such
phenomena, that is most often not feasible. We defined and tested a
simple approximation allowing the study of coronal phenomena while
taking into account a parametrised chromospheric reflectivity. We
addressed the problems of the transmission of the surface rotation to
the corona and that of the generation of polar plumes by Alfvén waves
[1, 2]. We found that a high (yet partial) effective chromospheric
reflectivity is required to properly describe the angular momentum
balance in the corona and the way the surface differential rotation
is transmitted upwards. Alfvén wave-driven polar plumes maintain
their properties for a wide range of values for the reflectivity,
but they become bursty (and eventually disrupt) when the limit of
total reflection is attained.
Title: On the kinetic Alfven Wave
Authors: Parashar, Tulasi Nandan; Velli, Marco; Goldstein, Bruce
Bibcode: 2013shin.confE.124P
Altcode:
The Kinetic Alfven Wave (KAW) has been at the center stage of attention
recently because of hints coming from observations and simulations that
this could be the dominant mode at the solar wind kinetic scales. We
explore the parameter space (β, m_e/m_i, k, θ) to find possible
regions where we can have the possibility of a KAW and compare it to
the existing limits in literature. Then we setup the KAW as an initial
condition in a hybrid code. We present some preliminary results related
to the nonlinear decay of KAWs and plasma heating.
Title: Solar Orbiter and Solar Probe Plus: science goals and mission
synergies
Authors: Velli, M. M.
Bibcode: 2013AGUSMSH41A..01V
Altcode:
The magnetic field is fundamental to solar activity and shapes
the interplanetary environment, as clearly shown by the full
three dimensional monitoring of the heliosphere provided by the
measurements of the Helios, Ulysses, SOHO, ACE, Wind, STEREO and Voyager
spacecraft. Magnetic fields are also the source for coronal heating and
the very existence of the solar wind; produced by the sun's dynamo and
emerging into the corona magnetic fields become a conduit for waves,
act to store energy, and then propel plasma into the heliosphere in
the form of Coronal Mass Ejections (CMEs). Transformation of magnetic
energy is also the source solar energetic particle events. The
way in which solar convective energy couples to magnetic fields to
produce the multifaceted heliosphere is at the heart of Solar Orbiter
and Solar Probe Plus exploration. After discussing the respective
science objectives, I will highlight the exciting perspectives for
discovery provided by these missions to the inner heliosphere. Tests
of current theoretical models will be described and focus areas for
further numerical and theoretical efforts illustrated in the light of
the potential synergestic observations from Solar Orbiter and Solar
Probe Plus.
Title: Exploring wave propagation in the outer solar corona using
the Accelerating Expanding Box
Authors: Tenerani, A.; Velli, M. M.
Bibcode: 2013AGUSMSH31B..04T
Altcode:
Magnetic field depressions are ubiquitous in the solar wind. In
situ observations provide evidence of such drops in the magnetic
field magnitude at different latitudes (from the ecliptic plane up
to near-polar latitudes) and in a wide range of radial distances
from the sun (0.3 - 17 AU). In addition, a detailed study shows the
presence of such magnetic structures to be an intrinsic property of the
solar wind and to have well defined and recognized features, e.g., the
anti-correlation of magnetic pressure with thermal plasma pressure. In
spite of the well-documented observations, a controversial issue is to
understand how these magnetic field depressions are generated and where,
and if they are stable or not. Two major paradigms are usually invoked
to interpret these magnetic structures, namely, the mirror instability
on the one hand, and solitary waves on the other hand. Besides these
two possible interpretations, we propose here a self-consistent study
of the evolution of magneto-hydrodynamic (MHD) waves by showing first
results within the fluid model of the "accelerating expanding box". The
aim is to highlight the role of the solar wind expansion in both the
temporal and spatial evolution of MHD waves propagating out from the
lower corona, and how the inhomogeneity, which sets in because of the
radial expansion of the solar wind, act to modify the properties of
the waves themselves.
Title: Proton thermal energetics in the solar wind: Helios reloaded
Authors: Hellinger, Petr; TráVníček, Pavel M.; Štverák, Štěpán;
Matteini, Lorenzo; Velli, Marco
Bibcode: 2013JGRA..118.1351H
Altcode:
The proton thermal energetics in the slow solar wind between 0.3 and
1 AU is reinvestigated using the Helios 1 and 2 data, complementing a
similar analysis for the fast solar wind [Hellinger et al., 2011]. The
results for slow and fast solar winds are compared and discussed
in the context of previous results. Protons need to be heated in the
perpendicular direction with respect to the ambient magnetic field from
0.3 to 1 AU. In the parallel direction, protons need to be cooled at 0.3
AU, with a cooling rate comparable to the corresponding perpendicular
heating rate; between 0.3 and 1 AU, the required cooling rate decreases
until a transition to heating occurs: by 1 AU the protons require
parallel heating, with a heating rate comparable to that required
to sustain the perpendicular temperature. The heating/cooling rates
(per unit volume) in the fast and slow solar winds are proportional to
the ratio between the proton kinetic energy and the expansion time. On
average, the protons need to be heated and the necessary heating rates
are comparable to the energy cascade rate of the magnetohydrodynamic
turbulence estimated from the stationary Kolmogorov-Yaglom law at 1 AU;
however, in the expanding solar wind, the stationarity assumption for
this law is questionable. The turbulent energy cascade may explain
the average proton energetics (although the stationarity assumption
needs to be justified) but the parallel cooling is likely related to
microinstabilities connected with the structure of the proton velocity
distribution function. This is supported by linear analysis based on
observed data and by results of numerical simulations.
Title: Formation of the Coronal Cloud Prominences Inside Magnetic
Funnels
Authors: Panasenco, Olga; Velli, Marco; Martin, Sara F.
Bibcode: 2013enss.confE..94P
Altcode:
We describe observations of coronal cloud prominences with the Solar
Dynamics Observatory and STEREO. Observations of this phenomenon
during the month of September 2012, in similar locations but over
widely separated periods, are used to investigate the reasons for
the appearance of coronal cloud prominces at different times in the
same location. In particular, we focus on the large scale structure of
the background magnetic field. Using a potential field source-surface
extrapolation to compute the coronal field from photospheric maps, we
find that coronal cloud prominences always form after filament eruptions
and CMEs have occurred nearby. The location of the cloud prominence
coincides with a magnetic field region which appears to be open but
rapidly expanding, an open field with a funnel structure. Part of the
plasma from the neighboring eruption falling back towards the sun is
captured and accumulates in these field regions of strong expansion
of the field. The plasma suspension at heights of 0.3 Rs, coinciding
with the largest gradients in the field naturally lead to a diamagnetic
hypothesis for the force counteracting gravity. We study the evolution
of the funnel-like open fields during several solar rotations and find
a direct relation between funnels and the presence of coronal clouds
at great heights in the solar corona.
Title: Solar Tornado Prominences: Plasma Motions Along Filament Barbs
Authors: Panasenco, Olga; Velli, Marco; Martin, Sara F.; Rappazzo,
Franco
Bibcode: 2013enss.confE..93P
Altcode:
Recent high-resolution observations from the Solar Dynamic Observatory
(SDO) have reawakened interest in the old and fascinating phenomenon
of solar tornado prominences. This class of prominences was first
introduced by E. Pettit in 1932, who studied them over many years
up to 1950. High resolution and high cadence multi-wavelength data
obtained by SDO reveal that the tornado-like properties of these
prominences are mainly an illusion due to projection effects. We
show that counterstreaming plasma motions with projected velocities
up to +/- 45 km/sec along the prominence spine and barbs create a
tornado-like impression when viewed at the limb. We demonstrate that
barbs are often rooted at the intersection between 4-5 supergranular
cells. We discuss the observed oscillations along the vertical parts
of barbs and whether they may be related to vortex flows coming
from the convection downdrafts at the intersection of supergranules
(and possibly smaller convective cells) in the photosphere and their
entrained magnetic field. The unwinding of magnetic threads near the
photosphere via reconnection might be a source of the waves which are
observed as oscillations in prominence barbs.
Title: Proton Temperature Anisotropy and Magnetic Reconnection in
the Solar Wind: Effects of Kinetic Instabilities on Current Sheet
Stability
Authors: Matteini, L.; Landi, S.; Velli, M.; Matthaeus, W. H.
Bibcode: 2013ApJ...763..142M
Altcode: 2012arXiv1212.2101M
We investigate the role of kinetic instabilities driven by a
proton anisotropy on the onset of magnetic reconnection by means of
two-dimensional hybrid simulations. The collisionless tearing of a
current sheet is studied in the presence of a proton temperature
anisotropy in the surrounding plasma. Our results confirm that
anisotropic protons within the current sheet region can significantly
enhance/stabilize the tearing instability of the current. Moreover,
fluctuations associated with linear instabilities excited by large
proton temperature anisotropies can significantly influence the
stability of the plasma and perturb the current sheets, triggering
the tearing instability. We find that such a complex coupling leads
to a faster tearing evolution in the T_\perp >T_\Vert regime when
an ion-cyclotron instability is generated by the anisotropic proton
distribution functions. On the contrary, in the presence of the
opposite anisotropy, fire-hose fluctuations excited by the unstable
background protons with T_\Vert are not able to efficiently destabilize
current sheets, which remain stable for a long time after fire-hose
saturation. We discuss possible influences of this novel coupling on
the solar wind and heliospheric plasma dynamics.
Title: Foreword
Authors: Burgess, David; Drake, James; Marsch, Eckart; Velli, Marco;
von Steiger, Rudolf; Zurbuchen, Thomas H.
Bibcode: 2013mspc.book....1B
Altcode:
No abstract at ADS
Title: Ion Kinetics in the Solar Wind: Coupling Global Expansion to
Local Microphysics
Authors: Matteini, Lorenzo; Hellinger, Petr; Landi, Simone;
Trávníček, Pavel M.; Velli, Marco
Bibcode: 2013mspc.book..373M
Altcode:
No abstract at ADS
Title: Solar Wind Models from the Chromosphere to 1 AU
Authors: Hansteen, Viggo H.; Velli, Marco
Bibcode: 2013mspc.book...89H
Altcode:
No abstract at ADS
Title: Multi-scale Physics in Coronal Heating and Solar Wind
Acceleration
Authors: Burgess, David; Drake, James; Marsch, Eckart; von Steiger,
Rudolf; Velli, Marco; Zurbuchen, Thomas
Bibcode: 2013mspc.book.....B
Altcode:
No abstract at ADS
Title: Hybrid Expanding Box Description of the Accelerating Solar
Wind: Mirror Force Effects
Authors: Parashar, T. N.; Velli, M. M.; Goldstein, B. E.
Bibcode: 2012AGUFMSH53A2259P
Altcode:
The expanding box model (Grappin et. al., PRL 1993) describes
the effects of the spherical expansion of the solar wind on the
thermodynamics, turbulence and kinetic properties of a plasma parcel
moving outwards from the sun. It has been used to study, e.g., the
evolution of Alfvén waves in MHD and the preferential heating of
ions kinetically. If the average solar wind speed is constant, the
expanding box equations can safely ignore higher order effects due to
the mirror force which particles feel in the inhomogeneous large scale
heliospheric magnetic field. However, in the acceleration region of
the solar wind, such effects can be important. We include this higher
order effect in the expanding box equations and study the evolution
of particle distribution functions with expansion. The mirror force
accelerates (along the mean field) particles with higher perpendicular
thermal velocities and decelerates the ones with lower perpendicular
thermal velocities, distorting the distribution function as the plasma
accelerates outwards. Here we discuss the effects of this competition
between the mirror force effects and expansion effects in the presence
of multiple ion species.
Title: Nonlinear Dynamics of Turbulent Coronal Heating Mechanisms:
Thermodynamics, Energy Storage and Release
Authors: Einaudi, G.; Dahlburg, R.; Rappazzo, A. F.; Velli, M. M.
Bibcode: 2012AGUFMSH33D2255E
Altcode:
Thermodynamics of the magnetically confined solar corona
are investigated with fully compressible three-dimensional
magnetohydrodynamic simulations. Coronal loops field lines' footpoints
are shuffled by space-filling homogeneous photospheric motions giving
rise to turbulent nonlinear dynamics. Field-aligned current sheets
continuously form and dissipate depositing energy at the small-scales
where the heating occurs. Previous studies show that current sheets
thickness is orders of magnitude smaller than current state of
the art observational resolution (~700 km). Therefore to understand
coronal heating and its observed properties we need to understand the
thermodynamics of such a system where energy is deposited at unresolved
small-scales. We show that at observationally sub-resolution scales
temperature is highly structured and non-homogeneously distributed. In
this multi-thermal highly dynamical system, hotter and cooler plasma
strands are found one next to the other at sub-resolution scales, and
only a fraction of the coronal mass and volume gets heated at each
time. The basal coronal heating described above is characterized by
a limited storage of magnetic energy and numerous small releases of
energy, so-called nanoflares. Reduced magnetohydrodynamic simulations
show that such system stores a considerably higher magnetic energy when
localized photospheric motions are applied. Its dynamics, mechanisms
of subsequent release of the stored energy in micro- or larger flares,
and the impact on coronal dynamics will be discussed.
Title: The Solar Wind From Pseudostreamers And Their Immediate
Environment
Authors: Panasenco, O.; Velli, M. M.; Panasenco, A.; Lionello, R.
Bibcode: 2012AGUFMSH53A2257P
Altcode:
Beyond the very large-scale relationship of fast solar wind streams
to coronal holes, the connection between coronal structures and their
solar wind counterparts remains largely mysterious. Here we study the
three-dimensional expansion of the solar wind from open field lines in
the immediate neighborhood of coronal pseudostreamers, including the
pseudostreamer spine. To this effect, we use a PFSS extrapolation of
the photospheric magnetic field out to 2.5 solar radii, and assume the
field expands radially beyond that. Different types of pseudostreamers
exist, with a complex inner structure which depends on the number
of polarities embedded in the closed regions below. In addition
pseudostreamers may also harbor filament channels, often occurring
in pairs (twin filament channels). In the latter case, the strongly
sheared field of the channel magnetic structures and the skew of the
coronal arcade above the channels dictate the way the coronal field
expands in the neighborhood of pseudostreamers. Here we integrate the
time dependent 1D MHD equations along the PFSS extrapolated magnetic
field, in the presence of gravity, and including the effect of an
Alfvén wave pressure term, and determine the dependence of wind
speed on different types of observed pseudostreamers. There is no
simple relationship between pseudostreamers and wind speed, rather the
resulting wind type is a function of the global coronal environment,
including the height of the pseudostreamer null point, the presence
or absence of filament channels, and therefore the expansion of the
coronal magnetic field in the neighborhood of the pseudostreamer spine.
Title: Role of kinetic instabilities driven by temperature anisotropy
in the evolution of current sheets and magnetic reconnection
Authors: Matteini, L.; Velli, M. M.; Landi, S.; Matthaeus, W. H.
Bibcode: 2012AGUFMSH51B2249M
Altcode:
We investigate the role of kinetic instabilities driven by proton
anisotropies on the onset and evolution of magnetic reconnection
by means of 2-D hybrid simulations. The collisionless tearing of a
current sheet is studied confirming that anisotropic protons within the
current sheet region can significantly enhance/stabilize the tearing
instability of the current. Moreover, fluctuations associated with
linear instabilities excited by large proton temperature anisotropies
can significantly influence the stability of the plasma and perturb
the current sheets, triggering the tearing instability. Such coupling
leads to faster tearing evolution when an ion-cyclotron instability
is triggered by anisotropic proton distribution functions with large
perpendicular temperatures. If instead the parallel temperature is
sufficiently large compared to the perpendicular temperature, fire hose
fluctuations excited by the unstable background protons are not able
to efficiently destabilize current sheets, which remain stable for a
long time after fire hose saturation. We discuss possible influences of
this novel coupling on the solar wind and heliospheric plasma dynamics.
Title: Proton Energetics in the Solar Wind: Helios Reloaded
Authors: Hellinger, P.; Travnicek, P. M.; Stverak, S.; Matteini, L.;
Velli, M. M.
Bibcode: 2012AGUFMSH53C..07H
Altcode:
The proton thermal energetics in the solar wind between 0.3 and 1 AU
is re-investigated using the Helios 1 and 2 data. Heating and cooling
rates are evaluated for the slow and fast solar wind and compared
with estimates of the energy cascade rate of the magnetohydrodynamic
turbulence estimated from the Kolmogorov-Yaglom law at 1 AU. The
observed heating/cooling rates are also compared with the results of
numerical kinetic simulations. Possible influence of the interaction
between fast and slow solar wind streams on the proton energetics is
also discussed.
Title: Magnetic reconnection, shear flow and the axial filament
channel magnetic field
Authors: Velli, M. M.; Rappazzo, F.; Panasenco, O.
Bibcode: 2012AGUFMSH33D2251V
Altcode:
The same processes leading to coronal heating also structure the global
corona. Among these one of the most fascinating is the formation
of filament channels. Here we discuss the formation and evolution
of the axial magnetic field of filament channels as a result of
the photospheric transport of magnetic field footpoints with flows
converging to the neutral line leading to magnetic reconnection as
well as some shearing of the magnetic field along the neutral line.The
focus here is not on the large-scale 3D simulation of the formation
of prominences or filaments but on the magnetic reconnection process
itself, and its dependence on photospheric motions, stratification,
and shear. Previous numerical simulations and models are used as a
guide to setup numerical simulations and analytical calculations of
tearing and reconnection leading to the enhancement of the coronal
axial magnetic field. We generalize previous work by taking full
account of recent advances in MHD turbulence and reconnection theory
(plasmoid instability, reconnection rates, current sheet thicknesses)
to associate the rate of enhancement of the axial magnetic field with
observed photospheric flows and the consequent matter flows through
the chromosphere up into the corona along the filament.
Title: Interchange Reconnection in a Turbulent Corona
Authors: Rappazzo, A. F.; Matthaeus, W. H.; Ruffolo, D. J.; Servidio,
S.; Velli, M. M.
Bibcode: 2012AGUFMSH32A..03R
Altcode:
Magnetic reconnection at the interface between coronal holes and
loops, so-called interchange reconnection, can release the hotter,
denser plasma from magnetically confined regions into the heliosphere,
contributing to the formation of the highly variable slow solar wind. In
the prevailing ``standard'' view the interchange process is thought to
develop in null points (with B=0) preferably at the apex of streamers or
pseudo-streamers, near Y and X-points, from where slow solar wind flows
would originate. This standard model does not meet recent observations
of slow wind streams from the edges of active regions, that suggest
that slow streams are not limited to apex-regions near neutral points
(B=0). Furthermore in order to account for the slow wind diffusion (~
30 degrees) observed in situ around the Heliospheric Current Sheet,
within the standard model framework one has to posit that the slow wind
would originate from a small fraction, with a complex topology, of the
whole coronal hole-loop boundary, namely narrow channels (supposedly at
observationally sub-resolution scales) linking coronal holes. However,
coronal heating models, with magnetic field lines shuffled by convective
motions, show that reconnection can occur continuously in unipolar
magnetic field regions with no neutral points. We propose that a similar
alternate interchange mechanism operating near boundaries between
open and closed regions induces a continual stochastic rearrangement
of connectivity everywhere along the open-closed boundary. We examine
a reduced magnetohydrodynamic model of a simplified unipolar interface
region between open and closed corona. This boundary is not stationary,
becomes fractal, and field lines change connectivity continuously,
becoming alternatively open and closed. This model suggests that slow
wind may originate everywhere along coronal loop-hole boundaries,
a possibility that has major implications for coronal heating and
models of the slow solar wind, and accounts naturally, simply and in
an elegant way both for the observed diffusion of the slow wind around
the heliospheric current sheet and for flows at edges of active regions.
Title: Solar Wind Models from the Chromosphere to 1 AU
Authors: Hansteen, Viggo H.; Velli, Marco
Bibcode: 2012SSRv..172...89H
Altcode: 2012SSRv..tmp...35H
Recent models of the fast solar wind are characterized by low coronal
electron temperatures while proton, α-particle, and minor ion
temperatures are expected to be quite high and generally anisotropic,
including large temperatures perpendicular to the magnetic field
and parallel beams. This entails that the electric field should be
relatively unimportant and that solar wind outflows with both high
asymptotic flow speeds but maintaining a low mass flux should be a
natural outcome of plasma expansion along open polar magnetic field
lines. In this chapter we will explain why such changes with respect
to the classical, electron thermally driven solar wind have come about
and outline the most important remaining concerning the astrophysics of
coronal winds. The progress we have seen in the last decade is largely
due observations made with instruments onboard Ulysses (McComas et
al. in Space Sci. Rev. 72:93, 1995) and SOHO (Fleck et al. in The SOHO
Mission, Kluwer, Dordrecht, 1995). These observations have spawned a
new understanding of solar wind energetics, and the consideration of
the chromosphere, corona, and solar wind as a unified system. We will
begin by giving our own, highly biased, "pocket history" of solar wind
theory highlighting the problems that had to be resolved in order to
make the original Parker formulation of thermally driven winds conform
with observational results. Central to this discussion are questions
of how the wind's asymptotic flow speed and mass flux are set, but we
will also touch upon higher order moments such as the ion and electron
temperatures and heat fluxes as well as the possible role of Alfvén
waves and particle effects in driving the solar wind outflow. Solar
wind scaling laws will be discussed in the context of the origin of
slow and fast wind streams.
Title: Ion Kinetics in the Solar Wind: Coupling Global Expansion to
Local Microphysics
Authors: Matteini, Lorenzo; Hellinger, Petr; Landi, Simone;
Trávníček, Pavel M.; Velli, Marco
Bibcode: 2012SSRv..172..373M
Altcode: 2011SSRv..tmp..363M; 2011SSRv..tmp...60M; 2011SSRv..tmp..128M;
2011SSRv..tmp...45M; 2011SSRv..tmp..204M
We discuss selected ion kinetic processes relevant in the context
of the expanding solar wind. We focus on the role of wave-wave and
wave-particle interactions, plasma instabilities and Coulomb collisions
on the overall kinetic evolution of ions. We review recent results
from the hybrid expanding box model, which enables the coupling of
the large scale effects of the solar wind expansion to the microscale
kinetics of ions. We discuss how different plasma processes develop and
influence each other during the expansion, as well their role in the
shaping of ion distribution functions, and we compare the simulation
results with the observed trends in the solar wind.
Title: Foreword
Authors: Burgess, David; Drake, James; Marsch, Eckart; Velli, Marco;
von Steiger, Rudolf; Zurbuchen, Thomas H.
Bibcode: 2012SSRv..172....1B
Altcode: 2012SSRv..tmp...85B
No abstract at ADS
Title: Interchange Reconnection in a Turbulent Corona
Authors: Rappazzo, A. F.; Matthaeus, W. H.; Ruffolo, D.; Servidio,
S.; Velli, M.
Bibcode: 2012ApJ...758L..14R
Altcode: 2012arXiv1209.1388R
Magnetic reconnection at the interface between coronal holes and loops,
the so-called interchange reconnection, can release the hotter, denser
plasma from magnetically confined regions into the heliosphere,
contributing to the formation of the highly variable slow solar
wind. The interchange process is often thought to develop at the apex
of streamers or pseudo-streamers, near Y- and X-type neutral points,
but slow streams with loop composition have been recently observed
along fanlike open field lines adjacent to closed regions, far from
the apex. However, coronal heating models, with magnetic field lines
shuffled by convective motions, show that reconnection can occur
continuously in unipolar magnetic field regions with no neutral
points: photospheric motions induce a magnetohydrodynamic turbulent
cascade in the coronal field that creates the necessary small scales,
where a sheared magnetic field component orthogonal to the strong
axial field is created locally and can reconnect. We propose that a
similar mechanism operates near and around boundaries between open
and closed regions inducing a continual stochastic rearrangement of
connectivity. We examine a reduced magnetohydrodynamic model of a
simplified interface region between open and closed corona threaded
by a strong unipolar magnetic field. This boundary is not stationary,
becomes fractal, and field lines change connectivity continuously,
becoming alternatively open and closed. This model suggests that slow
wind may originate everywhere along loop-coronal-hole boundary regions
and can account naturally and simply for outflows at and adjacent to
such boundaries and for the observed diffusion of slow wind around
the heliospheric current sheet.
Title: Reflection and dissipation of Alfvén waves in interstellar
clouds
Authors: Pinto, C.; Verdini, A.; Galli, D.; Velli, M.
Bibcode: 2012A&A...544A..66P
Altcode: 2012arXiv1207.4518P
Context. Supersonic nonthermal motions in molecular clouds are often
interpreted as long-lived magnetohydrodynamic (MHD) waves. The
propagation and amplitude of these waves is affected by local
physical characteristics, most importantly the gas density and the
ionization fraction.
Aims: We study the propagation, reflection
and dissipation of Alfvén waves in molecular clouds deriving the
behavior of observable quantities such as the amplitudes of velocity
fluctuations and the rate of energy dissipation.
Methods: We
formulated the problem in terms of Elsässer variables for transverse
MHD waves propagating in a one-dimensional inhomogeneous medium,
including the dissipation due to collisions between ions and neutrals
and to a nonlinear turbulent cascade treated in a phenomenological
way. We considered both steady-state and time-dependent situations
and solved the equations of the problem numerically with an iterative
method and a Lax-Wendroff scheme, respectively.
Results: Alfvén
waves incident on overdense regions with density profiles typical of
cloud cores embedded in a diffuse gas suffer enhanced reflection in
the regions of the steepest density gradient, and strong dissipation
in the core's interior. These effects are especially significant
when the wavelength is intermediate between the critical wavelength
for propagation and the typical scale of the density gradient. For
larger wave amplitudes and/or steeper input spectra, the effects
of the perpendicular turbulent cascade result in a stronger energy
dissipation in the regions immediately surrounding the dense core.
Conclusions: The results may help to interpret the sharp decrease
of line width observed in the environments of low-mass cloud cores in
several molecular transitions.
Title: Turbulent coronal heating mechanisms: coupling of dynamics
and thermodynamics
Authors: Dahlburg, R. B.; Einaudi, G.; Rappazzo, A. F.; Velli, M.
Bibcode: 2012A&A...544L..20D
Altcode: 2012arXiv1208.2459D
Context. Photospheric motions shuffle the footpoints of the strong axial
magnetic field that threads coronal loops, which gives rise to turbulent
nonlinear dynamics that are characterized by the continuous formation
and dissipation of field-aligned current sheets in which energy is
deposited at small-scales and the heating occurs. Previous studies
showed that the current sheet thickness is several orders of magnitude
smaller than present-day state-of-the-art observational resolution (~700
km).
Aims: To understand coronal heating and correctly interpret
observations it is crucial to study the thermodynamics of such a system
in which energy is deposited at unresolved small-scales.
Methods:
Fully compressible three-dimensional magnetohydrodynamic simulations
were carried out to understand the thermodynamics of coronal heating
in the magnetically confined solar corona.
Results: We show
that temperature is highly structured at scales below observational
resolution. It is also nonhomogeneously distributed so that only a
fraction of the coronal mass and volume is heated at each time.
Conclusions: This is a multi-thermal system in which hotter and cooler
plasma strands are also found next to each other at sub-resolution
scales and exhibit a temporal dynamics.
Title: On the origin of the 1/f spectrum in the heliosphere
Authors: Verdini, Andrea; Velli, Marco; Roland, Grappin; Rui, Pinto
Bibcode: 2012cosp...39.2077V
Altcode: 2012cosp.meet.2077V
We present results of numerical simulations in which the low frequency
1/f spectrum is a natural outcome of the 2D turbulent dynamics inside
the sub-Alfvenic solar wind. Forcing with a high frequency spectrum
at the chromosphere we find a spectrum with distinct perpendicular
and parallel properties outside the Alfvenic critical point (about 19
rsun). The perpendicular spectrum is a power-law with a slope close
to the Kolmogorov 5/3. The parallel spectrum has a double-power-law
with slope approximately -1 and -2 at low and high frequencies
respectively. The low frequencies extends up to periods of about a day,
depending on the turbulence strength, while the frequency break depends
more on the the input spectrum at the chromosphere. The formation of the
1/f spectrum is based on the recycling of Alfvenic turbulence between
the transition region and the Alfvenic critical point, a mechanism
that works more efficiently for weak turbulence. This example points
out how long-living (or large scale) structures can be formed in the
sub-Alfvenic solar wind, without having solar origin.
Title: The Expanding Box Model in ECHO: Application to the Parametric
Decay of Alfvén Waves in the Fast Solar Wind
Authors: Del Zanna, L.; Landi, S.; Matteini, L.; Velli, M.
Bibcode: 2012ASPC..459..196D
Altcode:
The nonlinear evolution and the decay instability of monochromatic
Alfvén waves in the fast solar wind is studied through MHD numerical
simulations taking into account the effects of the radial expansion of
the background plasma. This is achieved by means of the expanding box
model, a local approach which allows to maintain Cartesian coordinates
and periodical boundary conditions. In this contribution we discuss the
implementation of the model in the ECHO code and we present preliminary
results of the decay instability in the presence of radial expansion
effects.
Title: Numerical Simulation of DC Coronal Heating
Authors: Dahlburg, Russell; Velli, Marco; Einaudi, Giorgio; Rappazzo,
Franco
Bibcode: 2012cosp...39..391D
Altcode: 2012cosp.meet..391D
MHD turbulence has long been proposed as a mechanism for the heating
of coronal loops in the framework of the Parker scenario for coronal
heating. So far most of the studies have focused on its dynamical
properties without considering its thermodynamical and radiative
features because of the very demanding computational requirements. In
this paper we extend our previous research to the compressible regime
by using HYPERION, a new parallelized, viscoresistive, three-dimensional
compressible MHD code. HYPERION employs a Fourier collocation -- finite
difference spatial discretization, and uses a third-order Runge-Kutta
temporal discretization. Dissipative terms in the energy equation,
resulting from the coronal dynamics induced by appropriate photospheric
motions, represent heating terms which balance the thermal conduction
parallel to the DC magnetic field and the radiative emission. The
computed temperature and density profiles exhibit temporal and spatial
intermittency. The resulting efficient heating mechanism leads to
properties similar to the chromosphere--transition region--corona
system.
Title: Where do we stand in understanding fast solar wind
acceleration?
Authors: Velli, Marco
Bibcode: 2012cosp...39.2066V
Altcode: 2012cosp.meet.2066V
Ulysses and SOHO/UVCS were instrumental in confirming the origin of
fast solar wind streams in solar coronal holes. In this paper I will
try to assess where our understanding of fast solar wind acceleration
really stands, illustrating new observations and the most recent
models involving reflection-driven Alfvenic turbulence, Alfven waves
and compressible interactions, and coronal plumes and microstream
shear. Since Alfvenic turbulence is often considered "exotic" I will
also try to give an understandable, yet precise, description of the
large scale interaction of Alfven waves in the solar wind, including
turbulence phenomenologies. While models predicated on turbulence come
the closest to describing the fast solar wind correctly, a number of
issues, from the precise dissipation mechanisms, to the relative roles
of helium and minor ions, to the formation and role of the ubiquitous
electron strahl in the fast wind still remain far from resolution.
Title: Nasa's Solar Probe Plus Mission and Implications for the
Theoretical Understanding of the Heliosphere
Authors: Velli, Marco
Bibcode: 2012cosp...39.2065V
Altcode: 2012cosp.meet.2065V
Solar Probe Plus (SPP), one of the most challenging missions to
understand the origins of the Heliosphere, will carry a payload
consisting of plasma and energetic particle detectors, elec- tromagnetic
field antennas and magnetometers, and a white light imager, to the
unexplored regions extending from 70 to 8.5 solar radii (0.3 to 0.05
AU) from the photosphere of the Sun. Solar Probe Plus's goals are to
understand the extended heating of the solar corona and acceleration
of the solar wind,the origins of solar wind structures including
high and low speed streams, and the origins of energetic particle
acceleration in Coronal Mass Ejections and CMEs. In addition, combined
measurements from the white light imager and the EM field antennas
will allow the first direct measurements of dust deep in the inner
solar system. This presentation will provide a broad context for the
mission objectives and measurements and illustrate the likely progress
SPP will bring to the understanding of the Heliosphere, stellar winds,
and the fundamental physics of particle acceleration, reconnection,
collisionless shocks and turbulence in space and astrophysical plasmas.
Title: Pseudostreamers and Twin Filaments in the Solar Corona
Authors: Panasenco, Olga; Velli, Marco
Bibcode: 2012shin.confE.163P
Altcode:
Pseudostreamer configurations appear in globally unipolar regions
above multiple polarity reversal boundaries, and are a generic feature
which seems to be important for coronal physics. On small scales
pseudostreamer configurations can support jets, or polar plumes. On
large scales, some of these polarity reversal boundaries can be filament
channels, and when this is the case they always occur as twin filament
channels often containing twin filaments of the same chirality. The
magnetic structure of pseudostreamers for cases with and without twin
filaments lying at their base, as reconstructed with a PFSS model,
is significantly different. Branches of pseudostreamers on opposite
sides of the separatrix surface diverge when filaments are present,
in association with the strong horizontal component of the field
present in filament channels. Here we analyze possible magnetic field
configurations of the complete pseudostreamer system and study the
links between its separate parts, which include open field lines of
pseudostreamers, filament channels, filaments, cavities, overlying
filament arcades. Following the dynamical changes in the coronal
pseudostreamer, we analyze the twin filament eruption due to new
flux emergence and how the topology of the pseudostreamer gradually
changes during the pre-eruptive and erupting phases. The presence
of well developed filaments of the same chirality at the base of
pseudostreamers implies sheared fields, which in the PFSS model are
current-free, but are more generally consistent with the presence of
a vertical pseudostreamer field-aligned current sheet. We discuss the
3D magnetic topology of the filament, filament cavity and overlying
filament arcades for these twin systems and its implications on the
theories for filaments, filament eruptions and CMEs.
Title: Observations and models of solar wind acceleration
Authors: Velli, Marco
Bibcode: 2012shin.confE.109V
Altcode:
We review different models for the acceleration of the fast solar
wind, with particular emphasis on the role of waves and turbulence and
possible observables for solar probe plus and solar orbiter which may
help differentiate among different theories. The talk has a pedagogical
aspect, summarizing modern MHD turbulence and the wave-structures and
small scale length generation debate.
Title: Sympathetic Eruptive Events and Pseudostreamers
Authors: Panasenco, Olga; Titov, Viacheslav; Mikić, Zoran; Török,
Tibor; de Toma, Giuliana; Velli, Marco
Bibcode: 2012shin.confE.162P
Altcode:
Sequences of apparently coupled CMEs triggered by sympathetic eruptions
of solar filaments are usually observed when the initial coronal
magnetic configuration above the source region contains at least
one coronal pseudostreamer. We study in detail an example of such a
sympathetic event observed on 27-28 July 2011 by SDO and STEREO. This
involved five filaments and caused four individual filament eruptions
and one partial eruption. The eruptions were closely synchronized,
even though some occurred at widely separated locations. In an attempt
to identify a plausible reason of such a synchronization, we study the
large-scale structure of the background PFSS magnetic fields, computed
from the observed photospheric magnetic field (SDO/HMI) during the
appropriate time period. We investigate the magnetic connectivities in
these configurations by calculating and analyzing the distributions of
the so-called squashing factor at the photospheric and source-surface
boundaries, as well as other cross-sections at different heights. This
allows us to get a comprehensive understanding of the underlying
structural skeleton of the magnetic configuration. In particular,
our analysis reveals two pseudostreamer magnetic configurations in the
region where the eruptions occurred. Of special interest to us are the
magnetic null points and separators located at the intersection of the
separatrix domes and curtains of the pseudostreamers. We assume that
magnetic reconnection induced by the first eruption at these locations
played likely a major role in establishing the postulated link between
the different eruptions in sequence. The close relationship between the
sympathetic eruptions and pseudostreamer configurations are supported
by a statistical study covering the SDO era (2010-2012).
Title: Near the Boundary of the Heliosphere: A Flow Transition Region
Authors: Opher, M.; Drake, J. F.; Velli, M.; Decker, R. B.; Toth, G.
Bibcode: 2012ApJ...751...80O
Altcode:
Since April of 2010, Voyager 1 has been immersed in a region of near
zero radial flows, where the solar wind seems to have stopped. The
existence of this region contradicts current models that predict
that the radial flows will go to zero only at the heliopause. These
models, however, do not include the sector region (or include it in
a kinematic fashion), where the solar magnetic field periodically
reverses polarity. Here we show that the presence of the sector region
in the heliosheath, where reconnection occurs, fundamentally alters
the flows, giving rise to a Flow Transition Region (FTR), where the
flow abruptly turns and the radial velocity becomes near zero or
negative. We estimate, based on a simulation, that at the Voyager 1
location, the thickness of the FTR is around 7-11 AU.
Title: Pseudostreamers and Twin Filaments in the Solar Corona
Authors: Panasenco, Olga; Velli, M.
Bibcode: 2012AAS...22020212P
Altcode:
Pseudostreamer configurations appear in globally unipolar regions
above multiple polarity reversal boundaries, and are a generic feature
which seems to be important for coronal physics. On small scales
pseudostreamer configurations can support jets, or polar plumes. On
large scales, some of these polarity reversal boundaries can be filament
channels, and when this is the case they always occur as twin filament
channels often containing twin filaments of the same chirality. The
magnetic structure of pseudostreamers for cases with and without twin
filaments lying at their base, as reconstructed with a PFSS model,
is significantly different. Branches of pseudostreamers on opposite
sides of the separatrix surface diverge when filaments are present,
in association with the strong horizontal component of the field
present in filament channels. Here we analyze possible magnetic field
configurations of the complete pseudostreamer system and study the
links between its separate parts, which include open field lines of
pseudostreamers, filament channels, filaments, cavities, overlying
filament arcades. Following the dynamical changes in the coronal
pseudostreamer, we analyze the twin filament eruption due to new
flux emergence and how the topology of the pseudostreamer gradually
changes during the pre-eruptive and erupting phases. The presence
of well developed filaments of the same chirality at the base of
pseudostreamers implies sheared fields, which in the PFSS model are
current-free, but are more generally consistent with the presence of
a vertical pseudostreamer field-aligned current sheet. We discuss the
3D magnetic topology of the filament, filament cavity and overlying
filament arcades for these twin systems and its implications on the
theories for filaments, filament eruptions and CMEs.
Title: On the Origin of the 1/f Spectrum in the Solar Wind Magnetic
Field
Authors: Verdini, Andrea; Grappin, Roland; Pinto, Rui; Velli, Marco
Bibcode: 2012ApJ...750L..33V
Altcode: 2012arXiv1203.6219V; 2012arXiv1203.6219G
We present a mechanism for the formation of the low-frequency 1/f
magnetic spectrum based on numerical solutions of a shell-reduced
MHD model of the turbulent dynamics inside the sub-Alfvénic solar
wind. We assign reasonably realistic profiles to the wind speed
and the density along the radial direction, and a radial magnetic
field. Alfvén waves of short periodicity (600 s) are injected at the
base of the chromosphere, penetrate into the corona, and are partially
reflected, thus triggering a turbulent cascade. The cascade is strong
for the reflected wave while it is weak for the outward propagating
waves. Reflection at the transition region recycles the strong turbulent
spectrum into the outward weak spectrum, which is advected beyond
the Alfvénic critical point without substantial evolution. There,
the magnetic field has a perpendicular power-law spectrum with slope
close to the Kolmogorov -5/3. The parallel spectrum is inherited from
the frequency spectrum of large (perpendicular) eddies. The shape
is a double power law with slopes of ~= - 1 and -2 at low and high
frequencies, respectively, with the position of the break depending on
the injected spectrum. We suggest that the double power-law spectrum
measured by Helios at 0.3 AU, where the average magnetic field is not
aligned with the radial (contrary to our assumptions), results from
the combination of such different spectral slopes. At low frequency
the parallel spectrum dominates with its characteristic 1/f shape,
while at higher frequencies its steep spectral slope (-2) is masked
by the more energetic perpendicular spectrum (slope -5/3).
Title: Coronal Holes, Filament Channels And Filaments: Observations
Of The Self-organization Of The Coronal Magnetic Field Over Solar
Cycles 23 And 24
Authors: Panasenco, Olga; Martin, S. F.; Velli, M.; Berger, M. A.
Bibcode: 2012AAS...22020202P
Altcode:
The aim of this work is to understand the relationship between coronal
holes, coronal hole boundaries and one of the other main features
of the coronal magnetic field, namely filament channels, regions
of highly sheared magnetic fields overlying photospheric polarity
reversal boundaries. The well-developed filament channel is a necessary
ingredient for filament formation. Polar coronal holes and polar crown
filament channels always seem to exist together, and even during periods
of weakest activity, when nothing indicates the presence of polar crown
channels on the solar disk, polar crown prominences appear at the limb
proving their existence. Does a similar symbiotic relationship exist
also for other coronal holes? There is some indication that for middle
and low latitude coronal holes different configurations occur depending
on the polarity of the hole relative to the pole. If the polarity is
the same, then a coronal pseudostreamer configuration can form with
twin filament channels at its base, while if the polarity is opposite,
the right conditions for the development of the filament channels
and filaments following the hemispheric helicity rule arise. Using
SOHO/EIT and MDI, STEREO/EUVI, SDO/AIA and HMI instruments we trace the
formation and mutual evolution of coronal holes and their symbiotic
filament channels up to and including filament formations, eruptions
and subsequent reformations during period 1998-2012 (solar cycles 23
and 24).
Title: Parametric decay of large-amplitude Alfvén waves: MHD and
hybrid simulations
Authors: Del Zanna, L.; Matteini, L.; Landi, S.; Velli, M.
Bibcode: 2012AIPC.1436...12D
Altcode:
Parametric instabilities have often been invoked to explain some of the
features of Alfvénic turbulence as observed in the (fast) solar wind
plasma, namely the gradual reduction of cross helicity and its final
saturation with heliocentric distance. Moreover, this instability could
also be responsible for direct plasma heating and creation of transverse
small-scale Alfvénic fluctuations, as required by recent models for
coronal heating and solar wind acceleration. Here we discuss numerical
simulations of the long-term nonlinear evolution of Alfvén waves, both
in the MHD and hybrid regimes, performed by the plasma theory group
in Florence in the last decade and we show, for the first time, 2-D
MHD simulations of the parametric decay of oblique arc-polarized waves.
Title: Observations and simulations of the sympathetic eruptions on
2010 August 1
Authors: Torok, T.; Mikic, Z.; Panasenco, O.; Titov, V. S.; Reeves,
K. K.; Velli, M.; Linker, J. A.; de Toma, G.
Bibcode: 2012EGUGA..14.3270T
Altcode:
During the rise of the new solar cycle, the Sun has produced a number
of so-called sympathetic eruptions, i.e., eruptions that occur close in
time in different source regions. While it has become clear in recent
years that in many of such events the individual eruptions must be
magnetically connected, the exact nature of these connections is not
yet understood. A particularly beautiful case, which consisted of half
a dozen individual eruptions, was observed by STEREO and SDO on 2010
August 1. Here we focus on a subset of two large, consecutive filament
eruptions that were preceded by a nearby CME. We first summarize the
main features of these events and then present 3D MHD simulations
that were designed to model such a chain of eruptions. The simulations
suggest that the two filament eruptions were triggered by two successive
reconnection events, each of which was induced by the previous eruption,
and thus provide a new mechanism for sympathetic eruptions.
Title: Coronal heating in coupled photosphere-chromosphere-coronal
systems: turbulence and leakage
Authors: Verdini, A.; Grappin, R.; Velli, M.
Bibcode: 2012A&A...538A..70V
Altcode: 2011arXiv1111.5359V
Context. Coronal loops act as resonant cavities for low-frequency
fluctuations that are transmitted from the deeper layers of the solar
atmosphere. These fluctuations are amplified in the corona and lead
to the development of turbulence that in turn is able to dissipate
the accumulated energy, thus heating the corona. However, trapping
is not perfect, because some energy leaks down to the chromosphere
on a long timescale, limiting the turbulent heating.
Aims: We
consider the combined effects of turbulence and energy leakage from
the corona to the photosphere in determining the turbulent energy
level and associated heating rate in models of coronal loops, which
include the chromosphere and transition region.
Methods: We use a
piece-wise constant model for the Alfvén speed in loops and a reduced
MHD-shell model to describe the interplay between turbulent dynamics
in the direction perpendicular to the mean field and propagation along
the field. Turbulence is sustained by incoming fluctuations that are
equivalent, in the line-tied case, to forcing by the photospheric shear
flows. While varying the turbulence strength, we systematically compare
the average coronal energy level and dissipation in three models with
increasing complexity: the classical closed model, the open corona,
and the open corona including chromosphere (or three-layer model),
with the last two models allowing energy leakage.
Results: We
find that (i) leakage always plays a role. Even for strong turbulence,
the dissipation time never becomes much lower than the leakage time,
at least in the three-layer model; therefore, both the energy and
the dissipation levels are systematically lower than in the line-tied
model; (ii) in all models, the energy level is close to the resonant
prediction, i.e., assuming an effective turbulent correlation time
longer than the Alfvén coronal crossing time; (iii) the heating rate is
close to the value given by the ratio of photospheric energy divided by
the Alfvén crossing time; (iv) the coronal spectral range is divided
in two: an inertial range with 5/3 spectral slope, and a large-scale
peak where nonlinear couplings are inhibited by trapped resonant modes;
(v) in the realistic three-layer model, the two-component spectrum
leads to a global decrease in damping equal to Kolmogorov damping
reduced by a factor urms/Vac where
Vac is the coronal Alfvén speed.
Title: Ion distributions in the fast solar wind and associated
kinetic instabilities: Ulysses observations
Authors: Matteini, L.; Hellinger, P.; Goldstein, B. E.; Landi, S.;
Velli, M. M.
Bibcode: 2011AGUFMSH53B2041M
Altcode:
We investigate properties of ions in the fast solar wind using
Ulysses observations and we compare the results with linear
theory predictions. An analysis of ion distribution functions,
which are characterized by temperature anisotropy and differential
velocities, based on drifting bi-Maxwellians (Goldstein et al. 2010)
is reported. The stability of the plasma, composed by the core and
beam proton populations and the alpha particles, is investigated with
respect to kinetic instabilities driven by temperature anisotropies
and/or by drift velocities between different species. We find that
while the total global distribution of protons appears constrained
by a fire hose instability, in agreement with previous studies,
the core of distributions is anisotropic with the perpendicular
temperature that is larger then the parallel one, thus possibly
exciting an ion-cyclotron or mirror instability. At the same time,
signatures of ion-beam instabilities are found, suggesting that such
instabilities play a role in the regulation of the ion drifts during
the solar wind expansion. These Ulysses observations suggest that
wave-particle interactions driven by kinetic instabilities are most
of the time at work in the fast solar wind, influencing the plasma
thermodynamics and providing also a possible explanation for recent
magnetic field spectra observations (Wicks et al. 2010).
Title: Reconnection-Driven Alfven (RDA) Waves in the Solar Corona
Authors: Edmondson, J. K.; Lynch, B. J.; DeVore, C. R.; Velli, M.
Bibcode: 2011AGUFMSH31A1990E
Altcode:
The mechanisms for heating the corona and accelerating the solar wind
remain active areas of solar and heliospheric research. Currently,
two relatively distinct paradigms confront these problems: a
waves/turbulence paradigm, and a magnetic reconnection/dissipation
paradigm. The fact that magnetic reconnection generates Alfvén
waves suggests that the heating of the low-beta corona and the
acceleration of the wind are likely the result of both paradigms
operating simultaneously. This research aims to unify the two
competing viewpoints. We present 3D MHD simulations of magnetic
reconnection/dissipation processes in the solar corona that provide a
self-consistent mechanism for producing the energy injection spectrum
required by the waves/turbulence paradigm. We analyze the simulations
to quantify the amount of magnetic energy released in the reconnection
process and the material and energy fluxes carried by large-amplitude,
nonlinear, reconnection-driven Alfvén (RDA) waves. In addition to
isolating the RDA waves, characterizing their physical properties,
and investigating their evolution in a low-beta, gravitationally
stratified model corona, we examine their time-dependent material and
energy outflows and discuss their observable consequences in the corona
and inner heliosphere.
Title: Flow Transition Region in the Heliosheath
Authors: Opher, M.; Drake, J. F.; Velli, M.; Toth, G.
Bibcode: 2011AGUFMSH11A1908O
Altcode:
The tilt between the solar rotation and magnetic axes creates a
sector region. Recently, we argued that the magnetic field in the
sector region in the heliosheath has reconnected (Opher et al. 2011)
and is filled with magnetic structures disconnected from the sun,
called "bubbles". Here we show, that the sector region affects the
flows in the heliosheath such as to create a region where the flow
abruptly turns and the radial flow is near zero or negative. We dub
this the flow transition region (FTR). The FTR is formed due to several
effects that we have explored. The sector region in the heliosheath
defines two flows: the flow within the sector region (region 1)
behaves like an un-magnetized flow while the flow outside the sector
(region 2) is connected to the larger heliosphere through the laminar
magnetic field. The region 1 flow is dominantly affected by the blunt
heliopause ahead of it and is mostly radial. As the flow streamlines
approach the heliopause they turn abruptly, creating the FTR.This
region didn't exist in previous simulations with no sectors where the
flows downstream of the termination shock turn almost immediately to
the sides and to higher latitudes. The thickness of FTR varies and is
thinner in the southern hemisphere. We estimate, based on a recent 3D
MHD simulation (Opher et al. 2011) that at the Voyager 1 location the
thickness of FTR is 10-12AU. The simulations accurately reproduce the
Voyager 1 flows. Since 2010 Voyager 1 has been immersed in the FTR,
based on the negligible flows detected (Krimigis et al. 2011). If no
other temporal dependent effects change the overall structure of the
heliosphere, Voyager 1 is expected to cross the heliopause in the
next 3-5 years. The FTR is much narrower in the southern hemisphere
and Voyager 2 is expected to enter that region in the next couple years.
Title: Coronal Holes and Filaments: Life in Symbiosis
Authors: Panasenco, O.; Velli, M.; Martin, S. F.; Berger, M. A.
Bibcode: 2011AGUFMSH12A..05P
Altcode:
The aim of this work is to understand the relationship between
coronal holes, coronal hole boundaries and one of the other main
features of the coronal magnetic field, namely filament channels,
regions of highly sheared magnetic fields overlying photospheric
polarity reversal boundaries. The well developed filament channel
is a necessary ingredient for the filament formation. Polar coronal
holes and polar crown filament channels always seem to exist together,
and even during periods of weakest activity, when nothing indicates
the presence of polar crown channels on the solar disk, polar crown
prominences appear at the limb proving their existence. Does a similar
symbiotic relationship exist also for other coronal holes? There is
some indication that for middle and low latitude coronal holes different
configurations occur depending on the polarity of the hole relative to
the pole. If the polarity is the same, then a coronal pseudostreamer
configuration can form with twin filament channels at its base, while if
the polarity is opposite, the right conditions for the development of
the filament channels and filaments following the hemispheric helicity
rule arise. Using STEREO/EUVI, SDO/AIA and HMI instruments we trace the
formation and mutual evolution of coronal holes and their symbiotic
filament channels up to and including filament formations, eruptions
and subsequent reformations during many solar rotations in 2010-2011.
Title: Observations of the White Light Corona from Solar Orbiter
and Solar Probe Plus
Authors: Howard, R. A.; Thernisien, A. F.; Vourlidas, A.; Plunkett,
S. P.; Korendyke, C. M.; Sheeley, N. R.; Morrill, J. S.; Socker,
D. G.; Linton, M. G.; Liewer, P. C.; De Jong, E. M.; Velli, M. M.;
Mikic, Z.; Bothmer, V.; Lamy, P. L.
Bibcode: 2011AGUFMSH43F..06H
Altcode:
The SoloHI instrument on Solar Orbiter and the WISPR instrument on Solar
Probe+ will make white light coronagraphic images of the corona as the
two spacecraft orbit the Sun. The minimum perihelia for Solar Orbiter
is about 60 Rsun and for SP+ is 9.5 Rsun. The wide field of view of the
WISPR instrument (about 105 degrees radially) corresponds to viewing
the corona from 2.2 Rsun to 20 Rsun. Thus the entire Thomson hemisphere
is contained within the telescope's field and we need to think of
the instrument as being a traditional remote sensing instrument and
then transitioning to a local in-situ instrument. The local behavior
derives from the fact that the maximum Thomson scattering will favor
the electron plasma close to the spacecraft - exactly what the in-situ
instruments will be sampling. SoloHI and WISPR will also observe
scattered light from dust in the inner heliosphere, which will be an
entirely new spatial regime for dust observations from a coronagraph,
which we assume to arise from dust in the general neighborhood of about
half way between the observer and the Sun. As the dust grains approach
the Sun, they evaporate and do not contribute to the scattering. A
dust free zone has been postulated to exist somewhere inside of 5 Rsun
where all dust is evaporated, but this has never been observed. The
radial position where the evaporation occurs will depend on the
precise molecular composition of the individual grains. The orbital
plane of Solar Orbiter will gradually increase up to about 35 degrees,
enabling a very different view through the zodiacal dust cloud to test
the models generated from in-ecliptic observations. In this paper we
will explore some of the issues associated with the observation of
the dust and will present a simple model to explore the sensitivity
of the instrument to observe such evaporations.
Title: Solar Probe Plus exploration of the solar corona and inner
heliosphere
Authors: Velli, M. M.; Bale, S.; Fox, N. J.; Howard, R. A.; Kasper,
J. C.; Szabo, A.
Bibcode: 2011AGUFMSH43F..08V
Altcode:
Solar Probe Plus (SPP) will be the first mission to directly explore
the sub-Alfvénic corona measuring in-situ the plasma, magnetic
fields, electromagnetic waves and energetic particles and imaging
the scattered light in the source regions of the heliosphere. Solar
Probe Plus objectives are to understand coronal heating and wind
acceleration for both slow and fast solar wind streams, determine
the structure and dynamics of the plasma and magnetic fields at the
sources of the solar wind, and explore the mechanisms that accelerate
and transport energetic particles. This talk describes SPP objectives
and the measurement strategies to achieve the objectives in detail. It
will also illustrate how Solar Probe Plus' investigation of the inner
heliosphere will enhance the scientific return of other missions,
such as Solar Orbiter (SO): SPP and SO will together provide our
most detailed understanding of the processes by which the dynamically
interacting plasmas and magnetic fields escape the corona to become
the solar wind.
Title: A Model for Magnetically Coupled Sympathetic Eruptions
Authors: Török, T.; Panasenco, O.; Titov, V. S.; Mikić, Z.; Reeves,
K. K.; Velli, M.; Linker, J. A.; De Toma, G.
Bibcode: 2011ApJ...739L..63T
Altcode: 2011arXiv1108.2069T
Sympathetic eruptions on the Sun have been observed for several decades,
but the mechanisms by which one eruption can trigger another remain
poorly understood. We present a three-dimensional MHD simulation that
suggests two possible magnetic trigger mechanisms for sympathetic
eruptions. We consider a configuration that contains two coronal flux
ropes located within a pseudo-streamer and one rope located next to
it. A sequence of eruptions is initiated by triggering the eruption of
the flux rope next to the streamer. The expansion of the rope leads
to two consecutive reconnection events, each of which triggers the
eruption of a flux rope by removing a sufficient amount of overlying
flux. The simulation qualitatively reproduces important aspects of the
global sympathetic event on 2010 August 1 and provides a scenario for
the so-called twin filament eruptions. The suggested mechanisms are
also applicable for sympathetic eruptions occurring in other magnetic
configurations.
Title: Who Needs Turbulence?. A Review of Turbulence Effects in the
Heliosphere and on the Fundamental Process of Reconnection
Authors: Matthaeus, W. H.; Velli, M.
Bibcode: 2011SSRv..160..145M
Altcode: 2011SSRv..tmp..222M; 2011SSRv..tmp..146M; 2011SSRv..tmp..273M;
2011SSRv..tmp...77M
The significant influences of turbulence in neutral fluid hydrodynamics
are well accepted but the potential for analogous effects in
space and astrophysical plasmas is less widely recognized. This
situation sometimes gives rise to the question posed in the title;
"Who need turbulence?" After a brief overview of turbulence effects
in hydrodynamics, some likely effects of turbulence in solar and
heliospheric plasma physics are reviewed here, with the goal of
providing at least a partial answer to the posed question.
Title: The evolution of turbulent fluctuations in the solar wind and
their dynamical role: what can we learn from models and simulations?
Authors: Velli, Marco
Bibcode: 2011shin.confE.110V
Altcode:
I will discuss what theoretical advances are required to make progress
in understanding the evolution of turbulence in the solar wind,
and in particular alfven wave turbulence and the evolution of the
spectrum. I will consider models that start from the solar corona
and include simplified interactions, such as shell models, and full
numerical simulations limited to the supersonic solar wind, such as the
expanding box models. The talk hopes to stimulate discussion of what
we are hoping to understand concerning the nature of turbulent solar
wind fluctuations, including their role in heating and accelerating
the solar wind.
Title: A model for sympathetic eruptions
Authors: Torok, Tibor; Panasenco, O.; Titov, V. S.; Mikic, Z.; Velli,
M.; Linker, J.; De Toma, G.
Bibcode: 2011shin.confE.125T
Altcode:
Apart from single eruptions originating in localized source regions,
the Sun sometimes produces so-called sympathetic events, which consist
of several individual eruptions occurring almost simultaneously
in different source regions. The close temporal correlation of the
individual eruptions in such events indicates a causal link between
them, but the mechanisms by which one eruption can trigger another
one remain largely a mystery. A particularly beautiful example
of a global sympathetic event was recently observed by the Solar
Dynamics Observatory (SDO) on 1 August 2010. It included a small
filament eruption and CME that was shortly after followed by the
nearby subsequent eruptions of two large adjacent (twin) filaments,
indicating that these three eruptions were physically connected. A
coronal potential field extrapolation reveals that the twin filaments
were located in the lobes of a so-called pseudostreamer prior to
their eruptions. Here we present a 3D MHD simulation of the
successive eruption of two magnetic flux ropes in such a pseudostreamer
configuration. The two eruptions are triggered by the simulated eruption
of a third flux rope in the vicinity of the pseudostreamer. The
simulation qualitatively reproduces the CME and subsequent twin
filament eruption on 1 August 2010 and suggests that these events
were indeed physically connected. Furthermore, it provides a generic
scenario for the frequently observed twin filament eruptions in coronal
pseudostreamers and suggests a mechanism by which such eruptions can
be triggered in the first place. Our results thus provide an important
step for a better understanding of sympathetic eruptions.
Title: Coronal Plumes in the Fast Solar Wind
Authors: Velli, Marco; Lionello, Roberto; Linker, Jon A.; Mikić, Zoran
Bibcode: 2011ApJ...736...32V
Altcode:
The expansion of a coronal hole filled with a discrete number of
higher density coronal plumes is simulated using a time-dependent
two-dimensional code. A solar wind model including an exponential
coronal heating function and a flux of Alfvén waves propagating both
inside and outside the structures is taken as a basic state. Different
plasma plume profiles are obtained by using different scale heights for
the heating rates. Remote sensing and solar wind in situ observations
are used to constrain the parameter range of the study. Time dependence
due to plume ignition and disappearance is also discussed. Velocity
differences of the order of ~50 km s-1, such as those
found in microstreams in the high-speed solar wind, may be easily
explained by slightly different heat deposition profiles in different
plumes. Statistical pressure balance in the fast wind data may be
masked by the large variety of body and surface waves which the higher
density filaments may carry, so the absence of pressure balance in the
microstreams should not rule out their interpretation as the extension
of coronal plumes into interplanetary space. Mixing of plume-interplume
material via the Kelvin-Helmholtz instability seems to be possible
within the parameter ranges of the models defined here, only at large
distances from the Sun, beyond 0.2-0.3 AU. Plasma and composition
measurements in the inner heliosphere, such as those which will become
available with Solar Orbiter and Solar Probe Plus, should therefore
definitely be able to identify plume remnants in the solar wind.
Title: Magnetohydrodynamic turbulent cascade of coronal loop
magnetic fields
Authors: Rappazzo, A. F.; Velli, M.
Bibcode: 2011PhRvE..83f5401R
Altcode: 2010arXiv1005.1640R
The Parker model for coronal heating is investigated
through a high resolution simulation. An inertial
range is resolved where fluctuating magnetic energy
EM(k⊥)∝k⊥-2.7
exceeds kinetic energy
EK(k⊥)∝k⊥-0.6.
Increments scale as
δbℓ≃ℓ-0.85andδuℓ≃ℓ+0.2
with velocity increasing at small scales, indicating that magnetic
reconnection plays a prime role in this turbulent system. We show that
spectral energy transport is akin to standard magnetohydrodynamic
(MHD) turbulence even for a system of reconnecting current sheets
sustained by the boundary. In this new MHD turbulent cascade, kinetic
energy flows are negligible while cross-field flows are enhanced,
and through a series of “reflections” between the two fields,
cascade more than half of the total spectral energy flow.
Title: 3d Mhd Simulation Of Sympathetic Eruptions On 1 August 2010
Authors: Torok, Tibor; Panasenco, O.; Titov, V.; Mikic, Z.; Reeves,
K.; Velli, M.; Linker, J.; de Toma, G.
Bibcode: 2011SPD....42.0908T
Altcode: 2011BAAS..43S.0908T
Apart from single eruptions originating in localized source regions, the
Sun sometimes produces so-called sympathetic events, which consist of
several individual eruptions occurring almost simultaneously
in different source regions. The close temporal vicinity of the
individual eruptions in such events indicates the existence of
a causal link between them, but the mechanisms by which one eruption
can trigger another one remain largely a mystery. A particularly
beautiful example of a global sympathetic event was recently observed
by the Solar Dynamics Observatory (SDO) on 1 August 2010. It included
a small filament eruption and CME that was closely followed by the
eruptions of two large adjacent (twin) filaments, indicating that these
three eruptions were physically connected. A coronal potential field
extrapolation revealed that the twin filaments were located in the
lobes of a so-called pseudostreamer prior to their eruptions. Here we
present a 3D MHD simulation of the successive eruption of two magnetic
flux ropes in such a pseudostreamer configuration. The two eruptions are
triggered by the simulated eruption of a third flux rope in the vicinity
of the pseudostreamer. The simulation qualitatively reproduces the CME
and subsequent twin filament eruption on 1 August 2010 and suggests that
these events were indeed physically connected. Furthermore, it provides
a generic scenario for the frequently observed twin filament eruptions
in coronal pseudostreamers and suggests a mechanism by which such
eruptions can be triggered in the first place. Our results thus provide
an important step for a better understanding of sympathetic eruptions.
Title: MHD Simulations of Coronal Plumes
Authors: Lionello, Roberto; Velli, M.; Linker, J. A.; Mikic, Z.
Bibcode: 2011SPD....42.1807L
Altcode: 2011BAAS..43S.1807L
The expansion of a coronal hole filled with a discrete number of
higher density coronal plumes is simulated using a time-dependent 2D
code. A solar wind model including an exponential coronal heating
function and a flux of Alfven waves propagating both inside and
outside the structures is taken as a basic state. Different plasma
plume profiles are obtained by using different scale heights for the
heating rates. Remote sensing and solar wind in situ observations are
used to constrain the parameter range of the study. Time dependence
due to plume ignition and disappearance is also discussed. Velocity
differences of the order of 50 km/s, such as those found in microstreams
in the high-speed solar wind, may be easily explained by slightly
different heat deposition profiles in different plumes. Statistical
pressure balance in the fast wind data may be masked by the large
variety of body and surface waves which the higher density filaments
may carry, so the absence of pressure balance in the microstreams
should not rule out their interpretation as the extension of coronal
plumes into interplanetary space. Mixing of plume-interplume material
via the Kelvin-Helmholtz instability seems to be possible, within the
parameter ranges of the models defined here, only at large distances
from the Sun, beyond 0.2-0.3 AU. Plasma and composition measurements
in the inner heliosphere, such as those which will become available
with Solar Orbiter and Solar Probe Plus, should therefore definitely
be able to identify plume remnants in the solar wind.
Title: Dynamic Current Sheet Formation and Evolution with Application
to Inter-(Super)granular Flow Lanes and Quasi-Homologous Jet Activity
Authors: Edmondson, Justin K.; Velli, M.
Bibcode: 2011SPD....42.1748E
Altcode: 2011BAAS..43S.1748E
The coronal magnetic field structure is an immensely complex system
constantly driven away from equilibrium by global drivers such as
photospheric flow, flux emergence/cancellation at the lower boundary,
helicity injection and transport, etc. In low-beta plasma systems,
such as solar corona, the Maxwell stresses dominate forces and
therefore the system dynamics. General Poynting stress injection
(i.e., flux injection, helicity injection, translational motions,
or any combination thereof) results in (possibly large) geometric
deformations of the magnetic field, such that the Maxwell stresses
distribute as uniformly as possible, constrained by the distorted
geometry and topology of the bounding separatricies. Since the
topological connectivity is discontinuous across these separatrix
surfaces, the magnetic stresses will be discontinuous there as well,
manifesting as current sheets within the field. The solar magnetic
field undergoes major geometric expansion passing from the photosphere,
through the chromosphere, into the corona. No matter the specific
details, a mixed polarity distribution at the lower boundary and
the divergence-free condition require invariant topological features
such as an X-line and separatricies to exist between fields emanating
from separate regions of the photosphere. We present the results of
fully-3D numerical simulations of a simplified low-beta model of this
field expansion. A symmetric injection of Maxwell stresses into this
geometry inflates strongly line-tied fields, generating a region of
large current densities and magnetic energy dissipation. Elsewhere the
injected stresses accumulate along the existing separatricies. There
is no evidence of reconnection dynamics until after the initial
left-right parity is broken. Once the symmetry breaks, the X-line
deforms explosively into a Syrovatskii-type current sheet, leading to
a succession of quasi-homologous jet dynamics. The bursty-oscillations
of these jets occur as the stresses within the low-lying arcades are
alternately relived by reconnection. These results have applications
to jet activity in the low-corona, and general lower-coronal boundary
dynamics.
Title: Magnetic Structure of Twin Filaments Inside Pseudostreamers
Authors: Panasenco, O.; Velli, M. M.
Bibcode: 2010AGUFMSH51A1663P
Altcode:
Among the large scale coronal structures, pseudostreamers appear in
unipolar regions above multiple polarity reversal boundaries. Some of
these polarity reversal boundaries can be filament channels, and when
this is the case they always occur as twin filament channels often
containing twin filaments. The magnetic structure of pseudostreamers
with and without twin filaments lying at their base is significantly
different. Branches of pseudostreamers on opposite sides of the
separatrix surface diverge when filaments are present. Here we analyze
possible current and magnetic field configurations of the complete
pseudostreamer system and study the links between its separate parts,
which include open field lines of pseudostreamers, filament channels,
filaments, cavities, overlying filament arcades. The presence of well
developed filaments of the same chirality at the base of pseudostreamers
implies the presence of a vertical current sheet, which divides and
repeal branches of the pseudostreamer field lines in 3D. We discuss the
3D magnetic topology of the filament, filament cavity and overlying
filament arcades for these twin systems and its implications on the
theories for filaments an filament eruptions.
Title: Parametric Decay of Obliquely Propagating Alfvén Waves:
Transverse Coupling and Proton Parallel Acceleration
Authors: Matteini, L.; Landi, S.; Del Zanna, L.; Velli, M. M.;
Hellinger, P.
Bibcode: 2010AGUFMSH51F..06M
Altcode:
We present two-dimensional hybrid simulations of the evolution of
large amplitude shear Alfvén waves in oblique propagation with
respect to the ambient magnetic field. As in the case of parallel
propagation, Alfvén waves are unstable to parametric decay resulting
in the excitation of ion-acustic modes and of backward propagating
Alfvén waves. Ion-acoustic modes, due to non-linear trapping,
support the acceleration of protons with formation of a beam along
the mean magnetic field. In contrast to the parallel propagating case,
oblique shear Alfvén modes allow for the generation of a broad band
spectrum of coupled Alfvénic and compressive modes (also in oblique
propagation), selected by the resonance condition for the three-wave
coupling. This leads to a direct coupling of the parent waves to small
scales, supporting a strong transverse magnetic field modulation,
once saturation is attained.
Title: Current Sheet Formation and Reconnection Dynamics in the
Closed Corona Due to Intragranular Flow Lanes
Authors: Edmondson, J. K.; Velli, M. M.; DeVore, C. R.
Bibcode: 2010AGUFMSH54C..02E
Altcode:
Current sheet formation and reconnection have been shown to be very
important processes in coronal heating and dynamics. The formation
of current discontinuities and therefore the reconnection dynamics
in a low-beta, force-free system depend on two major influences: the
structure of the driving flow fields, and the overall magnetic field
geometry. Photospheric magnetograms show magnetic field footpoint
elements moving along intragranular flow lanes. The magnetic field
undergoes a major expansion as it passes from the photosphere, through
the chromosphere, into the corona. We model this field expansion
geometry and intragranular flow lanes with an initially analytic
potential field. We simulate the closed corona using the Parker
ansatz of line-tying the field at the upper and lower boundaries. The
flows imposed at the photosphere are confined to two relatively thin
lanes marking the footprints of the expanding field. We calculate the
system evolution in fully 3D MHD, finding the current discontinuities
forming along the topological separatricies of the magnetic field,
as well as quasi-separatrix layers forming with the large velocity
gradients - exactly as expected. Reconnection occurs across the current
discontinuities, shifting flux between the different domains, reducing
the free magnetic energy, while increasing the field complexity. The
injected stresses do not accumulate significantly before reconnection,
thus there are no major explosive energy release, only an average
steady dissipation.
Title: Understanding heliospheric origins with Solar Probe Plus
Authors: Velli, M. M.
Bibcode: 2010AGUFMSH33C..08V
Altcode:
The magnetic field is fundamental to solar activity and shapes
the interplanetary environment, as clearly shown by the full
three dimensional monitoring of the heliosphere provided by the
measurements of the Helios, Ulysses, SOHO, ACE, Wind, STEREO and Voyager
spacecraft. Magnetic fields are also the source for coronal heating and
the very existence of the solar wind; produced by the sun’s dynamo and
emerging into the corona magnetic fields become a conduit for waves,
act to store energy, and then propel plasma into the heliosphere in
the form of Coronal Mass Ejections (CMEs). Transformation of magnetic
energy is also the source solar energetic particle events. The way in
which solar convective energy couples to magnetic fields to produce
the multifaceted heliosphere is at the heart of the Solar Probe Plus
exploration. This contribution highlights the exciting perspectives for
discovery provided by the SPP investigation of the sub-Alfvénic corona.
Title: On the competition between radial expansion and Coulomb
collisions in shaping the electron velocity distribution function:
Kinetic simulations
Authors: Landi, S.; Matteini, L.; Pantellini, F.; Velli, M. M.
Bibcode: 2010AGUFMSH34A..05L
Altcode:
We present self consistent kinetic simulations where we explore
the effects of electron-electron and electron-proton collisions
against the radial expansion of a supersonic wind on the electron
velocity distribution function. The combined effects of collisions and
expansion naturally generate a two electron populations: a collisional
dominated core and an almost collisionless collimated "halo". Several
properties of the electron velocity distribution function are compared
with spacecraft data: in particular we find that relative densities,
drift velocities, temperatures gradients of the two populations are
consistent to what observed in the real solar wind. A discussion on
the electron heat flux properties is also presented.
Title: The Integrated Science Investigation of the Sun (ISIS):
Energetic Particle Measurements for the Solar Probe Plus Mission
Authors: Scherrer, J.; McComas, D. J.; Christian, E. R.; Cummings,
A. C.; Desai, M. I.; Giacalone, J.; Hill, M. E.; Krimigis, S. M.;
Livi, S. A.; McNutt, R. L.; Mewaldt, R. A.; Mitchell, D. G.; Matthaeus,
W. H.; Roelof, E. C.; von Rosenvinge, T. T.; Schwadron, N. A.; Stone,
E. C.; Velli, M. M.; Wiedenbeck, M. E.
Bibcode: 2010AGUFMSH11B1621S
Altcode:
One of the major goals of NASA’s Solar Probe Plus (SPP) mission is
to determine the mechanisms that accelerate and transport high-energy
particles from the solar atmosphere out into the heliosphere. During
the height of solar activity, which occurs roughly once every 11 years,
processes such as coronal mass ejections and solar flares release huge
quantities of energized matter, magnetic fields and electromagnetic
radiation into space. These high-energy particles, known as solar
energetic particles or SEPs, present a serious radiation threat to
human explorers living and working outside low-Earth orbit and to
technological assets such as communications and scientific satellites
in space. This talk describes the Integrated Science Investigation of
the Sun (ISIS) - Energetic Particle Instrument suite. ISIS measures
key properties such as intensities, energy spectra, composition,
and angular distributions of the low-energy suprathermal source
populations, as well as the more hazardous, higher energy particles
ejected from the Sun. By making the first-ever direct measurements
of the near-Sun regions where the acceleration takes place, ISIS will
provide the critical measurements that, when integrated with other SPP
instruments and with solar and interplanetary observations, will lead
to a much deeper understanding of the Sun and major drivers of solar
system space weather.
Title: Coronal Loops Dynamics and Photospheric Forcing Patterns
Authors: Rappazzo, A. F.; Velli, M. M.
Bibcode: 2010AGUFMSM51C1846R
Altcode:
We present a series of numerical simulations aimed at understanding
the nature of the dynamics and the magnetic reconnection taking place
in the Parker model for coronal heating. A coronal loop is studied
via reduced magnetohydrodynamics simulations in Cartesian geometry. A
uniform and strong magnetic field threads the volume between the two
photospheric planes, where a forcing in the form of a velocity field is
applied. It is commonly thought that the topology of the photospheric
driver should strongly influence the dynamics of a coronal loop, and
that the magnetic field lines anchored to the photospheric planes should
passively follow their footpoints motions. In this picture the electric
currents should develop along neighboring field lines whose footpoints
have a relative shear motion. In previous works we have identified
MHD turbulence as the physical process that transports energy from
the scale of photospheric motions to the small dissipative scales
where magnetic reconnection takes place. Here we present a series
of simulations aimed at understanding if the MHD turbulent dynamics
are due to the complexity of the imposed photospheric forcing or if
they rather originate from the intrinsic nonlinear properties of the
system. To this effect we apply a few ``ordered'' photospheric forcings
in the form of a 1D shear flow pattern and various combinations of
symmetric vortices. In all cases initially the magnetic field that
develops in the coronal loop is a simple map of the photospheric
velocity field. This initial configuration is unstable to some kind of
instability (a multiple tearing, a kink, etc.) that develops islands
with X and O points in the planes orthogonal to the axial field. Once
the nonlinear stage sets in the system evolution is characterized by a
regime of MHD turbulence dominated by magnetic energy. A well developed
power law in energy spectra is observed and the magnetic field never
returns to the simple initial state mapping the photospheric flow. The
formation of X and O points in the planes orthogonal to the axial
field allows the continued and repeated formation and dissipation of
small scale current sheets where the plasma is heated. We conclude that
the observed turbulent dynamics are not induced by the complexity of
the pattern that the magnetic field lines footpoints follow but they
rather stem from the inherent nonlinear nature of the system. Adding
that the total dissipation rate is independent from the Reynolds number
at sufficiently high values indicates that the magnetic reconnection
taking place is very likely turbulent and its properties will be
analyzed more in depth in future works.
Title: Imaging the Solar Wind with SoloHI
Authors: Howard, R. A.; Vourlidas, A.; Plunkett, S. P.; Korendyke,
C. M.; McMullin, D. R.; Liewer, P. C.; Velli, M. M.; Solohi
Bibcode: 2010AGUFMSH11B1627H
Altcode:
Imaging outflows in the corona have been observed with the SOHO/LASCO
instrument, since 1996. With the launch of the STEREO mission in
2006, these outflows can be followed into the heliosphere, with the
SECCHI/Heliospheric Imager. For the Solar Orbiter mission, we have
proposed an instrument called the Solar Orbiter Heliospheric Imager
(SoloHI) to be able to image the solar wind and the density fluctuations
in the wind and thus provide the link between the in-situ and remote
sensing measurements. The nature of the Thomson scattering process
integrates along a particular look direction but the scattering is a
maximum on the Thomson sphere - the locus of points that form a right
angle between the look direction and the solar vector. The experience
from SECCHI/HI shows that the density fluctuations are easily visible
and can be tracked back into the low corona, enabling a coupling
between the solar wind plasma crossing the spacecraft and the source
region in the corona. However, the SECCHI/HI observations have low
cadence and long integration times. As a new observing mode for SoloHI,
we have implemented a capability to readout a subset of the image at
a time cadence of about 1 second. Thus small scale fluctuations can
be observed in addition to the large scale fluctuations observed from
SECCHI/HI. This will enable us to determine the spectral index of the
density fluctuations over an unprecedented range of heights (from 5 to
135 Rsun) to compare with the in-situ determinations of the spectral
index. This may indicate whether the fluctuations are generated close
to the sun and convected out by the solar wind or are generated within
the solar wind.
Title: Parametric decay of linearly polarized shear Alfvén waves
in oblique propagation: One and two-dimensional hybrid simulations
Authors: Matteini, Lorenzo; Landi, Simone; Del Zanna, Luca; Velli,
Marco; Hellinger, Petr
Bibcode: 2010GeoRL..3720101M
Altcode:
The parametric instability of a monochromatic shear Alfvén wave
in oblique propagation with respect the ambient magnetic field is
investigated in a kinetic regime, performing one-dimensional (1-D) and
two-dimensional (2-D) hybrid simulations. The parallel component of the
mother wave is found to be subject to a parametric decay which excites
an ion-acoustic wave along the magnetic field and a backward propagating
daughter shear Alfvén wave, as in the instability for a purely parallel
mother wave. At the same time, the acoustic wave generation supports the
acceleration of a velocity beam in the ion distribution function, due
to the non-linear trapping of protons. Moreover, the instability leads
to the generation of broad band oblique spectra of coupled Alfvénic
and compressive modes with variable perpendicular wavevectors, and,
as a consequence, the magnetic field after saturation is characterized
by a strong transverse modulation.
Title: Shear Photospheric Forcing and the Origin of Turbulence in
Coronal Loops
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.
Bibcode: 2010ApJ...722...65R
Altcode: 2010arXiv1003.3872R
We present a series of numerical simulations aimed at understanding
the nature and origin of turbulence in coronal loops in the framework
of the Parker model for coronal heating. A coronal loop is studied
via reduced magnetohydrodynamic (MHD) simulations in Cartesian
geometry. A uniform and strong magnetic field threads the volume
between the two photospheric planes, where a velocity field in the
form of a one-dimensional shear flow pattern is present. Initially,
the magnetic field that develops in the coronal loop is a simple map of
the photospheric velocity field. This initial configuration is unstable
to a multiple tearing instability that develops islands with X and O
points in the plane orthogonal to the axial field. Once the nonlinear
stage sets in the system evolution is characterized by a regime of MHD
turbulence dominated by magnetic energy. A well-developed power law
in energy spectra is observed and the magnetic field never returns to
the simple initial state mapping the photospheric flow. The formation
of X and O points in the planes orthogonal to the axial field allows
the continued and repeated formation and dissipation of small-scale
current sheets where the plasma is heated. We conclude that the observed
turbulent dynamics are not induced by the complexity of the pattern
that the magnetic field-line footpoints follow but they rather stem
from the inherent nonlinear nature of the system.
Title: Kinetics of parametric instabilities of Alfvén waves:
Evolution of ion distribution functions
Authors: Matteini, Lorenzo; Landi, Simone; Velli, Marco; Hellinger,
Petr
Bibcode: 2010JGRA..115.9106M
Altcode: 2010JGRA..11509106M
Using numerical simulations in a hybrid regime, we studied the
evolution of large-amplitude Alfvén waves subject to modulational
and decay instabilities, including the effects of ion kinetics. We
considered both a monochromatic and incoherent spectrum of waves,
different wave polarizations and amplitudes, and different plasma
regimes, ranging from β < 1 to β > 1. We found in all cases
that ion dynamics affects the instability evolution and saturation;
as a feedback, wave-particle interactions provide a nonlinear trapping
of resonant particles that importantly change the properties of the
ion velocity distribution functions. In particular, we observed a
proton acceleration along the magnetic field and in some cases the
formation of a parallel velocity beam traveling faster than the rest of
the distribution. For the range of parameters used in our simulations,
the fundamental ingredient in generating an ion beam is observed to be
the parallel electric field carried by the density fluctuations driven
by the ion-acoustic modes generated by the parametric instabilities.
Title: Non-radial and Non-coaligned Propagation of Erupting Filaments
and CMEs
Authors: Panasenco, Olga; Velli, Marco
Bibcode: 2010shin.confE.134P
Altcode:
In the initial phases of a prominence eruption, the filament material
is seen to 'roll' sideways compared to the local vertical or radial
direction. The degree of the non-radial motion of a CME and the
degree and direction of the rolling motion of the filament plasma are
evidence of the global and local force imbalances occurring during
the eruption. Differences in the force at different positions and the
consequent deflections will produce the non-coaligned propagation of
erupting filament and corresponding CME. Generally speaking, above
and beyond the tendency of the eruption to move towards the weak
magnetic regions surrounding the null points above the structure,
there will also be a lateral deflection due to the lack of symmetry
in the local magnetic fields around the polarity reversal boundary
associated with the corresponding filament channel. We show examples of
lateral deflection due to the presence of a coronal hole on one side
of the filament channel and present magnetic intensity maps from pfss
modeling of the pre-existing coronal field. Though the pfss model by
definition has zero magnetic stresses, the intensity map gives a good
indication of where the forces will be strongest when the currents
associated with the eruption come into play.
Title: Formation and Reconnection of Three-Dimensional Current Sheets
in the Solar Corona
Authors: Edmondson, Justin K.; Antiochos, S. K.; DeVore, C.; Velli,
M.; Zurbuchen, T. H.
Bibcode: 2010AAS...21640701E
Altcode: 2010BAAS...41..859E
Current-sheet formation and magnetic reconnection are believed to
be the basic physical processes responsible for much of the activity
observed in astrophysical plasmas, such as interchange reconnection at
the boundaries between coronal holes and helmet streamers in the Sun's
corona. We investigate these processes for a magnetic configuration
consisting of a uniform background field and an embedded line dipole,
a topology that is expected to be ubiquitous in the corona. This
magnetic system is driven by a uniform horizontal flow applied
at the line-tied photosphere. Although both the initial field and
the driver are translationally symmetric, the resulting evolution
is calculated using a fully three-dimensional magnetohydrodynamic
(3D MHD) simulation with adaptive mesh refinement that resolves the
current sheet and reconnection dynamics in detail. The advantage of
our approach is that it allows us to apply directly the vast body
of knowledge gained from the many studies of 2D reconnection to the
fully 3D case. We find that a current sheet forms in close analogy to
the classic Syrovatskii 2D mechanism, but the resulting evolution is
different than expected. The current sheet is globally stable, showing
no evidence for a disruption or a secondary instability even for aspect
ratios as high as 80:1. The global evolution generally follows the
standard Sweet-Parker 2D reconnection model except for an accelerated
reconnection rate at a very thin current sheet, due to the tearing
instability and the formation of magnetic islands. An interesting
conclusion is that despite the formation of fully 3D structures at small
scales, the system remains close to 2D at global scales. We discuss
the implications of our results for observations of the solar corona.
Title: The ADAHELI solar mission: Investigating the structure of
Sun's lower atmosphere
Authors: Berrilli, F.; Bigazzi, A.; Roselli, L.; Sabatini, P.; Velli,
M.; Alimenti, F.; Cavallini, F.; Greco, V.; Moretti, P. F.; Orsini,
S.; Romoli, M.; White, S. M.; ADAHELI Team; Ascani, L.; Carbone, V.;
Curti, F.; Consolini, G.; Di Mauro, M. P.; Del Moro, D.; Egidi, A.;
Ermolli, I.; Giordano, S.; Pastena, M.; Pulcino, V.; Pietropaolo, E.;
Romano, P.; Ventura, P.; Cauzzi, G.; Valdettaro, L.; Zuccarello, F.;
ADAHELI Team
Bibcode: 2010AdSpR..45.1191B
Altcode: 2010AdSpR..45.1191A
ADAHELI (ADvanced Astronomy for HELIophysics) is a small-class
(500 kg) low-budget (50 MEuro) satellite mission for the study of
the solar photosphere and the chromosphere and for monitoring solar
flare emission. ADAHELI's design has completed its Phase-A feasibility
study in December 2008, in the framework of ASI's (Agenzia Spaziale
Italiana) 2007 "Small Missions" Program (calling for two missions at
50 MEeuros each, plus the launch budget). ADAHELI's main purpose is
to explore Sun's lower atmosphere in the near-infrared, a region so
far unexplored by solar observations from space. ADAHELI will carry
out observations of the solar photosphere and of the chromosphere at
high-temporal rate and high spatial and spectral resolutions. ADAHELI
will contribute to the understanding of Space Weather through the
study of particle acceleration during flares. A radiometer operating
in the millimeter radio band will continuously monitor the solar disk,
throughout the spacecraft's life time. ADAHELI's baseline instruments
are a 50-cm high-resolution telescope operating in the visible and
the near-infrared, and a lightweight full-disk radiometer operating at
millimeter wavelengths (90 GHz). The core of the telescope's focal plane
suite is the spectral imager based on two Fabry-Perot interferometers,
flying for the first time on a solar mission. The instrument
will return fast-cadence, full bi-dimensional spectral images at
high-resolution, thus improving on current slit-scan, mono-dimensional
architectures. Moreover, the possibility of working in polarized light
will enable full 3D magnetic field reconstruction on the photosphere
and the chromosphere. An optional instrumental package is also being
proposed to further extend ADAHELI's scope: a full-disk telescope for
helioseismology based on a double Magneto-Optical Filter, a Neutral
Particle Analyzer for magnetospheric research, an Extreme Ultraviolet
imaging and spectro-radiometry instrument. These options fall outside
the prescribed budget. ADAHELI, flying a Sun-Synchronous orbit at 800
km, will perform continuous, long-duration (4-h), daily acquisitions,
with the possibility of extending them up to 24 h. ADAHELI's operating
life is two years, plus one extension year. Launch would be nominally
planned for 2014.
Title: An MHD Model with Wave Turbulence Driven Heating and Solar
Wind Acceleration
Authors: Lionello, Roberto; Linker, J. A.; Mikic, Z.; Riley, P.;
Velli, M.
Bibcode: 2010AAS...21630301L
Altcode:
The mechanisms responsible for heating the Sun's corona and accelerating
the solar wind are still being actively investigated. However, it
is largely accepted that photospheric motions provide the energy
source and that the magnetic field must play a key role in the
process. Three-dimensional MHD models have traditionally used an
empirical prescription for coronal heating (e.g., Lionello et al. 2009),
together with WKB Alfven wave acceleration of the solar wind. Recently,
attention has been focused on wave turbulence driven models (e.g.,
Cranmer et al. 2007; Cranmer 2010) in which the heating and solar
wind acceleration by Alfven waves are included self-consistently. We
will demonstrate the initial implementation of this idea in an MHD
model based on turbulent cascade heating in the closed-field regions
(Rappazzo et al. 2007, 2008), and Alfven wave turbulent dissipation
in open field regions (Verdini & Velli 2007, 2010).
Title: Coupling Photosphere and Corona: Linear and Turbulent Regimes
Authors: Verdini, A.; Grappin, R.; Velli, M.
Bibcode: 2010AIPC.1216...28V
Altcode: 2009arXiv0912.1497V
In a recent work Grappin et al. [1] have shown that low- frequency
movements can be transmitted from one footpoint to the other along a
magnetic loop, thus mimicking a friction effect of the corona on the
photosphere, and invalidating the line-tying approximation. We consider
here successively the effect of high frequencies and turbulent damping
on the process. We use a very simple atmospheric model which allows to
study analytically the laminar case, and to study the turbulent case
both using simple phenomenological arguments and a more sophisticated
turbulence model [2]. We find that, except when turbulent damping is
such that all turbulence is damped during loop traversal, coupling
still occurs between distant footpoints, and moreover the coronal
field induced by photospheric movements saturates at finite values.
Title: Turbulence, Energy Transfers and Reconnection in Compressible
Coronal Heating Field-line Tangling Models
Authors: Dahlburg, R. B.; Rappazzo, A. F.; Velli, M.
Bibcode: 2010AIPC.1216...40D
Altcode: 2009arXiv0912.1063D
MHD turbulence has long been proposed as a mechanism for the heating
of coronal loops in the framework of the Parker scenario for coronal
heating. So far most of the studies have focused on its dynamical
properties without considering its thermodynamical and radiative
features, because of the very demanding computational requirements. In
this paper we extend this previous research to the compressible regime,
including an energy equation, by using HYPERION, a new parallelized,
viscoresistive, three-dimensional compressible MHD code. HYPERION
employs a Fourier collocation-finite difference spatial discretization,
and uses a third-order Runge-Kutta temporal discretization. We show that
the implementation of a thermal conduction parallel to the DC magnetic
field induces a radiative emission concentrated at the boundaries, with
properties similar to the chromosphere-transition region-corona system.
Title: On the role of wave-particle interactions in the evolution
of solar wind ion distribution functions
Authors: Matteini, Lorenzo; Landi, Simone; Velli, Marco; Hellinger,
Petr
Bibcode: 2010AIPC.1216..223M
Altcode:
We investigate the role of kinetic effects in the solar wind
expansion using 1-D numerical hybrid simulations. The analysis of
proton distribution functions in the solar wind shows a non-adiabatic
evolution and suggests that several kinetic processes are at work
during the expansion. From simulation studies wave-particle and
wave-wave interactions, as cyclotron heating and non-linear trapping
due to parametric instabilities, are found to play an important role on
constraining the proton temperature anisotropy and generating secondary
velocity beams. We report results from hybrid comoving simulations
that self-consistently retain and describe these processes. We
find that cyclotron interactions control the evolution of the proton
temperature anisotropy with distance providing a perpendicular heating
which contrasts the adiabatic cooling caused by the expansion. At the
same time ion-acoustic modes driven by parametric effects produce
a velocity beam in the proton distribution function. The resulting
proton distribution functions are reasonable agreement with those
observed in situ.
Title: Solar Wind Acceleration: Mechanisms and Scaling Laws
Authors: Velli, Marco
Bibcode: 2010AIPC.1216...14V
Altcode:
The basic conservation laws for mass, momentum and energy along
flow tubes from the sun to the Earth, together with observational
constraints coming from remote sensing observations of the corona and
in-situ measurements of the wind may be used to derive scaling laws
relating the asymptotic solar wind properties to the source regions
on the sun. This paper reviews and compares theories of solar wind
acceleration based on such conservation principles, stressing the
different approximations involved.
Title: Coupling Photosphere and Corona: Linear and Turbulent Regimes
Authors: Verdini, Andrea; Grappin, Roland; Velli, Marco
Bibcode: 2010cosp...38.2836V
Altcode: 2010cosp.meet.2836V
In numerical modeling, corona is often stuck directly on the photosphere
where movements are prescribed, thus fully reflecting Alfven waves
running along closed loops. In reality, reflection is not complete. We
consider here for the first time the combined effects of finite
reflection and turbulence which both control the coronal magnetic
energy injected by the photospheric velocity shear. The loop model
used is that of linear Alfven wave propagation along a 1D loop model,
defined by a piece-wise constant Alfven speed (chromosphere and corona),
and a turbulence shell model in the perpendicular direction.
Title: A Turbulence-Driven Model for Heating and Acceleration of
the Fast Wind in Coronal Holes
Authors: Verdini, A.; Velli, M.; Matthaeus, W. H.; Oughton, S.;
Dmitruk, P.
Bibcode: 2010ApJ...708L.116V
Altcode: 2009arXiv0911.5221V
A model is presented for generation of fast solar wind in coronal holes,
relying on heating that is dominated by turbulent dissipation of MHD
fluctuations transported upward in the solar atmosphere. Scale-separated
transport equations include large-scale fields, transverse Alfvénic
fluctuations, and a small compressive dissipation due to parallel shears
near the transition region. The model accounts for proton temperature,
density, wind speed, and fluctuation amplitude as observed in remote
sensing and in situ satellite data.
Title: Turbulent heating and cooling of coronal loops
Authors: Buchlin, Eric; Bradshaw, Stephen J.; Cargill, Peter J.;
Velli, Marco
Bibcode: 2010cosp...38.2834B
Altcode: 2010cosp.meet.2834B
In the solar corona, MHD turbulence is likely to be the process
producing small scales, at which heating processes become efficient and
are able to sustain the high coronal temperatures. However, as these
small scales are too small to be observable, comparison between models
and observations rely on indirect consequences of the heating. For this
reason we build a model of a coronal loop including at the same time
the heating processes (anisotropic turbulence driven by Alfvén waves),
the cooling processes (convection, conduction, and radiation based on
atomic physics), and the forward-modeling of spectroscopic observable
variables (such as the evolution of UV spectral line profiles). We show
that including the feedback of the cooling on the heating processes is
important in such models. The heating is intermittent and sufficient
to heat the loop at temperatures of more than a million degrees, with
realistic values of the amplitude of the forcing (corresponding to
motions of the photospheric footpoints of the loop). We discuss the
importance of small scale heating in the corona and the relevance of
its description by our model.
Title: Plasma Motions in Prominences Observed by Hinode/SOT
Authors: Panasenco, O.; Velli, M.
Bibcode: 2009ASPC..415..196P
Altcode:
We analyze the plasma motions inside prominences observed by Hinode/SOT
during 2006-2007 with focus on the two spectacular examples from
25 April 2007 in Hα line and 30 November 2006 in the CaH line. It
is now well-known that most filaments (prominences on the limb) are
composed of fine threads of similar dimensions. Recent observations of
counter-streaming motions together with oscillations along the threads
provide strong evidence that the threads are field aligned. To more
correctly interpret the nature of observed downward flows of dense and
cool plasma as well as the upward dark flows of less dense plasma,
we take into account the geometry of the prominence structures and
the viewing angle. Basic plasma physical considerations lead one to
conclude that the magnetic field for the SOT observations considered
above must be mainly orthogonal to the plane of the sky.
Title: Weak Magnetohydrodynamic Turbulence and Coronal Heating and
Acceleration (Invited)
Authors: Velli, M. M.; Rappazzo, A. F.
Bibcode: 2009AGUFMSM41C..01V
Altcode:
We will describe the characteristic state of the confined
coronal magnetic field as a special case of magnetically dominated
magnetohydrodynamic turbulence, where the free energy in the transverse
magnetic field is continuously cascaded to small scales, even though
the overall kinetic energy is small. The coronal turbulence problem
is defined by the photospheric boundary conditions, and these play a
role in determining the prevalence of local/vs non-local interactions
in the turbulent cascade. Simulations for various forcing models and
different coronal loop plasma parameters will be presented together
with a phenomenological model describing the anomalous turbulence
scaling laws found.
Title: Kinetics of the Solar Wind Expansion: Wave-Particle
Interactions and Ion Distribution Functions
Authors: Matteini, L.; Landi, S.; Velli, M.; Hellinger, P.
Bibcode: 2009AGUFMSH51C..06M
Altcode:
We investigate the kinetics of ions in the solar wind plasma performing
simulations with a hybrid expanding box model which includes the main
properties of the solar wind expansion and their effects on the ion
distributions. At the same time the model self-consistently takes
into account the non-adiabatic evolution of waves and particles due
to wave-particle and wave-wave interactions. In this work we extend
our previous results and we report 2D simulations which include
also non-parallel propagation. Processes like cyclotron resonances,
instabilities driven by a temperature anisotropy, non-linear evolution
of Alfvén waves and parametric instabilities are found to play a
role on shaping the ion distribution functions, producing non-thermal
properties as velocity beams and anisotropy. The level of Alfvenic
fluctuations and the properties of the initial spectra of waves in
the simulations control the amount of perpendicular heating provided
along expansion and it is regulated in order to match solar wind
observations. Results are then compared with in situ measurements at
various heliocentric distances.
Title: Turbulence in the Sub-Alfvénic Solar Wind Driven by Reflection
of Low-Frequency Alfvén Waves (Invited)
Authors: Verdini, A.; Velli, M. M.; Buchlin, E.
Bibcode: 2009AGUFMSH51C..07V
Altcode:
We study the formation and evolution of a turbulent spectrum
of Alfvén waves driven by reflection off the solar wind density
gradients, starting from the coronal base up to 17 solar radii, well
beyond the Alfvénic critical point, and using a 2D shell model to
describe nonlinear interactions. We find that the turbulent spectra
are influenced by the nature of reflected waves. Close to the base,
these give rise to a flatter and steeper spectrum for the outgoing
and reflected waves respectively. At higher heliocentric distance both
spectra evolve toward an asymptotic Kolmogorov spectrum. The turbulent
dissipation is found to account for at least half of the heating
required to sustain the background imposed solar wind and its shape is
found to be determined by the reflection-determined turbulent heating
below 1.5 solar radii. Therefore reflection and reflection-driven
turbulence are shown to play a key role in the acceleration of the
fast solar wind and origin of the turbulent spectrum found at 0.3 AU
in the heliosphere.
Title: Shell to Shell Energy Fluxes versus Force-free Magnetic Field
Configurations in Coronal Heating Field-lines Tangling Models
Authors: Rappazzo, A. F.; Velli, M.
Bibcode: 2009AGUFMSM43B1769R
Altcode:
Coronal loops are threaded by a strong magnetic field. As they are so
strongly magnetically dominated a first simplification in modeling
their dynamics is to neglect the velocity field. This is in fact
supposed to be very small in comparison with the Alfvén velocity
associated with the DC magnetic field. Setting the velocity equal to
zero u=0 leads to a static force-free solution for the magnetic field
in the MHD equations. The overall dynamics are then supposed to evolve
through a series of equilibria, where each equilibrium configuration
is successively destabilized by magnetic reconnection. We simulate
the Parker problem in the framework of reduced MHD, where a coronal
loop is modeled as an elongated Cartesian box threaded by a strong
magnetic field, whose footpoints are stirred by a velocity mimicking
photospheric motions. We confirm that the velocity and magnetic field
fluctuations induced in the computational box are very small compared
with the strong axial magnetic field, and that velocity fluctuations
are smaller than magnetic fluctuations. Both energy spectra develop
well-defined power-laws. We show that the presence of a small but
finite velocity field allows for transfers of energy among shells in
Fourier space, that would be impossible if u=0 exactly. A staggering
difference with 3-periodic MHD simulations with no average magnetic
field is that energy flows along the velocity field are negligible,
while the cross-field (between shells of u and b) energy flows
contribute equally as the flows among shells of b. Locality and
non-locality will be quantitatively discussed.
Title: Turbulence in the Sub-Alfvénic Solar Wind Driven by Reflection
of Low-Frequency Alfvén Waves
Authors: Verdini, A.; Velli, M.; Buchlin, E.
Bibcode: 2009ApJ...700L..39V
Altcode: 2009arXiv0905.2618V
We study the formation and evolution of a turbulent spectrum of Alfvén
waves driven by reflection off the solar wind density gradients,
starting from the coronal base up to 17 solar radii, well beyond
the Alfvénic critical point. The background solar wind is assigned
and two-dimensional shell models are used to describe nonlinear
interactions. We find that the turbulent spectra are influenced by the
nature of the reflected waves. Close to the base, these give rise to
a flatter and steeper spectrum for the outgoing and reflected waves,
respectively. At higher heliocentric distance both spectra evolve
toward an asymptotic Kolmogorov spectrum. The turbulent dissipation
is found to account for at least half of the heating required to
sustain the background imposed solar wind and its shape is found to be
determined by the reflection-determined turbulent heating below 1.5
solar radii. Therefore, reflection and reflection-driven turbulence
are shown to play a key role in the acceleration of the fast solar wind
and origin of the turbulent spectrum found at 0.3 AU in the heliosphere.
Title: Three-dimensional evolution of magnetic and velocity shear
driven instabilities in a compressible magnetized jet
Authors: Bettarini, Lapo; Landi, Simone; Velli, Marco; Londrillo,
Pasquale
Bibcode: 2009PhPl...16f2302B
Altcode: 2009arXiv0906.5383B
The problem of three-dimensional combined magnetic and velocity shear
driven instabilities of a compressible magnetized jet modeled as a plane
neutral/current double vortex sheet in the framework of the resistive
magnetohydrodynamics is addressed. The resulting dynamics given by
the stream+current sheet interaction is analyzed and the effects of
a variable geometry of the basic fields are considered. Depending
on the basic asymptotic magnetic field configuration, a selection
rule of the linear instability modes can be obtained. Hence, the
system follows a two-stage path developing either through a fully
three-dimensional dynamics with a rapid evolution of kink modes
leading to a final turbulent state, or rather through a driving
two-dimensional instability pattern that develops on parallel planes
on which a reconnection+coalescence process takes place.
Title: Magnetohydrodynamic Simulations of Plumes in the Solar Wind.
Authors: Lionello, Roberto; Velli, M.; Linker, J. A.; Mikić, Z.
Bibcode: 2009SPD....40.1408L
Altcode:
Coronal plumes are raylike structures that are seen extending for many
solar radii above the limb of the Sun in white light observations. They
are associated with localized areas of mixed-polarity photospheric
magnetic field within the predominantly unipolar coronal holes and play
an important role in the origin of the fast solar wind. In the past, the
expansion of plumes into the heliosphere has been studied extensively
with pressure balanced models. We have used our computational MHD code
that includes thermal conduction, radiation losses, and coronal heating
to study self-consistently the dynamics of plumes in the solar wind. To
generate the plumes, we have prescribed heating functions exponentially
decreasing in radius as in Wang (1990). Funding provided by NASA Solar
and Heliospheric Physics Program.
Title: Reflection Driven MHD Turbulence in the Solar Atmosphere
and Wind
Authors: Verdini, A.; Velli, M.; Buchlin, E.
Bibcode: 2009EM&P..104..121V
Altcode: 2008EM&P..tmp...33V
Alfvénic turbulence is usually invoked and used in many solar wind
models (Isenberg and Hollweg, 1982, J. Geophys. Res. 87:5023;
Tu et al. 1984, J. Geophys. Res. 89:9695; Hu et al. 2000,
J. Geophys. Res. 105:5093; Li 2003, Astron. Astrphys. 406:345; Isenberg
2004, J. Geophys. Res. 109:3101) as a process responsible for the
transfer of energy, released at large scale in the photosphere, towards
small scale in the corona, where it is dissipated. Usually an initial
spectrum is prescribed since the closest constraint to the spectrum is
given by Helios measurements at 0.3 AU. With this work we intend to
study the efficiency of the reflection as a driver for the nonlinear
interactions of Alfvén waves, the development of a turbulent spectrum
and its evolution in the highly stratified solar atmosphere inside
coronal holes. Our main finding is that the perpendicular spectral slope
changes substantially at the transition region because of the steep
density gradient. As a result a strong turbulent heating occurs, just
above the transition region, as requested by current solar wind models.
Title: Magnetic and Velocity Shear Driven Instabilities in the
Heliospheric Plasma
Authors: Bettarini, L.; Landi, S.; Velli, M.; Londrillo, P.
Bibcode: 2009EM&P..104..135B
Altcode: 2008EM&P..tmp...45B
We have addressed the problem of combined magnetic and velocity shear
driven instabilities in the context of the heliospheric plasma. New
high-order numerical methods have been used to analyze the instability
dynamics of the heliospheric current-sheet interacting with the
structure determined by the slow component of the solar wind on the
solar equatorial plane above the helmet streamers. Preliminary results
are presented.
Title: POLAR investigation of the Sun—POLARIS
Authors: Appourchaux, T.; Liewer, P.; Watt, M.; Alexander, D.;
Andretta, V.; Auchère, F.; D'Arrigo, P.; Ayon, J.; Corbard, T.;
Fineschi, S.; Finsterle, W.; Floyd, L.; Garbe, G.; Gizon, L.; Hassler,
D.; Harra, L.; Kosovichev, A.; Leibacher, J.; Leipold, M.; Murphy,
N.; Maksimovic, M.; Martinez-Pillet, V.; Matthews, B. S. A.; Mewaldt,
R.; Moses, D.; Newmark, J.; Régnier, S.; Schmutz, W.; Socker, D.;
Spadaro, D.; Stuttard, M.; Trosseille, C.; Ulrich, R.; Velli, M.;
Vourlidas, A.; Wimmer-Schweingruber, C. R.; Zurbuchen, T.
Bibcode: 2009ExA....23.1079A
Altcode: 2008ExA...tmp...40A; 2008arXiv0805.4389A
The POLAR Investigation of the Sun (POLARIS) mission uses a combination
of a gravity assist and solar sail propulsion to place a spacecraft
in a 0.48 AU circular orbit around the Sun with an inclination of 75°
with respect to solar equator. This challenging orbit is made possible
by the challenging development of solar sail propulsion. This first
extended view of the high-latitude regions of the Sun will enable
crucial observations not possible from the ecliptic viewpoint or from
Solar Orbiter. While Solar Orbiter would give the first glimpse of
the high latitude magnetic field and flows to probe the solar dynamo,
it does not have sufficient viewing of the polar regions to achieve
POLARIS’s primary objective: determining the relation between the
magnetism and dynamics of the Sun’s polar regions and the solar cycle.
Title: PHOIBOS: probing heliospheric origins with an inner boundary
observing spacecraft
Authors: Maksimovic, M.; Velli, M.
Bibcode: 2009ExA....23.1057M
Altcode: 2008ExA...tmp...30M
The earth is immersed in a hot, rarefied, energetic flow of particles
and electromagnetic fields originating from the Sun and engulfing the
entire solar system, forming the heliosphere. The existence of the
solar wind has been established for almost 50 years now, and abundant
data has been accumulated concerning both its average properties and
the intermittent, violent energetic manifestations known as Coronal
Mass Ejections which often impact the earth’s magnetosphere (causing
geomagnetic storms and aurorae). The mystery of how the solar corona is
heated and the solar wind is accelerated remains unsolved, however,
because of the large gap in our knowledge of the inner region of
the heliosphere, inside the orbit of mercury. The PHOIBOS mission,
with a perihelion at 4 Rs, by accessing the regions where
energy in the coronal plasma is channeled from internal, magnetic
and turbulent energy into bulk energy of the solar wind flow aims to
solve the question of why the Sun has a hot corona and produces a solar
wind. The PHOIBOS mission builds on previous Solar Probe studies, but
provides an alternative orbit scenario avoiding a Jupiter encounter in
favor of multiple Venus encounters and SEP systems to work its way close
to the Sun in a gradual manner, providing a much vaster data return.
Title: A MHD-turbulence model for solar corona
Authors: Romeou, Z.; Velli, M.; Einaudi, G.
Bibcode: 2009AdSpR..43..612R
Altcode:
The disposition of energy in the solar corona has always been
a problem of great interest. It remains an open question how the
low temperature photosphere supports the occurence of solar extreme
phenomena. In this work, a turbulent heating mechanism for the solar
corona through the framework of reduced magnetohydrodynamics (RMHD)
is proposed. Two-dimensional incompressible long time simulations of
the average energy disposition have been carried out with the aim to
reveal the characteristics of the long time statistical behavior of a
two-dimensional cross-section of a coronal loop and the importance of
the photospheric time scales in the understanding of the underlying
mechanisms. It was found that for a slow, shear type photospheric
driving the magnetic field in the loop self-organizes at large scales
via an inverse MHD cascade. The system undergoes three distinct
evolutionary phases. The initial forcing conditions are quickly
“forgotten” giving way to an inverse cascade accompanied with
and ending up to electric current dissipation. Scaling laws are being
proposed in order to quantify the nonlinearity of the system response
which seems to become more impulsive for decreasing resistivity. It is
also shown that few, if any, qualitative changes in the above results
occur by increasing spatial resolution.
Title: Tearing and velocity shear driven instabilities in the
heliospheric plasmas: three-dimensional simulations.
Authors: Landi, S.; Velli, M.
Bibcode: 2009MSAIS..13...39L
Altcode:
We have studied magnetic reconnection and shear flow instabilities
using a new-compact algorithm recently designed, which combines
high-order techniques both in space and time, to follow magnetic
reconnection developing in current-sheets, together with shock-capturing
capabilities, able to handle field discontinuities often developing
in a compressible plasma. In particular we have followed the
three-dimensional non-linear evolution of the tearing instability and
its transition towards a turbulent state, and we have also investigated
the acceleration properties of a sheared flow where a current sheet
is embedded, a model for the acceleration of the slow solar wind above
helmet streamers.
Title: The ADAHELI (ADvanced Astronomy for HELIophysics) solar mission
Authors: Berrilli, F.; Velli, M.; Roselli, L.; Bigazzi, A.; ADAHELI
Team
Bibcode: 2009MmSAI..80..251B
Altcode:
The ADAHELI (ADvanced Astronomy for HELIOphysics) small mission
is an Italian project for the investigation of solar activity and
dynamics proposed in the framework of ASI Italian Space Agency Small
Missions Program call. It is devoted to the study of photospheric and
chromospheric structure and dynamics. The scientific payload consists of
ISODY, a telescope with a focal plane suite operating in the visible
and near infrared, and MIOS, a 90GHz radiometer. The telescope is
equipped with a spectro-polarimetric imager, based on Fabry-Perot
interferometers, a broad band imager, and an image stabilization
system. The 90GHz radiometer is a full-disk instrument exploiting an
antenna with an angular aperture of about ±25 arcmin. Proposals for
optional instruments are under evaluation: DIMMI-2h, a double channel
magneto-optical filter based full disk imager, EISR a dual channel
spectrometer operating in the EUV, and NPA, an in-situ Neutral Particle
Analyzer to detect ENA. We shortly present the scientific objectives
of the mission, the scientific payload packages and a possible mission
orbit. Possible dates for the launch are 2012 and 2014, both during
the declining phase of solar activity.
Title: Solar MHD: An Introduction
Authors: Chiuderi, C.; Velli, M.
Bibcode: 2009LNP...778...45C
Altcode:
The Universe is filled by plasmas. In fact, it can be reasonably
estimated that more than 95% of (standard) cosmic matter is found in the
plasma state. Given this basic fact, it is apparent that plasma physics
is the basic tool to understand the mechanisms that are at work in
the astrophysical context and to interpret the observations. The Earth
represents a notable exception as far as the presence of natural plasmas
is concerned, a lucky circumstance for living beings. In practice,
terrestrial plasmas are almost exclusively produced during electrical
discharges, such as lightning. If plasmas are almost absent on Earth
and in the low-altitude atmosphere, they start to be the dominant
state of matter immediately beyond the ionosphere, the magnetosphere,
and the whole heliosphere that includes the entire solar system made
up of plasmas. The Sun, like the other stars, is made up of ionized
gas almost everywhere.
Title: Proton Cyclotron Heating and Beam Generation in the Solar Wind
Authors: Matteini, L.; Landi, S.; Velli, M.; Hellinger, P.
Bibcode: 2008AGUFMSH43A1649M
Altcode:
We present results from hybrid expanding simulations of the solar
wind plasma. We investigate the role of kinetic processes in shaping
the proton distribution function along the wind expansion in the
prensence of an initial spectrum of Alfvén waves. We find that
both wave-particle and wave-wave interactions play a role in the
ion evolution, in particular waves interact with protons through
ion-cyclotron resonace and non-linear trapping due to the growth of
parametric instabilities. Cyclotron interactions control the evolution
of the temperature anisotropy providing a perpendicular heating which
contrasts the adiabatic cooling caused by the expansion. Ion-acoustic
modes driven by parametric effects produce a velocity beam in the
particle distribution function. We discuss and compare our results
with direct solar wind observations between 0.3 and 1 AU, and we find
that the resulting proton distribution functions are in reasonable
agreement with Helios data.
Title: Magnetic Reconnection in the Solar Streamer Belt as a Source
of the Slow Solar Wind
Authors: Rappazzo, A. F.; Velli, M.; Liewer, P.; Lionello, R.
Bibcode: 2008AGUFMSH51B1601R
Altcode:
The slow component of the solar wind is conjectured to originate in
and around the solar streamer belt. The region beyond the cusp of
an helmet streamer is characterized by the presence of a current
sheet embedded in a plasma flow, and plasma density enhancements
accelerating radially outward have been observed by the Large-Angle
Spectrometric Coronagraph (LASCO) instrument on board the Solar and
Heliospheric Observatory (SOHO). In the present work we investigate
the stability of such configuration. Due to the coupling with the
Kelvin-Helmholtz instability, magnetic reconnection can in fact
give rise to the formation of density enhanced magnetic islands that
accelerate outward. We have previously investigated this scenario in
2D cartesian simulations. In the present work we explore the behavior
of such a system in spherical geometry. Global 3D MHD simulations have
found that the velocity, at a fixed radius, grows from a slow value at
the current sheet towards higher values towards the polar regions. The
steady-state which was reached showed the bimodal characteristic of
the solar wind, but the slow component did not show its characteristic
variability. We present 2D numerical simulations, performed with the
SAIC MHD spherical code (MAS), of the region beyond the cusp of an
helmet streamer from 1 R⊙ up to 20 R⊙.
Title: Turbulence and reconnection in coronal heating field line
tangling models.
Authors: Velli, M.; Rappazzo, F.; Dahlburg, R.; Einaudi, G.
Bibcode: 2008AGUFMSH43A1637V
Altcode:
In previous work, we studied the Parker field line tangling problem
for coronal heating comprehensively via longtime high-resolution
simulations of the dynamics of a coronal loop in cartesian geometry
within the framework of reduced magnetohydrodynamics (RMHD). Although
the efficient turbulent cascade prevents the magnetic field lines
from becoming strongly entangled, current sheets are continuously
formed and dissipated. Current sheets are the result of the nonlinear
cascade that transfer energy from the scale of convective motions
down to the dissipative scales, where it is finally converted to
heat and/or particle acceleration. A picture is then realized, where
both slightly entangled magnetic field lines and current sheets are
present. Here we consider simpler forcing models and higher resolution
simulations and substantiate previous scalings for coronal heating,
while also discussing critical angles, secondary instabilities and
double inertial ranges.
Title: Linear and non linear tearing and Kelvin-Helmholtz
driven instabilities in current-sheets with velocity shears:
three-dimensional compressive MHD simulations.
Authors: Landi, S.; Bettarini, L.; Velli, M.
Bibcode: 2008AGUFMSH42A..07L
Altcode:
Magnetic shear driven instabilities play a major role both in the
dynamics of astrophysical objects and, in particular, in the evolution
of several structures in the heliosphere. Although tearing-driven
dynamics in two dimensions are relatively well understood, in three
dimensions the overall dynamics can be highly complex due to the
onset of secondary instabilities. The presence of sheared flows,
and the resulting stream plus current-sheet interaction, adds to
this complexity. Considering two different perturbed equilibrium
configurations of a current-sheet, a pressure-balanced and a force-free
configuration, we present the three dimensional evolution of a
tearing instability driven current- sheet in the presence of velocity
shears, in the framework of compressible and resistive MHD. The large
scale structure of the initial configuration determines the linear
and non linear evolution of the system: primary (resistive and/or
Kelvin-Helmholtz like) modes are selected according to the geometry of
the magnetic field, secondary instability development depends on the
initial equilibrium configuration with the strongest modes characterized
by a specific direction in Fourier space. The competition between
primary and secondary modes determines the global plasma structure in
the non linear regime and, in all cases, the magnetic energy spectrum
is observed to be highly anisotropic.
Title: Fine-Structured Plasma Flows in Prominences
Authors: Panasenco, O.; Velli, M.; Landi, S.
Bibcode: 2008AGUFMSH41A1613P
Altcode:
Plasmas in prominences (filaments against the disk) exhibit a very
wide spectrum of different kind of motions. Here we analyze the plasma
motions inside prominences observed by Hinode/SOT during 2006-2007 with
focus on two spectacular examples from 25 April 2007 in Halpha line
and 30 November 2006 in CaH line and then carry out some simulations
of the possible dynamics. Most filaments are composed of fine threads
of similar dimensions rooted in the chromosphere/photosphere. Recent
observations of counter-streaming motions together with oscillations
along the threads provide strong evidence that the threads are field
aligned. To more correctly interpret the nature of observed downward
flows of dense and cool plasma as well as the upward dark flows of
less dense plasma, we take into account the geometry of the prominence
structures and the viewing angle. The dark upflows exhibit turbulent
patterns such as vortex formation and shedding that are consistent
with the motions predicted by instabilities of the interchange
type. Sometimes an appearance of dark motions is generated by dark
voids opened in the prominence sheet after initiation of nearby
downflow streams, implying mass drainage in the downflows. Based on
304 A observations, there is more filament mass in prominences than is
visible in either the Halpha or CaH lines. The source of the downward
moving plasma may be located either higher above the visible upper edge
of the prominence or on the far end of the prominence spine. The bright
downward motions of the more cool and dense plasma may be partly due
to the counter-streaming motion along the magnetic fields lines and
also to the presence of Rayleigh-Taylor type or ballooning/interchange
instabilities in the upper regions of the prominence. Transverse motions
of filament threads caused by magnetic instabilities constantly provide
the conditions for reconnection in the low part of the corona and the
chromosphere. We suggest that the combination of flows along field
lines, shear, and unstable stratification may provide the answers to
the intriguingly elegant motions seen in prominences.
Title: Turbulence in anisotropic heliospheric plasmas
Authors: Buchlin, E.; Verdini, A.; Cargill, P. J.; Velli, M.
Bibcode: 2008sf2a.conf..547B
Altcode:
An alternative approach to Direct Numerical Simulations (DNS)
of Magnetohydrodynamics (MHD) is presented, providing insight
into the statistical properties of highly-turbulent, intermittent,
anisotropic MHD turbulence: a set of shell-models coupled by Alfvén
waves travelling along the axial magnetic field and which interact
non-linearly, producing perpendicular fluctuations of the fields
at small scales. This model can be applied to different physical
situations; we present the cases of heating in solar coronal loops,
and of turbulence in open coronal regions at the base of the solar wind.
Title: Stationary spherically symmetric supersonic winds and
accretion: from Parker to Bondi and back
Authors: Velli, Marco
Bibcode: 2008APS..DPPNM7003V
Altcode:
Although we have known the solar wind is supersonic for almost 50 years
now, it is little known that the structure of the stationary spherically
symmetric solar wind solutions found by Parker is fundamentally
connected to the Bondi solutions for spherically symmetric accretion. In
this talk I will describe how, for the simpler case of isothermal
flows, changes in the relative pressure jump between the coronal
base and distant medium produce changes in the resulting stationary
flow. The pressure jump between coronal base and interstellar medium
(ISM) functions as a control parameter in terms of which stationary
flows display a hysteresis-type cycle with two catastrophy points:
as the pressure of the ISM increases, the termination shock moves
closer towards the coronal base, but when the shock position reaches
the critical point, the flow collapses into supersonic accretion with
a shock below the critical point. If the pressure of the ISM then
decreases again, the flow can evolve continuously into subsonic breeze
accretion, but the flow evolves back into a state characterized by a
supersonic shocked wind, once the pressure difference corresponding to
a static coronal stratification is exceeded. Numerical simulations are
shown which confirm this scenario and illustrate the important role
boundary conditions play in fluid flows around astrophysical objects.
Title: The ADAHELI Solar Mission
Authors: Berrilli, F.; Velli, M.; Roselli, L.; Bigazzi, A.; Moretti,
P. F.; Romoli, M.; Orsini, S.; Cavallini, F.; Greco, V.; Carbone,
V.; Consolini, G.; Di Mauro, M. P.; Ermolli, I.; Pietropaolo, E.;
Romano, P.; Ventura, P.; White, S. M.; Zuccarello, F.; Cauzzi, G.;
Valdettaro, L.
Bibcode: 2008ESPM...12..6.6B
Altcode:
ADAHELI (Advanced Astronomy for HELIOphysics) is an Italian Space
project for the investigation of solar photospheric and chromospheric
dynamics, via high-resolution spectro-polarimetric observations in the
near-infrared spectral range. The mission has been financed for phase
A study in the framework of ASI Italian Space Agency Small Missions
Program call of September 2007. Four fields have been selected
to highlight the specific benefits of ADAHELI scientific payload: 1)
Photospheric and chromospheric dynamics and structure, 2) Emergence and
evolution of solar active regions and solar irradiance, 3) Chromospheric
and corona heating and turbulence, 4) Solar flares in the millimeter
wavelength region. The principal science instrument, ISODY, is
a 50 cm solar telescope equipped with an innovative Focal Plane Suite
composed of a spectro-polarimetric imager, based upon two Fabry-Perot
interferometers operating in the NIR regions around 845nm and 1083nm, a
broad band imager, and a correlation tracker used as image stabilization
system. Designed Mission Profiles for ADAHELI intend to achieve
continuous high-spectral and spatial resolution observations of the
Sun for a routine duration of 4 hours with a goal to be extended to
24 hours. ADAHELI also carries MIOS, a millimeter wavelengths
radiometer operating at around 90 GHz for flare detection. The
ADAHELI payload's instrument suite integrates and complements, without
overlap, the present major objectives of ESA, NASA and the International
Living with a Star program, in particular Solar Dynamics Observatory,
PICARD, Solar Orbiter, and the Solar Probe missions. Proposals
for optional instruments are also under evaluation: DIMMI-2h, a
double channel MOF based full disk imager operating at 589nm and
770nm, allowing high temporal resolution velocity and magnetic field
measurements; EISR a two channel spectrometer operating in the 50-130
nm wavelength range, and NPA, an in-situ Neutral Particle Analyzer
to detect Energetic Neutral Atoms (ENA). Science objectives related
to optional instruments include: solar high and low-degree p modes
oscillations, EUV solar structures and variability, solar gravitational
red-shift measurement, detection of ENA originating from the plasma
region in the Earth's magnetosphere and undergoing reflection from
the Earth's atmosphere.
Title: Spectroscopic Hinode Observables from Turbulent Heating and
Cooling of Coronal Loops
Authors: Buchlin, É.; Cargill, P. J.; Bradshaw, S. J.; Velli, M.
Bibcode: 2008ASPC..397...83B
Altcode:
We present a model of coronal loop turbulence allowing the fast
computation of heating in a loop at high Reynolds numbers. We
also consider the coupling of both heating and cooling processes
in loops, including for the first time a feedback of the cooling
on the heating: the heating computed by the shell-models used as an
input of a hydrodynamic model of a loop with thermal conduction and a
self-consistent treatment of radiation. We forward-model spectroscopic
variables that can be compared to Hinode observations.
Title: Alfvénic Turbulence and the Acceleration of the Fast
Solar Wind
Authors: Verdini, A.; Velli, M.; Buchlin, E.
Bibcode: 2008ESPM...12.3.69V
Altcode:
Alfvenic turbulence is usually invoked and used in many solar wind
models (Isenberg & Hollweg 1982, Tu et al. 1984, Hu et al. 2000,
Li 2003, Isenberg 2004) as a process responsible for the transfer of
energy released at large scales in the photosphere towards small scales
in the corona, where it is dissipated. Usually an initial spectrum is
prescribed since its closest constraint is given by Helios measurements
at 0.3 AU. With this work we intend to study the efficiency of the
reflection as a driver for the nonlinear interactions of Alfven waves,
the eventual development of a turbulent spectrum and its evolution in
the highly stratified solar atmosphere inside coronal holes. We
start imposing an upcoming flux of Alfven waves in a limited range
of perpendicular wave numbers, at the base of the corona. Open
boundary conditions allow the reflected waves to leave the domain form
below and to be advected by the solar wind outside the top boundary. The
nonlinear interaction in planes perpendicular to that of propagation
(assumed to be radial) are treated with a 2D shell model, so that large
Reynolds numbers are reached. Continuous interactions of counter
propagating waves form a turbulent spectrum in the low corona, before
the sonic point, in very short timescales (compared to the propagation
timescales). Both the location and the value of the maximum of
the dissipation (per unit mass) scale with the rms amplitude of the
velocity fluctuations at the coronal base (delta u), while they are
less sensitive to the frequency of the input flux of Alfven waves,
provided it is small enough to power the turbulent cascade by means of
reflection. For values of delta u in agreement with observational
constraints, the turbulent dissipation achieves levels capable of
sustaining a fast solar wind, with the maximum dissipation located at 2
solar radii, just below the sonic point. Despite the back reaction
of the solar wind is not taken into account, this model shows that,
under reasonable assumptions, a turbulent spectrum forms in the corona
and it is able to sustain the heating and acceleration of the fast
solar wind. Finally, the scaling laws obtained with this simplified 2D
turbulence can be further constrained in order to include this mechanism
of reflection driven turbulence in more complex solar wind models.
Title: Nonlinear Dynamics of the Parker Scenario for Coronal Heating
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.; Dahlburg, R. B.
Bibcode: 2008ApJ...677.1348R
Altcode: 2007arXiv0709.3687R
The Parker or field line tangling model of coronal heating is studied
comprehensively via long-time high-resolution simulations of the
dynamics of a coronal loop in Cartesian geometry within the framework
of reduced magnetohydrodynamics. Slow photospheric motions induce
a Poynting flux which saturates by driving an anisotropic turbulent
cascade dominated by magnetic energy. In physical space this corresponds
to a magnetic topology where magnetic field lines are barely entangled;
nevertheless, current sheets (corresponding to the original tangential
discontinuities hypothesized by Parker) are continuously formed and
dissipated. Current sheets are the result of the nonlinear cascade
that transfers energy from the scale of convective motions (~1000
km) down to the dissipative scales, where it is finally converted
to heat and/or particle acceleration. Current sheets constitute the
dissipative structure of the system, and the associated magnetic
reconnection gives rise to impulsive "bursty" heating events at
the small scales. This picture is consistent with the slender loops
observed by state-of-the-art (E)UV and X-ray imagers which, although
apparently quiescent, shine brightly in these wavelengths with little
evidence of entangled features. The different regimes of weak and
strong magnetohydrodynamic turbulence that develop and their influence
on coronal heating scalings are shown to depend on the loop parameters,
and this dependence is quantitatively characterized: weak turbulence
regimes and steeper spectra occur in stronger loop fields and lead to
larger heating rates than in weak field regions.
Title: The Parker Scenario for Coronal Heating as an MHD Turbulence
Problem
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.
Bibcode: 2008ASPC..383..353R
Altcode: 2010arXiv1002.2631R
The Parker or field line tangling model of coronal heating is
investigated through long-time high-resolution simulations of the
dynamics of a coronal loop in cartesian geometry within the framework of
reduced magnetohydrodynamics (RMHD). Slow photospheric motions induce
a Poynting flux which saturates by driving an anisotropic turbulent
cascade dominated by magnetic energy and characterized by current sheets
elongated along the axial magnetic field. Increasing the value of the
axial magnetic field different regimes of MHD turbulence develop with
a bearing on coronal heating rates. In physical space magnetic field
lines at the scale of convection cells appear only slightly bended
in agreement with observations of large loops of current (E)UV and
X-ray imagers.
Title: Vertical plasma motions in prominence sheets observed by Hinode
Authors: Panasenco, Olga; Velli, Marco; Berger, Thomas
Bibcode: 2008cosp...37.2337P
Altcode: 2008cosp.meet.2337P
We analyze the approximately vertical motions inside prominence plasma
observed by Hinode on 25 April 2007 in Hα line and 30 November 2006 in
CaH line. Well-established observational facts are that all filaments
(prominences on the limb) are composed of fine threads of similar
dimensions, rooted in the photosphere and presumably tracing magnetic
field lines, and that continuous counter-streaming motions occur
along threads. We take into account the geometry of the prominence
sheet and the viewing angle to reduce possible projection effect and
more correctly interpret the nature of observational downward flows
of denser and cooler plasma as well as the upward flow of hotter
plasma which appears dark in the Hα and CaH spectral lines. The dark
upflows exhibit turbulent flow properties such as vortex formation and
shedding that are consistent with the properties of thermal starting
plumes. Sometimes an illusion of dark upward motion is generated by
rarefactions in the plasma sheet caused by the cooler denser downward
flows. On both dates, we suspect there is probably more filament mass
in the prominence that is visible in either the Hα or CaH lines. The
source of the downward moving plasma may be located either higher
above the visible upper edge of the prominence or on the far end of
the prominence spine. The bright downward motions of the more cool
and dense plasma may be partly due to the counter-streaming motion
along the magnetic fields lines, or it may be due to the presence of
rayleigh-taylor type or ballooning/interchange instabilities in the
upper regions of the prominence, which are then stabilized lower down
where the magnetic field is stronger and the plasma beta lower.
Title: Competing nonlinear mechanisms in the dynamics of current
sheet-stream interactions in the solar environment: 3D fluid and
kinetic simulations
Authors: Bettarini, Lapo; Landi, Simone; Lapenta, Giovanni, , Prof;
Londrillo, Pasquale; Velli, Marco
Bibcode: 2008cosp...37..270B
Altcode: 2008cosp.meet..270B
A proper insight of complex dynamics observed for several structures in
the Solar atmosphere as well as in the whole Heliospheric environment
can not disregard a detailed analysis of combined magnetic and
velocity shear driven instabilities. Such phenomena evolve according
to a wide range of lengthand time-scales so that both the fluid and
the kinetic modeling approach are required. We address the problem of
combined magnetic and velocity shear driven 3D instabilities of a plane
current-vortex sheet in a compressible situation and comparisons are
made between 3D high-order MHD and kinetic simulations performed to
follow the system throughout its linear and nonlinear regime. Several
ingredients turn out to be critical on the competition between a
two-dimension behavior and a fully three-dimension evolving plasma
system, such as the dependence on the background field geometry and
the interaction among the linearly-increasing modes that afterwards
determine either a well-developed single or a multiple stage
nonlinear regime. These are fundamental factors that can be invoked,
for instance, in triggering mechanisms of solar explosive phenomena
at the Chromospheric level, for a proper description of magnetic
reconnection processes both in the low atmosphere as well as in the
creation, acceleration and evolution of structures like the slow solar
wind, polar plumes and several other sheared jet-like structures in
the Heliosphere.
Title: ADAHELI: Investigating the structure of Sun's lower atmosphere
and solar irradiance
Authors: Bigazzi, Alberto; Velli, Marco; Berrilli, Francesco; Egidi,
Alberto; Alimenti, Federico; Roselli, Luca
Bibcode: 2008cosp...37..291B
Altcode: 2008cosp.meet..291B
A new solar Mission ADAHELI (ADvanced Astronomy for HELIophysics)
has just been approved for Phase A feasibility study by the Italian
Space Agency (ASI), on its call for two new "Small Missions". ADAHELI
will study the dynamics and structure of the solar photosphere and
chromosphere, in the Near Infrared (NIR) and address solar variability
issues in the NIR. A Millimiter Waves (mmW) interferometer will monitor
irradiance in selected bands in the range 60-120 GHz. The possibility
of an additional UV payload is also being addressed. ADAHELI's
Sun-synchronous orbit shall guarantee continuous observation of the
Sun during three years of operations, planned to start by 2011-2012,
possibly the peak of Solar Cycle XXIV.
Title: The evolution of the solar wind proton temperature anisotropy
from 0.3 to 2.5 AU
Authors: Matteini, L.; Hellinger, P.; Landi, S.; Pantellini, F.;
Maksimovic, M.; Velli, M.; Goldstein, B. E.; Marsch, E.
Bibcode: 2007AGUFMSH21A0287M
Altcode:
We report an analysis of the proton temperature anisotropy evolution
from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With
increasing distance, the fast wind data show a path in the parameter
space (βallel p,T\perp p/Tallel p), and the first part of the
trajectory is well described by an anticorrelation between the
temperature anisotropy T\perp p/Tallel p and the proton parallel
beta, while after 1 AU the evolution with distance in the parameter
space changes and the data result in agreement with the constraints
derived by a fire hose instability. The slow wind data show a more
irregular behavior, and in general it is not possible to recover a
single evolution path. However, on small temporal scale we find that
different slow streams populate different regions of the parameter
space, and this suggests that when considering single streams also
the slow wind follows some possible evolution path.
Title: Slow Solar Wind Formation Beyond the Cusp of an Helmet Streamer
Authors: Rappazzo, A. F.; Velli, M.; Liewer, P.; Lionello, R.; Mikic,
Z.; Einaudi, G.; Dahlburg, R.
Bibcode: 2007AGUFMSH21A0290R
Altcode:
The region beyond the cusp of an helmet streamer is characterized by
the presence of a current sheet embedded in a plasma flow. In previous
3D and 2D simulations the velocity has been found, at a fixed radius,
to grow from a slow value at the current sheet towards higher values
towards the polar regions. The steady-state which was reached showed
the bimodal characteristic of the solar wind, but the slow component
did not show its characteristic variability. The velocity profile of
the slow component is in fact not steady in time, and plasma density
enhancements have been observed by the Large-Angle Spectrometric
Coronagraph (LASCO) instrument on board the Solar and Heliospheric
Observatory (SOHO). We present numerical simulations, performed with
the SAIC MHD spherical code (MAS), of the region beyond the cusp of
an helmet streamer from 1 R\odot up to 20 R\odot. With a sufficiently
high resolution magnetic reconnection of the heliospheric current sheet
is observed, which leads to the formation of density-enhanced magnetic
islands that are accelerated radially outward. The reconnection process
also gives rise to an acceleration profile that is not steady in time.
Title: Nonlinear Dynamics of the Parker Scenario for Coronal Heating
Authors: Einaudi, G.; Rappazzo, A. F.; Velli, M.; Dahlburg, R.
Bibcode: 2007AGUFMSH51C..07E
Altcode:
The Parker field line tangling problem for coronal heating is
studied comprehensively via longtime high-resolution simulations
of the dynamics of a coronal loop in cartesian geometry within the
framework of reduced magnetohydrodynamics (RMHD). Slow photospheric
motions induce a Poynting flux that injects energy in the loop at the
scales of convective motions (~ 1,000 km). During the linear stage
the magnetic field and the currents grow linearly in time, until they
saturate by driving an anisotropic nonlinear turbulent cascade. Although
the efficient turbulent cascade prevents the magnetic field lines from
becoming strongly entangled, current sheets are continuously formed
and dissipated. We show that the current sheets are the result of the
nonlinear cascade that transfer energy from the scale of convective
motions down to the dissipative scales, where it is finally converted to
heat and/or particle acceleration. A picture is then realized, where
both slightly entangled magnetic field lines and current sheets are
present. Current sheets are the dissipative structure for this system,
and the associated magnetic reconnection gives rise to impulsive
"bursty" heating events. This picture is consistent with the slender
loops observed recently by HINODE which, although apparently quiescent,
present an X-ray emission and at the resolution scale (~ 800 km) do
not seem to reveal entangled features. We also show how the different
regimes of MHD turbulence in the system influence the scaling laws
for the small-scale energy deposition.
Title: The PHOIBOS Mission : Probing Heliospheric Origins with an
Inner Boundary Observing Spacecraft
Authors: Maksimovic, M.; Velli, M.
Bibcode: 2007AGUFMSH21A0281M
Altcode:
Fifty years after the Sputnik launch and the beginning of the Space
Physics era the time has come for the in-situ exploration of one of
the last frontiers in the solar system - the solar corona and inner
heliosphere. We present the PHOIBOS (Probing Heliospheric Origins
with an Inner Boundary Observing Spacecraft) concept, which has been
submitted to the ESA Cosmic Vision program. PHOIBOS is a mission of
exploration and discovery designed to make comprehensive measurements
in the never-observed region of the heliosphere from 0.3 AU to as
close as 3 solar radii from the Sun's surface. The primary scientific
goal of PHOIBOS will be to determine the structure and dynamics of
plasmas and magnetic fields in the outer solar atmosphere which give
rise to the corona, the solar wind and the heliosphere. The two main
characteristics of the PHOIBOS mission are : (i) use of retractable
solar panels that avoid the use of RTGs (Radioisotope Thermoelectric
Generators) and (ii) insertion in the final operational orbit (4 Rs
to 3.7 AU with an inclination up to about 60 deg.) by using electric
propulsion and Earth and Venus flybys. With these characteristics,
the PHOIBOS concepts fit very well within the new NASA "Solar Probe
Lite" study.
Title: Alfven Profile in the Lower Corona: Implications for Shock
Formation
Authors: Evans, R. M.; Opher, M.; Manchester, W. B.; Velli, M.;
Gombosi, T. I.
Bibcode: 2007AGUFMSH21A0286E
Altcode:
Recent events (e.g. Tylka et al. 2005) indicate that CME-driven shocks
can form at 1-3 solar radii and are responsible for the GeV/nucleon
energies observed in some ground level solar energetic particle
events. The formation of shocks depends crucially on the background
solar wind environment, in particular on the profile of the background
Alfvén speed in the corona. Significant strides have been made in
the effort to develop realistic models of CME events; however, there
is no consensus as to the profile of the Alfvén speed in the lower
corona. Here we provide an overview of ten state-of-the-art models,
which includes various methods to model magnetic field and density,
as well as different strategies for accelerating the solar wind. We
present the Alfvén speed profile for each model in the lower corona. We
find that the "valley" and "hump" structures anticipated by Mann et
al. (2003) are sometimes present, but in some models the Alfvén
profiles drop off quickly. We discuss the implications of these
profiles, such as whether it will allow a shock to form, dissipate,
and form again (i.e. multiple shocks). Our study indicates that it is
crucial to establish the Alfvén speed as a function of height before
determining if shocks can form in the lower corona.
Title: Solar Wind Acceleration and Heating
Authors: Velli, M.
Bibcode: 2007AGUFMSH22B..01V
Altcode:
The heating of the solar corona and acceleration of the solar
wind represent one of the fundamental problems in all of space
science. While different scenarios have been proposed to explain
the heating of magnetically confined and open regions of the corona,
they all rely on the transfer, storage and dissipation of the abundant
free energy present in photospheric convection. In this talk I will
focus specifically on models involving Alfvén waves which lead to the
solar wind as observed at 1 AU, discussing the difficulties in deriving
consistently both wave- spectrum evolution and solar wind distribution
functions. Using the example of reflection driven Alfvénic turbulence
in coronal holes and the solar wind, I will describe which kinds of
measurements are most likely to help advance our understanding of this
crucial problem.
Title: Evolution of the solar wind proton temperature anisotropy
from 0.3 to 2.5 AU
Authors: Matteini, Lorenzo; Landi, Simone; Hellinger, Petr; Pantellini,
Filippo; Maksimovic, Milan; Velli, Marco; Goldstein, Bruce E.;
Marsch, Eckart
Bibcode: 2007GeoRL..3420105M
Altcode:
We report an analysis of the proton temperature anisotropy evolution
from 0.3 to 2.5 AU based on the Helios and Ulysses observations. With
increasing distance the fast wind data show a path in the parameter
space (β $\parallel$ p , T $\perp$ p /T
$\parallel$ p ). The first part of the trajectory is well
described by an anticorrelation between the temperature anisotropy T
$\perp$ p /T $\parallel$ p and the proton
parallel beta, while after 1 AU the evolution with distance in the
parameter space changes and the data result in agreement with the
constraints derived by a fire hose instability. The slow wind data
show a more irregular behavior, and in general it is not possible to
recover a single evolution path. However, on small temporal scale
we find that different slow streams populate different regions of
the parameter space, and this suggests that when considering single
streams also the slow wind follows some possible evolution path.
Title: Heating of coronal loops: weak MHD turbulence and scaling laws
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.
Bibcode: 2007AIPC..932..342R
Altcode: 2010arXiv1002.2635R
To understand the nonlinear dynamics of the Parker scenario for coronal
heating, longtime high-resolution simulations of the dynamics of a
coronal loop in cartesian geometry are carried out. A loop is modeled
as a box extended along the direction of the strong magnetic field B0
in which the system is embedded. At the top and bottom plates, which
represent the photosphere, velocity fields mimicking photospheric
motions are imposed. We show that the nonlinear dynamics is
described by different regimes of MHD anisotropic turbulence, with
spectra characterized by intertial range power laws whose indexes range
from Kolmogorov-like values (~ 5/3) up to ~ 3. We briefly describe
the bearing for coronal heating rates.
Title: Profiles of heating in turbulent coronal magnetic loops
Authors: Buchlin, E.; Cargill, P. J.; Bradshaw, S. J.; Velli, M.
Bibcode: 2007A&A...469..347B
Altcode: 2007astro.ph..2748B
Context: The location of coronal heating in magnetic loops has been
the subject of a long-lasting controversy: does it occur mostly at the
loop footpoints, at the top, is it random, or is the average profile
uniform?
Aims: We try to address this question in model loops
with MHD turbulence and a profile of density and/or magnetic field
along the loop.
Methods: We use the Shell-Atm MHD turbulent
heating model described in Buchlin & Velli (2007, ApJ, 662, 701),
with a static mass density stratification obtained by the HydRad model
(Bradshaw & Mason 2003, A&A, 401, 699). This assumes the absence
of any flow or heat conduction subsequent to the dynamic heating.
Results: The average profile of heating is quasi-uniform, unless there
is an expansion of the flux tube (non-uniform axial magnetic field)
or the variation of the kinetic and magnetic diffusion coefficients
with temperature is taken into account: in the first case the heating
is enhanced at footpoints, whereas in the second case it is enhanced
where the dominant diffusion coefficient is enhanced.
Conclusions:
These simulations shed light on the consequences on heating profiles
of the complex interactions between physical effects involved in a
non-uniform turbulent coronal loop.
Title: Alfvén Waves and Turbulence in the Solar Atmosphere and
Solar Wind
Authors: Verdini, Andrea; Velli, Marco
Bibcode: 2007ApJ...662..669V
Altcode: 2007astro.ph..2205V
We solve the problem of propagation and dissipation of Alfvénic
turbulence in a model solar atmosphere consisting of a static
photosphere and chromosphere, transition region, and open corona and
solar wind using a phenomenological model for the turbulent dissipation
based on wave reflection. We show that most of the dissipation for
a given wave-frequency spectrum occurs in the lower corona, and the
overall rms amplitude of the fluctuations evolves in a way consistent
with observations. The frequency spectrum for a Kolmogorov-like slope
is not found to change dramatically from the photosphere to the solar
wind; however, it does preserve signatures of transmission throughout
the lower atmospheric layers, namely, oscillations in the spectrum at
high frequencies reminiscent of the resonances found in the linear
case. These may disappear once more realistic couplings for the
nonlinear terms are introduced or if time-dependent variability of
the lower atmospheric layer is introduced.
Title: Shell Models of RMHD Turbulence and the Heating of Solar
Coronal Loops
Authors: Buchlin, E.; Velli, M.
Bibcode: 2007ApJ...662..701B
Altcode: 2006astro.ph..6610B
A simplified nonlinear numerical model for the development
of incompressible magnetohydrodynamics in the presence of a
strong magnetic field B∥ and stratification, nicknamed
``Shell-Atm,'' is presented. In planes orthogonal to the mean field,
the nonlinear incompressible dynamics is replaced by two-dimensional
shell models for the complex variables u and b, allowing one to reach
large Reynolds numbers while at the same time carrying out sufficiently
long integrations to obtain good statistics at moderate computational
cost. The shell models of different planes are coupled by Alfvén waves
propagating along B∥. The model may be applied to open or
closed magnetic field configurations where the axial field dominates and
the plasma pressure is low; here we apply it to the specific case of a
magnetic loop of the solar corona heated by means of turbulence driven
by photospheric motions, and we use statistics for its analysis. The
Alfvén waves interact nonlinearly and form turbulent spectra in the
directions perpendicular and, through propagation, also parallel
to the mean field. A heating function is obtained and shown to be
intermittent; the average heating is consistent with values required
for sustaining a hot corona and is proportional to the aspect ratio of
the loop to the -1.5 power, and characteristic properties of heating
events are distributed as power laws. Cross-correlations show a delay
of dissipation compared with energy content.
Title: Heating of Coronal Loops: Weak MHD Turbulence and Scaling Laws.
Authors: Rappazzo, F.; Velli, M.; Einaudi, G.; Dahlburg, R.
Bibcode: 2007AGUSMSH23C..07R
Altcode:
We have investigated the nonlinear dynamics of the Parker Scenario
for coronal heating through Reduced MHD long-time high-resolution
simulations. A coronal loop is modeled as an elongated Cartesian box
embedded in uniform and strong axial magnetic field, whose footpoints
are convected by motions at the top and bottom planes, mimicking the
photospere. We unambiguously identify MHD anisotropic turbulence as
the physical mechanism responsible for the transport of energy from
the large scales, where energy is injected by photosperic motions,
to the small scales, where it is dissipated. This allows us to give
analytical estimates of the heating rate for coronal loops as a
function of the loop parameters, i.e. lengths, Alfvén velocity,
forcing intensity. The predicted heating rate is within the lower
range of observed active region and quiet-Sun coronal energy losses.
Title: Coronal Heating, Weak MHD Turbulence, and Scaling Laws
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.; Dahlburg, R. B.
Bibcode: 2007ApJ...657L..47R
Altcode: 2007astro.ph..1872R
Long-time high-resolution simulations of the dynamics of a coronal
loop in Cartesian geometry are carried out, within the framework of
reduced magnetohydrodynamics (RMHD), to understand coronal heating
driven by the motion of field lines anchored in the photosphere. We
unambiguously identify MHD anisotropic turbulence as the physical
mechanism responsible for the transport of energy from the large
scales, where energy is injected by photospheric motions, to the
small scales, where it is dissipated. As the loop parameters vary,
different regimes of turbulence develop: strong turbulence is found for
weak axial magnetic fields and long loops, leading to Kolmogorov-like
spectra in the perpendicular direction, while weaker and weaker regimes
(steeper spectral slopes of total energy) are found for strong axial
magnetic fields and short loops. As a consequence we predict that
the scaling of the heating rate with axial magnetic field intensity
B0, which depends on the spectral index of total energy
for given loop parameters, must vary from B3/20
for weak fields to B20 for strong fields at a
given aspect ratio. The predicted heating rate is within the lower
range of observed active region and quiet-Sun coronal energy losses.
Title: Understanding coronal heating and solar wind acceleration:
Case for in situ near-Sun measurements
Authors: McComas, D. J.; Velli, M.; Lewis, W. S.; Acton, L. W.;
Balat-Pichelin, M.; Bothmer, V.; Dirling, R. B.; Feldman, W. C.;
Gloeckler, G.; Habbal, S. R.; Hassler, D. M.; Mann, I.; Matthaeus,
W. H.; McNutt, R. L.; Mewaldt, R. A.; Murphy, N.; Ofman, L.; Sittler,
E. C.; Smith, C. W.; Zurbuchen, T. H.
Bibcode: 2007RvGeo..45.1004M
Altcode:
The solar wind has been measured directly from 0.3 AU outward,
and the Sun's atmosphere has been imaged from the photosphere out
through the corona. These observations have significantly advanced our
understanding of the influence of the Sun's varying magnetic field on
the structure and dynamics of the corona and the solar wind. However,
how the corona is heated and accelerated to produce the solar wind
remains a mystery. Answering these fundamental questions requires
in situ observations near the Sun, from a few solar radii (R S
) out to ~20 R S , where the internal, magnetic, and
turbulent energy in the coronal plasma is channeled into the bulk energy
of the supersonic solar wind. A mission to make such observations has
long been a top priority of the solar and space physics community. The
recent Solar Probe study has proven that such a mission is technically
feasible and can be accomplished within reasonable resources.
Title: Density and Magnetic Field Signatures of Interplanetary
1/f Noise
Authors: Matthaeus, W. H.; Breech, B.; Dmitruk, P.; Bemporad, A.;
Poletto, G.; Velli, M.; Romoli, M.
Bibcode: 2007ApJ...657L.121M
Altcode:
We investigate the occurrence of 1/f noise in the interplanetary
density and the magnetic field at varying heliocentric latitudes. The
characteristic spectral amplitudes can be found in Ulysses density
and magnetic data in the expected frequency ranges at all available
latitudes, ranging from the ecliptic plane to more than 80°. Average
spectra indicate a latitudinal variation, with a 1/f density signal
becoming more pronounced in higher latitude bands. Azimuthal spectral
analysis of solar magnetogram data using the SOHO Michelson Doppler
Interferometer also shows 1/f noise in the photospheric magnetic field,
most clearly at high latitude. Accordingly, we discuss possibilities
that the 1/f signal arises at varying altitudes, possibly surviving
coronal dynamics. This raises questions that may be addressed in future
studies using spectroscopic, white light, and radio scintillation data.
Title: A Reduced MHD Turbulence Numerical Approach On Coronal Loop
Heating: Deriving Scaling Laws
Authors: Romeou, Z.; Velli, M.; Einaudi, G.
Bibcode: 2007ESASP.641E..78R
Altcode:
We have carried out incompressible 2D Reduced MHD simulations to
extensively investigate the long time statistical behavior of a coronal
loop subject to magnetic forcing. We are interested in particular in
studying how dissipation and evolution of the 2D system depends on the
time scales accociated with the photospheric forcing. Our simulations of
the average energy dissipation and the spectral and spatial distribution
at a given time demonstrate the self-organization of the loop at large
scales via an inverse MHD cascade, the highly intermittent response
of the system indicated by strong peaks in the power dissipation and
the strong nonlinearity of the effect. To quantify the nonlinearity of
the response we derive for the time constant case scaling laws against
resistivity of the difference between the numerical solution and the
linear approximation as well as of the time it takes the system to
reach the peak after exceeding the linear approximation solution. We
finally compare the results with the full time dependent forcing case
and discuss the implications.
Title: A Magnetometer For The Solar Orbiter Mission
Authors: Carr, C. M.; Horbury, T. S.; Balogh, A.; Baumjohann, W.;
Bavassano, B.; Breen, A.; Burgess, D.; Cargill, P. J.; Brooker, N.;
Erdõs, G.; Fletcher, L.; Forsyth, R. J.; Giacalone, J.; Glassmeier,
K. -H.; Hoeksema, J. T.; Goldstein, M. L.; Lockwood, M.; Magnes, W.;
Masimovic, M.; Marsch, G.; Matthaeus, W. H.; Murphy, N.; Nakariakov,
V. M.; Pacheco, J. R.; Pincon, J. -L.; Riley, P.; Russell, C. T.;
Schwartz, S. J.; Szabo, A.; Thompson, M.; Vainio, R.; Velli, M.;
Vennerstrom, S.; Walsh, R.; Wimmer-Schweingruber, R.; Zank, G.
Bibcode: 2007ESASP.641E..41C
Altcode:
The magnetometer is a key instrument to the Solar Orbiter mission. The
magnetic field is a fundamental parameter in any plasma: a precise
and accurate measurement of the field is essential for understanding
almost all aspects of plasma dynamics such as shocks and stream-stream
interactions. Many of Solar Orbiter's mission goals are focussed
around the links between the Sun and space. A combination of in situ
measurements by the magnetometer, remote measurements of solar magnetic
fields and global modelling is required to determine this link and
hence how the Sun affects interplanetary space. The magnetic field
is typically one of the most precisely measured plasma parameters and
is therefore the most commonly used measurement for studies of waves,
turbulence and other small scale phenomena. It is also related to the
coronal magnetic field which cannot be measured directly. Accurate
knowledge of the magnetic field is essential for the calculation of
fundamental plasma parameters such as the plasma beta, Alfvén speed
and gyroperiod. We describe here the objectives and context of magnetic
field measurements on Solar Orbiter and an instrument that fulfils those
objectives as defined by the scientific requirements for the mission.
Title: A Semiempirical Magnetohydrodynamical Model of the Solar Wind
Authors: Cohen, O.; Sokolov, I. V.; Roussev, I. I.; Arge, C. N.;
Manchester, W. B.; Gombosi, T. I.; Frazin, R. A.; Park, H.; Butala,
M. D.; Kamalabadi, F.; Velli, M.
Bibcode: 2007ApJ...654L.163C
Altcode:
We present a new MHD model for simulating the large-scale structure
of the solar corona and solar wind under ``steady state'' conditions
stemming from the Wang-Sheeley-Arge empirical model. The processes
of turbulent heating in the solar wind are parameterized using a
phenomenological, thermodynamical model with a varied polytropic
index. We employ the Bernoulli integral to bridge the asymptotic solar
wind speed with the assumed distribution of the polytropic index on
the solar surface. We successfully reproduce the mass flux from Sun to
Earth, the temperature structure, and the large-scale structure of the
magnetic field. We reproduce the solar wind speed bimodal structure
in the inner heliosphere. However, the solar wind speed is in a
quantitative agreement with observations at 1 AU for solar maximum
conditions only. The magnetic field comparison demonstrates that the
input magnetogram needs to be multiplied by a scaling factor in order
to obtain the correct magnitude at 1 AU.
Title: Coronal heating and solar wind acceleration by turbulence
Authors: Verdini, A.; Velli, M.; Matthaeus, W. H.
Bibcode: 2006AGUFMSH11B..06V
Altcode:
Observations such as Spartan and SOHO UVCS have challenged ideas
for the acceleration of the solar wind by constraining models to
produce >1.5 Million K protons, several hundred km~s-1 radial
outflows, and >700 km~s-1 terminal speeds in the wind emanating
from polar coronal holes, with coronal electrons remaining cooler than
protons. Observed properties of the solar wind at 1AU and by Ulysses
provide additional constraints on these models. It was recognized
some time ago that these conditions probably require adding internal
energy in sufficient quantities at altitudes <1.5 R_\odot, but
the origin of this energy and its method of transport and conversion
to heat have remained unclear. The involvement of turbulence in this
process was suggested some time ago, but various issues regarding the
physics of cascade and dissipation have persisted and a wind model
compatible with magnetohydrodynamic theories of turbulence, including
the physics of low frequency anisotropic cascade, has not yet been
presented to our knowledge. Here we suggest some simplifications
and assumptions that allow a self-consistent treatment of the solar
wind acceleration problem. Numerical implementation of the coupled
solar wind- turbulence equations is described, and computations for
a super-radially expanding coronal hole show results for wind speed,
temperature, density, and cross helicity profiles that are promising
in comparison with known observational constraints.
Title: Parallel proton fire hose instability in the expanding solar
wind: Hybrid simulations
Authors: Matteini, Lorenzo; Landi, Simone; Hellinger, Petr; Velli,
Marco
Bibcode: 2006JGRA..11110101M
Altcode:
We report a study of the properties of the parallel proton fire hose
instability comparing the results obtained by the linear analysis,
from one-dimensional (1-D) standard hybrid simulations and 1-D
hybrid expanding box simulations. The three different approaches
converge toward the same instability threshold condition which is
in good agreement with in situ observations, suggesting that such
instability is relevant in the solar wind context. We investigate
also the effect of the wave-particle interactions on shaping the
proton distribution function and on the evolution of the spectrum
of the magnetic fluctuations during the expansion. We find that the
resonant interaction can provide the proton distribution function to
depart from the bi-Maxwellian form.
Title: Forced MHD Turbulence Simulations for Coronal Loop Heating
Authors: Romeou, Z.; Velli, M.; Einaudi, G.
Bibcode: 2006AIPC..848..105R
Altcode:
In this work we revisit the question of whether the assumption of
a turbulent photosphere provides an efficient mechanism for the
disposition of energy in the solar corona. Through a two-dimensional
incompressible MHD spectral code and appropriate analysis we
investigate the long time statistical behavior of a two-dimensional
cross section of a coronal loop. In particular we study the transition
to turbulence from a large scale quasi-stationary coherent forcing
analyzing the effects of the finite Reynolds and Lundquist numbers and
the role of noise in triggering resistive instabilities and subsequent
cascades. Simulations of the average energy dissipation and the spectral
and spatial distribution at a given time show the self-organization
of the loop at large scales via an inverse MHD cascade. To quantify
the nonlinearity of the response in the case of constant time forcing,
we derive scaling laws against resistivity of the difference between
the numerical solution and the linear approximation as well as of the
time it takes the system to reach the peak after exceeding the linear
approximation solution. We finally comment on the response of the
loop also on the most general case of time dependent random forcing
comparing with the first case.
Title: A Turbulence Model for Acceleration of the High Latitude Fast
Solar Wind
Authors: Verdini, A.; Dmitruk, P.; Matthaeus, W. H.; Oughton, S.;
Velli, M.
Bibcode: 2006ESASP.617E.150V
Altcode: 2006soho...17E.150V
No abstract at ADS
Title: Alfvén Waves and Turbulence in the Inner Corona
Authors: Verdini, A.; Buchlin, E.; Velli, M.
Bibcode: 2006ESASP.617E.115V
Altcode: 2006soho...17E.115V
No abstract at ADS
Title: On the Role of the Parallel Proton Fire Hose Instability in
the Expanding Solar Wind: Simulations and Observations
Authors: Matteini, L.; Landi, S.; Hellinger, P.; Velli, M.; Maksimovic,
M.; Pantellini, F.; Marsch, E.
Bibcode: 2006ESASP.617E.101M
Altcode: 2006soho...17E.101M
No abstract at ADS
Title: Heliospheric magnetic field polarity inversions driven by
radial velocity field structures
Authors: Landi, Simone; Hellinger, Petr; Velli, Marco
Bibcode: 2006GeoRL..3314101L
Altcode:
Magnetic field polarity inversions embedded in the predominantly
unipolar fast solar wind have been observed by the Ulysses spacecraft
at high latitudes. Such reversals have the nature of folded back field
lines which we suggest are generated by the interaction of standard
large amplitude, low frequency, Alfvénic turbulence with velocity
shears in the fast solar wind. We present 2D magnetohydrodynamic
simulations of a very low frequency and high amplitude Alfvén wave
propagating away from the sun embedded in a velocity shear structure
such as a microstream and show how reversals in the magnetic field
lines are generated naturally on a time-scale consistent with their
observation at Ulysses. The generated magnetic field and plasma signals
are similar to those observed. We discuss the role turbulence-stream
shear interactions might play in limiting differential velocities in
the asymptotic high speed solar wind.
Title: On Linear and Nonlinear Analysis of Jet and Current Sheet
Interactions in the Solar System: 2D Hybrid Compact Shock Capturing
Simulations
Authors: Bettarini, L.; Landi, S.; Londrillo, P.; Velli, M.
Bibcode: 2006ESASP.617E..52B
Altcode: 2006soho...17E..52B
No abstract at ADS
Title: Tearing and Kelvin-Helmholtz instabilities in the heliospheric
plasma
Authors: Bettarini, L.; Landi, S.; Rappazzo, F. A.; Velli, M.;
Opher, M.
Bibcode: 2006A&A...452..321B
Altcode:
We used 2.5D simulations to analyze the magnetohydrodynamic
instabilities arising from an initial equilibrium configuration
consisting of a plasma jet or wake in the presence of a magnetic
field with strong transverse gradients, such as those arising in the
solar wind. Our analysis extends previous results by considering both
a force-free equilibrium and a pressure-balance condition for a jet
in a plasma sheet, along with arbitrary angles between the magnetic
field and velocity field. In the force-free case, the jet/wake does
not contain a neutral sheet but the field rotates through the flow to
invert its polarity. The presence of a magnetic field component aligned
with the jet/wake destroys the symmetric nature of the fastest growing
modes, leading to asymmetrical wake acceleration (or, equivalently,
jet deceleration). In the case of a jet, the instability properties
depend both on the magnetic field and flow gradients, as well as on
the length of the jet. The results are applied to the post-termination
shock jet recently found in 3D global heliospheric simulations, where
our analysis confirms and explains the stability properties found in
such simulations.
Title: Solar Wind Acceleration Models in SWMF
Authors: Cohen, O.; Sokolov, I. V.; Velli, M.; Gombosi, T. I.
Bibcode: 2006AGUSMSH53A..04C
Altcode:
The choice of the solar wind model in numerical simulations of
the processes in the Solar Corona (SC), Inner Heliosphere (IH) and
Outer Heliosphere (OH) is a matter of a crucial importance. Since the
available theoretical models for the turbulent processes and turbulent
heating in the solar corona do not provide a reliable and quantitatively
accurate agreement with the observed parameters of the solar wind
at 1 AU, the preference should be given to semi-empirical models,
as a first step. In the Space Weather Modeling Framework (SWMF),
the solar wind heating and acceleration can be accounted in three
different ways. I) Apply the solar wind parameters at 2.5-3.5 Rs as
derived from Wang-Sheeley-Arge (WSA) model (using expansion factor)
or the Fisk model (using the ratio of the radial to the absolute
magnetic field on the solar surface) via the inner boundary condition
of the Inner Heliosphere (IH) module with constant polytropic index of
~1.5. II) Relate the semi-empirical data for the solar wind velocity
to the spatial distribution of the Bernoulli integral throughout
the solar corona and inner heliosphere by employ a varied polytropic
index model. This allows us to use gamma(r) varying from 1.1 on the
solar surface to 1.5 at larger heliocentric distances (~10 Rs). III)
Use the Alfven turbulence model. The coupled MHD-Turbulence model in
this case allows us to relate the empirical data of the solar wind at
1 AU with the turbulence parameters in the solar corona, such as the
Pointing flux at the solar surface. This more refined model can be
also coupled with the model for turbulence solar energetic particles
through the SWMF. All methods are semi-empirical, and are derived
from potential field extrapolation of magnetogram data. We present
the different methods and compare the results with solar wind data at
1 AU. The work is supported by the contract F014254 between the Jet
Propulsion Laboratory and the University of Michigan.
Title: Small-Scale Anisotropy and Intermittence in High- and
Low-Latitude Solar Wind
Authors: Bigazzi, A.; Biferale, L.; Gama, S. M. A.; Velli, M.
Bibcode: 2006ApJ...638..499B
Altcode: 2004astro.ph.12320B
We study low- and high-latitude fast solar wind data from the Ulysses
spacecraft from 1992 to 1994 using for the first time a systematic
method to analyze the anisotropic content of the magnetic field
fluctuations beyond second-order correlation functions. We investigate
all available frequencies, 1-10-6 Hz, for both high- and
low-latitude data sets in which mean magnetic field points parallel
and perpendicular to the mean flow, respectively, and we are able
to quantify the relative importance of the anisotropic versus the
isotropic fluctuations. We analyze, up to sixth order, longitudinal,
transverse, and mixed magnetic field correlations. Our results show
that strongly intermittent and anisotropic events persist even at
high frequencies/small scales, indicating the absence of a complete
recovery of isotropy. Our study shows for the first time the existence
of intermittent anisotropic contributions at all scales in solar
wind. Analyses of anomalous scaling of quantities that mix isotropic
and anisotropic fluctuations, like longitudinal structure functions,
may therefore be flawed by systematic uncontrolled errors. Anisotropic
scaling properties are compatible for high- and low-latitude data,
suggesting a universal behavior in spite of the different rate of
evolution of the fast solar wind streams in the two environments.
Title: Nonlinear interactions in coronal heating
Authors: Rappazzo, A. F.; Dahlburg, R. B.; Einaudi, G.; Velli, M.
Bibcode: 2006AdSpR..37.1335R
Altcode:
The dynamics of the solar corona as well as its very existence are due
to the dynamics of plasmas and magnetic fields which, at the global
scales of coronal loops, prominences and helmet streamers may be
described by magnetohydrodynamics. Here, we discuss the importance and
role of nonlinear interactions in the heating of the solar corona, which
relies on the transfer, storage and dissipation of the mechanical energy
present in photospheric motion [Einaudi, G., Velli, M., Phys. Plasmas
6, 4146, 1999]. Nonlinear interactions including the coupling of
coronal fields to the motions and emerging flux through the photosphere
determine both the rate of heating and the topology of coronal magnetic
fields. We present the first results of a 3D reduced MHD simulation that
models the small-scale magnetic activity of coronal flux tubes. The
equations are solved inside a box of dimensions l × l × L (axial
direction), with an aspect ratio L/ l of the order of 10. The box
is initially threaded by a constant sinusoidal velocity field at
one base (corresponding to one photospheric footpoint of the loop),
of amplitude 1 km/s, (the axial Alfvén speed is about 1000 km/s),
whereas the other footpoint is anchored, i.e., no photospheric motions
are present. In the transverse directions, periodicity is assumed. Our
numerical calculations show that the magnetic field lines change their
topology continuously and often reconnect at small scales, forming
typical coronal loop-like structures. Energy release events which
provide a steady supply of energy are associated with the reconnection.
Title: Nonlinear analysis of jet/wake and current sheet interactions
in the heliospheric plasma
Authors: Bettarini, L.; Landi, S.; Rappazzo, F.; Velli, M.; Opher, M.
Bibcode: 2006cosp...36.2383B
Altcode: 2006cosp.meet.2383B
The interactions between a stream and a current sheet is the starting
point to understand the dynamics and evolution of complex structures
in the Heliospheric region We used 2 5D simulations to analyze the
magnetohydrodynamic instabilities arising from an initial equilibrium
configuration consisting of a plasma jet or wake in the presence of a
magnetic field with strong transverse gradients such as those arising
in the solar wind both close to the Sun and far from it Our analysis
extends previous results by considering both a force-free equilibrium
and a pressure-balance condition for a jet in a plasma sheet along with
arbitrary angles between the magnetic field and velocity field In the
force-free case the jet wake does not contain a neutral sheet but the
field rotates through the flow to invert its polarity The presence
of a magnetic field component aligned with the jet wake destroys the
symmetric nature of the fastest growing modes leading to asymmetrical
wake acceleration or equivalently jet deceleration In the case of a jet
the instability properties depend both on the magnetic field and flow
gradients as well as on the length of the jet We applied our results to
the wake model of the solar wind on the solar equatorial plane above the
helmet streamer cusp considering arbitrary angles between the magnetic
field and the velocity field and to the post-termination shock jet
recently found in 3D global heliospheric simulations where our analysis
confirms and explains the stability properties found in such simulations
Title: Propagation and dissipation of Alfvén waves in stellar
atmospheres permeated by isothermal winds
Authors: Verdini, A.; Velli, M.; Oughton, S.
Bibcode: 2005A&A...444..233V
Altcode:
We investigate the nonlinear evolution of Alfvén waves in a radially
stratified isothermal atmosphere with wind, from the atmospheric
base out to the Alfvénic point. Nonlinear interactions, triggered
by wave reflection due to the atmospheric gradients, are assumed to
occur mainly in directions perpendicular to the mean radial magnetic
field. The nonlinear coupling between waves propagating in opposite
directions is modeled by a phenomenological term, containing an integral
turbulent length scale, which acts as a dissipative coefficient for
waves of a given frequency. Although the wind acceleration profile is
not determined self-consistently one may estimate the dissipation rate
inside the layer and follow the evolution of an initial frequency
spectrum. Reflection of low frequency waves drives dissipation
across the whole spectrum, and steeper gradients, i.e. lower coronal
temperatures, enhance the dissipation rate. Moreover, when reasonable
wave amplitudes are considered, waves of all frequencies damp at
the same rate and the spectrum is not modified substantially during
propagation. Therefore the sub-Alfvénic coronal layer acts differently
when waves interact nonlinearly, no longer behaving as a frequency
dependent filter once reflection-generated nonlinear interactions are
included, at least within the classes of models discussed here.
Title: ALFVÉN Waves in the Solar Atmosphere: a Nonlinear Model from
the Photosphere to 1 AU
Authors: Verdini, A.; Velli, M.; Oughton, S.
Bibcode: 2005ESASP.600E..42V
Altcode: 2005dysu.confE..42V; 2005ESPM...11...42V
No abstract at ADS
Title: Reduced MHD and Shell-Model Simulations of Coronal Heating
in Magnetized Loops: Scaling Laws.
Authors: Velli, M.; Rappazzo, F.; Buchlin, E.; Einaudi, G.
Bibcode: 2005AGUFMSH13B..03V
Altcode:
We present direct magnetohydrodynamic (MHD) simulations modeling the
heating of coronal loops in the solar atmosphere via the tangling
of coronal field lines by photospheric footpoint motions within the
framework of reduced MHD. We carry out long-time 3D simulations with
the highest resolutions to date and compare them to simpler shell-model
simulations, in which the non-linear couplings in wave-number space
are drastically simplified. The latter reach much larger Reynolds
numbers but can not describe the dynamics in physical space, which
is driven by the reconnection of induced coronal magnetic fields. In
the direct numerical simulations, we reach resolutions sufficient to
derive scaling properties with Reynolds numbers, loop length, and ratio
of photospheric velocity to coronal Alfven speeds. Line-tying of the
axial field lines plays a significant role by inhibiting coalescence and
inverse cascades in the loop cross-sections, which dominate dynamics
in 2D models. To examine the role of line-tying simulations including
gradients in the density from the photosphere to the corona are also
included. Shell-model calculations are carried out for much longer
time-scales, sufficient to calculate the statistical properties of
heating. The scaling properties derived from the shell models and from
reduced MHD are compared and contrasted and on this basis we discuss
the required role of emerging flux, neglected here, in coronal heating.
Title: Alfvén wave heating of heavy ions in the expanding solar wind:
Hybrid simulations
Authors: Hellinger, Petr; Velli, Marco; TráVníčEk, Pavel; Gary,
S. Peter; Goldstein, Bruce E.; Liewer, Paulett C.
Bibcode: 2005JGRA..11012109H
Altcode:
We present hybrid expanding box simulations of the interaction of
left-handed Alfvén waves with protons, alpha particles, and a tenuous
population of oxygen O5+. The Alfvén waves are initially
nonresonant with the ions, and the expansion brings them to the
cyclotron resonance with O5+ ions, then with alpha particles,
and finally with protons. The simulations show that O5+
ions are efficiently heated in the directions perpendicular to the
background magnetic field but are only slightly accelerated. Oxygen
scattering has a finite time span and saturates mainly due to the
marginal stabilization with respect to the oxygen cyclotron instability
generated by the temperature anisotropy. During the scattering,
oxygen ions are able to absorb only a limited amount of available
fluctuating energy and, for the parameters used in the simulations,
their presence has a minimum influence on alpha particles and protons.
Title: Alfvénic Turbulence and Micro-Stream Structure in the Polar
Solar Wind.
Authors: Landi, S.; Velli, M.; Hellinger, P.; Neugebauer, M.
Bibcode: 2005AGUFMSH53A1254L
Altcode:
We present 2D simulations of the interaction of an outwardly propagating
Alfvén wave spectrum with micro-streams in the expanding solar wind. In
previous work, we suggested that velocity shears in the developing
solar wind could be responsible for the high-latitude polarity reversals
observed by Ulysses. Here we carry out numerical experiments with more
realistic initial conditions. We suggest that Alfvénic turbulence
might play a role in determining the overall velocity differences in
a stream- or jet- dominated early fast wind, surviving further out as
the microstream structures, since turbulence may act as a sink for free
energy in velocity shears. The magnitude of velocity shears at large
distances from the sun is determined by the interplay of turbulent
dissipation and magnetic tension. We compare simulation results with
representative data from the Ulysses measurements.
Title: Diamagnetic and Expansion Effects on the Observable Properties
of the Slow Solar Wind in a Coronal Streamer
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.; Dahlburg, R. B.
Bibcode: 2005ApJ...633..474R
Altcode: 2010arXiv1002.3325R
The plasma density enhancements recently observed by the Large-Angle
Spectrometric Coronagraph (LASCO) instrument on board the Solar and
Heliospheric Observatory (SOHO) spacecraft have sparked considerable
interest. In our previous theoretical study of the formation and
initial motion of these density enhancements it is found that beyond
the helmet cusp of a coronal streamer the magnetized wake configuration
is resistively unstable, that a traveling magnetic island develops at
the center of the streamer, and that density enhancements occur within
the magnetic islands. As the massive magnetic island travels outward,
both its speed and width increase. The island passively traces the
acceleration of the inner part of the wake. In the present paper
a few spherical geometry effects are included, taking into account
both the radial divergence of the magnetic field lines and the average
expansion undergone by a parcel of plasma propagating outward, using the
expanding box model (EBM), and the diamagnetic force due to the overall
magnetic field radial gradients, the so-called melon-seed force. It is
found that the values of the acceleration and density contrasts can
be in good agreement with LASCO observations, provided the spherical
divergence of the magnetic lines starts beyond a critical distance from
the Sun and the initial stage of the formation and acceleration of the
plasmoid is due to the Cartesian evolution of MHD instabilities. This
result provides a constraint on the topology of the magnetic field in
the coronal streamer.
Title: Solar Probe: Humanity's First Visit to a Star (Invited)
Authors: McComas, D. J.; Velli, M.; Lewis, W. S.; Acton, L. W.;
Balat-Pichelin, M.; Bothmer, V.; Dirling, R. B.; Eng, D. A.; Feldman,
W. C.; Gloeckler, G.; Guhathakurtha, M.; Habbal, S. R.; Hassler, D. M.;
Mann, I.; Maldonado, H. M.; Matthaeus, W. H.; McNutt, R. L.; Mewaldt,
R. A.; Murphy, N.; Ofman, L.; Potocki, K. A.; Sittler, E. C.; Smith,
C. W.; Zurbuchen, T. H.
Bibcode: 2005ESASP.592..279M
Altcode: 2005ESASP.592E..42M; 2005soho...16E..42M
No abstract at ADS
Title: Tearing and Kelvin-Helmholtz Instabilities in the Heliospheric
Plasma
Authors: Bettarini, L.; Rappazzo, F. A.; Landi, S.; Velli, M.
Bibcode: 2005ESASP.592..589B
Altcode: 2005ESASP.592E.113B; 2005soho...16E.113B
No abstract at ADS
Title: Proton Fire Hose Instability in the Expanding Solar Wind
Authors: Matteini, L.; Landi, S.; Hellinger, P.; Velli, M.
Bibcode: 2005ESASP.592..503M
Altcode: 2005ESASP.592E..92M; 2005soho...16E..92M
No abstract at ADS
Title: Non Linear Evolution of Alfvén Waves in the SolarAtmosphere
Authors: Verdini, A.; Velli, M.; Oughton, S.
Bibcode: 2005ESASP.592..567V
Altcode: 2005ESASP.592E.108V; 2005soho...16E.108V
No abstract at ADS
Title: On the Origin of the Heliospheric Magnetic Field Polarity
Inversion at High Latitudes
Authors: Landi, S.; Hellinger, P.; Velli, M.
Bibcode: 2005ESASP.592..785L
Altcode: 2005soho...16E.162L; 2005ESASP.592E.162L
No abstract at ADS
Title: Solar Polar Imager: Observing Solar Activity from a New
Perspective
Authors: Alexander, D.; Sandman, A.; Liewer, P.; Ayon, J.; Goldstein,
B.; Murphy, N.; Velli, M.; Floyd, L.; Moses, D.; Socker, D.; Vourlidas,
A.; Garbe, G.; Suess, S.; Hassler, D.; Kosovichev, A.; Mewaldt, R.;
Neugebauer, M.; Ulrich, R.; Zurbuchen, T.
Bibcode: 2005ESASP.592..663A
Altcode: 2005soho...16E.131A; 2005ESASP.592E.131A
No abstract at ADS
Title: Shell-Model Simulations of MHD in a Solar Coronal Loop
Authors: Buchlin, É.; Velli, M.
Bibcode: 2005ESASP.592..153B
Altcode: 2005soho...16E..23B; 2005ESASP.592E..23B
No abstract at ADS
Title: Influence of the definition of dissipative events on their
statistics
Authors: Buchlin, E.; Galtier, S.; Velli, M.
Bibcode: 2005A&A...436..355B
Altcode: 2004astro.ph.11592B
A convenient and widely used method to study the turbulent plasma in
the solar corona is to examine statistics of properties of events
(or structures) associated to flares either in observations or
in numerical simulations. Numerous papers have followed such
a methodology, using different definitions of an event, but the
reasons behind the choice of a particular definition is very rarely
discussed. We give here a comprehensive set of possible event
definitions starting from a one-dimensional data set such as a
time-series of energy dissipation. Each definition is then applied
to a time-series of energy dissipation obtained from simulations
of a shell-model of magnetohydrodynamic turbulence, or from a new
model of coupled shell-models designed to represent a magnetic loop
in the solar corona. We obtain distributions of the peak dissipation
power, total energy, duration and waiting-time associated with each
definition. These distributions are then investigated and compared,
and the influence of the definition of an event on the statistics is
discussed. In particular, power-law distributions are more likely to
appear when using a threshold. The sensitivity of the distributions to
the definition of an event seems also to be weaker for events found in
a highly intermittent time series. Some implications for statistical
results obtained from observations are discussed.
Title: Shell-Model Simulations of MHD in a Solar Coronal Loop
Authors: Buchlin, E.; Velli, M.
Bibcode: 2005AGUSMSP14A..05B
Altcode:
Statistics may be necessary to keep a global view of the complexity of
astrophysical turbulence, in particular the effects of non-linear
interactions over a wide range of scales. However, from the
numerical point of view, a statistical approach to turbulence has the
contradictory needs for computing speed and for a good description
of the solutions of the MHD equations. This problem can be addressed
by simplified models like cellular automata or shell-models. In the
shell-models, the low number of well-chosen modes allows to keep
the most possible of the complex and non-linear physics of the MHD
equations while running sufficiently fast to produce statistics of
fields, of structures, and of "events". The model we present here is
designed to represent a magnetic loop in the solar corona. It consists
of a pile of shell-models, which allows to reach a wide range of
wavenumbers in cross-sections of the loop and model the non-linear
couplings between these modes. The shell-models are also coupled by
Alfvén waves propagating along the loop. We study the statistical
properties of intermittent energy dissipation and of the velocity and
magnetic fields produced by this model. These statistical properties
can be compared to statistics issued from observations, like structure
functions or events distributions.
Title: Coronal Heating Through Reduced MHD Turbulence
Authors: Rappazzo, F.; Velli, M.; Dahlburg, R.; Einaudi, G.
Bibcode: 2005AGUSMSP41A..04R
Altcode:
We present 3D reduced-MHD simulations modeling the heating of coronal
loops in the solar atmosphere via the tangling of coronal field
lines by random photospheric footpoint motions, which we represent
as eddies having a finite correlation time. The overall behaviour of
the system is sensitive to the intrinsic time-scale present, namely
Alfvén propagation time along the loop, dynamical transverse time and
photospheric forcing correlation time. The line-tying effect associated
with the Alfvén wave propagation along the loop and the reflective
photospheric boundary conditions limit the extent of the inverse
cascade of magnetic energy when compared to 2D approximations and
increases intermittency in both kinetic and magnetic energy absorption
and dissipation. The simulations show that the corona self-organizes
in response to the forcing in what we conjecture to be a state of
minimal dissipation compatible with the driving.
Title: Alfvén Waves and Shock Wave Formation at an X-Point Magnetic
Field Configuration
Authors: Landi, Simone; Velli, Marco; Einaudi, Giorgio
Bibcode: 2005ApJ...624..392L
Altcode:
We present MHD numerical simulations of the propagation of Alfvén
waves in inhomogeneous magnetic fields whose topology consists
of two-dimensional X-points threaded by a nonvanishing normal
component. The coupling of the waves with the background field gradients
leads to the development of fast-mode shock trains propagating normal
to the average normal magnetic field. The fronts occur with a frequency
matching that of the generating Alfvén wave. Energy in the original
Alfvén wave packet is channeled via mode transformation to the fast
waves and into the shocks, where it is dissipated at a rate independent
of the Reynolds number. Such X-point shock trains might therefore
play an important role in the heating of the solar corona plasma and,
more generally, in cosmic particle acceleration.
Title: Origin of Heliospheric Magnetic Field Polarity Inversion at
High Latitudes
Authors: Velli, M.; Landi, S.; Hellinger, P.; Winterhalter, D.
Bibcode: 2005AGUSMSH43A..11V
Altcode:
High latitude observations of the magnetic field by the Ulysses
spacecraft have shown a significance number of cases where the radial
magnetic field polarity is reversed with respect to the dominant
polarity of the coronal hole from which the wind emanates. Such
reversals have the nature of folded back magnetic field lines. It
has been suggested that such reversals are due to reconnection of
closed and open field lines in the lower corona which would launch
a large amplitude Alfvén wave into the solar wind. We suggested an
alternative mechanism for the generation of the polarity reversal,
namely, the coupling of standard large amplitude Alfvénic turbulence
in the low frequency regime propagating away from the sun with the
microstream shears observed in the high speed solar wind. Here we show
that pressure and density signals are similiar to those observed in
the data, and discuss the correlation of the reversals with the high
latitude microstream structure.
Title: Effects of a Tilted Heliospheric Current Sheet in the
Heliosheath
Authors: Opher, M.; Liewer, P.; Velli, M.; Gombosi, T.; Manchester,
W.; Dezeeuw, D.; Toth, G.
Bibcode: 2005AGUSMSH23A..07O
Altcode:
Effects of a Tilted Heliospheric Current Sheet in the Heliosheath
Recent observations indicate that Voyager 1, now beyond 90 AU, is in a
region unlike any encountered in it's 26 years of exploration. There
is currently a controversy as to whether Voyager 1 has already
crossed the Termination Shock, the first boundary of the Heliosphere
(Krimigis et al. 2003; McDonald et al. 2003, Burlaga et al. 2003). An
important aspect of this controversy is our poor understanding
of this region. The region between the Termination Shock and the
Heliopause, the Helisheath, is one of the most unknown regions
theoretically. In the Heliosheath magnetic effects are crucial,
as the solar magnetic field is compressed at the Termination Shock
by the slowing flow. Therefore, to accurately model the heliosheath
the inclusion of the solar magnetic field is crucial. Recently, our
simulations showed that the Heliosheath presents remarkable dynamics,
with turbulent flows and a presence of a jet flow at the current sheet
that is unstable due to magnetohydrodynamic instabilities (Opher et
al. 2003; 2004). We showed that to capture these phenomena, spatial
numerical resolution is a crucial ingredient, therefore requiring the
use of an adaptive mesh refinement (AMR). These previous works assumed
that the solar rotation and the magnetic axis were aligned. Here we
present including, for the first time, the tilt of the heliocurrent
sheet using a 3D MHD AMR simulation with BATS-R-US code. We discuss
the effects on the global structure of the Heliosheath, the flows,
turbulence and magnetic field structure. We access the consequences for
the observations measured by Voyager 1 since mid-2002. This intensive
computational run was done at the supercomputer Columbia at NASA/AMES
Title: Transverse oscillations in solar coronal loops induced by
propagating Alfvénic pulses
Authors: Del Zanna, L.; Schaekens, E.; Velli, M.
Bibcode: 2005A&A...431.1095D
Altcode: 2004astro.ph.11661D
The propagation and the evolution of Alfvénic pulses in the
solar coronal arcades is investigated by means of MHD numerical
simulations. Significant transverse oscillations in coronal loops,
triggered by nearby flare events, are often measured in EUV lines
and are generally interpreted as standing kink modes. However, the
damping times of these oscillations are typically very short (from
one to a few periods) and the physical mechanism responsible for
the decay is still a matter of debate. Moreover, the majority of the
observed cases actually appears to be better modeled by propagating,
rather than standing, modes. Here we perform 2.5D compressible MHD
simulations of impulsively generated Alfvén waves propagating in a
potential magnetic arcade (assumed as a simplified 2D loop model),
taking into account the stratification of the solar atmosphere with
height from the photosphere to the corona. The results show a strong
spreading of the initially localized pulses along the loop, due to the
variations in the Alfvén velocity with height, and correspondingly an
efficient damping of the amplitude of the oscillations. We believe that
simple explanations based on the effects of wave propagation in highly
inhomogeneous media may apply to the majority of the reported cases,
and that variations of the background density and Alfvén speed along
the loop should be considered as key ingredients in future models.
Title: Simplified Simulations of MHD in a Coronal Loop by Coupled
Shell-Models
Authors: Buchlin, É.; Velli, M.; Galtier, S.
Bibcode: 2004ESASP.575..120B
Altcode: 2004soho...15..120B
No abstract at ADS
Title: Nonlinear Evolution of a Turbulent Spectrum of Outwardly
Propagating ALFVÉN Waves in Solar and Stellar Coronae
Authors: Verdini, A.; Velli, M.; Oughton, S.
Bibcode: 2004ESASP.575..454V
Altcode: 2004soho...15..454V
No abstract at ADS
Title: Propagation of Alfvénic Pulses in Coronal Arcades
Authors: Del Zanna, L.; Velli, M.; Schaekens, Eveline
Bibcode: 2004ESASP.575..383D
Altcode: 2004soho...15..383D
No abstract at ADS
Title: Long Time Incompressible 2D MHD Simulations of Coronal Loop
Heating: the Role of Photospheric Time-Scales
Authors: Romeou, Z.; Velli, M.; Einaudi, G.
Bibcode: 2004ESASP.575..523R
Altcode: 2004soho...15..523R
No abstract at ADS
Title: Effects of a Tilted Heliospheric Current Sheet in the
Heliosheath: 3D MHD Modeling
Authors: Opher, M.; Liewer, P.; Velli, M.; Gombosi, T.; Manchester,
W.; Dezeeuw, D.; Toth, G.
Bibcode: 2004AGUFMSH42A..02O
Altcode:
Recent observations indicate that Voyager 1, now beyond 90 AU, is in
a region unlike any encountered in it's 26 years of exploration. There
is currently a controversy as to whether Voyager 1 has already crossed
the Termination Shock, the first boundary of the Heliosphere (Krimigis
et al. 2003; McDonald et al. 2003, Burlaga et al. 2003). An important
aspect of this controversy is our poor understanding of this region. The
region between the Termination Shock and the Heliopause, the Helisheath,
is one of the most unknown regions theoretically. In the Heliosheath
magnetic effects are crucial, as the solar magnetic field is compressed
at the Termination Shock by the slowing flow. Therefore, to accurately
model the Heliosheath the inclusion of the solar magnetic field is
crucial. Recently, our simulations showed that the Heliosheath presents
remarkable dynamics, with turbulent flows and a presence of a jet
flow at the current sheet that is unstable due to magnetohydrodynamic
instabilities (Opher et al. 2003; 2004). We showed that to capture
these phenomena, spatial numerical resolution is a crucial ingredient,
therefore requiring the use of an adaptive mesh refinement (AMR). These
previous works assumed that the solar rotation and the magnetic axis
were aligned. Here we present for the first time results including
the tilt of the heliocurrent sheet using a 3D MHD AMR simulation, with
BATS-R-US code. We discuss the effects on the global structure of the
Heliosheath, the flows, turbulence and magnetic field structure. We
assess the consequences for the observations measured by Voyager 1
since mid-2002.
Title: High Latitude Magnetic Field Polarity Reversals: A Solar
Source or In Situ Generation?
Authors: Landi, S.; Hellinger, P.; Velli, M.; Winterhalter, D.
Bibcode: 2004AGUFMSH34A..02L
Altcode:
High latitude observations of the magnetic field by the Ulysses
spacecraft have shown a significant number of cases where the radial
magnetic field polarity is reversed with respect to the dominant
polarity of the coronal hole from which the wind emanates. It has been
suggested that such reversals are due to reconnection of closed and open
field lines in the lower corona which would launch a large amplitude
Alfvén wave into the solar wind. Here we carry out 2.5D simulations
of such a wave and show that the kink in the field line tends to be
unstable, disappearing within a few dynamical time-scales. We suggest
an alternative mechanism for the generation of polarity reversals,
namely, the coupling of standard large amplitude Alfvén turbulence
propagating away from the sun with the micro-stream shears observed in
the high speed solar wind. Numerical simulations show that reversals
generated by shears in the wind tend to be stable, and the time-scale
required for such generation is compatible with the transport time
out to 1 AU and beyond. It remains to be seen whether any correlation
between microstream structure and polarity reversals can be found in
the Ulysses data.
Title: Heating and Acceleration of Minor Ions in the Expanding
Solar Wind
Authors: Hellinger, P.; Velli, M.; Travnicek, P.; Goldstein, B. E.;
Liewer, P. C.
Bibcode: 2004AGUFMSH51C0281H
Altcode:
We present hybrid simulations of the interaction of Alfven waves with
protons, alpha particles and a small abundance of oxygen 5+ using the
expanding box model. The simulations test the sweeping mechanism of
the heating and acceleration of the solar wind by cyclotron resonance
with Alfven waves. The numerical simulations indicate that oxygen (and
other minor ions) are efficiently heated in perpendicular direction
and accelerated but are able to absorb only a limited amount of
available energy in the Alfven waves. The presence of oxygen ions has
a minimal influence on alpha particles and protons. However, for the
parameters used in the simulations the heating and acceleration of
alpha particles and protons are not very efficient. We also explore
the role of the radial stretching which occurs in the acceleration
region of the wind. This amounts to an expansion of the box also
in the radial direction and leads to important parallel cooling. The
combination of the perpendicular heating by Alfven wave and the parallel
cooling induced by the radial expansion leads to the strong temperature
anisotropies of oxygen ions. The simulations are discussed within the
context of observations and theoretical models of the evolution of MHD
turbulence and ion thermodynamics in the outer corona and accelerating
solar wind.
Title: Dynamics of Forced MHD Turbulence and Coronal Heating
Authors: Rappazzo, A. F.; Dahlburg, R. B.; Einaudi, G.; Velli, M.
Bibcode: 2004AGUFMSH13A1152R
Altcode:
We present 3D MHD simulations modeling the heating of coronal
loops in the solar atmosphere via the tangling of coronal field
lines by photospheric footpoint motions. The overall behaviour of
the system is sensitive to the intrinsic time-scale present, namely
Alfvén propagation time along the loop, dynamical transverse time and
photospheric forcing correlation time. The line-tying effect associated
with the Alfvén wave propagation along the loop and the reflective
photospheric boundary conditions limit the extent of the inverse
cascade of magnetic energy when compared to 2D approximations and
increase intermittency in both kinetic and magnetic energy absorption
and dissipation. The simulations show that the corona self-organizes
in response to the forcing in what we conjecture to be a state of
minimal dissipation compatible with the driving.
Title: Heavy Element Abundances in the Heliospheric Plasma Sheet
Authors: Winterhalter, D.; Woo, R.; Velli, M.; Gloeckler, G.
Bibcode: 2004AGUFMSH31A1155W
Altcode:
Using sample crossings of the heliospheric plasma sheet by the Ulysses
spacecraft, we compare the abundances of heavy elements (Z > 2)
near and within the sheet with the abundances found generally in the
solar wind. The primary motivation for this study is to ascertain
first if the sheet abundances are in fact different from those in the
general solar wind, and, if so, to what extend the abundances reflect
those known to be at the sun. A close relationship would confirm the
sun to be the source of the plasma sheet, and may identify the source
region. Variations of the abundances measured in the sheet from those
near the sun will yield details of the transport mechanism.
Title: Simplified simulations of MHD
Authors: Buchlin, É.; Velli, Marco; Galtier, Sébastien; Vial,
Jean-Claude
Bibcode: 2004sf2a.conf...91B
Altcode: 2004sf2a.confE.334B
Because of the wide range of scales involved in MHD turbulence, a
statistical approach may become necessary to keep a global view of this
complex phenomenon. In particular, in the framework of the heating of
the solar corona, the smallest events are not directly detectable by the
current instruments but may be integrated to a statistical study. From
the numerical point of view, the contradictory needs for computing speed
and good description of MHD solutions may be addressed by simplified
models, which keep the most possible of the complex and non-linear
physics of the MHD equations but run sufficiently fast to produce
statistics of fields, of structures, and of "events". We propose
two such models which have been originally developed to represent
coronal loops (with forcing and Alfvén wave reflection at the loop's
foot-points), but which may be adapted to represent any region with
a dominant large-scale magnetic field. The first model consists of a
set of cellular automata, in which the non-linear terms of the MHD
equations are modelled by a threshold dynamics on current density
(Buchlin et al. A&A, 2003). In the second model, the cellular
automata are replaced by shell-models of MHD, so as to reach a greater
range of wavenumbers and to model more realistically the non-linear
couplings between modes at different scales. The results obtained
with these models will be presented and consequences of this study
for observational statistics and for theory of MHD turbulence will
be discussed.
Title: Magnetic Effects Change Our View of the Heliosheath
Authors: Opher, M.; Liewer, P. C.; Velli, M.; Gombosi, T. I.;
Manchester, W.; Dezeeuw, D. L.; Toth, G.; Sokolov, I.
Bibcode: 2004AIPC..719..105O
Altcode: 2004astro.ph..6184O
There is currently a controversy as to whether Voyager 1 has
already crossed the termination Shock, the first boundary of the
heliosphere. The region between the termination shock and the
heliopause, the heliosheath, is one of the most unknown regions
theoretically. In the heliosheath magnetic effects are crucial,
as the solar magnetic field is compressed at the termination shock
by the slowing flow. Recently, our simulations showed that the
heliosheath presents remarkable dynamics, with turbulent flows and
the presence of a jet flow at the current sheet that is unstable due
to magnetohydrodynamic instabilities. In this paper we review these
recent results, and present an additional simulation with constant
neutral atom background. In this case the jet is still present but with
reduced intensity. Further study, e.g., including neutrals and the tilt
of the solar rotation from the magnetic axis, is required before we can
definitively address how the heliosheath behaves. Already we can say
that this region presents remarkable dynamics, with turbulent flows,
indicating that the heliosheath might be very different from what we
previously thought.
Title: Magnetic Effects at the Edge of the Solar System: MHD
Instabilities, the de Laval Nozzle Effect, and an Extended Jet
Authors: Opher, M.; Liewer, P. C.; Velli, M.; Bettarini, L.; Gombosi,
T. I.; Manchester, W.; DeZeeuw, D. L.; Toth, G.; Sokolov, I.
Bibcode: 2004ApJ...611..575O
Altcode: 2004astro.ph..6182O
To model the interaction between the solar wind and the interstellar
wind, magnetic fields must be included. Recently, Opher et al. found
that by including the solar magnetic field in a three-dimensional
high-resolution simulation using the University of Michigan BATS-R-US
code, a jet-sheet structure forms beyond the solar wind termination
shock. Here we present an even higher resolution three-dimensional case
in which the jet extends for 150 AU beyond the termination shock. We
discuss the formation of the jet due to a de Laval nozzle effect and
its subsequent large-period oscillation due to magnetohydrodynamic
(MHD) instabilities. To verify the source of the instability, we
also perform a simplified two-dimensional geometry MHD calculation
of a plane fluid jet embedded in a neutral sheet with the profiles
taken from our three-dimensional simulation. We find remarkable
agreement with the full three-dimensional evolution. We compare both
simulations and the temporal evolution of the jet, showing that the
sinuous mode is the dominant mode that develops into a velocity-shear
instability with a growth rate of 5×10-9s-1=0.027
yr-1. As a result, the outer edge of the heliosphere presents
remarkable dynamics, such as turbulent flows caused by the motion of
the jet. Further study, including neutrals and the tilt of the solar
rotation from the magnetic axis, is required before we can definitively
address how this outer boundary behaves. Already, however, we can say
that the magnetic field effects are a major player in this region,
changing our previous notion of how the solar system ends.
Title: Magnetic Effects and our Changing View of the Heliosheath
Authors: Liewer, P. C.; Opher, M.; Velli, M.; Gombosi, T. I.;
Manchester, W.; DeZeeuw, D. L.; Toth, G.; Sokolov, I.
Bibcode: 2004AAS...204.7208L
Altcode: 2004BAAS...36R.799L
The Sun traveling through the interstellar medium carves out a
bubble of solar wind called the Heliosphere. Recent observations
indicate that Voyager 1, now beyond 90 AU, is in a region unlike
any encountered in it's 26 years of exploration. There is currently
a controversy as to whether or not Voyager 1 has already crossed the
Termination Shock, the first boundary of the Heliosphere (Krimigis et
al. 2003; McDonald et al. 2003, Burlaga et al. 2003). The controversy
stems from different interpretations of observations from several
instruments. Contributing to this controversy is our poor understanding
of the outer heliosphere. The region between the Termination Shock and
the Heliopause, the Heliosheath, is one of the most unknown regions
theoretically. In the Heliosheath magnetic effects are crucial, as
the solar magnetic field is compressed at the Termination Shock by the
slowing flow. Recently, our simulations showed that the Heliosheath is
remarkably dynamic, with turbulent flows resulting from an unstable
jet flow at the current sheet (Opher et al. 2003; 2004). In this
talk we review these recent results, and present additional results
from simulations of the unstable jet with a constant neutral atom
background. Further studies which include additional effects such
as the tilt between the solar rotation axis and the magnetic axis,
are required before we can definitively address the structure and
dynamics of the outer heliosphere. Already we can say that this region
presents remarkable dynamics, with turbulent flows, indicating that
the Heliosheath might be very different from what we previously thought.
Title: Learning from our Sun: The Interaction of Stellar with
Interstellar Winds
Authors: Opher, M.; Liewer, P. C.; Velli, M.; Gombosi, T. I.;
Manchester, W.; DeZeeuw, D. L.; Toth, G.; Sokolov, I. V.
Bibcode: 2004AAS...204.0303O
Altcode: 2004BAAS...36..671O
Stars have winds which interact with the interstellar medium. The
intensity of the winds can be 10 million times greater than that of
the solar wind. The magnetic fields of these stars can be orders of
magnitude greater than that of the Sun. The rotation periods can be
appreciably different from that of the Sun. A detailed description of
the interaction of stellar winds with the interstellar winds has never
been made. The interaction between the Sun and Interstellar Medium
creates three major structures: Termination Shock, Heliopause and
Bow Shock. Recently, we found (Opher et al. 2003, 2004) that beyond
the region where the solar wind become subsonic, the Termination
Shock, a jet-sheet structure forms in the equatorial plane of the
Sun rotation axis. This structure forms due to the compression of the
solar magnetic field by the interstellar wind. The structure of the
jet-sheet resembles a the "brim of a baseball cap"- it extends beyond
the Termination Shock for 150 AU (almost touching the Bow Shock) and
has a width of 10AU. This result is due to a novel application of a
state-of-art 3D Magnetohydrodynamic (MHD) code with a highly refined
grid (0.75 AU 4 orders of magnitude smaller than the physical dimensions
of the system). The jet-sheet is unstable and oscillates up and down
due to a velocity shear instability. We showed that the sinuous mode
is the dominant mode that develops into a velocity-shear-instability
with a growth rate of 0.027 years-1. We are the first to
predict the formation of this structure at the equatorial region in
the interaction of magnetized rotating star and an external wind (for
a stellar rotation and magnetic field axis aligned). In this work,
we extend our previous solar studies and investigate the effect in
other solar-like stars. We present the dependence of the jet-sheet
structure and the velocity-shear instability on the star mass-loss rate
and magnetic field. We discuss further applications to other stellar
wind interactions and the observational limits for the detection of
this structure.
Title: Aspects of nonlinear magnetohydrodynamics in the solar corona
Authors: Einaudi, G.; Rappazzo, A. F.; Velli, M.; Dahlburg, R. B.
Bibcode: 2004AIPC..703..193E
Altcode:
The solar corona is structured by the dynamics of plasmas and magnetic
fields, which, at the global scales of coronal loops, prominences and
helmet streamers may be described by magnetohydrodynamics. Here we will
discuss the importance and role of nonlinear interactions both in the
heating of the solar corona, which relies on the transfer, storage
and dissipation of the mechanical energy present in photospheric
motion, and in the acceleration of the slow solar wind above helmet
streamers. In the first example, nonlinear interactions including the
coupling of coronal magnetic fields to the velocity field and emerging
flux through the photosphere determine both the rate of heating and the
resulting coronal topology. In the second example, linear resistive
instabilities in develop nonlinearly to accelerate plasmoids into
the slow wind. Once plasmoids are generated, the melon-seed force due
to the overall magnetic field radial gradients is followed using an
Expanding Box Model.
Title: Simplified simulations of non-linear interactions in an
anisotropic plasma
Authors: Buchlin, E.; Velli, M.; Galtier, S.
Bibcode: 2004cosp...35.3555B
Altcode: 2004cosp.meet.3555B
Statistics may be necessary to keep a global view of the complexity of
astrophysical turbulence, in particular the effects of non-linear
interactions over a wide range of scales. However, from the
numerical point of view, a statistical approach to turbulence has the
contradictory needs for computing speed and for a good description
of the solutions of the MHD equations. This problem can be addressed
by simplified models, for example models with a reduced number of
well-chosen modes, which keep the most possible of the complex and
non-linear physics of the MHD equations but run sufficiently fast to
produce statistics of fields, of structures, and of "events". The model
we present here was orginally designed to represent a magnetic loop
in the solar corona, but may in fact help to understand turbulence in
any region with a dominant magnetic field ěc{B}_0. It consists of a
pile of shell-models, which allow to reach a wide range of wavenumbers
in the directions orthogonal to ěc{B}_0 and model the non-linear
couplings between these modes. The shell-models are also coupled by
Alfvén waves propagating along ěc{B}_0. We study the statistical
properties of energy dissipation and of the velocity and magnetic
fields produced by this model.
Title: Simulations of Wave Particle Interactions in the Expanding
Solar Wind in 1 and 2 Dimensions.
Authors: Velli, M.; Hellinger, P.; Goldstein, B.; Liewer, P.
Bibcode: 2003AGUFMSH21B0158V
Altcode:
We present hybrid simulations of the interaction of Alfvén and ion
cyclotron waves with protons and helium in the accelerating solar
wind using the expanding box model. We study how mirror force and
wave-particle interactions compete in shaping the distribution function
both for protons and minor ions for a range of initial fluctuation
spectra and propagation directions in 1 and 2 dimensions. The
simulations are discussed within the context of analytical and numerical
models of the evolution of MHD turbulence in the outer corona and
accelerating solar wind, with the aim of constraining the possible
initial conditions leading to the observed in situ evolution.
Title: MHD turbulence and the heating of astrophysical plasmas
Authors: Velli, Marco
Bibcode: 2003PPCF...45A.205V
Altcode:
Magnetohydrodynamic (MHD) turbulence plays a major role in the dynamics
and thermodynamics of astrophysical plasmas in many environments
and over a wide range of scales and parameters: primary examples are
the heating of stellar and accretion disk coronae, acceleration of
stellar winds, and star formation in molecular clouds. In the case
of the solar wind and corona in situ measurements and remote-sensing
observations have given the most detailed experimental knowledge of
the interplay between large-scale driving forces, the development of
a turbulent cascade, and the collisionless kinetics of dissipation,
than in any other natural magnetized plasma environment (with the
possible exception of the earth's magnetosphere). The questions of
coronal and solar wind acceleration will be reviewed here within the
general context of MHD turbulence and nonlinear interactions, from
the large-scale energy sources and driving to the dissipation scales
dominated by wave particle interactions, from the special role of
Alfvén waves to the naturally intermittent nature of coronal energy
release and solar flares.
Title: Magnetic Effects at the Edge of the Solar System: MHD
Instabilities, the de Laval nozzle effect and an Extended Jet
Authors: Opher, M.; Liewer, P. C.; Velli, M.; Gombosi, T.; Manchester,
W.; DeZeeuw, D.
Bibcode: 2003AAS...20313403O
Altcode: 2003BAAS...35.1421O
To model the interaction between the solar system and the interstellar
wind magnetic fields, ionized and neutral components besides cosmic
rays must be included. Recently (Opher et al. ApJL 2003) found, that
by including the solar magnetic field in an high resolution run with
the University of Michigan BATS-R-US code, a jet-sheet structure forms
beyond the Termination Shock. Here we discuss the formation of the jet
and its subsequent large period oscillation due to magnetohydrodynamic
instabilities. We perform in a simplified two dimensional geometry
resistive magnetohydrodynamic calculation of a plane fluid jet embedded
in a neutral sheet with the profiles taken from our simulation. We
find remarkable agreement with the full three dimensional evolution. We
present an even higher resolution three dimensional case where the jet
extends for 150AU beyond the Termination Shock. We compare the temporal
evolution of the jet showing that the sinuous mode is the dominant mode
that develops into a velocity-shear-instability with a growth rate of
5 × 10-9 sec-1=0.027 years-1. As a
result the outer edge of the heliosphere presents remarkable dynamics,
such as turbulence and flows caused by the motion of the jet. Further
study, e.g., including neutrals and the tilt of the solar rotation
from the magnetic axis, is required before we can definitively address
how this outer boundary behaves. Already, however, we can say that the
magnetic field effects are a major player in this region changing our
previous notion of how the solar system ends.
Title: Alfvén Wave Reflection and Turbulence in the Solar Corona
and Solar Wind
Authors: Verdini, A.; Velli, M.
Bibcode: 2003AGUFMSH21B0157V
Altcode:
We solve the equations for Alfvén wave propagation along the magnetic
field from the base of the solar corona into the solar wind using a
phenomenological term for nonlinear interactions and dissipation,
along the lines of Dmitruk et al. 2002. Wave reflection due to
the gradients in the Alfvén and solar wind speed is explicitly
taken into account as a source for the nonlinear interactions, and
regularity boundary conditions at the Alfvénic critical point are
imposed. Within the corona, most of the wave reflection and nonlinear
interactions occur close to the solar base, though the dependence on
outward wave amplitude is non-trivial. Models in which interacting
waves have comparable frequencies are considered as well as models
in which the interaction depends on the full spectrum of inward and
outward propagating modes. The relevance of Alfvén wave reflection as
a source for turbulent heating of coronal holes and the fast solar wind
is discussed, and our results are compared to previous work on the same
topic. Dmitruk,ÿP.; Matthaeus,ÿW.ÿH.; Milano,ÿL.ÿJ.; Oughton,ÿS.;
Zank,ÿG.ÿP.; Mullan,ÿD.ÿJ., 2002, ``Coronal Heating Distribution
Due to Low-Frequency, Wave-driven Turbulence", ApJ 575, 571.
Title: Magnetic Effects at the Edge of the Solar System: MHD
Instabilities, the de Laval nozzle effect and an Extended Jet
Authors: Opher, M.; Liewer, P.; Velli, M.; Bettarini, L.; Gombosi,
T. I.; Manchester, W.; Dezeeuw, D. L.; Toth, G.; Sokolov, I.
Bibcode: 2003AGUFMSH11C1114O
Altcode:
To model the interaction between the solar system and the interstellar
wind magnetic fields, ionized and neutral components besides cosmic
rays must be included. Recently (Opher et al. ApJL 2003) found, that
by including the solar magnetic field in an high resolution run with
the University of Michigan BATS-R-US code, a jet-sheet structure forms
beyond the Termination Shock. Here we discuss the formation of the jet
and its subsequent large period oscillation due to magnetohydrodynamic
instabilities. We perform in a simplified two dimensional geometry
resistive magnetohydrodynamic calculation of a plane fluid jet embedded
in a neutral sheet with the profiles taken from our simulation. We
find remarkable agreement with the full three dimensional evolution. We
present an even higher resolution three dimensional case where the jet
extends for 150AU beyond the Termination Shock. We compare the temporal
evolution of the jet showing that the sinuous mode is the dominant mode
that develops into a velocity-shear-instability with a growth rate of
5 x 10-9 sec-1=0.027 years-1. As a
result the outer edge of the heliosphere presents remarkable dynamics,
such as turbulence and flows caused by the motion of the jet. Further
study, e.g., including neutrals and the tilt of the solar rotation
from the magnetic axis, is required before we can definitively address
how this outer boundary behaves. Already, however, we can say that the
magnetic field effects are a major player in this region changing our
previous notion of how the solar system ends.
Title: Energy release in a turbulent three-dimensional corona
Authors: Dahlburg, R. B.; Einaudi, G.; Velli, M.; Linton, M. G.
Bibcode: 2003AdSpR..32.1131D
Altcode:
Recently a lot of theoretical evidence has emerged in support of
the hypothesis that coronal dissipation occurs in bursts at very
small spatial scales. In this picture, a large number of coherently
triggered, unobservable bursts is what appears as one of the many
observed solar events (e.g., flares, blinkers, flashes, etc.). Most
previous computational studies of this process have been limited to two
or two and one half spatial dimensions. In addition, an incompressible
model has been used. This is problematical, since the solar corona
is three-dimensional and compressible as well. Furthermore, it is
unclear how good an approximation reduced magnetohydrodynamics is
in the compressible situation. Here we present the first results
of our coronal dissipation calculations using a three-dimensional,
compressible model. We solve the MHD equations with CRUNCH3D, a
massively parallel, viscoresistive, three-dimensional compressible MHD
code. The code employs a Fourier collocation spatial discretization,
and uses a second-order Runge-Kutta temporal discretization. Published
by ElsevieiLtd on behalf of COSPAR.
Title: A Three-dimensional Model of the Solar Wind Incorporating
Solar Magnetogram Observations
Authors: Roussev, I. I.; Gombosi, T. I.; Sokolov, I. V.; Velli, M.;
Manchester, W., IV; DeZeeuw, D. L.; Liewer, P.; Tóth, G.; Luhmann, J.
Bibcode: 2003ApJ...595L..57R
Altcode:
We present a new compressible MHD model for simulating the
three-dimensional structure of the solar wind under steady state
conditions. The initial potential magnetic field is reconstructed
throughout the computational volume using the source surface method, in
which the necessary boundary conditions for the field are provided by
solar magnetogram data. The solar wind in our simulations is powered
by the energy interchange between the plasma and large-scale MHD
turbulence, assuming that the additional energy is stored in the
``turbulent'' internal degrees of freedom. In order to reproduce
the observed bimodal structure of the solar wind, the thermodynamic
quantities for the initial state are varied with the heliographic
latitude and longitude depending on the strength of the radial
magnetic field.
Title: A solar cellular automata model issued from reduced MHD
Authors: Buchlin, E.; Aletti, V.; Galtier, S.; Velli, M.; Vial, J. -C.
Bibcode: 2003AIPC..679..335B
Altcode:
A three-dimensional cellular automata (CA) model inspired by the reduced
magnetohydrodynamic equations is presented to describe impulsive events
generated along a coronal magnetic loop. It consists of a set of planes,
distributed along the loop, between which the information propagates
through Alfvén waves. Statistical properties in terms of power laws are
obtained in agreement with SoHO observations of X-ray bright points of
the quiet Sun. Physical meaning and limits of the model are discussed.
Title: Evolution of Wake Instabilities and the Acceleration of the
Slow Solar Wind: Melon Seed and Expansion Effects
Authors: Rappazzo, A. F.; Velli, M.; Einaudi, G.; Dahlburg, R. B.
Bibcode: 2003AIPC..679..371R
Altcode:
We extend previous 2D simulation studies of slow solar wind acceleration
due to the nonlinear evolution of the instability of the plasma/current
sheet above streamers. We include the effects of the melon-seed force
due to the overall magnetic field radial gradients on the plasmoid
formed by the instability, as well as the subsequent expansion effects
using the Expanding Box Model.
Title: Nonlinear evolution of large-amplitude Alfvén waves in
parallel and oblique propagation
Authors: del Zanna, Luca; Velli, Marco; Londrillo, Pasquale
Bibcode: 2003AIPC..679..566D
Altcode:
The stability of monochromatic large-amplitude Alfvén waves is
investigated via MHD numerical simulations. In a compressible medium,
such as the heliospheric environment, these waves are subject to the
parametric decay instability. The mother wave decays in a compressive
mode, that soon steepens and dissipates thermal energy, and in a
backscattered Alfvénic mode with lower amplitude and frequency, thus
starting an inverse cascade. This well known process is shown here to
be very robust, since it occurs basically unchanged regardless of the
dimensionality of the spatial domain and, above all, even linear or
arc-polarized waves in oblique propagation, most often found in solar
wind data, appear to behave in the same way. This physical process
could help to explain the observed radial decrease of cross helicity
in the fast polar wind, as measured by Ulysses.
Title: A simplified numerical model of coronal energy dissipation
based on reduced MHD
Authors: Buchlin, E.; Aletti, V.; Galtier, S.; Velli, M.; Einaudi,
G.; Vial, J. -C.
Bibcode: 2003A&A...406.1061B
Altcode: 2002astro.ph.12444B
A 3D model intermediate between cellular automata (CA) models and
the reduced magnetohydrodynamic (RMHD) equations is presented to
simulate solar impulsive events generated along a coronal magnetic
loop. The model consists of a set of planes distributed along a
magnetic loop between which the information propagates through Alfvén
waves. Statistical properties in terms of power-laws for energies and
durations of dissipative events are obtained, {and their} agreement
with X-ray and UV flares observations {is discussed}. The existence
of observational biases is {also} discussed.
Title: Distributions of Coronal Events: Simulations and Event
Definitions
Authors: Buchlin, Éric; Galtier, Sébastien; Velli, Marco; Vial,
Jean-Claude
Bibcode: 2003ANS...324..109B
Altcode: 2003ANS...324..P15B
No abstract at ADS
Title: Numerical simulations of wave-particle interactions in coronal
heating and solar wind acceleration
Authors: Velli, M.; Liewer, P. C.; Goldstein, B. E.
Bibcode: 2003SPD....34.0606V
Altcode: 2003BAAS...35..818V
We present hybrid simulations of the interaction of Alfvén and ion
cyclotron waves with protons and helium in the accelerating solar
wind using the expanding box model. We study how mirror force and
wave-particle interactions compete in shaping the distribution
function both for protons and minor ions for a range of initial
fluctuation spectra and propagation directions, and attempt to couple
the results back into a self-consistent solar wind acceleration
model. The simulations are discussed within the context of analytical
and numerical models of the evolution of MHD turbulence in the outer
corona and accelerating solar wind, with the aim of constraining the
possible initial conditions leading to the observed in situ evolution.
Title: Interpreting Coronagraph Data used Simulated White Light
Images and 3D MHD Models of CMEs
Authors: Liewer, P. C.; Opher, M.; Velli, M.; Manchester, W.; DeZeeuw,
D.; Gombose, T.; Roussev, I.; Sokolov, I.; Toth, G.; Powell, K.
Bibcode: 2003SPD....34.0511L
Altcode: 2003BAAS...35Q.816L
We use a 3D time-dependent MHD model of a CME to try to understand the
relationship between the CME structure and the bright features seen
in coronagraph images. Questions addressed include whether the bright
leading edge seen in LASCO coronagraph images of CMEs corresponds to
compressed coronal material or shocked solar wind. We will analyze
the evolution of the density and magnetic field as the CME propagates
for CMEs of various field strengths and initial speeds. Coronagraph
line-of-sight (LOS) images show 2D projections of the 3D density
structure of the CME. Synthetic coronagraph images will be computed
for the various CME cases to relate the structure to the LOS images. We
use the University of Michigan BATS-R-US time-dependent adaptive grid
MHD code to compute the CME evolution. The CME is created by inserting
a flux-rope CME into a steady-state solution for the corona. The flux
rope is anchored at both ends in the photosphere and embedded in a
helmet streamer; it is not initially in equilibrium. The subsequent
evolution of the flux rope - its expansion and propagation through the
corona to 1 AU - is computed self-consistently with the evolution of
the background corona and solar wind.
Title: The Formation of an Unstable Jet-Sheet at the Edge of the
Solar System
Authors: Opher, M.; Liewer, P.; Velli, M.; Gombosi, T.; Manchester,
W.; DeZeeuw, D.; Sokolov, I.; Toth, G.
Bibcode: 2003SPD....34.0604O
Altcode: 2003BAAS...35Q.818O
We find that the boundary between the solar system and the interstellar
medium an unstable jet-sheet forms. The jet is unstable and oscillates
up and down due to Kelvin-Helmholtz type instability. We use a
state-of-art 3D MHD code art with an adaptive grid mesh especially
designed to refine the region at the current sheet and in the region
between the termination shock and the heliopause. In the present study
we assume as a first approximation that the solar magnetic field and
rotation axis are aligned. We include in the model self-consistently
magnetic field effects in the interaction between the solar and
interstellar winds. Previous studies of this interaction had poorer
spatial resolution and did not include the solar magnetic field. We
present results from three different resolutions (ranging from 0.5AU to
6AU at the current sheet) and discuss the effect of resolution on the
characteristics of the jet such as strength and width. We show that in
order to resolve the jet, there is a need of a resolution higher than
3-4AU, the resolution used in previous studies. The neutrals interacting
with the plasma component by charge-exchange interactions can affect
the formation of the jet and we present results discussing their effect.
Title: Solar Wind Ten
Authors: Velli, Marco; Bruno, Roberto; Malara, Francesco; Bucci, B.
Bibcode: 2003AIPC..679.....V
Altcode:
No abstract at ADS
Title: Nonlinear processes in heliospheric plasma: models and
observations
Authors: Velli, M.; Einaudi, G.; Chiuderi, C.; Veltri, P. L.;
MM02242342 project Team; Betta, R.; Londrillo, P.; Rappazzo, F.;
Del Zanna, L.; Landi, S.; Malara, F.; Carbone, V.; Zimbardo, G.;
Primavera, L.; Greco, A.; Sorriso-Valvo, L.; Pommois, P.; Lepreti, F.
Bibcode: 2003MmSAI..74..425V
Altcode:
We present the scientific objectives of the research project `Nonlinear
processes in heliospheric plasma: models and observations', co-financed
by the Italian Ministry for Universities and Scientific Research, as
well as a summary of some of the results obtained. The objective of the
proposal was a detailed study of the nonlinear and dissipation-scale
dynamics of heliospheric plasmas. The project focused on the study of
wave propagation and properties of turbulence at the various scales,
from the macroscopic scales of the solar wind, down to the microscopic
scales of magnetic reconnection and turbulence dissipation, in its
two aspects of evolutionary internal dynamics, and its effects on the
transport of energetic particles of both heliospheric and extra-solar
origins (cosmic rays, interstellar neutrals ionized in the solar wind
as pickup ions).
Title: 3D MHD description of the region beyond the termination shock:
The behaviour of the Current Sheet
Authors: Opher, M.; Liewer, P.; Gombosi, T.; Manchester, W.; Dezeeuw,
D. L.; Powell, K.; Sokolov, I.; Toth, G.; Velli, M.
Bibcode: 2002AGUFMSH21A0485O
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A fully self consistent MHD study of the heliosheath region is carried
out, using BATSRUS, a three dimensional time dependent adaptive grid
magnetohydrodynamic (MHD) model. The heliosheath, located between
the termination shock and the heliopause, has not been studied in
detail. At the termination shock the solar wind passes from a supersonic
to a subsonic regime decelerating until it reaches the heliopause
where it is diverted to the heliotail. This region is intersected
in the equatorial plane (assuming a no-tilt for the dipole field)
by a current sheet as the solar magnetic field changes polarity. One
of the major questions is whether the current sheet remains at the
equatorial plane. The magnetic field of the solar wind is included. In
order to isolate the effects at this region we assumed no magnetic
field in the interstellar medium. We observe a much faster flow of the
current sheet, where the compressed azimuthal magnetic field is absent,
leading to large velocity shear. With BATSRUS, we were able to obtain
high resolution needed to analyze the behavior of this complicated
regime, in particular the stability of the current sheet. We report
the results and comment on the major processes responsible.
Title: Simulations of wave particle interactions in the expanding
solar wind in the presence of particle beams
Authors: Velli, M.; Liewer, P.; Goldstein, B.
Bibcode: 2002AGUFMSH12A0415V
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We use the Expanding Box Model, which has been updated to include
the effects of the mirror force, to carry out 1D simulations of
wave-particle interactions in the fast solar wind in the presence of
particle beams. The aim is to understand the effects of beams on wave
dissipation as well as the role of turbulence in the regulation and
possible generation of the proton beam in the fast solar wind.
Title: A Solar Cellular Automata Model Issued From Reduced MHD
Authors: Buchlin, E.; Aletti, V.; Galtier, S.; Velli, M.; Vial, J. -C.
Bibcode: 2002sf2a.conf..129B
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A three-dimensional cellular automata model inspired by the reduced
magnetohydrodynamic equations is presented to describe impulsive events
generated along a coronal magnetic loop. It consists of a set of planes,
distributed along the loop, between which the information propagates
through Alfven waves. Statistical properties in terms of power laws are
obtained in agreement with SoHO observations of X-ray bright points of
the quiet Sun. Physical meaning and limits of the model are discussed.
Title: Coronal heating through Alfven waves
Authors: Del Zanna, L.; Velli, M.
Bibcode: 2002AdSpR..30..471D
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Sustaining the hot solar corona above polar regions, where the fast
solar wind is accelerated, requires an energy flux of about 5 × 10
5 erg cm -2 s -1 whose source must
be the photospheric motions below. The precise way this energy is
transferred and damped remains an open question, though Alfvén waves
are the more natural candidates. Such waves are observed in situ in
the fast solar wind and they are believed to provide heating through
kinetic resonant dissipation. Recent observations suggesting strong
anisotropic heating of heavy ions in coronal holes seem to confirm
that this mechanism is at work in the corona too and thus Alfvén waves
must play a fundamental role there. However, in order for such waves to
dissipate efficiently in the corona, extremely small scales must form
because of the huge local magnetic Reynolds numbers. Hence, one must
either assume that waves are directly generated at the dissipation
scales, as suggested in models with chromospheric and transition
region reconnection, or small scales must be reached through dynamical
evolution. This should occur both thanks to the inhomogeneous coronal
magnetic fields (resonant absorption, phase mixing) and to nonlinear
wave-wave interactions. The traditional Kolmogorov-like cascade,
involving interactions between incompressible modes, is inhibited,
since for its development it requires waves propagating both upwards
and downwards in the atmosphere. Therefore, coupling to compressible
modes must play an important role, especially where strong transverse
gradients in the Alfvén velocity are not at disposal, such as in
coronal holes. A source for effective dissipation of upward propagating
Alfvén waves via steepening of generated magnetoacoustic modes is
provided by the parametric decay process, whose nonlinear stage will
be studied here in two and three spatial dimensions.
Title: Energy release in a turbulent three-dimensional solar corona
Authors: Einaudi, G.; Dahlburg, R.; Velli, M.; Linton, M.
Bibcode: 2002cosp...34E1266E
Altcode: 2002cosp.meetE1266E
Recently a lot of theoretical evidence has emerged in support of
the hypothesis that coronal dissipation occurs in bursts at very
small spatial scales. In this picture, a large number of coherently
triggered, unobservable bursts is what appears a one of the many
observed solar events (e.g., flares blinkers, flashes, etc.). Most
previous computational studies of this process have been limited to
two or two an half spatial dimensions. In addition, an incompressible
model has been used. This is problematical, since the solar corona
is three-dimensional and compressible as well. Furthermore, it is
unclear how good an approximation reduced magnetohydrodynamics is in
the compressible situation. Here we present the first results of our
new compressible MHD coronal dissipation calculations. We solve the
MHD equations with CRUNCH3D, a massively parallel, visco-resistive,
MHD code with compressibility and thermal conduction. The code employs
a Fourier collocation spatial discretization, and uses a second-order
RungeKutta temporal discretization. Most simulations we will discuss
here are run with 64 X 64 X 64 Fourier modes.
Title: Alfvén wave propagation and ion cyclotron interactions in
the expanding solar wind: One-dimensional hybrid simulations
Authors: Liewer, Paulett C.; Velli, Marco; Goldstein, Bruce E.
Bibcode: 2001JGR...10629261L
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We carry out one-dimensional hybrid simulations of Alfvén waves
propagating along the magnetic field in the presence of a mean radial
spherically expanding plasma outflow, representing fast solar wind
streams. The equations for particle ions of multiple species and
fluid electrons are solved using the Expanding Box Model, a locally
Cartesian representation of motion in spherical coordinates, in a frame
moving with the local average wind speed. The model gives a minimally
consistent description of the effects associated with such motion
on particle dynamics, e.g., the flux-conserving decrease of magnetic
field intensity and consequent decrease of cyclotron frequency with
increasing distance from the Sun. The cyclotron frequency decreases
faster than Alfvén wave frequency, allowing fluctuations below
the cyclotron frequency at smaller distance from the Sun to come
into cyclotron resonance at greater distances. The hybrid treatment
yields a fully self-consistent description of the consequent cyclotron
wave-particle interaction in a multi-ion plasma. We present results for
cases of monochromatic circularly polarized Alfvén waves propagating
radially outward and for initially well developed Alfvénic spectra
with and without alpha particles. When both alpha particles and
protons are present, the alpha particles, which come into resonance
first as the wind expands, are observed to be preferentially heated and
accelerated. For high beta (equal to ratio of ion pressure to magnetic
field pressure) the amount of alpha particles acceleration and heating
is limited by the available wave power. For low beta cases the amount
of heating and acceleration is limited, not by the wave power, but
by the depletion of the distribution function in the resonance region
by pitch-angle scattering. The implication of these results for solar
wind models is discussed.
Title: Hybrid simulations of wave-particle interactions in the
expanding solar wind: effects of the mirror force.
Authors: Velli, M.; Liewer, P. C.; Goldstein, B. E.
Bibcode: 2001AGUFMSH21A0728V
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We present hybrid simulations of wave-particle interactions in the
solar wind, including the effects of the mirror force on the proton and
alpha-particle distribution functions. In previous work, we showed how
hybrid simulations using the expanding box model were able to describe
particle distribution deformation due to wave absorption including
important phenomena such as frequency streaming of the wave-spectrum
due to the decay of Alfvén speed with distance from the sun (Liewer,
P.C., Velli, M. and Goldstein, B.E., JGR 2001 in press). In its original
formulation, gradients of a background radial or spiral magnetic field
within the box were neglected, and the global heliospheric magnetic
field was taken to be uniform at the box scale, with a time variability
due to the motion of the plasma. In the supersonic region of the wind,
we assumed the mirror force to be negligible, or rather, we assume
the mirror force to only contribute in defining the asymptotic wind
speed. However, the non-vanishing radial gradient of the magnetic
field will cause a force, within the box, which tends to distort
distribution functions. This force is uniform in space (at the scale
of the box), time-dependent via the decreasing magnetic field with
distance, and tends to accelerate particles with a larger than average
perpendicular temperature, and decelerate particles with a cooler than
average perpendicular temperature. This term significantly improves
the description of particle behaviour in our model, especially close
to the sun, as the mirror force will provide a secular deformation of
distribution functions, consistent with their radial streaming from
the sun, even in the absence of waves. Our new simulations will be
compared to previous results as well as semi-analytical kinetic models
of the solar wind.
Title: Hydrodynamics of the Solar Wind Expansion
Authors: Velli, M.
Bibcode: 2001Ap&SS.277..157V
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A pedagogical introduction to the classical hydrodynamic steady state
solutions for flows in a spherically symmetric atmosphere is presented
along with a detailed analysis of the stability of such flows and a
discussion of the Parker/Bondi phase diagram of solutions in the Mach
number - stellar radius plane. This leads naturally to the scenario
presented by Velli (1994) for wind-accretion transitions, which helps
to explain results from numerical simulations in many contexts and
gives a more comprehensive answer to the question of why the solar
wind and other steady state flows in spherical symetry must quite
generally become supersonic.
Title: Origin of the Slow Solar Wind
Authors: Dahlburg, R. B.; Einaudi, G.; Velli, M.
Bibcode: 2001AGUSM..SH21B05D
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The dynamical behaviour of a configuration consisting of a plane fluid
wake flowing in a neutral sheet is examined. Such a configuration
is assumed to model a section of the streamer belt where the typical
Alfven velocity exceeds the typical flow speed far away from the neutral
sheet, i.e. within some solar radii. The neutral sheet is cohexisitng
with a plasma sheet where the plasma beta is enhanced of two orders of
magnitude with respect to typical coronal values, due to a contemporary
increase of density and decrease of magnetic field. It is shown that
beyond the helmet cusp such a configuration is resistively unstable
and develops at the center of the streamer a magnetic island which is
accelerated outwards by a Kelvin-Helmholtz instability which develops
during the non-linear evolution of the resistive instability. The values
of density within the island is enhanced by the nonlinear dynamics,
producing a density contrast, depending on the local plasma beta,
of some percents. The resulting accelerated plasmoid passively traces
the acceleration of the inner part of the wake and it is claimed to
represent the moving coronal features of enhanced density observed
with LASCO instrument on the Solar and Heliospheric Observatory (SOHO).
Title: Constraints on high frequency wave heating of coronal holes
and the fast solar wind
Authors: Velli, M.; Liewer, P. C.; Goldstein, B. E.
Bibcode: 2001AGUSM..SH41B04V
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We present numerical simulations as well as model calculations of ion
heating via interaction with a spectrum of outward propagating Alfven
waves and compare them with phenomenological models of the solar
wind. Approximations for the total maximm amount of energy that may
be absorbed by minor ions, as well as the overall absorption spectrum
are given and compared to predictions from quasi-linear theory. The
simulations use the previously developed Expandng Box Model to provide
limits on the efficiency of cyclotron heating.
Title: Coronal Heating via MHD Turbulence
Authors: Einaudi, G.; Betta, R.; Velli, M.
Bibcode: 2001AGUSM..SP51C04E
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We present reduced MHD turbulence scenarios for coronal heating in a
coronal loop using the 2D cross section approximation (Einaudi and Velli
1999) and the complete boundary value problem 3D simulations. 2D runs
are carried out for longer times and greater resolution with different
types of forcing, allowing the exploration of the dependence of the
heating rates, statistical properties and scalings with the nature
of the photospheric forcing. 3D runs are carried out using only some
selected photospheric forcings. It is found that line-tying partially
inhibits the inverse cascade which dominates the 2D approximation. In
3D the overall behavior of the system is sensitive to the 3 time-scales
present in the system, namely propagation time along the loop, dynamical
transverse time and photospheric forcing correlation time. Coronal
heating scaling laws are reviewed in the light of simulation results
Title: Plasmoid Formation and Acceleration in the Solar Streamer Belt
Authors: Einaudi, Giorgio; Chibbaro, Sergio; Dahlburg, Russell B.;
Velli, Marco
Bibcode: 2001ApJ...547.1167E
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The dynamical behavior of a configuration consisting of a plane fluid
wake flowing in a current sheet embedded in a plasma sheet that is
denser than its surroundings is discussed. This configuration is a
useful model for a number of structures of astrophysical interest, such
as solar coronal streamers, cometary tails, the Earth's magnetotail
and Galactic center nonthermal filaments. In this paper, the results
are applied to the study of the formation and initial motion of the
plasma density enhancements observed by the Large-Angle Spectrometric
Coronagraph (LASCO) instrument onboard the Solar and Heliospheric
Observatory (SOHO) spacecraft. It is found that beyond the helmet cusp
of a coronal streamer, the magnetized wake configuration is resistively
unstable, that a traveling magnetic island develops at the center of
the streamer, and that density enhancements occur within the magnetic
islands. As the massive magnetic island travels outward, both its speed
and width increase. The island passively traces the acceleration of
the inner part of the wake. The values of the acceleration and density
contrasts are in good agreement with LASCO observations.
Title: Parametric decay of circularly polarized Alfvén waves:
Multidimensional simulations in periodic and open domains
Authors: Del Zanna, L.; Velli, M.; Londrillo, P.
Bibcode: 2001A&A...367..705D
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The nonlinear evolution of monochromatic large-amplitude circularly
polarized Alfvén waves subject to the decay instability is studied via
numerical simulations in one, two, and three spatial dimensions. The
asymptotic value of the cross helicity depends strongly on the
plasma beta: in the low beta case multiple decays are observed, with
about half of the energy being transferred to waves propagating in
the opposite direction at lower wave numbers, for each saturation
step. Correspondingly, the other half of the total transverse energy
(kinetic and magnetic) goes into energy carried by the daughter
compressive waves and to the associated shock heating. In higher
beta conditions we find instead that the cross helicity decreases
monotonically with time towards zero, implying an asymptotic balance
between inward and outward Alfvénic modes, a feature similar to
the observed decrease with distance in the solar wind. Although the
instability mainly takes place along the propagation direction, in
the two and three-dimensional case a turbulent cascade occurs also
transverse to the field. The asymptotic state of density fluctuations
appears to be rather isotropic, whereas a slight preferential cascade
in the transverse direction is seen in magnetic field spectra. Finally,
parametric decay is shown to occur also in a non-periodic domain with
open boundaries, when the mother wave is continuously injected from one
side. In two and three dimensions a strong transverse filamentation is
found at long times, reminiscent of density ray-like features observed
in the extended solar corona and pressure-balanced structures found
in solar wind data.
Title: Observations and Models of Coronal Heating
Authors: Malara, F.; Velli, M.
Bibcode: 2001IAUS..203..456M
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The theoretical basis and observational evidence for possible
scenarios of coronal heating are reviewed, with particular emphasis
on the interpretative uncertainties involved in the detection of
low energy bursts, or nanoflares. From a theoretical point of view,
it appears that the presence of complex topologies in the coronal
magnetic field is crucial both both for the triggering of discrete
small-scale events and efficient wave dissipation. At low energies,
the distinction between AC/DC heating may be blurred, since the power
output from dissipating waves or small scale current sheets can be
bursty and intermittent. The interplay of these phenomena is discussed
within the unifying framework of MHD turbulence, and the importance
of progress in forward modelling from theories to observables in the
understanding of sub-resolution physics is underlined.
Title: Microscale Structures on the Quiet Sun and Coronal Heating
Authors: Aletti, V.; Velli, M.; Bocchialini, K.; Einaudi, G.;
Georgoulis, M.; Vial, J. -C.
Bibcode: 2000ApJ...544..550A
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We present some results concerning transient brightenings on the quiet
Sun, based on data from the Extreme-Ultraviolet Imaging Telescope on
board the Solar and Heliospheric Observatory. Histograms of intensity
are found to be well fitted by χ2 distributions for
small values of the intensity, while at high intensities power-law
distributions are always observed. Also, the emission presents the
same statistical properties when the resolution is downgraded by local
averaging; i.e., it appears to be self-similar down to the resolution
scale of the instruments. These properties are characteristic of
the emission from a forced turbulent system whose dissipation scale
is much smaller than the pixel dimension. On the basis of the data
presented as well as other published results and our present theoretical
understanding of MHD turbulence, we discuss the realism of the nanoflare
scenario of coronal heating.
Title: Hybrid simulations of preferential heating of heavy ions in
the solar wind
Authors: Liewer, Paulett C.; Velli, Marco; Goldstein, Bruce E.
Bibcode: 2000AIPC..528..274L
Altcode: 2000atep.conf..274L
We present results from the first fully self-consistent 1D hybrid
(kinetic ions/fluid electrons) simulations of the preferential
heating of alphas and heavier minor ions by a flat spectrum of
Alfvén-ion cyclotron waves in a collisionless plasma. We find that the
simulations reproduce the observed solar wind scaling T~M for alphas
and heavier minor ions when the alphas and the minor ions have equal
charge to mass ratios, q/M, and equal initial thermal velocities,
Vth=(T/M)1/2. This scaling is interpreted as a result of
the basic physics: the time evolution of the Vlasov/Maxwell system
without collisions depends only on the ratio q/M and not q or M
separately. Because this result follows from the basic nature of the
physical model, the T~M scaling would be obtained for any spectrum
of waves. For minor ions with q/M different from the alphas but equal
initial thermal velocities, the final thermal velocity is seen to vary
by +/-50% from that of the alphas in the simulations presented here. .
Title: Proton, Helium and Minor Ion Interactions with Circularly
Polarized Alfven and Ion-cyclotron waves in the Expanding Solar Wind:
Hybrid Simulations
Authors: Velli, M.; Liewer, P. C.; Goldstein, B. E.
Bibcode: 2000SPD....31.0233V
Altcode: 2000BAAS...32..816V
We present simulations of parallel propagating Alfvén waves in the
accelerating solar wind and their interactions with protons, alpha
particles, and minor ions using an expanding box hybrid code (Liewer
et al., 1999). In this model, the average solar wind flow speed is a
given external function, and the simulation domain follows a plasma
parcel as it expands both in the radial and transverse directions
accordingly: the decrease of Alfvén speed and density with distance
from the Sun are taken into account self-consistently. It is therefore
possible to carry out a detailed study of frequency drifting and the
coming into resonance with the waves at different radial locations
of particles with differing charge to mass ratios. Simulations of
monochromatic waves as well as waves with well-developed spectra are
presented for plasmas with one, two and three ion species. We observe
preferential heating and acceleration of protons and minor ions. Under
some conditions, we obtain the scaling observed in coronal hole solar
wind: the heavy ion temperature is proportional to its mass (Liewer
et al., 2000). A comparison with predictions from models based on such
quasi-linear or linear analyses will also be presented. P. C. Liewer,
M. Velli and B. E. Goldstein, in Solar Wind Nine, S. Habbal, R. Esser,
J. V. Hollweg, P. A. Isenberg, eds., (AIP Conference Proceedings 471,
1999) 449. P. C. Liewer, M. Velli, and B. E. Goldstein, in Proc. ACE
2000 Conference (2000) to be published.
Title: The distribution of flares, statistics of magnetohydrodynamic
turbulence and coronal heating
Authors: Einaudi, G.; Velli, M.
Bibcode: 1999PhPl....6.4146E
Altcode:
In this paper theoretical evidence in favor of the hypothesis that
coronal dissipation occurs in bursts at very small spatial scales is
presented. Each individual burst, though unobservable and energetically
insignificant, is thought to represent the building block of coronal
activity. In this framework, a large number of coherently triggered
bursts is what appears as one of the many observed solar atmospheric
events (i.e., blinkers, heating events, explosive events, flashes,
microflares, flares,...). Histograms of such events, when computed,
in terms of total energy, duration and peak luminosity appear to
display power-law behavior. Simulations of the energy dissipation
in the simplest possible forced magnetohydrodynamic (MHD) system,
admitting reconnection events, indeed displays such kind of behavior:
dissipative events of varying intensity, size and duration may be
defined, whose distributions follow power laws. The meaning of cellular
automaton models, introduced to describe the power-law statistics
of observed energetic events on the Sun, i.e., solar flares, is then
discussed. Finally, a minimal set of constraints necessary to render
such automaton models more relevant for the description of dynamic
phenomena described by magnetohydrodynamic equations is introduced.
Title: Hybrid simulations of wave propagation and ion cyclotron
heating in the expanding solar wind
Authors: Liewer, Paulett C.; Velli, Marco; Goldstein, Bruce E.
Bibcode: 1999AIPC..471..449L
Altcode: 1999sowi.conf..449L
We present results from hybrid (particle ions, fluid electrons)
simulations of the evolution of Alfvén waves close to the ion
cyclotron frequency in the solar wind, which take into account
the basic properties of the background solar wind flow, i.e., the
spherical expansion and the consequent decrease in magnetic field and
cyclotron frequency with increasing distance from the Sun. We follow
the evolution of a plasma parcel in a frame of reference moving with
the solar wind using a 1D expanding box hybrid model; use of the hybrid
model yields a fully self-consistent treatment of the resonant cyclotron
wave-particle interaction. This model is related to a previous MHD model
(1), which allows the use of a simple Cartesian geometry with periodic
boundary conditions. The use of stretched expanding coordinates in
directions transverse to the mean radial solar wind flow naturally
introduces an anisotropic damping effect on velocity and magnetic
field. We present results for the evolution of a 1/k spectrum of
circularly polarized Alfvén wave propagating radially. Initially,
most of the wave energy is below the cyclotron frequency for both the
alpha particles and protons. As the wind expands, the wave frequencies
decrease more slowly than the cyclotron frequencies and the waves
come into cyclotron resonance. When only protons are present,
significant perpendicular heating is observed as the dominant wave
frequencies approach the proton cyclotron frequency. When both alphas
and protons are present, the alphas, which come into resonance first,
are observed to be preferentially heated and accelerated. In both cases,
the cyclotron damping leads to a steepening of the wave spectrum.
Title: Coronal plumes and the expansion of pressure balanced
structures in the fast solar wind
Authors: Del Zanna, Luca; Velli, Marco
Bibcode: 1999AIPC..471..385D
Altcode: 1999sowi.conf..385D
The expansion of a coronal hole filled with a discrete number of higher
density filaments (plumes) in overall pressure balance with the ambient
medium is described within the thin flux tube approximation. The
resulting solar wind model extends the results of Parker (1964)
and Velli et al. (1994) to non-isothermal temperature profiles and
includes a flux of Alfvén waves propagating both inside and outside
the structures. Remote sensing and solar wind in situ observations are
used to constrain the parameter range of our study. Possible candidates
to be the remnants of plumes in the polar high-speed solar wind are
pressure balanced structures (PBS) and/or microstreams.
Title: Propagation and disruption of Alfvénic solitons in the
expanding solar wind
Authors: Velli, M.; Buti, B.; Goldstein, B. E.; Grappin, R.
Bibcode: 1999AIPC..471..445V
Altcode: 1999sowi.conf..445V
The evolution of large amplitude circularly polarized Alfvén solitons
in the solar wind is described via dispersive MHD simulations in the
expanding box model. Solar wind expansion causes the soliton amplitude
to decay, initially following the standard WKB law, which together
with the plasma cooling causes the propagation speed to decrease as
well. At long times, the soliton disrupts, giving rise to relative
density fluctuations which are enhanced as compared to those which
develop when evolution is within the framework of homogeneous MHD.
Title: Coronal plumes and the expansion of pressure-balanced
structures in the fast solar wind
Authors: Casalbuoni, Sara; Del Zanna, Luca; Habbal, Shadia R.;
Velli, Marco
Bibcode: 1999JGR...104.9947C
Altcode:
The expansion of a coronal hole filled with a discrete number of
higher-density filaments (plumes) in overall pressure balance with the
ambient medium is described within the thin flux tube approximation. The
resulting solar wind model extends the results of Parker [1964]
and Velli et al. [1994] to nonisothermal temperature profiles and
includes a flux of Alfvén waves propagating both inside and outside
the structures. Remote sensing and solar wind in situ observations
are used to constrain the parameter range of our study. Close to the
Sun, the precise plasma parameters are fundamental in determining the
relative position of the critical points, which are found by means
of an iterative procedure because the flows from the two regions are
coupled. At greater distances the filling factor of the higher-density
regions may vary largely, and streams which are either faster or
slower than the wind arising from the ambient hole may result,
depending on the temperature differences and on the flux of Alfvén
waves assumed in the two regions. Velocity differences of the order
of ~50 kms-1, such as those found in microstreams in the
high-speed solar wind, might be thus easily explained by reasonable
fluctuation amplitudes at the Sun, although the natural candidates
for plumes at large heliocentric distances are more likely to be the
so-called pressure-balanced structures.
Title: Alfvénic Turbulence and Wave Propagation in the Corona
and Heliosphere
Authors: Velli, M.
Bibcode: 1999LNP...536..198V
Altcode: 1999nmwt.conf..198V
The properties of Alfvén wave propagation through the solar corona
and heliosphere are reviewed, with particular emphasis on the role of
large scale radial gradients in determining the variation of amplitude
with distance. Some comments on the role of photospheric vorticity as
a source for Alfvén waves are presented, 1-D and 2-D MHD couplings
of the waves as they climb out of the coronal density gradient are
described (parametric decay, sound wave generation), while the final
section is devoted to the 1-D propagation of Alfvén type solitons in
the solar wind. Throughout, the focus is on the relevance of theory
to the turbulent fluctuations measured in the wind and remote sensing
observations of the corona: many puzzles and problems are highlighted.
Title: Alfvén Wave Generation in Photospheric Vortex Filaments,
Macrospicules, and "Solar Tornadoes"
Authors: Velli, Marco; Liewer, Paulett
Bibcode: 1999SSRv...87..339V
Altcode:
The properties of Alfvén waves generated in the photosphere by field
lines trapped in vortex sinks and propagating upwards through the
transition region and corona are discussed and contrasted to those
of waves generated via reconnection in transition region explosive
events, or rather via untwisting reconnecting flux tubes. An outline
for future simulations and theoretical advances necessary to understand
the dynamics of spicules and macrospicules is described, and a detailed
search for photospheric velocity patterns underlying macrospicules
is suggested.
Title: Hybrid Simulations of Wave Propagation and Ion Cyclotron
Heating in the Expanding Solar Wind
Authors: Liewer, Paulett; Velli, Marco; Goldstein, Bruce
Bibcode: 1999SSRv...87..257L
Altcode:
We present results from hybrid (particle ions, fluid electrons)
simulations of the evolution of Alfvén waves close to the ion
cyclotron frequency in the solar wind, which take into account the
basic properties of the background solar wind flow, i.e., the spherical
expansion and the consequent decrease in magnetic field and cyclotron
frequency with increasing distance from the Sun. We follow the evolution
of a plasma parcel in a frame of reference moving with the solar wind
using a 1D expanding box hybrid model; use of the hybrid model yields a
fully self-consistent treatment of the resonant cyclotron wave-particle
interaction. This model is related to a previous MHD model (Velli
et al. 1992), which allows the use of a simple Cartesian geometry
with periodic boundary conditions. The use of stretched expanding
coordinates in directions transverse to the mean radial solar wind
flow naturally introduces an anisotropic damping effect on velocity and
magnetic field. We present results for the case of a single circularly
polarized Alfvén wave propagating radially outward. Initially, the
wave is below the cyclotron frequency for both the alpha partcles and
protons. As the wind expands, the wave frequency (as seen in the solar
wind frame) decreases more slowly than the cyclotron frequencies and
the wave comes into resonance. With only protons, heating occurs as
the wave frequency approaches the proton cyclotron frequency. With
both alphas and protons, the alphas, which come into resonance first,
are observed to be preferentially heated and accelerated.
Title: Ideal kink instabilities in line-tied coronal loops
Authors: Baty, H.; Einaudi, G.; Lionello, R.; Velli, M.
Bibcode: 1998A&A...333..313B
Altcode:
We investigate the nonlinear development of ideal kink instabilities
in a line-tied coronal loop, using a three dimensional numerical
code. In order to understand how the equilibrium loop properties affect
nonlinear evolution, various different initial magnetic equilibria
are considered. In most cases, a fine-scale magnetic field structure
is shown to develop. However, the corresponding electric current
structure depends sensitively on the initial equilibrium: the initial
magnetic twist profile, the loop length, and the nature of the outer
potential region. If there are resonant regions at the loop apex where
the radial component of the linear perturbed magnetic field vanishes,
a current concentration develops there in the subsequent non linear
phase. Otherwise current concentrations may develop as a consequence
of the effect of line-tying. The ensuing resistive evolution of the
system and the impact on coronal activity are discussed.
Title: The Expansion of Coronal Plumes in the Fast Solar Wind
Authors: Del Zanna, L.; von Steiger, R.; Velli, M.
Bibcode: 1998SSRv...85..349D
Altcode:
Coronal plumes are believed to be essentially magnetic features: they
are rooted in magnetic flux concentrations at the photosphere and are
observed to extend nearly radially above coronal holes out to at least
15 solar radii, probably tracing the open field lines. The formation of
plumes itself seems to be due to the presence of reconnecting magnetic
field lines and this is probably the cause of the observed extremely
low values of the Ne/Mg abundance ratio.
Title: Statistical Properties of Magnetic Activity in the Solar Corona
Authors: Georgoulis, Manolis K.; Velli, Marco; Einaudi, Giorgio
Bibcode: 1998ApJ...497..957G
Altcode:
The long-time statistical behavior of a two-dimensional section of
a coronal loop subject to random magnetic forcing is presented. The
highly intermittent nature of dissipation is revealed by means of
magnetohydrodynamic (MHD) turbulence numerical simulations. Even with a
moderate magnetic Reynolds number, intermittency is clearly present in
both space and time. The response of the loop to the random forcing,
as described either by the time series of the average and maximum
energy dissipation or by its spatial distribution at a given time,
displays a Gaussian noise component that may be subtracted to define
discrete dissipative events. Distribution functions of both maximum
and average current dissipation, for the total energy content, the
peak activity, and the duration of such events are all shown to display
robust scaling laws, with scaling indices δ that vary from δ ~= -1.3
to δ ~= -2.8 for the temporal distribution functions, while δ ~=
-2.6 for the overall spatial distribution of dissipative events.
Title: Dynamical response of a stellar atmosphere to pressure
perturbations: numerical simulations
Authors: Del Zanna, L.; Velli, M.; Londrillo, P.
Bibcode: 1998A&A...330L..13D
Altcode:
The time dependent reactions of an isothermal spherically symmetric
stellar atmosphere to perturbations of the external (interstellar)
pressure are analysed by means of computer simulations. The system is
seen to evolve, through the phases of wind, breeze, accretion and back,
according to an hysteresis type cycle with two catastrophe points:
the value of the external pressure relative to a static atmosphere and
that corresponding to the fastest (critical) breeze. This behaviour is
proved to be due to the instability of the outflow breeze solutions
(due to their unfavourable stratification), while subsonic accretion
is stable. A crucial factor of this instability is the position of
the outer boundary: if this is placed too close to the base of the
atmosphere the inflow/outflow breeze stability is reversed. These
simulations confirm a scenario first proposed by Velli (1994).
Title: Nonlinear Magnetohydrodynamic Evolution of Line-tied Coronal
Loops
Authors: Lionello, Roberto; Velli, Marco; Einaudi, Giorgio; Mikić,
Zoran
Bibcode: 1998ApJ...494..840L
Altcode:
Simulations of the nonlinear evolution of the m = 1 kink mode
in magnetic flux tubes with line-tying boundary conditions are
presented. The initial structure of the flux tube is intended to
model a solar coronal loop that either has evolved quasi-statically
through sequences of equilibria with increasing twist due to the
application of localized photospheric vortex flows or has emerged with
a net current through the photosphere. It is well known that when the
twist exceeds a critical value that depends on its radial profile and
on the loop length, the loop becomes kink unstable. The nonlinear
evolution of the instability is followed using a three-dimensional
MHD code in cylindrical geometry, in different types of magnetic field
configurations, with the common property that the current is confined
within the same radius, so that the magnetic field is potential
in the external regions. The differences reside in the net axial
current carried by the structure, ranging from a vanishing current
(corresponding to an outer axial potential field) to a high current
(corresponding to an outer almost azimuthal potential field). It
is shown that, during the nonlinear phase of the instability, loops
develop current sheets and, consequently, their evolution becomes
resistive with the occurrence of magnetic reconnection. The dependence
of the topology of the currents at saturation on the initial magnetic
structure, the details of the reconnection phenomenon, and the resistive
dissipation mechanism are examined. Finally, the impact of the results
on the understanding of coronal activity is discussed.
Title: Heliospheric Plasma Physics: an Introduction
Authors: Velli, Marco
Bibcode: 1998LNP...507..217V
Altcode: 1998sspt.conf..217V
A selection of topics in plasma physics and hydrodynamics relevant
to the heliosphere is presented. The first three sections cover
basic defining properties of a plasma, the essentials of particle
orbit theory, including conservation of magnetic moment and the
various drifts, and an introduction to kinetic theory, with an
heuristic derivation of relaxation times. The fourth section is
devoted to the hydrodynamic description of the solar wind, and is a
pedagogical introduction to Parker's theory as well as to the methods
of hydrodynamic and plasma stability. In the fifth and final section
we return to the kinetic description of the solar wind plasmas and
explore recent versions of the collisionless, or exospheric, models
of wind acceleration, and compare their predictive merits and faults
with those of the more widely studied fluid theories vis à vis in
situ and remote observations of the corona and wind.
Title: MHD models of coronal plumes
Authors: Del Zanna, L.; Hood, A.; Velli, M.; von Steiger, R.
Bibcode: 1998ESASP.421..359D
Altcode: 1998sjcp.conf..359D
No abstract at ADS
Title: The Expansion of Coronal Plumes in the Fast Solar Wind
Authors: del Zanna, L.; von Steiger, R.; Velli, M.
Bibcode: 1998sce..conf..349D
Altcode:
No abstract at ADS
Title: Alfvén waves in the solar corona and solar wind
Authors: Velli, M.; Pruneti, F.
Bibcode: 1997PPCF...39..317V
Altcode:
No abstract at ADS
Title: Magnetic Flux Tubes at 3 Au?
Authors: Parenti, S.; Velli, M.; Poletto, G.; Suess, S. T.; McComas,
D. J.
Bibcode: 1997SoPh..174..329P
Altcode:
We present an analysis of plasma and magnetic field data acquired by
the Ulysses spacecraft on May 1994. Our study is motivated by the
result of Poletto et al. (1996) who found some evidence for a peak
in the power spectrum of magnetic pressure at a frequency ν ≈
2 × 10−5 Hz, during that period. A re-evaluation of
the plasma pressure power spectrum, on the basis of better data than
used in the previous work, gives only marginal evidence for a peak at
that frequency. If both spectra had excess power in the same spectral
range, one might hypothesize that the Pressure Balanced Structures
(PBS) detected in the data trace periodically distributed coronal
structures which maintain their identity up to large distances. A
careful data analysis, however, shows that this interpretation is
hardly tenable. Hence, we consider the alternative hypotheses that
the observed PBS are either a bundle of magnetic flux tubes, with no
characteristic periodicity, in pressure equilibrium with the ambient,
or the manifestation, at large distances, of waves generated close to
the Sun. To prove the latter case, we made a test simulation of the
evolution with heliocentric distance of an ensemble of Alfvén and
slow mode waves, generated close to the Sun, and show that structures
similar to those we analyzed may form in the interplanetary medium. Our
simulations also seem to show that together with PBS, magnetic holes,
frequently observed in the Ulysses data, could also originate from the
nonlinear evolution of large amplitude slow waves in quasi-perpendicular
propagation. We conclude that the observed PBS most likely arise via
an in situ generation mechanism, rather than being remnants of solar
structures.
Title: Acoustic waves in isothermal winds in the vicinity of the
sonic point.
Authors: Grappin, R.; Cavillier, E.; Velli, M.
Bibcode: 1997A&A...322..659G
Altcode:
We study the propagation of acoustic waves incident on the base
of a stellar wind and the back-reaction on the mean flow, in the
spherically symmetric, isothermal case, both analytically and via direct
simulations of the Navier-Stokes equations. We consider successively
the quasi-linear inviscid case and the nonlinear dissipative case
(shocks). We show that wave reflection is small everywhere even when
the WKB approximation breaks down, and conjecture that the same result
could hold for radial Alfven waves in a spherically symmetric wind. We
show that, after a transient acceleration, outward propagating waves
lead to a lower mean wind velocity than in the unperturbed wind, so
that the average velocity may become negative below the sonic point,
the difference with the standard result that Lagrangian-mean velocities
are higher in presence of waves being explained by the drift between
reference frames. We propose that negative average velocities might
provide a test for the presence of compressive waves close to the
sun. We conjecture that, for MHD fluctuations, the net effect of
the wave pressure on the wind velocity depends on the importance of
compressive components, and that this might play a role in the observed
correlation between the mean solar wind velocity and the level of the
compressive component in the wave spectrum.
Title: Kink Modes and Current Sheets in Coronal Loops
Authors: Velli, M.; Lionello, R.; Einaudi, G.
Bibcode: 1997SoPh..172..257V
Altcode: 1997ESPM....8..257V
We present simulations of the non-linear evolution of the m=1 kink mode
in line-tied coronal loops. We focus on the structure of the current
concentrations which develop as a consequence of the instability in two
different types of magnetic field configuration, one containing a net
axial current and the other with a vanishing total axial current. In the
first case, current sheets develop one third of the way from footpoint
to loop apex (where the non-linear kink mode folds on itself) within
the body of the current channel, while in the second case the current
sheet develops at the loop apex at the interface between the current
containing channel and the outer axial potential field. In both cases
line-tying, while playing a stabilizing role in the linear theory, acts
as a destabilizing agent for the non-linear resistive evolution. The
unwrapping of magnetic field lines in the vanishing axial current
model appears to be consistent with the geometry of compact recurrent
loop flares.
Title: Alfvén wave propagation at x-points and shock wave formation
in the solar corona
Authors: Velli, M.; Landi, S.; Einaudi, G.
Bibcode: 1997AIPC..385..211V
Altcode: 1997recs.conf..211V
The dynamics of the development of extremely small scales in magnetic
fields is crucial to understand the heating and energetic manifestations
of the high temperature plasma of the solar corona. Here we illustrate
what could be an essential aspect of the cascade of magnetic energy
to small scales via numerical simulations of the propagation of
(shear) Alfvén waves in a magnetic field with an x-point geometry
in 2.5 D. The coupling of the waves with the background field leads
to the development of fast-mode shocks whose number depends on the
ratio of the Alfvén wave frequency to the intrinsic frequency of the
x-point. Though the x-point is essential to shock wave formation,
dissipation occurs within the shocks which sweep the whole plasma
volume. The shocks might also play an important role in modifying
particle acceleration around the x-point.
Title: Parametric Decay of Large Amplitude Alfven Waves in the
Solar Atmosphere
Authors: Pruneti, F.; Velli, M.
Bibcode: 1997ESASP.404..623P
Altcode: 1997cswn.conf..623P
No abstract at ADS
Title: MHD Turbulence and Statistics of Energy Release in the
Solar Corona
Authors: Georgoulis, M.; Velli, M.; Einaudi, G.
Bibcode: 1997ESASP.404..401G
Altcode: 1997cswn.conf..401G
No abstract at ADS
Title: Magnetic reconnection in solar coronal loops
Authors: Einaudi, G.; Lionello, R.; Velli, M.
Bibcode: 1997AdSpR..19.1875E
Altcode:
Simulations of the evolution of kink modes in line-tied coronal loops
are presented which demonstrate the occurrence of magnetic reconnection
in the non-linear stage of the instability. In loops which do not carry
a net axial current (and are confined by a potential purely axial field)
the reconnection is limited to the initial current-carrying channel
and no overall loss of confinement is observed. In loops which carry
a net current on the other hand, reconnection progressively involves
field lines at greater and greater distances from the axis and even
regions where the field was initially potential, leading to a total
disruption of the magnetic field topology.
Title: Statistical Properties of Magnetic Activity in the Solar Corona
Authors: Georgoulis, M. K.; Einaudi, G.; Velli, M.
Bibcode: 1997jena.confE..38G
Altcode:
A long-time statistical analysis of a two-dimensional section of a
coronal loop has been carried out. The highly intermittent nature of
the spatiotemporal evolution of the system has been revealed by means of
Magnetohydrodynamic (MHD) Turbulence numerical simulations. Albeit the
moderate magnetic Reynolds number, intermittency is strikingly present
both in space and in time. This type of behaviour might physically
motivate statistical theories to describe the long-term evolution of
a turbulent corona, provided that such an environment is a driven
dissipative nonlinear dynamical system. The coronal loop is driven
by a random spatiotemporal magnetic forcing, which induces a noise
component in the resulting timeseries. If this component is properly
subtracted, the obtained spatiotemporal evolution can be statistically
described in terms of robust scaling laws, occurring in the distribution
functions of both maximum and average current dissipation for the
total energy content, the peak activity and the duration of the events
obtained. Adopting low-beta and large-aspect-ratio conditions for
the coronal loop, we emphasize that, higher spatial resolution could
well give rise both to localized equipartition, and to the emergence
of super-Dreicer electric fields built-up in the vicinity of strong,
intense current sheets.
Title: Coronal heating, nanoflares, and MHD turbulence
Authors: Velli, M.
Bibcode: 1996AIPC..382...28V
Altcode:
Coronal heating is at the origin of the X-ray emission and mass loss
from the Sun and many other stars. While different scenarios have
been proposed to explain the heating of magnetically confined and
open regions of the corona, they all rely on the transfer, storage
and dissipation of the abundant energy present in photospheric
motions. Here we focus on theories which rely on magnetic fields
and electric currents both for the energy transfer and storage in
the corona. The dissipation of this energy, whether in the form of
reconnection in current sheets (nanoflares ?) or the dissipation of
MHD waves, depends crucially on the development of extremely small
scales in the coronal magnetic field, where kinetic effects are likely
to be fundamental. The question of whether coronal heating and flares
may be viewed respectively as the macroscopic, low-energy average and
the high-energy, temporally intermittent aspect of the same underlying
driven, dissipative, turbulent system is also addressed, with emphasis
placed on the main observational and theoretical stumbling blocks in
the way of a confirmation or disproof of such a conjecture.
Title: High amplitude waves in the expanding solar wind plasma
Authors: Schmidt, J. M.; Velli, M.; Grappin, R.
Bibcode: 1996AIPC..382..315S
Altcode:
We simulated the 1 D nonlinear time-evolution of high-amplitude Alfvén,
slow and fast magnetoacustic waves in the solar wind propagating outward
at different angles to the mean magnetic (spiral) field, using the
expanding box model. The simulation results for Alfvén waves and fast
magnetoacustic waves fit the observational constraints in the solar wind
best, showing decreasing trends for energies and other rms-quantities
due to expansion and the appearance of inward propagating waves as
minor species in the wind. Inward propagating waves are generated
by reflection of Alfvén waves propagating at large angles to the
magnetic field or they coincide with the occurrence of compressible
fluctuations. It is the generation of sound due to ponderomotive forces
of the Alfvén wave which we can detect in the latter case. For slow
magnetoacustic waves we find a kind of oscillation of the character
of the wave between a sound wave and an Alfvén wave. This is the
more, the slow magnetoacustic wave is close to a sound wave in the
beginning. On the other hand, fast magnetoacustic waves are much more
dissipated than the other wave-types and their general behaviour is
close to the Alfvén. The normalized cross-helicity σc
is close to one for Alfvén-waves and this quantity is decreasing
slightly when density-fluctuations are generated. σc
decreases significantly when the waves are close to perpendicular
propagation. Then, the waves are close to quasi-static structures.
Title: Energy Release in a Turbulent Corona
Authors: Einaudi, G.; Velli, M.; Politano, H.; Pouquet, A.
Bibcode: 1996ApJ...457L.113E
Altcode:
Numerical simulations of a two-dimensional section of a coronal loop
subject to random magnetic forcing are presented. The forcing models
the link between photospheric motions and energy injection in the
corona. The results show the highly intermittent spatial distribution
of current concentrations generated by the coupling between internal
dynamics and external forcing. The total power dissipation is a rapidly
varying function of time, with sizable jumps even at low Reynolds
numbers, and is caused by the superposition of magnetic dissipation
in a number of localized current sheets. Both spatial and temporal
intermittency increase with the Reynolds number, suggesting that the
turbulent nature of the corona can physically motivate statistical
theories of solar activity.
Title: MHD waves and turbulence in the polar regions of the
heliosphere
Authors: Velli, M.
Bibcode: 1996ASPC..109..451V
Altcode: 1996csss....9..451V
No abstract at ADS
Title: Long Time Statistics of Magnetically Driven MHD Turbulence,
Solar Flares and Coronal Heating
Authors: Chiuden, C.; Velli, M.; Einaudi, C.; Pouquet, A.
Bibcode: 1996mpsa.conf...45C
Altcode: 1996IAUCo.153...45C
No abstract at ADS
Title: Waves and streams in the expanding solar wind
Authors: Grappin, R.; Velli, M.
Bibcode: 1996JGR...101..425G
Altcode:
The expanding box model (EBM) allows the simulation of the evolution
of compressible MHD turbulence within the expanding solar wind,
taking into account the basic properties of expansion. Using the EBM
we follow the evolution of waves within a compressive stream shear and
magnetic sector structure in the range of 0.1 to 1 AU from the Sun. We
analyze the physical processes which lead in these simulations to the
modulation and erosion of the wave component, combined with WKB and
non-WKB processes due to expansion. A strong erosion by stream shear
corresponds indeed to one of the observed regimes in the solar wind;
however, we are unable to reproduce the regime which holds during
solar minimum, in which the correlation between large-scale stream
structure and turbulence remains high independently from distance to
the Sun. The main point of disagreement with observations concerns the
energy spectrum (it is difficult to generate and sustain small-scale
turbulence with an Alfvénic wave band present, and even more so
in an expanding medium); the main point of agreement concerns the
statistics of density fluctuations, which are independent of distance,
and matches the observed amplitudes both within slow and fast wind. At
the same time, small scales appear to be dominated in the simulations
by compressible effects, which contradicts popular ideas on solar
wind turbulence.
Title: Coronal heating, nanoflares and MHD turbulence
Authors: Velli, M.
Bibcode: 1995sowi.conf...28V
Altcode:
Coronal heating is at the origin of the X-ray emission and mass
loss from the sun and many other stars. While different scenarios
have been proposed to explain the heating of magnetically confined
and open regions of the corona, they must all rely on the transfer,
storage and dissipation of the abundant energy present in photospheric
motions. Here we focus on theories which rely on magnetic fields
and electric currents both for the energy transfer and storage in
the corona. The dissipation of this energy, whether in the form of
reconnection in current sheets (nanoflare?) or the dissipation of MHD
waves, depends crucially on the development of extremely small scales
in the coronal magnetic field, where kinetic effects are likely to
be fundamental. The question of whether coronal heating and flares
may be viewed respectively as the macroscopic, low-energy average and
the high-energy, temporally intermittent aspect of the same underlying
driven, dissipative, turbulent system is also addressed, with emphasis
placed on the main observational and theoretical stumbling blocks in
the way of a confinement or disproof of such a conjecture.
Title: What determines the direction of minimum variance of the
magnetic field fluctuations in the solar wind?
Authors: Grappin, R.; Velli, M.
Bibcode: 1995sowi.conf...76G
Altcode:
The solar wind is not an isotropic medium; two symmetry axis are
provided, first the radial direction (because the mean wind is radial)
and second the spiral direction of the mean magnetic field, which
depends on heliocentric distance. Observations show very different
anisotropy directions, depending on the frequency waveband; while
the large-scale velocity fluctuations are essentially radial, the
smaller scale magnetic field fluctuations are mostly perpendicular
to the mean field direction, which is not the expected linear (WkB)
result. We attempt to explain how these properties are related, with
the help of numerical simulations.
Title: High amplitude waves in the expanding solar wind plasma
Authors: Schmidt, J. M.; Velli, M.; Grappin, R.
Bibcode: 1995sowi.conf...77S
Altcode:
We simulated the 1-D nonlinear time-evolution of high-amplitude Alfven,
slow and fast magnetoacoustic waves in the solar wind propagating
outward at different angles to the mean magnetic (spiral) field,
using the expanding box model. The simulation results for Alfven waves
and fast magnetoacustic waves fit the observational constraints in
the solar wind best, showing decreasing trends for energies and other
rms-quantities due to expansion and the appearance of inward propagating
waves as minor species in the wind. Inward propagating waves are
generated by reflection of Alfven waves propagating at large angles to
the magnetic field or they coincide with the occurrence of compressible
fluctuations. In our simulations, fast and slow magnetoacoustic waves
seem to have a level in the density-fluctuations which is too high
when we compare with the observations. Furthermore, the evolution
of energies for slow magnetoacoustic waves differs strongly from the
evolution of fluctuation energies in situ.
Title: Coronal plumes and final scale structure in high speed solar
wind streams
Authors: Velli, M.; Habbal, S. R.; Esser, R.
Bibcode: 1994SSRv...70..391V
Altcode:
We present a solar wind model which takes into account the possible
origin of fast solar wind streams in coronal plumes. We treat coronal
holes as being made up of essentially 2 plasma species, denser,
warmer coronal plumes embedded in a surrounding less dense and cooler
medium. Pressure balance at the coronal base implies a smaller magnetic
field within coronal plumes than without. Considering the total coronal
hole areal expansion as given, we calculate the relative expansion
of plumes and the ambient medium subject to transverse pressure
balance as the wind accelerates. The magnetic flux is assumed to
be conserved independently both within plumes and the surrounding
coronal hole. Magnetic field curvature terms are neglected so the
model is essentially one dimensional along the coronal plumes, which
are treated as thin flux-tubes. We compare the results from this
model with white-light photographs of the solar corona and in-situ
measurements of the spaghetti-like fine-structure of high-speed winds.
Title: From Supersonic Winds to Accretion: Comments on the Stability
of Stellar Winds and Related Flows
Authors: Velli, M.
Bibcode: 1994ApJ...432L..55V
Altcode:
For nearly vanishing values of the interstellar pressure a supersonic
flow connecting via a shock to the instellar medium is the only
stationary state describing the extension of a hot corona into space. We
show here that in terms of the relative pressure jump between the
coronal base and distant medium the stationary flow solutions follow
an hysteresis-type cycle with two catastrophy points: as the pressure
of the interstellar medium increases, the termination shock moves
closer toward the stellar surface, but when the shock position reaches
the sonic point the wind collapses into supersonic accretion with a
shock below the critical point. If the pressure of the interstellar
medium decreases again, or the pressure at the coronal base increases,
the flow can evolve continuously into breeze (everywhere subsonic)
accretion, but the flow evolves back into a state characterized by a
supersonic shocked wind, once the pressure difference corresponding
to a static stratification is exceeded.
Title: Nanoflares and current sheet dissipation
Authors: Einaudi, G.; Velli, M.
Bibcode: 1994SSRv...68...97E
Altcode:
Energy must be continually supplied to the solar corona to maintain
both its average temperature and its high energy manifestations. The
energy is supplied by photospheric motions and the magnetic field
acts both to transmit this energy to the corona and as the furnace
in which the energy is stored. The means by which the energy is
dissipated and transformed into the actual forms we observe is the
activation of current sheets. We conjecture here the properties of such
current sheets as derived by both energetical arguments and numerical
evidence of the self-organization of a system of currents in a highly
turbulent medium. The consequences of the appearance of spatial and
temporal intermittency on the different aspects of solar acitvity are
also discussed.
Title: Alfven waves in the solar corona and solar wind
Authors: Velli, M.
Bibcode: 1994AdSpR..14d.123V
Altcode: 1994AdSpR..14..123V
In situ solar wind measurements of MHD turbulence first showed, 20
years ago, that Alfvén waves propagating away from the sun are a
dominant component, at least in high speed streams at solar minimum,
with sufficient energy to explain the heating of the distant solar
wind. Here we discuss some aspects of the propagation of these waves
upward from the solar coronal base, where they are presumably generated,
with particular emphasis on the effects of the large scale gradients
on the transmission, the development of turbulence and wave dissipation.
Title: Wave-Based Heating Mechanisms for the Solar Corona (Invited)
Authors: Malara, F.; Velli, M.
Bibcode: 1994scs..conf..443M
Altcode: 1994IAUCo.144..443M
No abstract at ADS
Title: Coronal heating mechanisms.
Authors: Einaudi, G.; Velli, M.
Bibcode: 1994LNP...432..149E
Altcode: 1994LNPM...11..149E
Thermal energy must be continually supplied to the solar corona to
maintain its 106K temperature. In the first part of this
paper the authors review the efforts which have been made in the past
twenty years to find a viable mechanism to explain coronal heating,
with special emphasis on the conditions of applicability of the
existing theories and on the possibility that coronal heating may be
intimately linked to other manifestations of solar activity such as
solar flares. The interplay of the different aspects of solar activity
is discussed within the unifying framework of MHD turbulence.
Title: Properties of the solar wind
Authors: Velli, Marco; Grappin, Roland
Bibcode: 1993AdSpR..13i..49V
Altcode: 1993AdSpR..13...49V
Solar wind observations reveal a wealth of variations in physical
properties on a wide range of temporal and spatial scales. While the
largest spatial scales are dominated by the average radial expansion
of the plasma and the magnetic sector structure, significant amounts of
energy are present also in the so called mesoscale fluctuations between
several hours and one minute, which may be interpreted as examples of
MHD turbulence. This paper attempts to summarize, from an observational
and theoretical point of view, our present knowledge of the dynamical
interactions between solar wind structures at the different scales.
Title: Compressible MHD turbulence in the interplanetary medium.
Authors: Velli, Marco; Grappin, R.
Bibcode: 1993ppcn.conf..185V
Altcode:
The authors present results from numerical simulations of the evolution
of nonlinear waves and compressible MHD turbulence in the solar wind,
taking into account the basic properties of the background flow, namely
the solar wind expansion. They follow the evolution of a plasma parcel
in a comoving frame of reference. The main features of the expansion
may be introduced in a simple cartesian geometry with periodic
boundary conditions by expanding the equations in a small parameter,
the angular dimensions of the box as viewed from the sun. The box is
seen to expand in the plane normal to the radial direction and one
follows the expansion by using stretched coordinates in this plane. The
expansion naturally introduces an anisotropic damping effect on velocity
and magnetic field, as well as the Parker-like rotation of the average
magnetic field. The authors show how this effects the evolution of an
initially circularly polarized large amplitude Alfvén wave in oblique
propagation. Finally they present preliminary results for evolution
in 2-D and discuss their relevance to solar wind in situ measurements.
Title: Nonlinear wave evolution in the expanding solar wind
Authors: Grappin, Roland; Velli, Marco; Mangeney, André
Bibcode: 1993PhRvL..70.2190G
Altcode:
We report here on a numerical model allowing direct numerical
simulations of magnetohydrodynamic fluctuations advected by the
expanding solar wind. We show that the expansion of the plasma delays
and possibly freezes the turbulent evolution, but that it also triggers
the nonlinear evolution of otherwise stable (Alfvén) waves, which
can thus release their energy in the wind.
Title: On the propagation of ideal, linear Alfven waves in radially
stratified stellar atmospheres and winds
Authors: Velli, M.
Bibcode: 1993A&A...270..304V
Altcode:
The propagation of Alfven waves through isothermal, radially stratified,
spherically symmetrical models of stellar atmospheres and winds is
discussed. The transmission coefficient for the waves is calculated as a
function of frequency, magnetic field base intensity, surface gravity,
and atmospheric temperature. When a wind is present, the wave energy
flux is no longer conserved, but the conservation of the wave-action
flux allows the definition of an analogous transmission coefficient,
giving the relative amount of waves reaching the super-Alfvenic regions
of the wind. It is shown that for HF waves the transmission coefficient
for static and wind models is identical, while for LF waves the
presence of a wind enhances the transmission considerably. The latter
are however totally reflected asymptotically, far from the stellar
surface, a behavior which is reminiscent of the observed evolution of
the 'Alfvenicity' of turbulence in the solar wind. Recent isotropic
models for Alfvenic turbulence which display the same qualitative
behavior are compared to the LF limit of the linear equations. It is
argued that models for the mass loss of cool giants and supergiants
which properly treat the reflection of Alfven waves might overcome
the difficulties inherent in standard (WKB) wave-driven models.
Title: Alfvén wave propagation in the solar corona and inner
heliosphere.
Authors: Velli, M.
Bibcode: 1993wpst.conf...71V
Altcode:
The author discusses some aspects of the propagation of Alfvén waves
upward from the solar coronal base, where they are presumably generated,
with particular emphasis on the effects of the large scale gradients
on the transmission, the development of turbulence and wave dissipation.
Title: MHD turbulence and solar wind dynamics.
Authors: Velli, M.
Bibcode: 1993wpst.conf..153V
Altcode:
Solar wind observations reveal a wealth of variations in physical
properties on a wide range of temporal and spatial scales. The author
summarizes the present knowledge of the dynamical interactions between
the solar wind structures at different scales.
Title: Coronal and solar wind Alfvén wave propagation.
Authors: Velli, M.
Bibcode: 1992ESASP.344...53V
Altcode: 1992spai.rept...53V
The transmission of Alfven waves through the solar corona and into the
solar wind is discussed, for isothermal models with a radial magnetic
field. For comparison, the transmission through a static, spherical
isothermal corona is also calculated.
Title: MHD turbulence in an expanding atmosphere
Authors: Velli, M.; Grappin, R.; Mangeney, A.
Bibcode: 1992AIPC..267..154V
Altcode: 1992ecsa.work..154V
The evolution of MHD fluctuations advected by the solar wind is
profoundly affected by the spherical wind expansion: the latter
in an important source of anisotropy, the solar plasma becoming
increasingly stretched in the transverse direction as it recedes from
the sun. As a consequence, the nonlinear evolution of compressive
velocity fluctuations in the transverse direction is ``frozen'' at
a finite time in the low frequency limit; at higher frequencies the
evolution, e.g., of Alfvén waves, is easily predicted only in the
linear limit. In the fully three-dimensional and nonlinear case,
numerical simulations are necessary: here we present preliminary
numerical results from simulations of MHD turbulence in a plasma volume
expanding with the wind.
Title: Alfven wave propagation in the solar atmosphere and models
of MHD turbulence in the solar wind
Authors: Velli, M.; Grappin, R.; Mangeney, A.
Bibcode: 1992sws..coll..569V
Altcode:
The propagation of Alfven waves along a purely radial magnetic field in
the solar atmosphere is discussed, with particular emphasis on the role
of the Alfvenic critical point in determining the transmission of the
waves into the wind. Models for the evolution of Alfvenic turbulence
are compared to the low-frequency limit of the linear equations.
Title: "Alfvénic" versus "standard" turbulence in the solar wind.
Authors: Grappin, R.; Velli, M.; Mangeney, A.
Bibcode: 1991AnGeo...9..416G
Altcode: 1991AnG.....9..416G
The authors study the variation of the properties of turbulence
with stream structure, on time scales of hours and minutes, in the
inner heliosphere at solar minimum. Between fast hot streams, this
turbulence is found to show many properties typical of "standard" weakly
compressible magnetohydrodynamic (MHD) turbulence such as excess of
turbulent magnetic energy and a relative level of density fluctuation
approximately equal to the turbulent Mach number squared. They discuss
whether or not the more peculiar properties of Alfvénic turbulence,
found within fast streams, represent some genuinely different state
of MHD turbulence which might not relax towards standard turbulence
at large distances from the sun and the ecliptic plane. The Ulysses
spacecraft data should allow these possibilities to be distinguished.
Title: MHD turbulence in the solar wind.
Authors: Mangeney, A.; Grappin, R.; Velli, M.
Bibcode: 1991gamp.conf..327M
Altcode:
The basic characteristics of the wind necessary to understand the
in situ measurements are described. The authors discuss how and to
what extent the data in a medium frequency range support the idea of
evolution of MHD turbulence and discuss the specificity of Alfvénic
turbulence versus "standard" MHD turbulence in the context of the
solar wind.
Title: Magnetohydrodynamic Turbulence in the Solar Wind
Authors: Mangeney, A.; Grappin, R.; Velli, M.
Bibcode: 1991assm.conf..327M
Altcode:
No abstract at ADS
Title: Waves from the sun?
Authors: Velli, M.; Grappin, R.; Mangeney, A.
Bibcode: 1991GApFD..62..101V
Altcode:
Satellite observations of solar wind turbulence in the low frequency
MHD domain show highly variable properties with respect to time and
distance from the sun: one of the markers of this variability is the
degree of "Alfvenicity", which characterizes the relative level of
quasi-incompressible waves propagating away from the sun. To answer
the question of the origin and the evolution of this wave spectrum
one must investigate the propagation of MHD fluctuations through the
highly inhomogeneous and spherically expanding solar wind. Here we
discuss some aspects both of the linear propagation before the critical
point and of recent models for the evolution of the turbulence in the
supersonic regions of the wind.
Title: Solar wind expansion effects on the evolution of hydromagnetic
turbulence in the interplanetary medium
Authors: Velli, Marco; Grappin, Roland; Mangeney, André
Bibcode: 1990CoPhC..59..153V
Altcode:
The effects of the large scale gradients generated by the solar wind
expansion are taken into account to estimate the turbulent flux from
nonlinear interactions among inward and outward propagating Alfvén
waves. It is shown that even if all inward propagating waves are lost
in the acceleration region of the wind, nonlinear couplings survive
because of scattering effects from the large scale gradients. A new
phenomenology is proposed which involves interactions among purely
outgoing waves (mediated by secondary scattered incoming fluctuations)
and leads to a power spectrum scaling k-α, α ~ 1, close to
what is found in the long-wavelength domain of solar wind fluctuations
near the sun. In more general situations, when a nonnegligible amount
of ordinary incoming waves are present, the spectral index is determined
by the competition between the different contributions to the nonlinear
flux, so that the spectral index may vary with wavelength, from α ~
1 for the largest scales, to α ~ 1.5-1.7 for the small scales.
Title: The ideal MHD stability of line-tied coronal loops: A truncated
Fourier series approach
Authors: de Bruyne, P.; Velli, M.; Hood, A. W.
Bibcode: 1990CoPhC..59...55D
Altcode:
The stability behaviour of a line-tied cylindrically symmetric
coronal loop is investigated using a general method presented by Velli,
Einaudi and Hood. The plasma perturbation in the linearised equation of
motion is expanded in a truncated Fourier series in the poloidal and
axial directions, and the resulting system of ordinary differential
equations for the radial displacement is solved as an eigenvalue
problem. The eigenvalue, be it the critical loop length or the growth
rate, is found to converge rapidly with the order of the truncation
(approximately as N-2, where N is the number of terms in
the Fourier series). Results for the non-force-free uniform-twist
field of Gold and Hoyle are presented and compared with a previous
study based on the energy principle. The instability threshold for
the m = 1 kink mode and the m = ∞ balloning mode are found to be of
the same order of magnitude when substantial pressure gradients are
present. Preliminary results for higher-m modes indicate that they
yield instability thresholds comparable to the ballooning threshold
for smaller gradients too.
Title: Velli, Grappin, and Mangeney reply
Authors: Velli, M.; Grappin, R.; Mangeney, A.
Bibcode: 1990PhRvL..64.2592V
Altcode:
No abstract at ADS
Title: Ideal Kink Instabilities in Line-tied Coronal Loops: Growth
Rates and Geometrical Properties
Authors: Velli, M.; Einaudi, G.; Hood, A. W.
Bibcode: 1990ApJ...350..428V
Altcode:
A detailed analysis of the ideal kink instability in line-tied
cylindrically symmetric coronal loops is presented. Using a rapidly
converging Fourier series expansion technique, the growth rate, as well
as the eigenfunction, of ideal m = 1 kink modes is calculated for two
topologically distinct models of force-free static MHD equilibria: one
in which all the magnetic field lines are connected to the photosphere
and one presenting a polarity inversion surface. The growth rates
depend crucially on the loop length. Loops of the former type are found
to be more unstable, and possess higher growth rates, than loops of
the latter type, which are unstable to sausage-tearing modes and may
also be unstable to m = 1 resistive kink modes. Applications of these
results to the structure of coronal loops are presented.
Title: Boundary Effects on the Magnetohydrodynamic Stability of a
Resistive Plasma
Authors: Velli, M.; Einaudi, G.; Hood, A. W.
Bibcode: 1990ApJ...350..419V
Altcode:
A general method for studying the resistive MHD stability of plasma
configurations where boundary effects are of crucial importance and
can be expressed as additional constraints on a periodic system is
presented and applied to the case of line-tied cylindrically symmetric
coronal loops. The eigenvalue equations obtained are a generalization
of the Freidberg and Hewett equations, to which they reduce when
the loop length is made infinite. An application to tearing modes is
described which shows that in a finite geometry, tearing takes place
at the center of the configuration, corresponding to the vertex of
coronal loops. Applications to other configurations of astrophysical
interest are described.
Title: The Effect of Large Scale Gradients on the Evolution of
Alfvénic Turbulence in the Solar Wind.
Authors: Velli, M.; Grappin, R.; Mangueney, A.
Bibcode: 1990ppsa.conf..115V
Altcode:
No abstract at ADS
Title: Linear stability of line-tied coronal loops.
Authors: de Bruyne, P.; Velli, M.; Hood, A. W.
Bibcode: 1990PDHO....7..142D
Altcode: 1990dysu.conf..142D
The ideal linear MHD stability of line-tied 1-D coronal loops is
investigated. It is shown that an extended Suydam criterion, obtained
from a local analysis, provides a necessary condition for stability
of the global kink mode.
Title: Turbulent cascade of incompressible unidirectional Alfvén
waves in the interplanetary medium
Authors: Velli, Marco; Grappin, Roland; Mangeney, Andre
Bibcode: 1989PhRvL..63.1807V
Altcode:
The large-scale inhomogeneity of the solar wind is taken into account to
estimate the turbulent flux due to nonlinear interactions among purely
outward-traveling waves. The nonlinear interactions are mediated by
secondary, incoming waves generated by the linear coupling of the
dominant species to the large-scale gradients. A quasistationary
self-similar turbulent cascade is possible, with a spectrum scaling
as k-1, close to what is found in the low-frequency range
of solar-wind fluctuations near to the sun.
Title: Resistive Tearing in Line-Tied Magnetic Fields - Slab Geometry
Authors: Velli, M.; Hood, A. W.
Bibcode: 1989SoPh..119..107V
Altcode:
The resistive tearing-mode instability of a current carrying plasma
sheet is investigated including the stabilising photospheric line-tying
boundary conditions. This end condition prohibits a single Fourier mode
and so requires a series expansion in harmonics of the fundamental
sheet excitation. Equilibria in which there exist field lines that
do not connect to the photosphere are unstable provided the ratio of
the sheet length to characteristic transverse scale is smaller than
a critical value that depends on the equilibrium profile. Line-tying
has a strong stabilising effect on the fundamental periodic mode. That
tearing mode harmonic which develops close to the instability threshold,
leads to a configuration with one X point and one 0 point. Its linear
growth rate follows the usual constant-ψ scaling with resistivity γ
∼ S-3/5, where S is the magnetic Reynolds number.
Title: Can resistive kink instabilities drive simple loop flares?
Authors: Velli, M.; Emaudi, G.; Hood, A. W.
Bibcode: 1989sasf.confP.305V
Altcode: 1988sasf.conf..305V; 1989IAUCo.104P.305V
A detailed analysis of the kink instability in finite length
(inertially line-tied), cylindrically symmetric coronal loops is
presented. The correct line-tying boundary conditions within the
framework of ideal and resistive magnetohydrodynamics are discussed,
and the growth rates of unstable modes and corresponding eigenfunctions
are calculated. Resistive kink modes are found to be unstable in
configurations where the axial magnetic field undergoes an inversion,
resistive effects being confined to a small region around the loop
vertex.
Title: On the MHD Stability of the M=1 Kink Mode in Finite Length
Coronal Loops
Authors: Velli, M.; Hood, A. W.; Einaudi, G.
Bibcode: 1989ESASP.285..105V
Altcode: 1989rsp..conf..105V
A general method for studying the ideal and resistive MHD stability of
plasma configurations with line-tying is presented, and applied to the
case of the M=1 kink mode in coronal loops. The method consists in a
truncated Fourier series approach applied to the linearized equations
of motion, and is found to converge rapidly with the order of the
truncation. Models of the boundary conditions at the corona-photosphere
interface are discussed, and the growth rates of unstable modes are
calculated for equilibrium profiles with an without a reversal in the
field component connecting to the photosphere. The relevance of these
modes to compact loop flares is assessed.
Title: Resistive Ballooning Modes in Line-Tied Coronal Fields -
Part Two
Authors: Velli, M.; Hood, A. W.
Bibcode: 1987SoPh..109..351V
Altcode:
The resistive stability of coronal loops to perturbations with short
wavelength across the magnetic field is analysed, taking full account
of the line tying effect due to the presence of the photosphere. The
results presented are similar to those previously obtained for arcades:
configurations with a pressure profile decreasing with distance from
the loop axis at some point are found to be always unstable, the growth
rate γ increasing monotonically with the wavenumber (n) and scaling
approximately as γ ∼ (n2Dr)1/3
in the limit of large n.
Title: Resistive ballooning modes in line-tied coronal arcades.
Authors: Velli, Marco
Bibcode: 1986NASCP2442..461V
Altcode: 1986copp.nasa..461V
The equations describing the linear evolution of resistive ballooning
modes are obtained by using a modified WKB expansion in the short
perpendicular wavelength, while variations of the perturbations along
the field are described by a slowly varying amplitude, on which the
tying boundary conditions are imposed. In general, given an equilibrium,
there are certain ranges of magnetic surfaces for which the system
predicts instability even without dissipation. The main conclusion is
that within the resistive MHD approximation cylindrically symmetric
arcades with pressure falling with radius are unstable to resistive
localized modes; the growth rates, close to ideal marginal stability,
are large, so that it would appear that energy could be released during
10 to 100 Alfven times. The wavelength of the modes is expected to be
limited by the ion gyroradius, when stabilizing drift effects must be
taken into account. The nonlinear evolution of resistive ballooning
modes should be studied to assess their overall relevance to the
violent and rapidly evolving phenomena observed on the sun.
Title: Resistive Ballooning Modes in Line-Tied Coronal Fields -
Part One - Arcades
Authors: Velli, M.; Hood, A. W.
Bibcode: 1986SoPh..106..353V
Altcode:
The stability of coronal arcades to localized resistive interchange
modes in the ballooning ordering, including photospheric line tying,
is investigated. It is found that the anchoring of magnetic footpoints
in the photosphere is not sufficient to stabilise ballooning modes, once
resistivity is taken into account. All configurations with a pressure
profile decreasing from the arcade axis at some point are unstable,
a purely growing mode being excited. The dependence of the growth rate
γ on the parameter Rm−1 ∼ k2η, where η is
the resistivity and k the wavenumber in a direction perpendicular to
the equilibrium field, can be described by a power law with varying
index: for small values of k2 and an ideally stable field
one finds γ ∼ Rm−1. As k2 is increased or
marginal stability is approached one finds γ ∼ Rm−1/3. T
implications of these localised instabilities to the temporal evolution
and overall energy balance of arcade structures in the solar corona
is discussed.