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Author name code: viall
ADS astronomy entries on 2022-09-14
author:"Viall, Nicholeen M."
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Title: Defining the Middle Corona
Authors: West, Matthew J.; Seaton, Daniel B.; Wexler, David B.;
Raymond, John C.; Del Zanna, Giulio; Rivera, Yeimy J.; Kobelski,
Adam R.; DeForest, Craig; Golub, Leon; Caspi, Amir; Gilly, Chris R.;
Kooi, Jason E.; Alterman, Benjamin L.; Alzate, Nathalia; Banerjee,
Dipankar; Berghmans, David; Chen, Bin; Chitta, Lakshmi Pradeep; Downs,
Cooper; Giordano, Silvio; Higginson, Aleida; Howard, Russel A.; Mason,
Emily; Mason, James P.; Meyer, Karen A.; Nykyri, Katariina; Rachmeler,
Laurel; Reardon, Kevin P.; Reeves, Katharine K.; Savage, Sabrina;
Thompson, Barbara J.; Van Kooten, Samuel J.; Viall, Nicholeen M.;
Vourlidas, Angelos
2022arXiv220804485W Altcode:
The middle corona, the region roughly spanning heliocentric altitudes
from $1.5$ to $6\,R_\odot$, encompasses almost all of the influential
physical transitions and processes that govern the behavior of
coronal outflow into the heliosphere. Eruptions that could disrupt
the near-Earth environment propagate through it. Importantly, it
modulates inflow from above that can drive dynamic changes at lower
heights in the inner corona. Consequently, this region is essential
for comprehensively connecting the corona to the heliosphere and for
developing corresponding global models. Nonetheless, because it is
challenging to observe, the middle corona has been poorly studied by
major solar remote sensing missions and instruments, extending back to
the Solar and Heliospheric Observatory (SoHO) era. Thanks to recent
advances in instrumentation, observational processing techniques,
and a realization of the importance of the region, interest in the
middle corona has increased. Although the region cannot be intrinsically
separated from other regions of the solar atmosphere, there has emerged
a need to define the region in terms of its location and extension
in the solar atmosphere, its composition, the physical transitions
it covers, and the underlying physics believed to be encapsulated by
the region. This paper aims to define the middle corona and give an
overview of the processes that occur there.
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Title: New Insights into the First Two PSP Solar Encounters Enabled
by Modeling Analysis with ADAPT-WSA
Authors: Wallace, Samantha; Jones, Shaela I.; Arge, C. Nick; Viall,
Nicholeen M.; Henney, Carl J.
2022ApJ...935...24W Altcode: 2022arXiv220513010W
Parker Solar Probe's (PSP's) unique orbital path allows us to observe
the solar wind closer to the Sun than ever before. Essential to
advancing our knowledge of solar wind and energetic particle formation
is identifying the sources of PSP observations. We report on results for
the first two PSP solar encounters derived using the Wang-Sheeley-Arge
(WSA) model driven by Air Force Data Assimilative Photospheric Flux
Transport (ADAPT) model maps. We derive the coronal magnetic field
and the 1 R <SUB>⊙</SUB> source regions of the PSP-observed solar
wind. We validate our results with the solar wind speed and magnetic
polarity observed at PSP. When modeling results are very reliable,
we derive time series of model-derived spacecraft separation from the
heliospheric current sheet, magnetic expansion factor, coronal hole
boundary distance, and photospheric field strength along the field
lines estimated to be connected to the spacecraft. We present new
results for Encounter 1, which show time evolution of the far-side
mid-latitude coronal hole that PSP corotates with. We discuss how this
evolution coincides with solar wind speed, density, and temperature
observed at the spacecraft. During Encounter 2, a new active region
emerges on the solar far side, making it difficult to model. We show
that ADAPT-WSA output agrees well with PSP observations once this active
region rotates onto the near side, allowing us to reliably estimate the
solar wind sources retrospectively for most of the encounter. We close
with ways in which coronal modeling enables scientific interpretation
of these encounters that would otherwise not have been possible.
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Title: The Solaris Solar Polar MIDEX-Class Mission Concept: Revealing
the Mysteries of the Sun's Poles
Authors: Hassler, Donald M.; Harra, Louise K.; Gibson, Sarah; Thompson,
Barbara; Gusain, Sanjay; Berghmans, David; Linker, Jon; Basu, Sarbani;
Featherstone, Nicholas; Hoeksema, J. Todd; Viall, Nicholeen; Newmark,
Jeffrey; Munoz-Jaramillo, Andres; Upton, Lisa A.
2022cosp...44.1528H Altcode:
Solaris is an exciting, innovative & bold mission of discovery to
reveal the mysteries of the Sun's poles. Solaris was selected for Phase
A development as part of NASA's MIDEX program. Solaris builds upon
the legacy of Ulysses, which flew over the solar poles, but Solaris
provides an entirely new feature remote sensing, or IMAGING. Solaris
will be the first mission to image the poles of the Sun from ~75
degrees latitude and provide new insight into the workings of the
solar dynamo and the solar cycle, which are at the foundation of our
understanding of space weather and space climate. Solaris will also
provide enabling observations for improved space weather research,
modeling and prediction with time series of polar magnetograms and
views of the ecliptic from above, providing a unique view of the
corona, coronal dynamics, and CME eruption. To reach the Sun's poles,
Solaris will first travel to Jupiter, and use Jupiter's gravity to
slingshot out of the ecliptic plane, and fly over the Sun's poles
at ~75 degrees latitude. Just as our understanding of Jupiter &
Saturn were revolutionized by polar observations from Juno and Cassini,
our understanding of the Sun will be revolutionized by Solaris.
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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.
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.
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Title: Relating the variability of the middle corona to the structure
of the slow solar wind
Authors: Higginson, Aleida; DeVore, C. Richard; Antiochos, Spiro;
Viall, Nicholeen
2022cosp...44.1320H Altcode:
The recent revolution in heliospheric measurements, brought about by
NASAʼs Parker Solar Probe and ESAʼs Solar Orbiter, has shown that
processes in the middle corona can influence the structure and dynamics
of the solar wind across spatial scales. Understanding the formation
of the young solar wind structures currently being measured by Parker
Solar Probe and Solar Orbiter is now essential. Numerical calculations
have shown that magnetic field dynamics at coronal hole boundaries
in the middle corona, in particular interchange reconnection driven
by photospheric motions, can be responsible for the dynamic release
of structured slow solar wind, including along huge separatrix-web
(S-Web) arcs formed by pseudostreamers. Quantifying the plasma and
magnetic variability along the heliospheric current sheet and these
S-Web arcs is crucial to furthering our understanding of how coronal
magnetic field dynamics can influence the slow solar wind throughout the
heliosphere. Here we present fully dynamic, 3D numerical calculations of
a coronal hole boundary driven continuously by realistic photospheric
motions at its base. We consider our simulation results within the
context of Parker Solar Probe and Solar Orbiter, and make predictions
for the structure and variability of the young slow solar wind.
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Title: Investigating Solar Wind Formation in the Inner Corona Using
ADAPT-WSA
Authors: Wallace, Samantha; Young, Peter; Arge, Charles; Viall,
Nicholeen; Jones, Shaela
2022cosp...44.1321W Altcode:
Several fundamental outstanding questions in heliophysics pertain to
the genesis and energization of the solar wind - both of which are
driven by physical processes that largely occur in the inner solar
corona. Recent and upcoming missions enable more direct measurements
of the inner corona; however, the use of a model is required to bridge
in situ and remote observations to investigate how the solar wind was
formed. We present results from aggregate work that support this claim,
where we use the Wang-Sheeley-Arge (WSA) model driven by Air Force
Data Assimilative Photospheric Flux Transport (ADAPT) time-dependent
photospheric field maps to connect in situ solar wind observations from
various spacecraft (e.g., PSP, SolO, ACE, Helios) to their source
regions at 1 Rs. We show results in which we apply our modeling
to test solar wind formation theories (e.g., reconnection/S-web,
waves-turbulence, expansion factor), and to characterize the solar wind
from specific sources (e.g., active region vs. quiet Sun coronal hole
boundaries, deep inside coronal holes). We discuss several current and
former collaborations, including connecting PSP in situ measurements
to remote composition measurements from Hinode/EIS, and identifying
the sources of transient eruptions observed at PSP. We close with how
ADAPT-WSA is currently supporting both the PSP and SolO missions.
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Title: Exploring Structures and Flows with NASA's under-construction
PUNCH mission
Authors: DeForest, Craig; Gibson, Sarah; Thompson, Barbara;
Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall, Nicholeen;
Cranmer, Steven; de Koning, Curt
2022cosp...44.1077D Altcode:
The Polarimeter to UNify the Corona and Heliosphere is a NASA Small
Explorer to image the corona and heliosphere as parts of a single
system. PUNCH comprises four ~50kg smallsats, each carrying one imaging
instrument, that work together to form a single "virtual coronagraph"
with a 90° field of view, centered on the Sun. Scheduled for joint
launch with NASA's SPHEREx mission, PUNCH starts its two-year prime
science phase in 2025. PUNCH will generate full polarized image
sequences of Thomson-scattered light from free electrons in the corona
and young solar wind, once every four minutes continuously. This
enables tracking the young solar wind and turbulent structures within
it as they disconnect from the Sun itself, as well as large transients
such as CMEs, CIRs, and other shocks within the young solar wind. A
student-contributed X-ray spectrometer (STEAM) will address questions
of coronal heating and flare physics. We present motivating science,
expected advances, mission status, and how to get involved with PUNCH
science now.
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Title: Expected results for the cradle of the Solar Wind with the
Polarimeter to UNify the Corona and Heliosphere (PUNCH)
Authors: DeForest, Craig; Gibson, Sarah; De Koning, Curt A.; Thompson,
Barbara; Malanushenko, Anna; Desai, Mihir; Elliott, Heather; Viall,
Nicholeen; Cranmer, Steven
2022cosp...44.1324D Altcode:
The Polarimeter to UNify the Corona and Heliosphere is a NASA Small
Explorer to image the corona and heliosphere as parts of a single
system. Imaging the corona and heliosphere together from a constellation
of four synchronized smallsats, PUNCH will — starting in 2025 —
provide a unique window on global structure and cross-scale processes
in the outer corona and young solar wind. PUNCH science is informed
by, and complements, the results of PSP and Solar Orbiter; and will
synergize with PROBA3/ASPIICS. We present early prototype results from
STEREO/SECCHI and current preparation work to enable PUNCH science
when data arrive, discuss anticipated results from the deeper-field,
higher time resolution imaging that PUNCH will provide, and describe
how to get involved with PUNCH science now.
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Title: Remote Sensing of Turbulence and Solar Wind Structure with
the PUNCH mission
Authors: DeForest, Craig; Gibson, Sarah; Matthaeus, William; Viall,
Nicholeen
2022cosp...44.1212D Altcode:
The Polarimeter to UNify the Corona and Heliosphere is a mission to
observe the corona and the inner heliosphere as a unified system. PUNCH
will produce continuous images of the solar wind and corona between
1.5° and 45° from the Sun, over a two year prime science mission
scheduled to start in early 2025. PUNCH uses visible sunlight scattered
by free electrons in the corona, to track density structures in the
corona and solar wind. We will describe PUNCH's unique 3D imaging
capability, mission structure, and anticipated results measuring the
development of large-scale turbulence, and the large- and meso-scale
structure of the solar wind itself.
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Title: Periodic Solar Wind Structures Observed in Measurements of
Elemental and Ionic Composition in situ at L1
Authors: Gershkovich, Irena; Lepri, Susan T.; Viall, Nicholeen M.;
Matteo, Simone Di; Kepko, Larry
2022ApJ...933..198G Altcode:
Mesoscale periodic structures observed in solar wind plasma serve as an
important diagnostic tool for constraining the processes that govern
the formation of the solar wind. These structures have been observed
in situ and in remote data as fluctuations in proton and electron
density. However, only two events of this type have been reported
regarding the elemental and ionic composition. Composition measurements
are especially important in gaining an understanding of the origin
of the solar wind as the composition is frozen into the plasma at the
Sun and does not evolve as it advects through the heliosphere. Here,
we present the analysis of four events containing mesoscale periodic
solar wind structure during which the Iron and Magnesium number
density data, measured by the Solar Wind Ion Composition Spectrometer
(SWICS) on board the Advanced Composition Explorer spacecraft, are
validated at statistically significant count levels. We use a spectral
analysis method specifically designed to extract periodic signals from
astrophysical time series and apply it to the SWICS 12 minute native
resolution data set. We find variations in the relative abundance
of elements with low first ionization potential, mass dependencies,
and charge state during time intervals in which mesoscale periodic
structures are observed. These variations are linked to temporal or
spatial variations in solar source regions and put constraints on the
solar wind formation mechanisms that produce them. Techniques presented
here are relevant for future, higher-resolution studies of data from
new instruments such as Solar Orbiter's Heavy Ion Sensor.
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Title: Scattered light in the Hinode/EIS and SDO/AIA instruments
measured from the 2012 Venus transit
Authors: Young, Peter R.; Viall, Nicholeen M.
2022arXiv220709538Y Altcode:
Observations from the 2012 transit of Venus are used to derive empirical
formulae for long and short-range scattered light at locations on
the solar disk observed by the Hinode Extreme ultraviolet Imaging
Spectrometer (EIS) and the Solar Dynamics Observatory Atmospheric
Imaging Assembly (AIA) instruments. Long-range scattered light
comes from the entire solar disk, while short-range scattered light
is considered to come from a region within 50" of the region of
interest. The formulae were derived from the Fe XII 195.12 A emission
line observed by EIS and the AIA 193 A channel. A study of the weaker Fe
XIV 274.20 A line during the transit, and a comparison of scattering
in the AIA 193 A and 304 A channels suggests the EIS scattering
formula applies to other emission lines in the EIS wavebands. Both
formulae should be valid in regions of fairly uniform emission such as
coronal holes and quiet Sun, but not faint areas close (around 100")
to bright active regions. The formula for EIS is used to estimate the
scattered light component of Fe XII 195.12 for seven on-disk coronal
holes observed between 2010 and 2018. Scattered light contributions of
56% to 100% are found, suggesting that these features are dominated
by scattered light, consistent with earlier work of Wendeln \&
Landi. Emission lines from the S X and Si X ions - formed at the same
temperature as Fe XII and often used to derive the first ionization
potential (FIP) bias from EIS data - are also expected to be dominated
by scattered light in coronal holes.
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Title: On Differentiating Multiple Types of ULF Magnetospheric Waves
in Response to Solar Wind Periodic Density Structures
Authors: Di Matteo, S.; Villante, U.; Viall, N.; Kepko, L.; Wallace, S.
2022JGRA..12730144D Altcode:
Identifying the nature and source of ultra-low frequencies (ULF) waves
(f ⪅ 4 mHz) at discrete frequencies in the Earth's magnetosphere is
a complex task. The challenge comes from the simultaneous occurrence of
externally and internally generated waves, and the ability to robustly
identify such perturbations. Using a recently developed robust spectral
analysis procedure, we study an interval that exhibited in magnetic
field measurements at geosynchronous orbit and in-ground magnetic
observatories both internally supported and externally generated ULF
waves. The event occurred on 9 November 2002 during the interaction of
the magnetosphere with two interplanetary shocks that were followed
by a train of 90 min solar wind periodic density structures. Using
the Wang-Sheeley-Arge model, we mapped the source of this solar wind
stream to an active region and a mid-latitude coronal hole just prior
to crossing the Heliospheric current sheet. In both the solar wind
density and magnetospheric field fluctuations, we separated broad
power increases from enhancements at specific frequencies. For the
waves at discrete frequencies, we used the combination of satellite and
ground magnetometer observations to identify differences in frequency,
polarization, and observed magnetospheric locations. The magnetospheric
response was characterized by: (a) forced breathing by periodic solar
wind dynamic pressure variations below ≈1 mHz, (b) a combination of
directly driven oscillations and wave modes triggered by additional
mechanisms (e.g., shock and interplanetary magnetic field discontinuity
impact, and substorm activity) between ≈1 and 4 mHz, and (c) largely
triggered modes above ≈4 mHz.
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Title: A Strategy for a Coherent and Comprehensive Basis for
Understanding the Middle Corona
Authors: West, M. J.; Seaton, D. B.; Alzate, N.; Caspi, A.; DeForest,
C. E.; Gilly, C. R.; Golub, L.; Higginson, A. K.; Kooi, J. E.; Mason,
J. P.; Rachmeler, L. A.; Reeves, K. K.; Reardon, K.; Rivera, Y. J.;
Savage, S.; Viall, N. M.; Wexler, D. B.
2022heli.conf.4060W Altcode:
We describe a strategy for coherent and comprehensive observations
needed to achieve a fundamental understanding of the middle solar
corona.
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Title: Relating Solar Wind Variability to the Magnetic Topology of
its Coronal Source Region
Authors: Lynch, Benjamin; Viall, Nicholeen; Higginson, Aleida; Zhao,
Liang; Lepri, Susan; Sun, Xudong
2021AGUFMSH32B..07L Altcode:
The global magnetic configuration of the solar corona directly
determines the structure of the solar wind outflow and decades of
in-situ observations have shown that the solar wind properties reflect
the coronal conditions and magnetic structure of its origin. Connecting
the solar wind observed throughout the heliosphere to its origins
in the solar corona is a fundamental science objective of the Parker
Solar Probe and Solar Orbiter missions and one of the central aims of
heliophysics. The slow solar wind exhibits significant variability on
relatively short timescales, from minutes to days. This short-term
variability in the magnetic field, bulk plasma, and composition
properties of the slow wind likely results from magnetic reconnection
processes in the extended solar corona. We present a detailed analysis
of the solar wind from 2003 April 15 to May 13, corresponding to
Carrington Rotation 2002. We identify regions of enhanced variability
and composition signatures and demonstrate their relationship to the
large-scale magnetic topology of the solar corona. Specifically,
there are four pseudostreamer wind intervals and two heliospheric
current sheet crossings (and an ICME) which all exhibit enhanced
alpha-to-proton ratios and signatures in the charge states of carbon,
oxygen, and iron. We investigate whether other turbulence properties
of these intervals, e.g. the Alfvenicity, Hm-PVI structures, etc,
can be related to coronal topological features such as the S-Web arcs
of the pseudostreamers or the heliospheric current sheet/plasma sheet
crossings. We discuss these results in the context of upcoming PSP
and SolO joint observational campaigns.
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Title: Towards a Coherent View of the Sun/Corona/Heliosphere:
Combining Remote Sensing Data Products with PSP In Situ Measurements
Authors: Alzate, Nathalia; Morgan, Huw; Seaton, Daniel; Thompson,
Barbara; Nieves-Chinchilla, Teresa; Di Matteo, Simone; Viall, Nicholeen
2021AGUFMSH24C..02A Altcode:
In situ (IS) heliospheric measurements and remote sensing
observations contribute crucial information to our understanding of
the Sun/Corona/Heliosphere as a single system. In Situ measurements by,
e.g., Parker Solar Probe (PSP), are detailed and precise measurements of
physical observables (e.g., magnetic field components), which cannot
be gained from remote observations. Remote sensing observations,
in turn, provide the large-scale context, which is absent from
the in situ data taken at one point in space. Therefore, to
understand the Sun/Corona/Heliosphere system, we need to effectively
establish a link between in situ measurements and remote sensing
observations by characterizing structure and plasma properties of
the inner corona. Additionally, we need to resolve the line-of-sight
limitations of white-light (WL) coronagraph and Extreme Ultraviolet
(EUV) observations to properly identify the location of structures and
their temporal density changes. Previous studies have identified outward
propagating density variations in the solar wind (on timescales of hours
up to ~3 days) that have a plasma composition of coronal origin and that
can be traced down through the field of view of STEREO/COR2 (~2.5-15
Rs). Our advanced image processing techniques can reveal structures
(on various timescales) in both EUV and WL data providing continuous
tracking of brightness enhancements from the coronal base out to the
radial extended corona. Our most recent work using STEREO/COR1 and
GOES-R/SUVI observations has proven crucial in linking the low to high
corona and has facilitated the interpretation of PSP data. Further, our
time-dependent rotational tomography of coronal data yields empirically
derived coronal density distribution directly comparable to PSP
measurements at perihelion. We present our current work that combines
PSP data with remote sensing EUV/WL observations of the corona, via the
use of coronal rotational tomography from SOHO/LASCO and STEREO/COR2
observations, which provides the capabilities to reconstruct features
in the solar wind and subsequently study the evolution between EUV/WL
and in situ of the plasma flows that give rise to them.
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Title: Solar Wind Driven Ultra-Low Frequency Waves Properties and
Effects on Radiation Belt Electrons Loss
Authors: Di Matteo, Simone; Viall, Nicholeen; Kepko, Larry; Breneman,
Aaron; Halford, Alexa; Villante, Umberto
2021AGUFMSM44A..05D Altcode:
The Ultra-Low Frequency (ULF) waves in the Earth's magnetosphere
are created by a variety of external and internal generation
mechanisms. Among possible solar wind driving processes, magnetospheric
forced breathing due to solar wind periodic density structures
(PDSs) with size scales of the order of the magnetosphere cavity are
important for fluctuations at frequencies below ~4 mHz. In the rest
frame of a spacecraft or Earth, the PDS length scale and the solar
wind velocity determine the apparent frequency of the PDSs and the
associated dynamic pressure fluctuations. These structures directly
drive magnetospheric field oscillations at similar frequencies and
may also trigger additional ULF waves. These magnetospheric field
fluctuations can cause the loss of radiation belt electrons directly,
e.g., via loss cone modulation, or indirectly, e.g., via inward radial
transport to a region of larger loss cone or triggering the growth of
other magnetospheric processes such as cyclotron waves, which in turn
scatter electrons causing an increase in the precipitation loss. In this
work, we applied a recently developed spectral analysis procedure, based
on the multitaper method, to identify periodicity in solar wind density
and magnetospheric field at the geostationary orbit and ground. The
analysis is also extended to indirect measurements of radiation belt
electron losses in the form of bremsstrahlung X-rays from both the
first and second campaigns of the Balloon Array for Radiation belt
Relativistic Electron Losses (BARREL) mission. We discuss case events
in which we used the combination of satellites and ground magnetometer
observations to characterize ULF wave differences in frequency,
polarization, and preferential magnetospheric locations. We also discuss
preliminary results of electron losses in terms of duration, location,
and observed periodicity. We show that both externally driven (by
PDSs) and internally supported (e.g., Field Line Resonance) waves occur
simultaneously. In addition, while both internally and externally driven
ULF waves are important for radiation belt dynamics, we find instances
of BARREL X-ray counts that closely follow the solar wind driver.
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Title: Understanding Solar Eruptions, Solar Wind Formation, and how
the Sun Connects to the Heliosphere through a Polar Perspective
Authors: Viall, Nicholeen; Gibson, Sarah; Hassler, Don; Newmark,
Jeffrey; Seaton, Daniel; Downs, Cooper
2021AGUFMSH34D..01V Altcode:
A major limitation to our understanding of how the Sun connects to the
heliosphere is due to our ecliptic bias: all remote observations of the
Sun and corona have been made from the ecliptic. The ecliptic viewpoint
by itself can never capture the global corona and its connection
to the heliosphere. The ecliptic view has large uncertainties in
measurements of the polar magnetic fields and has limited ability
to measure longitudinal coronal structure. A polar perspective can
provide new ways to test theories of a host of solar and heliospheric
physics problems, from the quiescent processes involved in solar
wind formation, up through transient solar eruptions and coronal
mass ejections (CMEs). Because the structure and strength of the
polar photospheric magnetic fields shape the corona and provide key
input to coronal and heliospheric models, measuring and tracking
the evolution of the polar magnetic fields provides the bones of the
corona-heliosphere connection as well as information on the storage
and release of explosive energy. Images of the corona in EUV and white
light provide the coronal counterpart to the photospheric magnetic
field measurements for connecting the Sun to the heliosphere. They
capture global coronal connectivity and interactions, longitudinal
expansion and structure, and the effects of co-rotation. Since CMEs
tend to deflect toward the equator, a polar view captures essentially
all Earth-and planet-directed CMEs from a view perpendicular to their
direction of propagation. Overall, the discovery space for a polar
imager is enormous. We describe progress on these topics that can
be expected with Solar Orbiter, which will get to 30 degrees orbital
inclination in the extended mission. We also discuss the unique science
that can be done by continuous imaging of the polar magnetic fields
and corona from above 70 degrees for at least a solar rotation, such
as proposed by the Solaris mission.
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Title: Understanding the Corona-Heliosphere Connection by Identifying
the Origins of In Situ Solar Wind Observations
Authors: Wallace, Samantha; Viall, Nicholeen; Arge, Charles
2021AGUFMSH24C..03W Altcode:
The corona and heliosphere are fundamentally connected by the solar
wind outflow and magnetic field emanating from the Sun. Thus, accurate
knowledge of the physical processes responsible for solar wind formation
(i.e., origin, release, and acceleration), and evolution as the wind
propagates are critical to understanding the corona and heliosphere as
a globally connected system. We present evidence from aggregate work
that demonstrates the importance of knowing the specific solar wind
source regions when investigating solar wind formation. Each study
makes use of the Wang-Sheeley-Arge (WSA) model driven by Air Force
Data Assimilative Photospheric Flux Transport (ADAPT) time-dependent
photospheric field maps to connect in situ solar wind observations
from various spacecraft (e.g., ACE, PSP) to their source regions at
1 Rs. We highlight one study which investigates the properties of
observed solar wind streams in relationship to their coronal streamer
source. Specifically, we test whether the coronal plasma characteristics
(e.g., hot/dense active region vs. cooler/more tenuous quiet Sun)
or the global magnetic structure (e.g., pseudostreamer vs. helmet
streamer) of the coronal streamer are more important in determining
the resulting solar wind characteristics.
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Title: The Multiview Observatory for Solar Terrestrial Science (MOST)
Authors: Gopalswamy, Nat; Kucera, Therese; Leake, James; MacDowall,
Robert; Wilson, Lynn; Kanekal, Shrikanth; Shih, Albert; Christe,
Steven; Gong, Qian; Viall, Nicholeen; Tadikonda, Sivakumar; Fung,
Shing; Yashiro, Seiji; Makela, Pertti; Golub, Leon; DeLuca, Edward;
Reeves, Katharine; Seaton, Daniel; Savage, Sabrina; Winebarger, Amy;
DeForest, Craig; Desai, Mihir; Bastian, Tim; Lazio, Joseph; Jensen,
P. E., C. S. P., Elizabeth; Manchester, Ward; Wood, Brian; Kooi,
Jason; Wexler, David; Bale, Stuart; Krucker, Sam; Hurlburt, Neal;
DeRosa, Marc; Pevtsov, Alexei; Tripathy, Sushanta; Jain, Kiran;
Gosain, Sanjay; Petrie, Gordon; Kholikov, Shukirjon; Zhao, Junwei;
Scherrer, Philip; Woods, Thomas; Chamberlin, Philip; Kenny, Megan
2021AGUFMSH12A..07G Altcode:
The Multiview Observatory for Solar Terrestrial Science (MOST) is a
comprehensive mission concept targeting the magnetic coupling between
the solar interior and the heliosphere. The wide-ranging imagery and
time series data from MOST will help understand the solar drivers and
the heliospheric responses as a system, discerning and tracking 3D
magnetic field structures, both transient and quiescent in the inner
heliosphere. MOST will have seven remote-sensing and three in-situ
instruments: (1) Magnetic and Doppler Imager (MaDI) to investigate
surface and subsurface magnetism by exploiting the combination of
helioseismic and magnetic-field measurements in the photosphere; (2)
Inner Coronal Imager in EUV (ICIE) to study large-scale structures
such as active regions, coronal holes and eruptive structures by
capturing the magnetic connection between the photosphere and the
corona to about 3 solar radii; (3) Hard X-ray Imager (HXI) to image
the non-thermal flare structure; (4) White-light Coronagraph (WCOR) to
seamlessly study transient and quiescent large-scale coronal structures
extending from the ICIE field of view (FOV); (5) Faraday Effect
Tracker of Coronal and Heliospheric structures (FETCH), a novel radio
package to determine the magnetic field structure and plasma column
density, and their evolution within 0.5 au; (6) Heliospheric Imager
with Polarization (HIP) to track solar features beyond the WCOR FOV,
study their impact on Earth, and provide important context for FETCH;
(7) Radio and Plasma Wave instrument (M/WAVES) to study electron beams
and shocks propagating into the heliosphere via passive radio emission;
(8) Solar High-energy Ion Velocity Analyzer (SHIVA) to determine spectra
of electrons, and ions from H to Fe at multiple spatial locations
and use energetic particles as tracers of magnetic connectivity; (9)
Solar Wind Magnetometer (MAG) to characterize magnetic structures at
1 au; (10) Solar Wind Plasma Instrument (SWPI) to characterize plasma
structures at 1 au. MOST will have two large spacecraft with identical
payloads deployed at L4 and L5 and two smaller spacecraft ahead of L4
and behind L5 to carry additional FETCH elements. MOST will build upon
SOHO and STEREO achievements to expand the multiview observational
approach into the first half of the 21st Century.
---------------------------------------------------------
Title: Periodic Structures in Solar Wind Composition Observed by
the ACE/SWICS Instrument: Event Studies and Superposed Epoch Analysis
Authors: Gershkovich, Irena; Lepri, Susan; Viall, Nicholeen; Di
Matteo, Simone
2021AGUFMSH25F2157G Altcode:
Periodic structures in the solar wind plasma are extremely abundant
within the ACE SWICS 12-minute-resolution composition data set. We have
demonstrated this statistically in our 2020 AGU Fall Meeting poster
[Gershkovich et al., 2020]. These structures, observed in-situ,
are indicators of variations in composition at the Corona being
imprinted onto the solar wind. Previous work showed that specific
periodic structures in proton and alpha density exist at statistically
significant levels and cannot be attributed to turbulence [Viall et al.,
2009]. Prior to our work, only one periodic event had been identified
in the ACE SWICS composition data [Kepko et al., 2016]. We are greatly
expanding upon knowledge in this area by analyzing 13 years (1998-2011)
of SWICS 12-minute composition data, looking at all ions and charge
states of sufficiently high data quality. Composition is excellent
for studying processes occurring at the Sun as it is frozen into the
solar wind and has no means to evolve as the plasma advects from the
solar atmosphere towards the point of observation. Here, we present the
results of a superposed epoch analysis consisting of several events and
examine trends in charge-state, mass and First Ionization Potential
(FIP) in the 24-hour windows before, during and after each event. We
compare the composition ratio and charge state data to previously
established typical coronal hole and streamer values and discuss the
patterns that emerge.
---------------------------------------------------------
Title: An Analysis of Spikes in Atmospheric Imaging Assembly
(AIA) Data
Authors: Young, Peter R.; Viall, Nicholeen M.; Kirk, Michael S.;
Mason, Emily I.; Chitta, Lakshmi Pradeep
2021SoPh..296..181Y Altcode: 2021arXiv210802624Y
The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory (SDO) returns high-resolution images of the solar atmosphere
in seven extreme ultraviolet (EUV) wavelength channels. The images
are processed on the ground to remove intensity spikes arising from
energetic particles hitting the instrument, and the despiked images
are provided to the community. In this article, a three-hour series of
images from the 171 Å channel obtained on 28 February 2017 was studied
to investigate how often the despiking algorithm gave false positives
caused by compact brightenings in the solar atmosphere. The latter
were identified through spikes appearing in the same detector pixel
for three consecutive frames. 1096 examples were found from the 900
image frames. These "three-spikes" were assigned to 126 dynamic solar
features, and it is estimated that the three-spike method identifies
19% of the total number of features affected by despiking. For any
ten-minute sequence of AIA 171 Å images there are around 37 solar
features that have their intensity modified by despiking. The features
are found in active regions, quiet Sun, and coronal holes and, in
relation to solar surface area, there is a greater proportion within
coronal holes. In 96% of the cases, the despiked structure is a compact
brightening with a size of two arcsec or less, and the remaining 4%
have narrow, elongated structures. By applying an EUV burst detection
algorithm, we found that 96% of the events could be classified as EUV
bursts. None of the spike events are rendered invisible by the AIA
processing pipeline, but the total intensity over an event's lifetime
can be reduced by up to 67%. Users are recommended to always restore
the original intensities in AIA data when studying short-lived or
rapidly evolving features that exhibit fine-scale structure.
---------------------------------------------------------
Title: Connecting the Low to the High Corona: A Method to Isolate
Transients in STEREO/COR1 Images
Authors: Alzate, Nathalia; Morgan, Huw; Viall, Nicholeen; Vourlidas,
Angelos
2021ApJ...919...98A Altcode: 2021arXiv210702644A
We present a method that isolates time-varying components from
coronagraph and extreme ultraviolet images, allowing substreamer
transients propagating within streamers to be tracked from the
low to the high corona. The method uses a temporal bandpass filter
with a transmission bandwidth of ~2.5-10 hr that suppresses both
high- and low-frequency variations in observations made by the
STEREO/SECCHI suite. We demonstrate that this method proves crucial
in linking features in the low corona, where the magnetic field is
highly nonradial, to their counterparts in the high corona, where the
magnetic field follows a radial path, through the COR1 instrument. We
also apply our method to observations by the COR2 and EUVI instruments
on board SECCHI and produce height-time profiles that reveal small
density enhancements, associated with helmet streamers propagating from
~1.2 R<SUB>⊙</SUB> out to beyond 5 R<SUB>⊙</SUB>. Our processing
method reveals that these features are common during the period
of solar minimum in this study. The features recur on timescales
of hours, originate very close to the Sun, and remain coherent out
into interplanetary space. We measure the speed of the features and
classify them as slow (a few to tens of kilometers per second) or fast
(~100 km s<SUP>-1</SUP>). Both types of features serve as an observable
tracer of a variable component of the slow solar wind to its source
regions. Our methodology helps overcome the difficulties in tracking
small-scale features through COR1. As a result, it proves successful
in measuring the connectivity between the low and high corona and in
measuring the velocities of small-scale features.
---------------------------------------------------------
Title: Understanding Heating in Active Region Cores through Machine
Learning. II. Classifying Observations
Authors: Barnes, W. T.; Bradshaw, S. J.; Viall, N. M.
2021ApJ...919..132B Altcode: 2021arXiv210707612B
Constraining the frequency of energy deposition in magnetically closed
active region cores requires sophisticated hydrodynamic simulations
of the coronal plasma and detailed forward modeling of the optically
thin line-of-sight integrated emission. However, understanding which
set of model inputs best matches a set of observations is complicated
by the need for any proposed heating model to simultaneously satisfy
multiple observable constraints. In this paper, we train a random
forest classification model on a set of forward-modeled observable
quantities, namely the emission measure slope, the peak temperature
of the emission measure distribution, and the time lag and maximum
cross-correlation between multiple pairs of AIA channels. We then use
our trained model to classify the heating frequency in every pixel of
active region NOAA 1158 using the observed emission measure slopes,
peak temperatures, time lags, and maximum cross-correlations, and are
able to map the heating frequency across the entire active region. We
find that high-frequency heating dominates in the inner core of the
active region while intermediate-frequency dominates closer to the
periphery of the active region. Additionally, we assess the importance
of each observed quantity in our trained classification model and find
that the emission measure slope is the dominant feature in deciding
with which heating frequency a given pixel is most consistent. The
technique presented here offers a very promising and widely applicable
method for assessing observations in terms of detailed forward models
given an arbitrary number of observable constraints.
---------------------------------------------------------
Title: Erratum: "On the Relationship between Magnetic Expansion Factor
and Observed Speed of the Solar Wind from Coronal Pseudostreamers"
(2020, ApJ, 898, 78)
Authors: Wallace, Samantha; Arge, C. Nick; Viall, Nicholeen;
Pihlström, Ylva
2021ApJ...919...68W Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Mesoscale Structure in the Solar Wind
Authors: Viall, N. M.; DeForest, C. E.; Kepko, L.
2021FrASS...8..139V Altcode:
Structures in the solar wind result from two basic mechanisms:
structures injected or imposed directly by the Sun, and structures
formed through processing en route as the solar wind advects outward
and fills the heliosphere. On the largest scales, solar structures
directly impose heliospheric structures, such as coronal holes imposing
high speed streams of solar wind. Transient solar processes can inject
large-scale structure directly into the heliosphere as well, such as
coronal mass ejections. At the smallest, kinetic scales, the solar
wind plasma continually evolves, converting energy into heat, and all
structure at these scales is formed en route. `Mesoscale' structures,
with scales at 1 AU in the approximate spatial range of 5 Mm -10,000
Mm and temporal range of 10 s - 7 hrs, lie in the orders of magnitude
gap between the two size-scale extremes. Structures of this size regime
are created through both mechanisms. Competition between the imposed and
injected structures with turbulent and other evolution leads to complex
structuring and dynamics. The goal is to understand this interplay
and to determine which type of mesoscale structures dominate the solar
wind under which conditions. However, the mesoscale regime is also the
region of observation space that is grossly under-sampled. The sparse
in situ measurements that currently exist are only able to measure
individual instances of discrete structures, and are not capable of
following their evolution or spatial extent. Remote imaging has captured
global and large scale features and their evolution, but does not yet
have the sensitivity to measure most mesoscale structures and their
evolution. Similarly, simulations cannot model the global system while
simultaneously resolving kinetic effects. It is important to understand
the source and evolution of solar wind mesoscale structures because they
contain information on how the Sun forms the solar wind, and constrains
the physics of turbulent processes. Mesoscale structures also comprise
the ground state of space weather, continually buffeting planetary
magnetospheres. In this paper we describe the current understanding
of the formation and evolution mechanisms of mesoscale structures in
the solar wind, their characteristics, implications, and future steps
for research progress on this topic.
---------------------------------------------------------
Title: Periodic Solar Wind Density Structures Observed with Parker
Solar Probe WISPR
Authors: Viall, N. M.; Vourlidas, A.; Howard, R.; Linton, M.; Kepko,
L.; Di Matteo, S.; Higginson, A. K.
2021AAS...23812305V Altcode:
Periodic trains of mesoscale structures in solar wind density have been
observed close to the Sun with in situ data from the Helios spacecraft,
as well as remotely in STEREO/COR2 and STEREO/HI1 white light imaging
data. While some periodic density structures may be a consequence of
the development of dynamics en route, many are remnants of the formation
and release of the solar wind, and thus provide important constraints on
solar wind models. 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. Here,
we report on the first observations of periodic trains of mesoscale
structures in solar wind density observed by the Wide-field Imager
for Parker Solar PRobe (WISPR). We describe our open-source Fourier
analysis and robust spectral background estimation technique used to
identify the periodic density structures. The observation of periodic
density structures so near to the Sun allows us to begin disentangling
how much structure is created during solar wind formation, versus how
much is due to evolution as the solar wind advects outward.
---------------------------------------------------------
Title: Evidence For Active Region Coronal Heating By Nanoflares
Based On Time-lag Measurements In EUV Light Curves From EIS
Authors: Brosius, J.; Viall, N.
2021AAS...23832813B Altcode:
The nanoflare model of solar coronal heating is based on the idea
that ubiquitous tiny, independent heating events occur on individual
sub-resolution strands within coronal loops. Each heating event raises
its strand plasma to temperatures (6-10 MK) that are greater than the
average active region temperature (about 2 MK). After the impulsive
energy release, the loop strand increases in density and cools by
conduction and radiation. The strand spends more time at higher density
in the radiative cooling phase than it does in any other phase of
the heating and cooling cycle. Thus, even when observed on spatial
scales larger than the unresolvable individual strands, the solar
atmosphere is expected to exhibit an overall cooling trend. Evidence
for this has been presented based on correlations among light curves
from AIA's six EUV channels. While this supports the nanoflare model
of coronal heating, AIA's lack of temperature fidelity means that
precise cooling information for small locations or single events are
less than conclusive. Here we report results from an investigation
of time-lag diagnostics based on EUV light curves derived from stare
spectra obtained with a new EIS study designed to investigate time
lags in non-flaring active regions. This study observes line emission
from ten successive ionization stages of iron (VIII-XVII, 0.45-4 MK),
as well as Fe XXIII (seen in flares and microflares) and lines formed
at lower temperatures. We find evidence of post-nanoflare cooling in
AR 12759 from 2.8 to 0.45 MK, but note that not all locations cool to
temperatures this low, possibly indicating a mixture of medium and
low frequency nanoflares. The AR periphery is cooler than its core,
and exhibits post-nanoflare cooling only from 1.7 to 1.4 MK, suggestive
of higher frequency nanoflares.
---------------------------------------------------------
Title: The Heating of the Solar Corona
Authors: Viall, Nicholeen M.; De Moortel, Ineke; Downs, Cooper;
Klimchuk, James A.; Parenti, Susanna; Reale, Fabio
2021GMS...258...35V Altcode:
No abstract at ADS
---------------------------------------------------------
Title: The Solar Wind
Authors: Rouillard, Alexis P.; Viall, Nicholeen; Pierrard, Viviane;
Vocks, Christian; Matteini, Lorenzo; Alexandrova, Olga; Higginson,
Aleida K.; Lavraud, Benoit; Lavarra, Michael; Wu, Yihong; Pinto, Rui;
Bemporad, Alessandro; Sanchez-Diaz, Eduardo
2021GMS...258....1R Altcode:
No abstract at ADS
---------------------------------------------------------
Title: Understanding Solar Wind Formation by Identifying the Origins
of In Situ Observations
Authors: Wallace, Samantha; Viall, Nicholeen M.; Arge, Charles N.
2021EGUGA..23.6200W Altcode:
Solar wind formation can be separated into three physical steps
- source, release, and acceleration - that each leave distinct
observational signatures on plasma parcels. The Wang-Sheeley-Arge (WSA)
model driven by Air Force Data Assimilative Photospheric Flux Transport
(ADAPT) time-dependent photospheric field maps now has the ability to
connect in situ observations more rigorously to their precise source at
the Sun, allowing us to investigate the physical processes involved in
solar wind formation. In this talk, I will highlight my PhD dissertation
research in which we use the ADAPT-WSA model to either characterize
the solar wind emerging from specific sources, or investigate the
formation process of various solar wind populations. In the first study,
we test the well-known inverse relationship between expansion factor
(fs) and observed solar wind speed (vobs) for solar wind that emerges
from a large sampling of pseudostreamers, to investigate if field line
expansion plays a physical role in accelerating the solar wind from this
source region. We find that there is no correlation between fs and vobs
at pseudostreamer cusps. In the second study, we determine the source
locations of the first identified quasiperiodic density structures
(PDSs) inside 0.6 au. Our modeling provides confirmation of these
events forming via magnetic reconnection both near to and far from the
heliospheric current sheet (HCS) - a direct test of the Separatrix-web
(S-web) theory of slow solar wind formation. In the final study, we use
our methodology to identify the source regions of the first observations
from the Parker Solar Probe (PSP) mission. Our modeling enabled us to
characterize the closest to the Sun observed coronal mass ejection (CME)
to date as a streamer blowout. We close with future ways that ADAPT-WSA
can be used to test outstanding questions of solar wind formation.
---------------------------------------------------------
Title: Power Spectral Density Background Estimate and Signal Detection
via the Multitaper Method
Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.
2021JGRA..12628748D Altcode:
We present a new spectral analysis method for the identification of
periodic signals in geophysical time series. We evaluate the power
spectral density with the adaptive multitaper method, a nonparametric
spectral analysis technique suitable for time series characterized
by colored power spectral density. Our method provides a maximum
likelihood estimation of the power spectral density background according
to four different models. It includes the option for the models to
be fitted on four smoothed versions of the power spectral density
when there is a need to reduce the influence of power enhancements
due to periodic signals. We use a statistical criterion to select
the best background representation among the different smoothing +
model pairs. Then, we define the confidence thresholds to identify
the power spectral density enhancements related to the occurrence of
periodic fluctuations (γ test). We combine the results with those
obtained with the multitaper harmonic F test, an additional complex
valued regression analysis from which it is possible to estimate the
amplitude and phase of the signals. We demonstrate the algorithm on
Monte Carlo simulations of synthetic time series and a case study of
magnetospheric field fluctuations directly driven by periodic density
structures in the solar wind. The method is robust and flexible. Our
procedure is freely available as a stand alone IDL code at <A
href="https://zenodo.org/record/3703168">https://zenodo.org/record/3703168</A>.
The modular structure of our methodology allows the introduction of
new smoothing methods and models to cover additional types of time
series. The flexibility and extensibility of the technique makes it
broadly suitable to any discipline.
---------------------------------------------------------
Title: Power spectrum power-law indices as a diagnostic of coronal
heating
Authors: Ireland, Jack; Bradshaw, Stephen; Kirk, Michael; Viall,
Nicholeen
2021cosp...43E1805I Altcode:
We investigate the coronal heating of active regions using time-series
analysis and hydrodynamic modeling. Viall & Klimchuk 2011, 2012,
2014, 2017 have shown that the timing of active region coronal emission
brightenings in multiple channels of Solar Dynamics Observatory
Atmospheric Imaging Assembly (SDO/AIA) approximates that derived
from simulations of a nanoflare-heated corona. Using Numerical
HYDrodynamic RADiative Emission Model for the Solar Atmosphere
(HYDRAD)-based simulations of AIA emission for an AR, Bradshaw &
Viall 2016 have shown that the timing of coronal emission brightenings
is dependent on the properties of the nanoflare energy distribution
and occurrence rate. Relatedly, Ireland et al. 2015 show that average
power spectra $P(f)$ (where $f$ is frequency) of time series of AIA
171Å and 193Å AR images are dominated by power laws, $P(f)\approx
f^{-z}$, $z>0$. This may be explainable by assuming a distribution
of exponentially decaying events of emission along the line-of-sight
which can also result in power-law power spectra. We present analyses
that test the hypothesis that a distribution of nanoflare events
causes both the emission power-law power spectrum in AIA time-series
as well as the observed brightening time-lags. Firstly, we show that
the power-law indices of Fourier power spectra of the same simulated
data described in Bradshaw & Viall 2016 depends on the frequency
of nanoflares used. Secondly, using the observational AIA time-series
data analyzed by Viall & Klimchuk (2013), we obtain and discuss the
correlations of the cross-channel time-lags with the power-law indices
of Fourier power spectra in each AIA channel. Finally, we discuss the
ability of power-law indices and time-lags together to constrain the
underlying nanoflare frequency distribution.
---------------------------------------------------------
Title: A Synthesis of First Results from Parker Solar Probe and
Solar Orbiter
Authors: Viall, Nicholeen
2021cosp...43E.930V Altcode:
Parker Solar Probe, which launched on August 12, 2018, has now
completed several orbits, traveling closer to the Sun than any
spacecraft before. Together with Solar Orbiter, which was launched
February 10, 2020, these missions provide a remarkable combination
of vantage points and measurements of the connection between the Sun
and the heliosphere. The instrument suites measure in situ thermal
plasma, energetic particles, magnetic and electric fields and waves,
and the remote measurements include measurements of the photospheric
magnetic field, extreme ultraviolet and X-ray emissions, and white
light emission from the corona and heliosphere. These data are already
yielding great advancements in the understanding of coronal heating,
solar wind formation and acceleration, and coronal mass ejections
and energetic particle acceleration. We describe first results from
Parker Solar Probe and Solar Orbiter on the Sun and how it creates
the heliosphere.
---------------------------------------------------------
Title: Investigating power law power spectra as a diagnostic of
nanoflare coronal heating in active regions
Authors: Ireland, J.; Bradshaw, S. J.; Viall, N. M.; Kirk, M. S.
2020AGUFMSH0370006I Altcode:
Power spectra of time series of synthetic AIA emission derived
from simulations of coronal heating in a realistic active region
geometry are analyzed. The synthetic AIA emissions are the same as
those described in Bradshaw & Viall 2016 ApJ, 821, 63. In those
simulations, low, intermediate and high frequency nanoflare occurrence
rates are postulated and the consequent synthetic AIA observations are
calculated. Bradshaw & Viall (2016) calculate the time lags between
hotter and cooler synthetic AIA channels derived via cross-correlation
of time series, and show that these timelags are broadly similar to
those derived from observational data. Power spectra of time series
of synthetic AIA emission are fit by a model P(f) = Af<SUP>-n</SUP> +
C, where f is frequency, n>0, A>C>0. For all six synthetic
AIA channels, it is shown that the fit power law index n depends
on AIA channel, spatial location, and the frequency of nanoflare
energy deposition. This suggests that power spectra of time series
of observational AIA data contain information on the frequency of
nanoflare energy deposition. We discuss the use of power law power
spectra as a possible diagnostic of nanoflare heating in observational
data. We demonstrated that the analysis of power spectra may provide
further constraints on the distribution of heating frequencies in
active regions, complementary to existing constraints derived from
other observables such as emission measure slopes and time lags.
---------------------------------------------------------
Title: Contemporary Analysis Methods for Coronagraph and Heliospheric
Imager Data
Authors: Thompson, B. J.; Attie, R.; Chhiber, R.; Cranmer, S. R.;
DeForest, C.; Gallardo-Lacourt, B.; Gibson, S. E.; Jones, S. I.;
Moraes Filho, V.; Reginald, N. L.; Uritsky, V. M.; Viall, N. M.
2020AGUFMSH031..05T Altcode:
Coronagraphs, polarimeters, and heliospheric imagers are providing
new insight into how structures in the solar wind form and develop as
they flow from the inner corona into the heliosphere. With this comes
a whole new frontier of physical observables in 3D, including kinetic
(velocity and acceleration), thermodynamic (density, temperature, and
shock boundary), and magnetic field properties. These measurements
inform and challenge models of global solar wind flow, turbulence,
and CME propagation. We will discuss recent advances in quantifying
physical properties of the corona and solar wind using coronagraph
and heliospheric imager data, and make predictions of what new models
and instrumentation (including the in-development PUNCH mission)
will bring us in the future.
---------------------------------------------------------
Title: A New Spectral Analysis Procedure for the Identification of
ULF Waves.
Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.
2020AGUFMSM0060002D Altcode:
In the analysis of time series, one of the major diagnostic tools
for the identification of periodic fluctuations is the frequency
domain characterization via the power spectral density (PSD). Periodic
signals manifest as enhancements relative to the continuous PSD. While
harmonic analysis to identify the occurrence of periodic variations
compared to a flat PSD are well established, there is a lack of
standard techniques to assess the significance of a periodicity
against colored noise, such as in the identification of ULF waves
with respect to the colored PSD typically found in geophysical time
series. Here, we present a new procedure based on the adaptive
multitaper method (an open source IDL version is available at <A
href="https://zenodo.org/record/3703168">https://zenodo.org/record/3703168</A>).
This sophisticated non-parametric spectral analysis approach, suitable
for colored PSD, provide an additional complex-valued regression model
from which it is possible to estimate the amplitude and phase of the
signals: the harmonic F test. A priori knowledge of the statistical
properties of the multitaper PSD estimates allows a robust maximum
likelihood fitting of different continuous PSD background models. Then,
the best representation is selected via objective statistical
criteria. Confidence thresholds are used to determine statistically
significant PSD enhancements, that, when combined with the harmonic F
test, provide robust estimates of the frequency of periodic oscillations
occurring in the time series. After testing the performance of this
method on Monte Carlo simulation of synthetic time series, we identify
ULF fluctuations in magnetospheric field observations at geostationary
orbit and in ground observatories. We present a brief tutorial of
our procedure, and we discuss our analysis of the ULF fluctuations
identified with our method.
---------------------------------------------------------
Title: Changes in Alpha-to-Proton Ratios During Periodic Solar Wind
Density Structures
Authors: Kepko, L.; Viall, N. M.
2020AGUFMSH0440030K Altcode:
Trains of mesoscale density structures in the solar wind are often
periodic, with length scales between ~100-1000 Mm, and frequencies
corresponding to their advected timescales of ~0.1-5 mHz. Previous
work has shown that periodic density structure (PDSs) occur at
recurrent scale sizes, and that those scale sizes are correlated with
the terminator signaling end of the solar cycle. However, only a few
event studies have examined the compositional changes associated with
PDSs. Compositional changes, such as the alpha-to-proton ratio, are
frozen into the plasma low in the corona, and so do not evolve as the
solar wind advects and fills the Heliosphere. Therefore, alpha-to-proton
ratio changes within PDSs provide critical insights into and constraints
on their generation - i.e., whether they are generated in situ during
transit to 1 AU, or created as part of solar wind formation. Using
25 years of Wind proton and alpha solar wind data measured at 90-s
cadence, we examine the occurrence distributions of periodic density
structures. Our analysis utilizes multiple temporal (between 6 and 48
hours) and length-scale (9-72 Gm) spectral analysis windows to fully
explore the distribution of alpha-to-proton ratio changes during PDSs
placing important constraints on their generation mechanism and the
formation of the solar wind.
---------------------------------------------------------
Title: Evidence of Solar Coronal Heating by Nanoflares Based on
Time-Lag Measurements in EUV Light Curves from EIS
Authors: Brosius, J. W.; Viall, N. M.
2020AGUFMSH0370004B Altcode:
The nanoflare model is a major contender to explain solar coronal
heating. The model is based on the idea that ubiquitous tiny,
independent heating events occur on individual sub-resolution strands
within coronal loops. Each heating event raises its strand plasma to
temperatures (6 - 10 MK) that are greater than the average active
region temperature (~2 MK). Compelling evidence for this mechanism
is pervasive faint emission at flare-like temperatures, such as that
detected in an active region by the EUNIS sounding rocket. After the
impulsive energy release, the loop strand cools by conduction and
radiation, during which it spends more time at higher density and
at colder temperatures than it does at hotter temperatures. Thus,
even when observed on spatial scales larger than the unresolvable
individual strands, the solar atmosphere is expected to exhibit an
overall cooling trend. Evidence for this cooling trend has been sought
and found based on correlations among light curves from AIA's six
EUV channels. While this provides further support for the nanoflare
model of coronal heating, AIA's lack of temperature fidelity means
that precise cooling information for small locations or single events
are less than conclusive. Here we report preliminary results from an
investigation of time-lag diagnostics based on EUV light curves from
Hinode/EIS spectra. We present results for non-flaring active regions
and quiet-sun areas derived from stare spectra obtained with several
different EIS studies that observe unblended emission lines formed
at temperatures that range from 0.14 to 14 MK. We performed time-lag
diagnostics on light curves of Fe XXIII, Fe XVII, Fe XVI, Fe XIV, S X,
Si VII, Mg VI, O IV, and other lines. For example, for a 2014 March 11
observing run on AR 12002, EIS observed fan loops that cooled slowly
between 1.4 and 0.6 MK on timescales of ~3000s; microflares that cooled
from 14 to 2 MK on timescales of ~1000 s; and core loops that cooled
from about 4 to 0.1 MK on timescales ~1500 s. Properties such as peak
temperature, the timescales of the cooling, and a determination of
whether the cooling is full or partial all provide valuable constraints
on nanoflares as a source of coronal heating.
---------------------------------------------------------
Title: On the Relationship between Magnetic Expansion Factor and
Observed Speed of the Solar Wind from Coronal Pseudostreamers
Authors: Wallace, S.; Arge, C. N.; Viall, N. M.; Pihlstrom, Y.
2020AGUFMSH041..06W Altcode:
For the past 30+ yr, the magnetic expansion factor ( f <SUB>s </SUB>)
has been used in empirical relationships to predict solar wind
speed ( v <SUB>obs</SUB>) at 1 AU based on an inverse relationship
between these two quantities. Coronal unipolar streamers (i.e.,
pseudostreamers) undergo limited field line expansion, resulting
in f <SUB>s </SUB>-dependent relationships to predict the fast wind
associated with these structures. However, case studies have shown
that the in situ observed pseudostreamer solar wind was much slower
than that derived with f <SUB>s </SUB>. To investigate this further,
we conduct a statistical analysis to determine if f <SUB>s </SUB>and v
<SUB>obs</SUB> are inversely correlated for a large sample of periods
when pseudostreamer wind was observed at multiple 1 au spacecraft
(i.e., ACE, STEREO-A/B). We use the Wang-Sheeley-Arge model driven
by Air Force Data Assimilative Photospheric Flux Transport (ADAPT)
photospheric field maps to identify 38 periods when spacecraft
observe pseudostreamer wind. We compare the expansion factor of the
last open field lines on either side of a pseudostreamer cusp with
the corresponding in situ measured solar wind speed. We find that
only slow wind ( v <SUB>obs</SUB> < 500 km s <SUP>−1</SUP>) is
associated with pseudostreamers and that there is not a significant
correlation between f <SUB>s </SUB>and v <SUB>obs</SUB> for these field
lines. This suggests that field lines near the open-closed boundary
of pseudostreamers are not subject to the steady-state acceleration
along continuously open flux tubes assumed in the f <SUB>s </SUB>-
v <SUB>obs</SUB> relationship. In general, dynamics at the boundary
between open and closed field lines such as interchange reconnection
will invalidate the steady-state assumptions of this relationship.
---------------------------------------------------------
Title: Signatures of Type III Solar Radio Bursts from Nanoflares:
Final Results
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2020AGUFMSH0430016C Altcode:
The heating mechanisms responsible for the million degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence o f reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full sized
flares is unknown. The lack of strong emission in hard X rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump on tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III radio bursts bursts are characteristically
known to exhibit high frequency drifts as the beam propagates through
a density gradient. The time lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We will
present the final results of our modeling efforts along with results
from application of the technique to actual radio observations from VLA
(Very Large Array), MWA (Murchison Widefield Array) and seeking data
from LOFAR (Low Frequency Array) as well.We are also using data from the
PSP (Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: The Coronal Diagnostic Experiment (CODEX)
Authors: Newmark, J. S.; Gopalswamy, N.; Kim, Y. H.; Viall, N. M.;
Cho, K. S. F.; Reginald, N. L.; Bong, S. C.; Gong, Q.; Choi, S.;
Strachan, L.; Yashiro, S.
2020AGUFMSH0280011N Altcode:
Understanding solar wind sources and acceleration mechanisms
is an overarching solar physics goal. Current models are highly
under-constrained due to the limitations of the existing data,
particularly in the ~3-10 Rs range. COronal Diagnostic EXperiment
(CODEX) is designed to deliver the first global, comprehensive
data sets that will impose crucial constraints and answer targeted
essential questions, including: Are there signatures of hot plasma
released into the solar wind from previously closed fields? What are
the velocities and temperatures of the density structures that are
observed so ubiquitously within streamers and coronal holes? <P />To
provide these crucial measurements, NASA's Goddard Space Flight Center,
in collaboration with the Korea Astronomy and Space Science Institute,
will develop a next-generation coronagraph for the International
Space Station. This imaging coronagraph uses multiple filters to obtain
simultaneous measurements of electron density, temperature, and velocity
within a single instrument. This will be the first time all three
have been measured simultaneously for this critical field-of-view,
and CODEX achieves these measurements multiple times a day.
---------------------------------------------------------
Title: Connecting the Low to High Corona: Tracking Outward Propagating
Small-Scale Structures Using EUV and Coronagraph Observations
Authors: Alzate, N.; Seaton, D. B.; Morgan, H.; Viall, N. M.
2020AGUFMSH0300010A Altcode:
Previous studies have identified density structures in the slow solar
wind that can be traced down through the field of view (FOV) of STEREO
A/COR2. Advanced image processing techniques can reveal small, faint,
formerly hidden structures in extreme ultraviolet (EUV) and white light
(WL) data providing continuous tracking of brightness enhancements
from the coronal base to the radial extended corona. Our most recent
work, carried out using STEREO/COR1 observations, proved crucial in
linking the low to the high corona. Our method for processing COR1,
which facilitates the tracking of small-scale outward propagating
structures, was applied to several time periods of observations during
Solar cycle 24 for the study of the sources of the slow solar wind. We
identified the outflows in ancillary data from multiple WL coronagraphs
as well as EUV instruments, including STEREO/EUVI and GOES-R/SUVI when
available. The outflows were tracked across the different FOVs from
carefully chosen datasets based on spacecraft position. The larger FOV
of SUVI enabled an uninterrupted view of outflows from the Sun through
images from corresponding coronagraph observations. In doing so, we
were able to link structures near the Sun where the magnetic field
structure is highly non-radial to their counterparts higher up where
the magnetic field follows a radial path. Our work opens the door for
studies of the origin of small-scale structures in the heliosphere
and their implications for the sources of the slow solar wind.
---------------------------------------------------------
Title: Inherent Spatial Scales of Solar Wind Periodic Structures
Found in ACE/SWICS Data
Authors: Gershkovich, I.; Lepri, S. T.; Viall, N. M.; Di Matteo, S.
2020AGUFMSH0440028G Altcode:
The existence of prevalent, statistically significant, periodic
structures in the in-situ observations of number density and solar wind
composition at 1 AU can meaningfully address outstanding questions
about solar wind formation and evolution. Past work [ Viall et al.,
2009] has revealed that specific periodicities in solar wind proton
and alpha density, as well as dayside magnetospheric oscillations,
exist and suggest that mechanisms beyond what can be attributed to
turbulence are responsible. Composition is frozen into the solar wind
and does not evolve as the plasma advects from the upper atmosphere of
the Sun, making it invaluable to studying the processes that form the
heliosphere. Although a periodic event has been identified in the ACE
SWICS data previously [ Kepko et al., 2016], we greatly expand upon
the scope of all prior work by analyzing 13 years (1998-2011) of SWICS
12-minute solar wind composition data, considering all ion species and
charge states with sufficient data quality. We now have a database of
validated windows, for a variety of ion species and charge states, which
have sufficient continuity, counts and low enough errors to be suitable
for any time series analysis. Spectral peaks identified by our algorithm
are required to pass both an amplitude and a harmonic F-test at a 90%
or greater confidence level. This ensures that rigorous statistical
significance criteria have been met. We are conducting an unprecedented
study of the relationships between the periodicities of different
ion densities and their dependence on First Ionization Potential
(FIP) bias, charge state, and mass. Here, we present statistical
results that identify generally prevalent periodic enhancements as
well as event studies that explore the spectral behaviors associated
with heliospheric current sheet crossings, ICME sheaths and stream
interaction regions. This analysis of composition variations informs
our understanding of solar wind formation and evolution.
---------------------------------------------------------
Title: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen M.; Kucera, Therese A.; Karpen, Judith T.
2020ApJ...905...15V Altcode:
We investigated the thermal properties of prominence formation using
time series analysis of Solar Dynamics Observatory's Atmospheric
Imaging Assembly (SDO/AIA) data. Here, we report the first time-lag
measurements derived from SDO/AIA observations of a prominence and its
cavity on the solar limb, made possible by AIA's different wave bands
and high time resolution. With our time-lag analysis, which tracks
the thermal evolution using emission formed at different temperatures,
we find that the prominence cavity exhibited a mixture of heating and
cooling signatures. This is in contrast to prior time-lag studies of
multiple active regions that chiefly identified cooling signatures
and very few heating signatures, which is consistent with nanoflare
heating. We also computed time lags for the same pairs of SDO/AIA
channels using output from a one-dimensional hydrodynamic model of
prominence material forming through thermal nonequilibrium (TNE). We
demonstrate that the SDO/AIA time lags for flux tubes undergoing TNE
are predicted to be highly complex, changing with time and location
along the flux tube, and are consistent with the observed time-lag
signatures in the cavity surrounding the prominence. Therefore, the
time-lag analysis is a sensitive indicator of the heating and cooling
processes in different coronal regions. The time lags calculated for
the simulated prominence flux tube are consistent with the behavior
deduced from the AIA data, thus supporting the TNE model of prominence
formation. Future investigations of time lags predicted by other models
for the prominence mass could be a valuable method for discriminating
among competing physical mechanisms.
---------------------------------------------------------
Title: Using Coronagraphs and Heliospheric Imagers to Answer the
Outstanding Questions of Solar Wind Physics
Authors: Viall, N. M.; Borovsky, J.
2020AGUFMSH0280003V Altcode:
As a part of the American Geophysical Union's Centennial celebration,
the Journal of Geophysical Research commissioned papers on the
Grand Challenges in the Earth and Space Sciences. We present our
Grand Challenge paper on nine outstanding questions of solar wind
physics that synthesizes input from the heliophysics community. These
involve questions about the formation of the solar wind, about the
inherent properties of the solar wind (and what the properties say
about its formation), and about the evolution of the solar wind. The
nine questions focus on (1) origin locations on the Sun, (2) plasma
release, (3) acceleration, (4) heavy-ion abundances and charge
states, (5) magnetic structure, (6) Alfven waves, (7) turbulence,
(8) distribution-function evolution, and (9) energetic-particle
transport. We address the aspects of these questions where progress is
being made with the coronagraphs and heliospheric imagers on current
missions such as Solar TErrestrial RElations Observatory (STEREO),
Parker Solar Probe, and Solar Orbiter. We conclude with the aspects that
require observations from the coronagraphs and heliospheric imagers on
the upcoming missions Polarimeter to Unify the Corona and Heliosphere
(PUNCH), COronal Diagnostic EXperiment (CODEX), as well as a mission
with a polar view, such as Solaris .
---------------------------------------------------------
Title: SPD_MTM: a spectral analysis tool for the SPEDAS framework
Authors: Di Matteo, Simone; Viall, Nicholeen; Kepko, Larry
2020zndo...3703168D Altcode:
This is a new spectral analysis method for the identification
of periodic signals in geophysical time series. We evaluate the
power spectral density with the adaptive multitaper method, a
sophisticated non-parametric spectral analysis technique suitable
for time series characterized by colored power spectral density. Our
method provides a maximum likelihood estimation of the power spectral
density background according to four different models. It includes the
option for the models to be fitted on four smoothed versions of the
power spectral density when there is a need to reduce the influence
of power enhancements due to periodic signals. We use a statistical
criterion to select the best background representation among the
different smoothing+model pairs. Then, we define the confidence
thresholds to identify the power spectral density enhancements
related to the occurrence of periodic fluctuations. We combine
the results with those obtained with the multitaper harmonic
F test, an additional complex-valued regression analysis from
which it is possible to estimate the amplitude and phase of the
signals. A complete description of the procedure is available at
https://doi.org/10.1002/essoar.10502619.2. This work is under review
for publication in JGR: Space Physics - Technical Reports: Methods.
---------------------------------------------------------
Title: Inherent Length Scales of Periodic Mesoscale Density Structures
in the Solar Wind Over Two Solar Cycles
Authors: Kepko, L.; Viall, N. M.; Wolfinger, K.
2020JGRA..12528037K Altcode:
It is now well established through multiple event and
statistical studies that the solar wind at 1 AU contains
contains periodic, mesoscale (L ∼ 100-1,000 Mm) structures
in the proton density. Composition variations observed
within these structures and remote sensing observations of
similar structures in the young solar wind indicate that at
least some of these periodic structures originate in the solar
atmosphere as a part of solar wind formation. Viall et al. (2008, <A
href="https://doi.org/10.1029/2007JA012881">https://doi.org/10.1029/2007JA012881</A>)
analyzed 11 years of data from the Wind spacecraft near L1
and demonstrated a recurrence to the observed length scales of
periodic structures in the solar wind proton density. In the time
since that study, Wind has collected 14 additional years of solar
wind data, new moment analysis of the Wind SWE data is available,
and new methods for spectral background approximation have been
developed. In this study, we analyze 25 years of Wind data collected
near L1 and produce occurrence distributions of statistically
significant periodic length scales in proton density. The
results significantly expand upon the Viall et al. (2008, <A
href="https://doi.org/10.1029/2007JA012881">https://doi.org/10.1029/2007JA012881</A>)
study and further show a possible relation of the length scales to solar
"termination" events.
---------------------------------------------------------
Title: On the Relationship between Magnetic Expansion Factor and
Observed Speed of the Solar Wind from Coronal Pseudostreamers
Authors: Wallace, Samantha; Arge, C. Nick; Viall, Nicholeen;
Pihlström, Ylva
2020ApJ...898...78W Altcode: 2020arXiv200716168W
For the past 30+ yr, the magnetic expansion factor (f<SUB>s</SUB>)
has been used in empirical relationships to predict solar wind
speed (v<SUB>obs</SUB>) at 1 au based on an inverse relationship
between these two quantities. Coronal unipolar streamers (i.e.,
pseudostreamers) undergo limited field line expansion, resulting
in f<SUB>s</SUB>-dependent relationships to predict the fast wind
associated with these structures. However, case studies have shown
that the in situ observed pseudostreamer solar wind was much slower
than that derived with f<SUB>s</SUB>. To investigate this further,
we conduct a statistical analysis to determine if f<SUB>s</SUB> and
v<SUB>obs</SUB> are inversely correlated for a large sample of periods
when pseudostreamer wind was observed at multiple 1 au spacecraft (i.e.,
ACE, STEREO-A/B). We use the Wang-Sheeley-Arge model driven by Air
Force Data Assimilative Photospheric Flux Transport (ADAPT) photospheric
field maps to identify 38 periods when spacecraft observe pseudostreamer
wind. We compare the expansion factor of the last open field lines on
either side of a pseudostreamer cusp with the corresponding in situ
measured solar wind speed. We find that only slow wind (v<SUB>obs</SUB>
< 500 km s<SUP>-1</SUP>) is associated with pseudostreamers and
that there is not a significant correlation between f<SUB>s</SUB>
and v<SUB>obs</SUB> for these field lines. This suggests that field
lines near the open-closed boundary of pseudostreamers are not subject
to the steady-state acceleration along continuously open flux tubes
assumed in the f<SUB>s</SUB>-v<SUB>obs</SUB> relationship. In general,
dynamics at the boundary between open and closed field lines such as
interchange reconnection will invalidate the steady-state assumptions
of this relationship.
---------------------------------------------------------
Title: Nine Outstanding Questions of Solar Wind Physics
Authors: Viall, Nicholeen M.; Borovsky, Joseph E.
2020JGRA..12526005V Altcode:
In situ measurements of the solar wind have been available for almost
60 years, and in that time plasma physics simulation capabilities
have commenced and ground-based solar observations have expanded into
space-based solar observations. These observations and simulations
have yielded an increasingly improved knowledge of fundamental
physics and have delivered a remarkable understanding of the solar
wind and its complexity. Yet there are longstanding major unsolved
questions. Synthesizing inputs from the solar wind research community,
nine outstanding questions of solar wind physics are developed and
discussed in this commentary. These involve questions about the
formation of the solar wind, about the inherent properties of the
solar wind (and what the properties say about its formation), and
about the evolution of the solar wind. The questions focus on (1)
origin locations on the Sun, (2) plasma release, (3) acceleration,
(4) heavy-ion abundances and charge states, (5) magnetic structure,
(6) Alfven waves, (7) turbulence, (8) distribution-function evolution,
and (9) energetic-particle transport. On these nine questions we offer
suggestions for future progress, forward looking on what is likely
to be accomplished in near future with data from Parker Solar Probe,
from Solar Orbiter, from the Daniel K. Inouye Solar Telescope (DKIST),
and from Polarimeter to Unify the Corona and Heliosphere (PUNCH). Calls
are made for improved measurements, for higher-resolution simulations,
and for advances in plasma physics theory.
---------------------------------------------------------
Title: Magnetic Origins of Cool Plasma in the Sun's Corona
Authors: Mason, E.; Antiochos, S.; Viall, N.
2020AAS...23610606M Altcode:
Much of solar physics research focuses on two questions: how the
corona's temperature becomes hundreds of times hotter than the surface,
and how the slow solar wind forms. Among the most fascinating phenomena
produced by coronal heating is coronal rain, in which plasma undergoes
rapid cooling (from roughly 10<SUP>6</SUP> to 10<SUP>3</SUP> K),
condenses, and falls to the surface. One proposed rain origin theory,
thermal nonequilibrium (TNE), posits a height restriction in coronal
heating. By studying condensations, physicists hope to better understand
coronal heating. Solar wind is often subdivided into fast and slow
wind. The former originates in coronal hole regions; slow wind's source,
however, is still under debate. One leading theory postulates that
it comes from coronal hole boundaries, where magnetic field lines
frequently reconnect. This research investigates the origins and
dynamics of coronal rain via study of recently-discovered structures
called raining null-point topologies, or RNPTs. RNPTs — the first
identification and characterization of which comprise part of this work
— are decaying active regions situated near coronal hole boundaries,
between 50-150 Mm in height. They are host to long periods of continuous
coronal rain formation, and provide insight into coronal heating, slow
solar wind origins, and coronal dynamics. We focus on identifying and
analyzing RNPTs' observational characteristics. We process and analyze
RNPT data using both the Solar Dynamics Observatory Atmospheric Imaging
Assembly and the Helioseismic and Magnetic Imager. Potential field
source-surface extrapolations that model the magnetic field in the
corona aid in the interpretation of the structures' topology. Results
indicate that RNPTs experience two rain-forming mechanisms, TNE and
interchange reconnection. The interchange reconnection is posited to
power much of the early bursts of coronal rain, which constitutes a
new rain-formation mechanism and allows for plasma from closed loops
to escape into the slow solar wind. Observations also show evidence
of partial condensations, which condense but do not fully cool.
---------------------------------------------------------
Title: The Solaris Solar Polar Mission
Authors: Hassler, Donald M.; Newmark, Jeff; Gibson, Sarah; Harra,
Louise; Appourchaux, Thierry; Auchere, Frederic; Berghmans, David;
Colaninno, Robin; Fineschi, Silvano; Gizon, Laurent; Gosain, Sanjay;
Hoeksema, Todd; Kintziger, Christian; Linker, John; Rochus, Pierre;
Schou, Jesper; Viall, Nicholeen; West, Matt; Woods, Tom; Wuelser,
Jean-Pierre
2020EGUGA..2217703H Altcode:
The solar poles are one of the last unexplored regions of the solar
system. Although Ulysses flew over the poles in the 1990s, it did
not have remote sensing instruments onboard to probe the Sun's polar
magnetic field or surface/sub-surface flows.We will discuss Solaris,
a proposed Solar Polar MIDEX mission to revolutionize our understanding
of the Sun by addressing fundamental questions that can only be answered
from a polar vantage point. Solaris uses a Jupiter gravity assist to
escape the ecliptic plane and fly over both poles of the Sun to >75
deg. inclination, obtaining the first high-latitude, multi-month-long,
continuous remote-sensing solar observations. Solaris will address key
outstanding, breakthrough problems in solar physics and fill holes in
our scientific understanding that will not be addressed by current
missions.With focused science and a simple, elegant mission design,
Solaris will also provide enabling observations for space weather
research (e.g. polar view of CMEs), and stimulate future research
through new unanticipated discoveries.
---------------------------------------------------------
Title: The Heliospheric Current Sheet and Plasma Sheet during Parker
Solar Probe's First Orbit
Authors: Lavraud, B.; Fargette, N.; Réville, V.; Szabo, A.; Huang,
J.; Rouillard, A. P.; Viall, N.; Phan, T. D.; Kasper, J. C.; Bale,
S. D.; Berthomier, M.; Bonnell, J. W.; Case, A. W.; Dudok de Wit,
T.; Eastwood, J. P.; Génot, V.; Goetz, K.; Griton, L. S.; Halekas,
J. S.; Harvey, P.; Kieokaew, R.; Klein, K. G.; Korreck, K. E.;
Kouloumvakos, A.; Larson, D. E.; Lavarra, M.; Livi, R.; Louarn, P.;
MacDowall, R. J.; Maksimovic, M.; Malaspina, D.; Nieves-Chinchilla,
T.; Pinto, R. F.; Poirier, N.; Pulupa, M.; Raouafi, N. E.; Stevens,
M. L.; Toledo-Redondo, S.; Whittlesey, P. L.
2020ApJ...894L..19L Altcode:
We present heliospheric current sheet (HCS) and plasma sheet (HPS)
observations during Parker Solar Probe's (PSP) first orbit around the
Sun. We focus on the eight intervals that display a true sector boundary
(TSB; based on suprathermal electron pitch angle distributions) with
one or several associated current sheets. The analysis shows that (1)
the main density enhancements in the vicinity of the TSB and HCS are
typically associated with electron strahl dropouts, implying magnetic
disconnection from the Sun, (2) the density enhancements are just about
twice that in the surrounding regions, suggesting mixing of plasmas from
each side of the HCS, (3) the velocity changes at the main boundaries
are either correlated or anticorrelated with magnetic field changes,
consistent with magnetic reconnection, (4) there often exists a layer
of disconnected magnetic field just outside the high-density regions, in
agreement with a reconnected topology, (5) while a few cases consist of
short-lived density and velocity changes, compatible with short-duration
reconnection exhausts, most events are much longer and show the
presence of flux ropes interleaved with higher-β regions. These
findings are consistent with the transient release of density blobs
and flux ropes through sequential magnetic reconnection at the tip of
the helmet streamer. The data also demonstrate that, at least during
PSP's first orbit, the only structure that may be defined as the HPS
is the density structure that results from magnetic reconnection,
and its byproducts, likely released near the tip of the helmet streamer.
---------------------------------------------------------
Title: Relating Streamer Flows to Density and Magnetic Structures
at the Parker Solar Probe
Authors: Rouillard, Alexis P.; Kouloumvakos, Athanasios; Vourlidas,
Angelos; Kasper, Justin; Bale, Stuart; Raouafi, Nour-Edine; Lavraud,
Benoit; Howard, Russell A.; Stenborg, Guillermo; Stevens, Michael;
Poirier, Nicolas; Davies, Jackie A.; Hess, Phillip; Higginson,
Aleida K.; Lavarra, Michael; Viall, Nicholeen M.; Korreck, Kelly;
Pinto, Rui F.; Griton, Léa; Réville, Victor; Louarn, Philippe;
Wu, Yihong; Dalmasse, Kévin; Génot, Vincent; Case, Anthony W.;
Whittlesey, Phyllis; Larson, Davin; Halekas, Jasper S.; Livi, Roberto;
Goetz, Keith; Harvey, Peter R.; MacDowall, Robert J.; Malaspina, D.;
Pulupa, M.; Bonnell, J.; de Witt, T. Dudok; Penou, Emmanuel
2020ApJS..246...37R Altcode: 2020arXiv200101993R
The physical mechanisms that produce the slow solar wind are still
highly debated. Parker Solar Probe's (PSP's) second solar encounter
provided a new opportunity to relate in situ measurements of the
nascent slow solar wind with white-light images of streamer flows. We
exploit data taken by the Solar and Heliospheric Observatory, the Solar
TErrestrial RElations Observatory (STEREO), and the Wide Imager on Solar
Probe to reveal for the first time a close link between imaged streamer
flows and the high-density plasma measured by the Solar Wind Electrons
Alphas and Protons (SWEAP) experiment. We identify different types of
slow winds measured by PSP that we relate to the spacecraft's magnetic
connectivity (or not) to streamer flows. SWEAP measured high-density and
highly variable plasma when PSP was well connected to streamers but more
tenuous wind with much weaker density variations when it exited streamer
flows. STEREO imaging of the release and propagation of small transients
from the Sun to PSP reveals that the spacecraft was continually impacted
by the southern edge of streamer transients. The impact of specific
density structures is marked by a higher occurrence of magnetic field
reversals measured by the FIELDS magnetometers. Magnetic reversals are
associated with much stronger density variations inside than outside
streamer flows. We tentatively interpret these findings in terms of
magnetic reconnection between open magnetic fields and coronal loops
with different properties, providing support for the formation of a
subset of the slow wind by magnetic reconnection.
---------------------------------------------------------
Title: Imaging the Solar Corona From Within
Authors: Hess, P.; Howard, R.; Vourlidas, A.; Bothmer, V.; Colaninno,
R.; DeForest, C.; Gallagher, B.; Hall, J. R.; Higginson, A.; Korendyke,
C.; Kouloumvakos, A.; Lamy, P.; Liewer, P.; Linker, J.; Linton, M.;
Penteado, P.; Plunkett, S.; Poirer, N.; Raouafi, N.; Rich, N.; Rochus,
P.; Rouillard, A.; Socker, D.; Stenborg, G.; Thernisien, A.; Viall, N.
2020AAS...23514907H Altcode:
Parker Solar Probe (PSP), launched, in August 2018 is humanity's
first probe of a stellar atmosphere. It will make measurements of
the near-Sun plasma from 'within' the outer corona with gradually
reduced perihelia from its first perihelia of 35 Rs in 2018-19 to 9.8
Rs in 2025. Here we report the results from the imaging observations
of the electron and dust corona, whe PSP was 35-54 Rs from the solar
surface, taken by the Wide-field Imager for Solar Probe (WISPR). The
spacecraft was near-corotating with the solar corona throughout the
observing window, which is an unprecedented situation for any type of
coronal imaging. Our initial analysis uncovers a long-hypothesized
depletion of the primordial dust orbiting near the Sun, reveals the
plasma structure of small-scale ejections, and provides a strict test
for validating model predictions of the large-scale configuration of
the coronal plasma. Thus, WISPR imaging allows the study of near-Sun
dust dynamics as the mission progresses. The high-resolution images
of small transients, largely unresolved from 1 AU orbits, unravel
the sub-structures of small magnetic flux ropes and show that the
Sun continually releases helical magnetic fields in the background
wind. Finally, WISPR's observations of the coronal streamer evolution
confirm the large-scale topology of the solar corona but they also
reveal that, as recently predicted, streamers are composed of yet
smaller sub-streamers channeling continual density fluctuations at
all visible scales.
---------------------------------------------------------
Title: The Importance of Periodic Density Structures Within Stream
Interaction Regions on Outer Zone Electrons
Authors: Kepko, L.; Viall, N. M.
2019AGUFMSM52A..04K Altcode:
While ULF waves are known to be important for radial transport of
radiation belt electrons, particularly in the outer zone, the current
state of the art relies on empirical relationships of ULF wave power
to geomagnetic indices (e.g., Kp) to model this transport. However,
ULF power departs significantly from these empirical representations
on short timescales, and empirical representations are often unable
to accurately describe the diffusion rates. One such departure is the
existence of periodic number density structures (PDSs) in the solar
wind, which are discrete (not broadband) oscillations, and that are not
included in parameterized models. Here we present six PDS events that
were embedded within the leading edge of stream interaction regions
(SIRs). The events ranged from strong SIRs, with a large jump in
velocity across the region and a forward shock, to weak SIRs, where the
velocity change was minor and no shock had yet developed. Despite the
range of velocity change, each event contained similar periodic density
structures that drove global, compressional (poloidal) ULF waves with
periods of 5-20 minutes, and had noticeable impacts on MeV electrons in
the outer zone. Similar periodic density structures have been observed
previously, but never in the context of SIRs, where their existence
may play an important role in magnetospheric particle acceleration,
loss, and transport, particularly for outer zone electrons that are
highly responsive to ULF wave activity. Since these directly driven
pulsations extend beyond the Pc5 band, their magnetospheric impacts
are underexplored, and we argue that studies examining magnetospheric
impacts of low frequency ULF pulsations should therefore not focus
exclusively on the Pc5 bandwidth. The association of stream interaction
regions (SIRs), which are known to have substantial impacts upon the
radiation belts, with periodic density structures likely enhances their
effectiveness. While our results demonstrate that there is driving
of energetic particles by periodic solar wind density structures, the
effects are not simple. There are global and local particle responses to
the interaction, and the response has time-dependent and quasi-static
aspects that are energy dependent, and should be taken into account
particularly when modeling such interactions.
---------------------------------------------------------
Title: Imaging the Solar Corona from Within: First Results from the
Parker Solar Probe Telescope
Authors: Howard, R. A.; Vourlidas, A.; Bothmer, V.; Colaninno, R. C.;
DeForest, C.; Gallagher, B.; Hall, J. R.; Hess, P.; Higginson, A. K.;
Korendyke, C.; Kouloumvakos, A.; Lamy, P.; Liewer, P. C.; Linker, J.;
Linton, M.; Penteado, P. F.; Plunkett, S. P.; Poirier, N.; Raouafi,
N.; Rich, N.; Rochus, P. L.; Rouillard, A. P.; Socker, D. G.; Stenborg,
G.; Thernisien, A.; Viall, N. M.
2019AGUFMSH11A..04H Altcode:
Parker Solar Probe (PSP) launched in August 2018 is humanity's
first probe of a stellar atmosphere. It will make measurements of
the near-Sun plasma from 'within' the outer corona with gradually
reduced perihelia from its first perihelia of 35 Rs in 2018-19 to 9.8
Rs in 2025. Here we report the results from the imaging observations
of the electron and dust corona, whe PSP was 35-54 Rs from the solar
surface, taken by the Wide-field Imager for Solar Probe (WISPR). The
spacecraft was near-corotating with the solar corona throughout the
observing window, which is an unprecedented situation for any type of
coronal imaging. Our initial analysis uncovers a long-hypothesized
depletion of the primordial dust orbiting near the Sun, reveals the
plasma structure of small-scale ejections, and provides a strict test
for validating model predictions of the large-scale configuration of
the coronal plasma. Thus, WISPR imaging allows the study of near-Sun
dust dynamics as the mission progresses. The high-resolution images
of small transients, largely unresolved from 1 AU orbits, unravel
the sub-structures of small magnetic flux ropes and show that the
Sun continually releases helical magnetic fields in the background
wind. Finally, WISPR's observations of the coronal streamer evolution
confirm the large-scale topology of the solar corona but they also
reveal that, as recently predicted, streamers are composed of yet
smaller sub-streamers channeling continual density fluctuations at
all visible scales.
---------------------------------------------------------
Title: The Properties of Periodic Mesoscale Density Structures in
the Solar Wind
Authors: Kepko, L.; Wolfinger, K.; Viall, N. M.
2019AGUFMSH43C3380K Altcode:
It is now well-established that the solar wind at 1 AU contains
periodic structures in the proton densities that are recurrent
at specific radial length-scales. Event studies using in situ
composition data and remote sensing images shows that many of these
structures originate deep within the solar corona, and advect with
the solar wind, as opposed to being generated by turbulence within
the solar wind during transit. Periodic density structures directly
drive global waves in Earth's magnetosphere, which can affect the
energetic particle populations in the radiation belts, making them
a potentially an important driver of space weather. Viall, Kepko,
and Spence (2008) previously analyzed 11 years of data from the Wind
satellite and found hints of a solar cycle dependence in the sets of
preferred length-scales, but they had insufficient data to examine long
term trends. Here, we utilize the longevity of the Wind spacecraft,
and new background estimation techniques, to extend the statistical
analysis of periodic solar wind density structures to cover more
than a full Hale cycle: a total of 24.5 years, from 1995 through May
of 2019, in order to probe which radial wavelengths preferentially
occur during the solar cycle. We required spectral peaks from the
density data to simultaneously pass a stringent combination of an
amplitude test and a harmonic F-test. We test three different noise
backgrounds: first-order auto-regressive, bending power law, and a
spectral running-average. We produce occurrence distributions from
the statistically significant length-scales. They display enhancements
above the background at persistent length-scales across the full span of
years covered, confirming the earlier results of Viall et al. 2008, but
with some intriguing differences that we will discuss. The solar cycle
dependence of preferred periodicities provides important constraints
on the formation of these periodic density structures. Furthermore, the
solar cycle-dependent statistics of these periodic density structures,
which can impact magnetospheric particle acceleration and transport,
can be used as inputs to space weather models.
---------------------------------------------------------
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.
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: Simulations of Thermal Nonequilibrium in Raining Null-Point
Topologies
Authors: Antiochos, S. K.; Mason, E. I.; Viall, N. M.
2019AGUFMSH53B3381A Altcode:
Coronal heating and the origins of slow solar wind remain central
open questions of solar physics. The recent discovery of raining
null-point topologies allows study of regions that hold implications
for both questions. We present observations from SDO AIA that show
persistent coronal condensations in null-point topologies formed by
decaying active regions located near coronal hole boundaries. Coronal
rain - catastrophically-cooled plasma precipitating along flux tubes
- can be used as a tracer of several physical processes to provide
insight into local heating and cooling dynamics. The rain forms in
two observationally-distinct ways: along the lower spine and null,
and within the closed loops under the fan surface. The former is
attributed to interchange reconnection, while the latter is due
to thermal nonequilibrium (TNE). TNE is caused by height-dependent
footpoint heating, which creates a runaway cooling affect far from
the loop base and triggers condensation. Using the one-dimensional
HYDrodynamic and RADiation solver code (HYDRAD, Bradshaw & Mason
2003), we model asymmetric flux tubes with a large expansion factor
where the loop apex occurs near the null point. A broad parameter study
of heating scale heights and heating rates show the ranges within which
rain could occur, and point to highly restricted coronal heating scale
heights in the decaying active regions. This study provides predictions
for Parker Solar Probe and Solar Orbiter observations.
---------------------------------------------------------
Title: Near-Sun observations of an F-corona decrease and K-corona
fine structure
Authors: Howard, R. A.; Vourlidas, A.; Bothmer, V.; Colaninno, R. C.;
DeForest, C. E.; Gallagher, B.; Hall, J. R.; Hess, P.; Higginson,
A. K.; Korendyke, C. M.; Kouloumvakos, A.; Lamy, P. L.; Liewer, P. C.;
Linker, J.; Linton, M.; Penteado, P.; Plunkett, S. P.; Poirier, N.;
Raouafi, N. E.; Rich, N.; Rochus, P.; Rouillard, A. P.; Socker, D. G.;
Stenborg, G.; Thernisien, A. F.; Viall, N. M.
2019Natur.576..232H Altcode:
Remote observations of the solar photospheric light scattered by
electrons (the K-corona) and dust (the F-corona or zodiacal light)
have been made from the ground during eclipses<SUP>1</SUP> and from
space at distances as small as 0.3 astronomical units<SUP>2-5</SUP> to
the Sun. Previous observations<SUP>6-8</SUP> of dust scattering have
not confirmed the existence of the theoretically predicted dust-free
zone near the Sun<SUP>9-11</SUP>. The transient nature of the corona
has been well characterized for large events, but questions still
remain (for example, about the initiation of the corona<SUP>12</SUP>
and the production of solar energetic particles<SUP>13</SUP>) and
for small events even its structure is uncertain<SUP>14</SUP>. Here
we report imaging of the solar corona<SUP>15</SUP> during the first
two perihelion passes (0.16-0.25 astronomical units) of the Parker
Solar Probe spacecraft<SUP>13</SUP>, each lasting ten days. The view
from these distances is qualitatively similar to the historical views
from ground and space, but there are some notable differences. At
short elongations, we observe a decrease in the intensity of the
F-coronal intensity, which is suggestive of the long-sought dust
free zone<SUP>9-11</SUP>. We also resolve the fine-scale plasma
structure of very small eruptions, which are frequently ejected from
the Sun. These take two forms: the frequently observed magnetic flux
ropes<SUP>12,16</SUP> and the predicted, but not yet observed, magnetic
islands<SUP>17,18</SUP> arising from the tearing-mode instability in
the current sheet. Our observations of the coronal streamer evolution
confirm the large-scale topology of the solar corona, but also reveal
that, as recently predicted<SUP>19</SUP>, streamers are composed of
yet smaller substreamers channelling continual density fluctuations
at all visible scales.
---------------------------------------------------------
Title: Study of Type III Solar Radio Bursts in Nanoflares
Authors: Chhabra, S.; Klimchuk, J. A.; Gary, D. E.; Viall, N. M.
2019AGUFMSH23C3337C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles (~ 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We are
applying the technique to actual radio observations from VLA (Very Large
Array), MWA (Murchison Widefield Array) and seeking data from LOFAR
(Low-Frequency Array) as well. We also plan to use data from the PSP
(Parker Solar Probe) to look for similar reconnection signatures in
the Solar Wind. Our goal is to determine whether nanoflares accelerate
energetic particles and to determine their properties. The results
will have important implications for both the particle acceleration
and reconnection physics.
---------------------------------------------------------
Title: Simultaneous occurrence of internally and externally driven
ULF waves in the magnetosphere.
Authors: Di Matteo, S.; Villante, U.; Viall, N. M.; Kepko, L.
2019AGUFMSM23F3277D Altcode:
Earth's magnetosphere is capable of supporting both internal and
externally driven ULF oscillations. In such a highly coupled system,
the identification of the processes that trigger ULF waves is a
complex task. Here, we discuss a unique interval of magnetospheric
ULF pulsations that occurred on November 9<SUP>th</SUP>, 2002
during the interaction with a complex interplanetary structure
(two interplanetary shocks followed by the heliospheric current
sheet). We use a joint analysis of interplanetary (WIND and GEOTAIL),
magnetospheric (GOES8 and GOES10) and ground observations, that enables
us to definitively identify both internal and external sources of ULF
waves. Following the second interplanetary shock, solar wind monitors
observed three consecutive periodic density structures (PDSs) with
a period of ≈90 minutes (0.2 mHz). These periodic structures then
directly drove globally coherent compressional magnetospheric field
oscillations observed at geostationary orbit and at middle and low
latitude ground observatories. In addition, immediately after the
impact of the shock onto the magnetosphere, a global oscillation
at ≈1.5 mHz was observed in the magnetosphere on the ground. The
concomitant occurrence of a ≈1.9 mHz wave at the same time in the
solar wind density suggests the stimulation of a global cavity mode
at the observed frequency. Additional magnetospheric field and solar
wind density oscillations were observed at ≈2.4-2.5 and ≈3.5 mHz,
concurrent with the 0.2 mHz oscillations. A signal at ≈5-6 mHz
was observed at geostationary orbit, primarily along the poloidal
component, with no clear counterpart in the solar wind. This higher
frequency oscillations suggests localized compressive oscillations,
possibly driven by magnetopause surface eigenmode or Kelvin-Helmholtz
instability. We conclude that internally and externally driven ULF
waves in the magnetosphere can occur simultaneously. In particular,
this event shows three regimes of interaction: at lower frequencies
(≈0.2 mHz) the ULF fluctuations are clearly externally driven; the
higher frequency oscillations (≈5-6 mHz) are of internal origin;
and the ULF waves at frequencies in between ≈1 and 5 mHz can be
triggered by both external and internal processes. Therefore, many
sources need to be considered before drawing conclusion on their origin.
---------------------------------------------------------
Title: Impacts of small coronal transients at Parker Solar Probe at
times of density increases and burst of magnetic switchbacks
Authors: Rouillard, A. P.; Kouloumvakos, A.; Vourlidas, A.; Raouafi,
N. E.; Lavraud, B.; Stenborg, G.; Kasper, J. C.; Bale, S.; Poirier,
N.; Howard, R. A.; Viall, N. M.; Lavarra, M.; Stevens, M. L.; Korreck,
K. E.; Case, A. W.; Whittlesey, P. L.; Larson, D. E.; Halekas, J. S.;
Livi, R.; Goetz, K.; Harvey, P.; MacDowall, R. J.; Malaspina, D.;
Pulupa, M.; Bonnell, J. W.; Dudok de Wit, T.
2019AGUFMSH12A..04R Altcode:
A subset at least of the slow solar wind is released in the form
of transients ejected continually along streamer rays. The physical
mechanisms responsible for these transient releases of dense material
are not yet fully understood. We exploit a period when the NASA
Solar-TErrestrial RElations Observatory-A (STEREO-A) was in orbital
quadrature with Parker Solar Probe (PSP) to track the release and
propagation of dense material from the corona to PSP. At the time
PSP had passed its second perihelion and was located near the Thomson
sphere of the inner Heliospheric Imager (HI-1) onboard STEREO-A. This
provided optimal observing conditions to track dense and therefore
bright structures from the corona to the Sun-approaching spacecraft. We
show that the streamers were continually ejecting bursts of dense
structures (so-called 'blobs') many of which exhibiting V-shapes that
are reminiscent of either magnetic kinks and/or the well-known back
ends of small magnetic flux ropes. The wide-angle imager on Parker
Solar Probe imaged similar structures at other locations of the
streamers during this second encounter suggesting a global nature
of this transient activity. We find evidence in STEREO ultraviolet
images for slow reconfigurations of the corona near the estimated source
regions of these structures but no one-to-one association is yet clearly
established between the lower and upper corona. The exploitation of
height-time maps ('J-maps') built from COR-2 and HI-1 images of the
solar wind allow us to track the dense features all the way to PSP. We
show that the spacecraft was repeatedly impacted by the southern edge
of these structures. The passage of the bright coronal material at
PSP is associated with clear density increases measured by the plasma
instrument as expected. We also find evidence that the impact of the
specific dense structures are correlated with a higher occurrence of
magnetic field reversals. Part of this work was funded by the European
Research Council through the project SLOW_SOURCE - DLV-819189
---------------------------------------------------------
Title: Tracking Outward Propagating Small-Scale Structures from EUVI
through COR1 and COR2
Authors: Viall, N. M.; Alzate, N.; Morgan, H.; Vourlidas, A.
2019AGUFMSH13A..07V Altcode:
The challenge of connecting slow solar wind variability to its source
at the Sun is primarily due to the limitations on observational
data of the low corona. Instrumental light scattering is higher in
coronagraph observations very close to the solar surface, resulting
in poor signal-to-noise ratios. Additionally, the non-radial nature
of the coronal structures in this region hampers the tracking of
small-scale structures in observations, since they produce weaker
signals. Our work focuses on developing and applying advanced image
processing techniques to solar imaging data in an effort to connect
the low corona to the high corona with a focus on the identification
of the sources of small-scale structures in the slow solar wind. The
inner coronagraph, COR1, onboard the STEREO spacecraft, observes the low
corona from ~1.1 to 4 R<SUB>s</SUB>, imaging the important connection
between the solar wind, its source, and the formation of the solar
wind structures. We have developed an approach for processing COR1
that allows the tracking of small-scale structures. The core process
is a bandpass filter of the data over time, where the signal from high
frequency noise is suppressed, as well as slowly evolving structures. We
applied this method to a 10-day period of observations during solar
minimum and compared them to observations from the imager EUVI and outer
coronagraph COR2. Height-time profiles reveal structures propagating
through the different fields of view, establishing a connection between
the low and high corona. Further analysis will allow us to characterize
these structures and determine occurrence frequencies, size scales,
formation height and mechanism. Our method for processing COR1 opens
the door to ~13 years of STEREO/COR1 data for studies in general of
the connection between the low and high corona, and specifically of
small-scale coronal structures.
---------------------------------------------------------
Title: Parker Solar Probe Observations of the Release of Density
Blobs and Flux Ropes at the Heliospheric Current Sheet
Authors: Lavraud, B.; Fargette, N.; Bale, S. D.; Bonnell, J. W.;
Case, A. W.; Dudok de Wit, T.; Eastwood, J. P.; Genot, V. N.; Goetz,
K.; Griton, L. S.; Halekas, J. S.; Harvey, P.; Huang, J.; Kasper,
J. C.; Klein, K. G.; Korreck, K. E.; Kouloumvakos, A.; Larson, D. E.;
Lavarra, M.; Livi, R.; Louarn, P.; MacDowall, R. J.; Maksimovic,
M.; Malaspina, D.; Nieves-Chinchilla, T.; Phan, T. D.; Pinto, R.;
Poirier, N.; Pulupa, M.; Raouafi, N. E.; Rouillard, A. P.; Stevens,
M. L.; Szabo, A.; Toledo-Redondo, S.; Viall, N. M.; Whittlesey, P. L.
2019AGUFMSH13C3442L Altcode:
We present Parker Solar Probe observations of several heliospheric
current sheet crossings during the first two encounters of the
mission. The data show a large variability in the internal structure
of the HCS. We focus in particular on the recurrent observation of
density enhancements, interleaved with magnetic field enhancements,
and which are deemed the signature of the sequential release of flux
ropes and blobs from the tip of the helmet streamers.
---------------------------------------------------------
Title: The Source, Significance, and Magnetospheric Impact of Periodic
Density Structures Within Stream Interaction Regions
Authors: Kepko, L.; Viall, N. M.
2019JGRA..124.7722K Altcode:
We present several examples of magnetospheric ultralow frequency
pulsations associated with stream interaction regions and demonstrate
that the observed magnetospheric pulsations were also present in
the solar wind number density. The distance of the solar wind
monitor ranged from just upstream of Earth's bow shock to 261
R<SUB>E</SUB>, with a propagation time delay of up to 90 min. The
number density oscillations far upstream of Earth are offset from
similar oscillations observed within the magnetosphere by the
advection timescale, suggesting that the periodic dynamic pressure
enhancements were time stationary structures, passively advecting with
the ambient solar wind. The density structures are larger than Earth's
magnetosphere and slowly altered the dynamic pressure enveloping Earth,
leading to a quasi-static and globally coherent "forced-breathing" of
Earth's dayside magnetospheric cavity. The impact of these periodic
solar wind density structures was observed in both magnetospheric
magnetic field and energetic particle data. We further show that
the structures were initially smaller-amplitude, spatially larger,
structures in the upstream slow solar wind and that the higher-speed
wind compressed and amplified these preexisting structures leading to
a series of quasiperiodic density structures with periods typically
near 20 min. Similar periodic density structures have been observed
previously at L1 and in remote images, but never in the context of solar
wind shocks and discontinuities. The existence of periodic density
structures within stream interaction regions may play an important
role in magnetospheric particle acceleration, loss, and transport,
particularly for outer zone electrons that are highly responsive to
ultralow frequency wave activity.
---------------------------------------------------------
Title: Understanding Heating in Active Region Cores through Machine
Learning. I. Numerical Modeling and Predicted Observables
Authors: Barnes, W. T.; Bradshaw, S. J.; Viall, N. M.
2019ApJ...880...56B Altcode: 2019arXiv190603350B
To adequately constrain the frequency of energy deposition in active
region cores in the solar corona, systematic comparisons between
detailed models and observational data are needed. In this paper, we
describe a pipeline for forward modeling active region emission using
magnetic field extrapolations and field-aligned hydrodynamic models. We
use this pipeline to predict time-dependent emission from active region
NOAA 1158 for low-, intermediate-, and high-frequency nanoflares. In
each pixel of our predicted multi-wavelength, time-dependent images,
we compute two commonly used diagnostics: the emission measure slope
and the time lag. We find that signatures of the heating frequency
persist in both of these diagnostics. In particular, our results
show that the distribution of emission measure slopes narrows and the
mean decreases with decreasing heating frequency and that the range
of emission measure slopes is consistent with past observational
and modeling work. Furthermore, we find that the time lag becomes
increasingly spatially coherent with decreasing heating frequency while
the distribution of time lags across the whole active region becomes
more broad with increasing heating frequency. In a follow-up paper,
we train a random forest classifier on these predicted diagnostics and
use this model to classify real observations of NOAA 1158 in terms of
the underlying heating frequency.
---------------------------------------------------------
Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.;
Gary, Dale E.
2019shin.confE..12C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate in
solving this problem [Klimchuk J.A., 2015 and references therein]. <P
/>Whether nanoflares accelerate energetic particles like full-sized
flares is unknown. The lack of strong emission in hard X-rays suggests
that the quantity of highly energetic particles is small. There could,
however, be large numbers of mildly energetic particles ( 10 keV). We
investigate such particles by searching for the type III radio bursts
that they may produce. If energetic electron beams propagating along
magnetic field lines generate a bump-on-tail instability, they will
produce Langmuir waves, which can then interact with other particles
and waves to give rise to emission at the local plasma frequency and
its first harmonic. Type III bursts are characteristically known
to exhibit high frequency drifts as the beam propagates through a
density gradient. The time-lag technique that was developed to study
subtle delays in light curves from different EUV channels [Viall &
Klimchuk 2012] can also be used to detect subtle delays at different
radio frequencies. We have modeled the expected radio emission from
nanoflares, which we used to test and calibrate the technique. We have
begun applying the technique to actual radio observations from VLA
(Very Large Array) and seeking data from MWA (Murchison Widefield Array)
as well. We also plan to use data from the PSP(Parker Solar Probe) to
look for similar reconnection signatures in the Solar Wind. Our goal is
to determine whether nanoflares accelerate energetic particles and to
determine their properties. The results will have important implications
for both the particle acceleration and reconnection physics."
---------------------------------------------------------
Title: Observations and Modelling of Condensation Formation at
Coronal Hole Boundaries
Authors: Mason, Emily; Antiochos, Spiro; Viall, Nicholeen; Macneice,
Peter; Bradshaw, Stephen
2019shin.confE..40M Altcode:
One of the primary mechanisms suggested for slow solar wind formation
is interchange reconnection. This tool for leveraging closed-loop
plasma into the heliosphere is believed to occur ubiquitously in the
corona, but has few definitive observational characteristics. We
present recent observations from SDO AIA and STEREO-A of frequent
condensations in small null-point topologies. These structures, termed
raining null-point topologies, result from decayed active regions
bordering on or entirely within coronal holes. These structures may have
unique S-Web fingerprints, aiding slow wind detection and prediction
capability. Our observations clearly show condensation formation at
the open-closed boundary, where interchange reconnection is widely
believed to occur. The condensations take the form of coronal rain,
catastrophically cooled plasma that is easy to track using remote
observations; their formation on apparently newly-opened coronal flux
tubes gives interchange reconnection a hallmark signature. We will also
present 1D hydrodynamic models for how these condensations can form
via interchange reconnection and thermal nonequilibrium. Due to the
nature of these null-point topologies and their proximity to coronal
holes, they share characteristics common to open-closed boundaries,
pseudostreamers, and active regions. Coordination between Parker Solar
Probe, DKIST, Solar Orbiter, etc. would provide a deeper understanding
of these ideal targets, which encompass such useful signatures for
slow wind investigation.
---------------------------------------------------------
Title: COHERENT: Studying the corona as a holistic environment
Authors: Caspi, Amir; Seaton, Daniel B.; Case, Traci; Cheung, Mark;
Cranmer, Steven; DeForest, Craig E.; de Toma, Giuliana; Downs, Cooper;
Elliott, Heather; Gold, Anne U.; Longcope, Dana; Savage, Sabrina L.;
Sullivan, Susan; Viall, Nicholeen; Vourlidas, Angelos; West, Matthew J.
2019shin.confE.241C Altcode:
The solar corona and the heliosphere must be part of a single
physical system, but because the dominant physical processes change
dramatically from the magnetically-dominated low corona, through the
sparsely-observed middle corona, and into the plasma flow-dominated
outer corona and heliospheric interface, unified frameworks to study
the corona as a whole are essentially nonexistent. Understanding how
physical processes shape and drive the dynamics of the corona as a
global system, on all spatiotemporal scales, is critical for solving
many fundamental problems in solar and heliospheric physics. However,
the lack of unifying observations and models has led to a fragmentation
of the community into distinct regimes of plasma parameter space,
largely clustering around regions where existing instrumentation has
made observations widely available and where models can be sufficiently
self-contained to be tractable. We describe COHERENT, the 'Corona as a
Holistic Environment' Research Network, a focused effort to facilitate
interdisciplinary collaborative research to develop frameworks for
unifying existing and upcoming observations, theory, models, and
analytical tools to study the corona as a holistic system.
---------------------------------------------------------
Title: Connecting the Low Corona to the High Corona: Outward
Propagating Small-Scale Transients Tracked from EUVI Through COR1
and COR2
Authors: Alzate, Nathalia; Viall, Nicholeen; Morgan, Huw; Vourlidas,
Angelos
2019shin.confE..59A Altcode:
Identification of the source of the slow solar wind has been hampered
by the complexity of plasma structures in the very low corona and data
limitations in terms of noise reduction. Application of state-of-the-art
image processing techniques has revealed very faint structures in
the low corona that have an impact on the structure of the extended
corona. These techniques, which overcome faint signals and noise in
the data, allow us to identify and characterize the sources of the slow
solar wind as we explore the variability of coronal density structures
over a distance range starting from the solar surface out to tens
of radii and beyond using data from the STEREO spacecraft. Having
successfully conquered the noise issues in the COR1 instrument, as
a proof of concept we revisited a 10-day period in January 2008 in
which density enhancements were previously identified as sources of the
slow solar wind in in situ, HI2 and COR2 data. Our preliminary results
show transients present in the EUVI, COR1 and COR2 FOVs, with several
transients propagating through at least two FOVs. Further analysis
will enable us to properly determine the exact formation heights of
ambient solar wind structures, formation mechanisms, and the features
in the low corona they connect to. We will include data from other
instruments to study different coronal configurations in order to
characterize solar wind structures low in the corona as a function
of coronal magnetic complexity, which will be an essential test for
theories of solar wind formation. Our method for processing COR1 data
opens the door to 12 years of data for studies of small-scale coronal
structures and their geoeffectiveness. This work is part of the broad
effort in Heliospheric physics to understand the different types of
transient structures created in the solar wind as it is formed.
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Title: Observations of Solar Coronal Rain in Null Point Topologies
Authors: Mason, E. I.; Antiochos, Spiro K.; Viall, Nicholeen M.
2019ApJ...874L..33M Altcode: 2019arXiv190408982M
Coronal rain is the well-known phenomenon in which hot plasma high in
the Sun’s corona undergoes rapid cooling (from ∼10<SUP>6</SUP> to
<10<SUP>4</SUP> K), condenses, and falls to the surface. Coronal rain
appears frequently in active region coronal loops and is very common
in post-flare loops. This Letter presents discovery observations,
which show that coronal rain is ubiquitous in the commonly occurring
coronal magnetic topology of a large (∼100 Mm scale) embedded bipole
very near a coronal hole boundary. Our observed structures formed when
the photospheric decay of active-region-leading-sunspots resulted in a
large parasitic polarity embedded in a background unipolar region. We
observe coronal rain to appear within the legs of closed loops well
under the fan surface, as well as preferentially near separatrices
of the resulting coronal topology: the spine lines, null point, and
fan surface. We analyze three events using SDO Atmospheric Imaging
Assembly observations in the 304, 171, and 211 Å channels, as well
as SDO Helioseismic and Magnetic Imager magnetograms. The frequency of
rain formation and the ease with which it is observed strongly suggests
that this phenomenon is generally present in null point topologies of
this size scale. We argue that these rain events could be explained
by the classic process of thermal nonequilibrium or via interchange
reconnection at the null; it is also possible that both mechanisms
are present. Further studies with higher spatial resolution data and
MHD simulations will be required to determine the exact mechanism(s).
---------------------------------------------------------
Title: Helios Observations of Quasiperiodic Density Structures in
the Slow Solar Wind at 0.3, 0.4, and 0.6 AU
Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.; Wallace, S.; Arge,
C. N.; MacNeice, P.
2019JGRA..124..837D Altcode:
Following previous investigations of quasiperiodic plasma density
structures in the solar wind at 1 AU, we show using the Helios1 and
Helios2 data their first identification in situ in the inner heliosphere
at 0.3, 0.4, and 0.6 AU. We present five events of quasiperiodic
density structures with time scales ranging from a few minutes to a
couple of hours in slow solar wind streams. Where possible, we locate
the solar source region of these events using photospheric field maps
from the Mount Wilson Observatory as input for the Wang-Sheeley-Arge
model. The detailed study of the plasma properties of these structures
is fundamental to understanding the physical processes occurring at
the origin of the release of solar wind plasma. Temperature changes
associated with the density structures are consistent with these
periodic structures developing in the solar atmosphere as the solar wind
is formed. One event contains a flux rope, suggesting that the solar
wind was formed as magnetic reconnection opened up a previously closed
flux tube at the Sun. This study highlights the types of structures that
Parker Solar Probe and the upcoming Solar Orbiter mission will observe,
and the types of data analyses these missions will enable. The data
from these spacecrafts will provide additional in situ measurements
of the solar wind properties in the inner heliosphere allowing,
together with the information of the other interplanetary probes,
a more comprehensive study of solar wind formation.
---------------------------------------------------------
Title: Timescales and radial lengthscales of quasi-periodic density
structures observed by the Helios probes
Authors: Di Matteo, S.; Viall, N. M.; Kepko, L.; Villante, U.
2019NCimC..42...20D Altcode:
Quasi-periodic density structures are important mesoscale structures
that constitute the slow solar wind. The associated periodicities play a
fundamental role in the understanding of the release processes of solar
coronal plasma and provide important constraints on models trying to
understand the origin of the slow solar wind. However, observational
restrictions have limited their study to coronagraph images near the
Sun and in situ measurements at 1 AU, preventing a clear connection
between in situ structures to their coronal sources. A first step
toward a better understanding of this problem is the analysis of
Helios measurements to probe the solar wind between 0.3 and 0.6
AU. Three instances of quasi-periodic density structures have been
identified showing characteristic timescales of ≈ 31-33 minutes and
≈ 112-120 minutes. Using the mean radial bulk speed corresponding to
the time intervals of the structures, the related radial lengthscales
are derived showing a characteristic value of ≈ 120-150 R_E.
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Title: Power spectrum power-law indices as a diagnostic of coronal
heating
Authors: Ireland, Jack; Viall, Nicholeen; Bradshaw, Stephen; Kirk,
Michael
2018csc..confE.119I Altcode:
We investigate the coronal heating of active regions by bringing
together novel data analysis techniques with hydrodynamic modeling in
a new and unique way. Viall & Klimchuk 2011, 2012, 2014, 2017 have
shown that the timing of active region coronal emission brightenings
in multiple channels of Solar Dynamics Observatory Atmospheric
Imaging Assembly (SDO/AIA) follows that expected from simulations
of a nanoflare-heated corona. Using Numerical HYDrodynamic RADiative
Emission Model for the Solar Atmosphere (HYDRAD)-based simulations of
AIA emission for an AR, Bradshaw & Viall 2016 have shown that the
timing of coronal emission brightenings is dependent on the properties
of the nanoflare energy distribution and occurrence rate. Relatedly,
Ireland et al. 2015 show that average power spectra P(f) (where f is
frequency) of time series of AIA 171Å and 193Å AR images are dominated
by power laws, P(f) f^{-z}, z>0. Ireland et al. 2015 show that a
distribution of exponentially decaying events of emission E along the
line-of-sight, where N(E) E^{-m} and the size of the emission depends
on its duration T such that E T^{k} creates a power law power spectrum
P(f) f^{-k(2-m)}. We present analyses that test the hypothesis that
a distribution of nanoflare events causes both the emission power-law
power spectrum in AIA time-series as well as the observed brightening
time-lags. Firstly, we show that the power-law indices of Fourier power
spectra of the same simulated data described in Bradshaw & Viall
2016 depends on the frequency of nanoflares used. Secondly, using the
same observational AIA time-series data analyzed by Viall & Klimchuk
(2012), we obtain correlations of the cross-channel time-lags with the
power-law indices of Fourier power spectra in each AIA channel. Finally,
the ability of power-law indices and time-lags together to constrain
the underlying nanoflare frequency distribution is discussed.
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Title: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen; Kucera, Therese; Karpen, Judith
2018csc..confE.124V Altcode:
We investigate prominence formation using time series analysis of
Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA)
data. We examine the thermal properties of forming prominences by
analyzing observed light curves using the same technique that we have
already successfully applied to active regions to diagnose heating
and cooling cycles. This technique tracks the thermal evolution using
emission formed at different temperatures, made possible by AIA's
different wavebands and high time resolution. We also compute the
predicted light curves in the same SDO/AIA channels of a hydrodynamic
model of thermal nonequilibrium formation of prominence material,
an evaporation-condensation model. In these models of prominence
formation, heating at the foot-points of sheared coronal flux-tubes
results in evaporation of material of a few MK into the corona followed
by catastrophic cooling of the hot material to form cool ( 10,000 K)
prominence material. We investigate prominences from different viewing
angles to evaluate possible line of sight effects. We demonstrate
that the SDO/AIA light curves for flux tubes undergoing thermal
nonequilibrium vary at different locations along the flux tube,
especially in the region where the condensate forms, and we compare
the predicted light curves with those observed.
---------------------------------------------------------
Title: Source and Effect of Mesoscale Solar Wind Structures in the
Inner Heliosphere
Authors: Viall, Nicholeen Mary
2018shin.confE..45V Altcode:
We discuss the mounting evidence from remote white light imaging data
and from in situ Helios data that a large fraction of the solar wind in
the inner heliosphere is comprised of mesoscale structures. Mesoscale
structures have spatial scales of 100-10,000 Mm and timescales of
tens of minutes to many hours, and they exhibit changes in the plasma
density, temperature and/or composition that are concurrent with
flux tube boundaries. Remote imaging shows that mesoscale structures
are already present as low as 2.5 Rs from the Sun - and possibly
lower. We will discuss the numerous possible sources and physical
processes involved, including magnetic reconnection, coronal jets,
waves, and filamentary structure from the low corona. It is likely
that multiple physical processes are involved. Regardless of their
source, the magnetic field changes in the mesoscale structures can
affect SEP propagation, and the dynamic pressure changes associated
with the density can drive global dynamics in the magnetospheres of
the terrestrial planets.
---------------------------------------------------------
Title: Observational Evidence of Coronal Magnetic Reconnection during
Quiescent Conditions
Authors: Viall, Nicholeen Mary
2018shin.confE..67V Altcode:
The slow solar wind is often released through magnetic reconnection
in the high corona. White light images of the corona show evidence of
the associated dipolarization flows collapsing back towards the solar
surface. In situ data in the inner heliosphere show composition and
temperature changes in solar wind structures with concurrent magnetic
field signatures - including flux ropes - indicating that closed
field plasma was released into the solar wind through magnetic field
reconnection. Magnetic reconnection is also likely important for heating
the solar corona, though direct evidence is difficult to find due to
instrumental constraints. Current observations suggest that Parker
Solar Probe will most often fly above the reconnection sites. Given the
existing methods used to identify magnetic reconnection, we will discuss
the types of signatures that Parker Solar Probe may see. This includes
(but not limited to) temperature changes, alpha/proton abundance
changes, flux ropes, magnetic connectivity changes, and Type III
radio bursts.
---------------------------------------------------------
Title: Tracing the Origins of the Solar Wind by Tracking Flows and
Disturbances in Coronagraph Data
Authors: Thompson, Barbara J.; Attie, Raphael; DeForest, Craig E.;
Gibson, Sarah E.; Hess Webber, Shea A.; Ireland, Jack; Kirk, Michael
S. F.; Kwon, Ryun Young; McGranaghan, Ryan; Viall, Nicholeen M.
2018shin.confE..47T Altcode:
The challenge of identifying transient motions in solar imagery has
been addressed in a number of ways. A variety of methods have been
developed to detect and characterize the motion and extent of coronal
mass ejections, for example. We discuss the adaptation of CME and
solar transient detection methods to trace smaller-scale perturbations
consistent with solar wind motions in the inner heliosphere (out to 10
RSun). We evaluate several methods, and compare the speed and structure
results to model predictions. In particular, we discuss how high-cadence
heliospheric imagery can be used to track small scale solar density
variations throughout the solar wind, serving as a proxy for in situ
velocity detection, but with global and continuous coverage.
---------------------------------------------------------
Title: Study of Type III Radio Bursts in Nanoflares
Authors: Chhabra, Sherry; Klimchuk, James A.; Viall, Nicholeen M.
2018shin.confE..18C Altcode:
The heating mechanisms responsible for the million-degree solar corona
remain one of the most intriguing problems in space science. It is
widely agreed, that the ubiquitous presence of reconnection events and
the associated impulsive heating (nanoflares) are a strong candidate
in solving this problem [Klimchuk J.A., 2015 and references therein].
---------------------------------------------------------
Title: The Highly Structured Outer Solar Corona
Authors: DeForest, C. E.; Howard, R. A.; Velli, M.; Viall, N.;
Vourlidas, A.
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 <SUP>-1</SUP> 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 <SUB>⊙</SUB>, 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: Timelag Analysis of Simulated Active Region Cores Heated
by Nanoflares
Authors: Barnes, Will; Bradshaw, Stephen J.; Viall, Nicholeen M.
2018tess.conf22403B Altcode:
The interpretation of remote sensing data in the context of the
underlying coronal heating mechansim is complicated by several factors,
including limited instrument sensitivity, nonequilibrium ionization
and multiple emitting structures along the line of sight. In this
presentation, we investigate observable signatures of impulsive
heating in active region NOAA 1158 through efficient hydrodynamic loop
models, magnetic field extrapolations, and advanced forward modeling
techniques. To compute synthetic observations for all EUV channels of
the Atmospheric Imaging Assembly (AIA), we calculate the emissivity
for the relevant ions using CHIANTI, fold this information through
the appropriate instrument response functions, and then map it to the
extrapolated field geometry. The timelag, the temporal offset which
gives the maximum cross-correlation between two channels, is calculated
in each pixel of our synthesized AIA observations. We investigate
the impact of three different parameters on the simulated timelags:
the frequency at which the loops are reheated, the assumption that the
ion populations are in equilibrium with the surrounding electrons, and
the orientation of the active region relative to the line of sight. We
make detailed comparisons to observed timelag maps of the same active
region. Additionally, our forward-modeling software has been developed
to be both modular and generally applicable. We briefly discuss how
this framework for synthesizing observations might be useful to the
larger solar physics community.
---------------------------------------------------------
Title: Tracking Flows and Disturbances in Coronagraph Data
Authors: Thompson, Barbara J.; Attie, Raphael; DeForest, Craig E.;
Gibson, Sarah E.; Hess Webber, Shea A.; Inglis, Anfew R.; Ireland,
Jack; Kirk, Michael S.; Kwon, RyunYoung; Viall, Nicholeen M.
2018tess.conf30922T Altcode:
The challenge of identifying transient motions in solar imagery has
been addressed in a number of ways. A variety of methods have been
developed to detect and characterize the motion and extent of coronal
mass ejections, for example. We discuss the adaptation of CME and
solar transient detection methods to trace smaller-scale perturbations
consistent with solar wind motions in the inner heliosphere (over 10
RSun). We evaluate several methods, and compare the speed and structure
results to model predictions. In particular, we discuss how high-cadence
heliospheric imagery can be used to track small scale solar density
variations throughout the solar wind, serving as a proxy for in situ
velocity detection, but with global and continuous coverage.
---------------------------------------------------------
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
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: Using Solar Wind Structures as a Rosetta Stone for
Understanding Solar Wind Formation
Authors: Viall, Nicholeen M.; Kepko, Larry; Antiochos, Spiro K.;
Higginson, Aleida Katherine; Vourlidas, Angelos; Lepri, Susan T.
2018tess.conf31702V Altcode:
In the inner heliosphere, the slow solar wind is often comprised of
mesoscale structures: structures with timescales of hours and length
scales of hundreds of mega meters. White light coronagraph data suggest
that these mesoscale structures are formed and embedded in the solar
wind within the first several solar radii above the solar surface,
which is still below even the closest approach of Parker Solar Probe at
nine solar radii. We argue that these mesoscale structures represent a
'Rosetta Stone' for using the embedded solar wind plasma signatures
to understand the fundamental release and acceleration of solar wind
plasma. We study events identified in data from current missions
to demonstrate how mesoscale structures can link dynamics observed
remotely in the lower corona with in situ observations. We discuss the
observations that Parker Solar Probe will make and how to capitalize
on this remote-to-in situ data connection.
---------------------------------------------------------
Title: From the Magnetosphere to the Sun: How we Used Waves in Earth's
Magnetosphere to Understand the Dynamic Nature of the Formation of
the Slow Solar Wind
Authors: Viall, Nicholeen M.
2018tess.conf31001V Altcode:
Constant buffeting of Earth's magnetosphere by the highly dynamic solar
wind produces a broad range of ULF oscillations in the mHz and sub mHz
range (timescales of minutes to hours). Such oscillations have been
observed for decades by in situ and ground-based observations. The
magnetospheric boundaries are sharp and capable of reflecting
oscillations, leading to the intriguing idea of cavity mode oscillations
(standing mode waves), in which the magnetospheric cavity acts like a
ringing bell, struck by the hammer that is the solar wind. As L1 solar
wind measurements became more prevalent, it became clear that there
were times when the magnetosphere was not ringing like a bell, but was
directly driven by quasi-periodic dynamic pressure structures in solar
wind that appeared to be ubiquitous - an unexpected finding. Yet,
while this discovery answered one question, a major new question
immediately arose: why are there quasi-periodic structures in the
solar wind in the first place? And with that question, my career in
heliophysics took a turn for the Sun to look for their source. In this
talk, I discuss my research connecting the waves in the magnetosphere to
quasi-periodic structures in the solar wind observed at L1, and further
connecting those to the dynamic nature of the corona and of solar wind
formation. We show that the slow solar wind in particular is released
through magnetic reconnection with characteristic time scales. The
result is quasi-periodic mesoscale structures in the slow solar
wind that retain the magnetic and plasma signatures of this process,
eventually driving magnetospheric pulsations several days later.
---------------------------------------------------------
Title: Dressing the Coronal Magnetic Extrapolations of Active Regions
with a Parameterized Thermal Structure
Authors: Nita, Gelu M.; Viall, Nicholeen M.; Klimchuk, James A.;
Loukitcheva, Maria A.; Gary, Dale E.; Kuznetsov, Alexey A.; Fleishman,
Gregory D.
2018ApJ...853...66N Altcode:
The study of time-dependent solar active region (AR) morphology and
its relation to eruptive events requires analysis of imaging data
obtained in multiple wavelength domains with differing spatial and
time resolution, ideally in combination with 3D physical models. To
facilitate this goal, we have undertaken a major enhancement of our
IDL-based simulation tool, GX_Simulator, previously developed for
modeling microwave and X-ray emission from flaring loops, to allow it
to simulate quiescent emission from solar ARs. The framework includes
new tools for building the atmospheric model and enhanced routines
for calculating emission that include new wavelengths. In this paper,
we use our upgraded tool to model and analyze an AR and compare the
synthetic emission maps with observations. We conclude that the modeled
magneto-thermal structure is a reasonably good approximation of the
real one.
---------------------------------------------------------
Title: Understanding Coronal Heating through Time-Series Analysis
and Nanoflare Modeling
Authors: Romich, Kristine; Viall, Nicholeen
2018AAS...23135911R Altcode:
Periodic intensity fluctuations in coronal loops, a signature of
temperature evolution, have been observed using the Atmospheric
Imaging Assembly (AIA) aboard NASA’s Solar Dynamics Observatory (SDO)
spacecraft. We examine the proposal that nanoflares, or impulsive bursts
of energy release in the solar atmosphere, are responsible for the
intensity fluctuations as well as the megakelvin-scale temperatures
observed in the corona. Drawing on the work of Cargill (2014) and
Bradshaw & Viall (2016), we develop a computer model of the energy
released by a sequence of nanoflare events in a single magnetic flux
tube. We then use EBTEL (Enthalpy-Based Thermal Evolution of Loops),
a hydrodynamic model of plasma response to energy input, to simulate
intensity as a function of time across the coronal AIA channels. We test
the EBTEL output for periodicities using a spectral code based on Mann
and Lees’ (1996) multitaper method and present preliminary results
here. Our ultimate goal is to establish whether quasi-continuous or
impulsive energy bursts better approximate the original SDO data.
---------------------------------------------------------
Title: Combining Remote and In Situ Observations with MHD models to
Understand the Formation of the Slow Solar Wind
Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.; Lepri, S. T.;
Vourlidas, A.; Linker, J.
2017AGUFMSH21C..05V Altcode:
Connecting the structure and variability in the solar corona to the
Heliosphere and solar wind is one of the main goals of Heliophysics
and space weather research. The instrumentation and viewpoints of
the Parker Solar Probe and Solar Orbiter missions will provide an
unprecedented opportunity to combine remote sensing with in situ data
to determine how the corona drives the Heliosphere, especially as it
relates to the origin of the slow solar wind. We present analysis of
STEREO coronagraph and heliospheric imager observations and of in
situ ACE and Wind measurements that reveal an important connection
between the dynamics of the corona and of the solar wind. We show
observations of quasi-periodic release of plasma into the slow solar
wind occurring throughout the corona - including regions away from the
helmet streamer and heliospheric current sheet - and demonstrate that
these observations place severe constraints on the origin of the slow
solar wind. We build a comprehensive picture of the dynamic evolution
by combining remote imaging data, in situ composition and magnetic
connectivity information, and MHD models of the solar wind. Our results
have critical implications for the magnetic topology involved in slow
solar wind formation and magnetic reconnection dynamics. Crucially,
this analysis pushes the limits of current instrument resolution and
sensitivity, showing the enormous potential science to be accomplished
with the Parker Solar Probe and Solar Orbiter missions.
---------------------------------------------------------
Title: Thermal Time Evolution of Non-Flaring Active Regions Determined
by SDO/AIA
Authors: Wright, Paul James; Hannah, Iain; Viall, Nicholeen; MacKinnon,
Alexander; Ireland, Jack; Bradshaw, Stephen
2017SPD....4840203W Altcode:
We present the pixel-level time evolution of DEM maps from SDO/AIA
data using two different methods (Hannah et al. 2012; Cheung et
al. 2015). These sets of Differential Emission Measure (DEM) maps
allow us to determine the slopes of the DEM throughout non-flaring
structures, and investigate how this changes with time, a crucial
parameter in terms of how these flux tubes are being heated. We
present this analysis on both real and synthetic data allowing us to
understand how robustly we can recover the thermal time evolution. As
this analysis also produces the time series in different temperature
bands we can further investigate the underlying heating mechanisms by
applying a variety of techniques to probe the frequency and nature of
the heating, such as time-lag analysis (Viall & Klimchuck 2012;
2016), power spectrum analysis (Ireland et al. 2015), and Local
Intermittency Measure (Dinkelaker & MacKinnon 2013a,b).
---------------------------------------------------------
Title: Diagnosing Coronal Heating in a Survey of Active Regions
using the Time Lag Method
Authors: Viall, Nicholeen; Klimchuk, James A.
2017SPD....4840202V Altcode:
In this paper we examine 15 different active regions observed with the
Solar Dynamics Observatory and analyze their nanoflare properties using
the time lag method. The time lag method is a diagnostic of whether
the plasma is maintained at a steady temperature, or if it is dynamic,
undergoing heating and cooling cycles. An important aspect of our
technique is that it analyses both observationally distinct coronal
loops as well as the much more prevalent diffuse emission surrounding
them. Warren et al. (2012) first studied these same 15 active regions,
which are all quiescent and exhibit a broad range of characteristics,
including age, total unsigned magnetic flux, area, hot emission, and
emission measure distribution. We find that widespread cooling is a
generic property of both loop and diffuse emission from all 15 active
regions. However, the range of temperatures through which the plasma
cools varies between active regions and within each active region,
and only occasionally is there full cooling from above 7 MK to well
below 1 MK. We find that the degree of cooling is not well correlated
with slopes of the emission measure distribution measured by Warren et
al. (2012). We show that these apparently contradictory observations
can be reconciled with the presence of a distribution of nanoflare
energies and frequencies along the line of sight, with the average
delay between successive nanoflare events on a single flux tube being
comparable to the plasma cooling timescale. Warren, H. P., Winebarger,
A. R., & Brooks, D. H. 2012, ApJ, 759, 141
---------------------------------------------------------
Title: Simulations and Observations of the Structured Variability
in the Slow Solar Wind
Authors: Lynch, Benjamin J.; Higginson, Aleida K.; Zhao, Liang; Viall,
Nicholeen; Lepri, Susan T.
2017SPD....4840401L Altcode:
In addition to the long-term heliospheric evolution on timescales of
months to years, the slow solar wind exhibits significant variability
on much shorter timescales—from minutes to days. This short-term
variability in the magnetic field, bulk plasma, and composition
properties of the slow solar wind likely results from magnetic
reconnection processes in the extended solar corona. Here, we continue
our analysis of the Higginson et al. (2017, ApJ 840, L10) numerical
MHD simulation to investigate the following sources of structured slow
solar wind variability. First, we examine the formation and evolution
of 3D “streamer blob” magnetic flux ropes from the cusp of the
helmet streamer belt by reconnection in the heliospheric current sheet
(HCS). Second, we examine the large-scale torsional Alfven wave that
propagates to high latitudes along the Separatrix-Web (S-Web) arc. We
argue that the in-situ Alfven wave signatures in our simulation should
be representative of the field and plasma signatures associated with
interchange reconnection process in the corona. Therefore, we predict
that streamer blob magnetic island flux ropes should be found primarily
near the HCS but the torsional Alfven wave signatures should be present
in both the streamer belt/HCS slow wind and in the slow wind in the
S-Web arcs of pseudostreamers. We present preliminary results of our
analysis of the field, plasma, and composition variability in select
intervals of slow solar wind in Carrington Rotation 2002 and show these
are in excellent agreement with the numerical simulation predictions.
---------------------------------------------------------
Title: A Survey of Nanoflare Properties in Active Regions Observed
with the Solar Dynamics Observatory
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2017ApJ...842..108V Altcode:
In this paper, we examine 15 different active regions (ARs) observed
with the Solar Dynamics Observatory and analyze their nanoflare
properties. We have recently developed a technique that systematically
identifies and measures plasma temperature dynamics by computing time
lags between light curves. The time lag method tests whether the
plasma is maintained at a steady temperature, or if it is dynamic,
undergoing heating and cooling cycles. An important aspect of our
technique is that it analyzes both observationally distinct coronal
loops as well as the much more prevalent diffuse emission between
them. We find that the widespread cooling reported previously for NOAA
AR 11082 is a generic property of all ARs. The results are consistent
with impulsive nanoflare heating followed by slower cooling. Only
occasionally, however, is there full cooling from above 7 MK to well
below 1 MK. More often, the plasma cools to approximately 1-2 MK
before being reheated by another nanoflare. These same 15 ARs were
first studied by Warren et al. We find that the degree of cooling is
not well correlated with the reported slopes of the emission measure
distribution. We also conclude that the Fe xviii emitting plasma that
they measured is mostly in a state of cooling. These results support the
idea that nanoflares have a distribution of energies and frequencies,
with the average delay between successive events on an individual flux
tube being comparable to the plasma cooling timescale.
---------------------------------------------------------
Title: Methods on Efficiently Relating Data from the Sun to In-situ
Data at L1: An Application to the Slow Solar Wind
Authors: McQuillan, Maria; Viall, Nicholeen
2017AAS...22933908M Altcode:
Understanding space weather has become increasingly important
as scientists and spacecraft extend their reach further into the
universe. The solar wind is highly ionized plasma that constantly
bombards the earth. It causes compression and relaxation in our
magnetosphere, and affects spacecraft and astronauts in outer
space. There are two types of solar wind, fast wind and slow wind. The
fast wind is considered to be steady in composition and speed, and
travels at speeds greater than 500 km/s. The slow solar wind is known
for being highly variable in composition and speed, and travels at
speeds less than 500 km/s. Fast solar wind originates from coronal
hole regions on the sun, while the slow solar wind’s origin is very
controversial. There are currently two types of theories for slow
solar wind. One theory involves wave heating dynamics, while the other
contends that slow solar wind originates from magnetic reconnection
that continually opens magnetic field lines. These models are currently
under-constrained with both types able to reproduce the long-term,
average behavior of the wind. To further constrain these models it
was necessary to research small scale structure in the solar wind,
however analyzing these structures pushes the limits of the current
instrument capabilities. We developed techniques that provide an
automated process to quickly generate results from multiple different
analysis techniques, allowing the user to compare data from STEREO’s
Heliospheric Imager (HI) and from data taken at L1. This increases
the efficiency and ability to relate data from the sun in HI and data
at Earth at L1. These techniques were applied to a study on the slow
solar wind which lead to possible evidence for the S-Web model.
---------------------------------------------------------
Title: On the Origin of the Slow Solar Wind: Periodic Plasma Release
from Pseudostreamers
Authors: Viall, N. M.; Kepko, L.; Antiochos, S. K.
2016AGUFMSH54A..05V Altcode:
We present observations of quasi-periodic release of plasma from
pseudostreamers, and demonstrate that these observations place
severe constraints on the origin of both the slow solar wind and
pseudostreamer dynamics. Though quasi-periodic release of slow solar
wind plasma is routinely observed in remote white light images, such
plasma release is often associated with the tips of helmet streamers and
the heliospheric current sheet. Helmet streamers and the heliospheric
current sheet are natural locations for magnetic reconnection to
occur, both in the form of complete disconnections and interchange
reconnection. However, pseudostreamers are not associated with the
heliospheric current sheet, and are predicted by some models to have
steady solar wind release. In contrast, in the S-web model of solar
wind formation, pseudostreamers and their magnetic extensions into
the heliosphere are also locations where slow solar wind is released
sporadically through magnetic reconnection. We present the first
observations demonstrating that quasi-periodic plasma release occurs
in pseudostreamers as well. We build a comprehensive picture of the
dynamics by combining remote-sensing data with in situ composition
and magnetic connectivity information. Our results have critical
implications for the magnetic topology of pseudostreamers and for
their reconnection dynamics. This analysis pushes the limits of current
instrument resolution and sensitivity, showing the enormous potential
science to be accomplished with Solar Probe Plus and Solar Orbiter.
---------------------------------------------------------
Title: Imaging the Top of the Solar Corona and the Young Solar Wind
Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer,
S. R.
2016AGUFMSH53A..05D Altcode:
We present the first direct visual evidence of the quasi-stationary
breakup of solar coronal structure and the rise of turbulence in
the young solar wind, directly in the future flight path of Solar
Probe. Although the corona and, more recently, the solar wind have both
been observed directly with Thomson scattered light, the transition from
the corona to the solar wind has remained a mystery. The corona itself
is highly structured by the magnetic field and the outflowing solar
wind, giving rise to radial "striae" - which comprise the familiar
streamers, pseudostreamers, and rays. These striae are not visible
in wide-field heliospheric images, nor are they clearly delineated
with in-situ measurements of the solar wind. Using careful photometric
analysis of the images from STEREO/HI-1, we have, for the first time,
directly observed the breakup of radial coronal structure and the rise
of nearly-isotropic turbulent structure in the outflowing slow solar
wind plasma between 10° (40 Rs) and 20° (80 Rs) from the Sun. These
observations are important not only for their direct science value,
but for predicting and understanding the conditions expected near SPP as
it flies through - and beyond - this final frontier of the heliosphere,
the outer limits of the solar corona.
---------------------------------------------------------
Title: Probing Prominence Formation with Time Series Analysis of
Models and AIA Data
Authors: Kucera, T. A.; Viall, N. M.; Karpen, J. T.
2016AGUFMSH43C2583K Altcode:
We present a observational and modeling study of the formation and
dynamics of prominence plasma, using a time series analysis of data
from the Solar Dynamic Observatory's Atmospheric Imaging Assembly
(SDO/AIA). The analysis consists of a diagnosis of heating and cooling
events by comparing the time profiles of emission formed at different
temperatures and observed by different AIA bands. We apply this
analysis both to prominences observed by AIA and to model runs from
the thermal non-equilibrium model in which heating at the foot-points
of sheared coronal flux-tubes results in evaporation of hot (a few MK)
material into the corona and subsequent catastrophic cooling of the
hot material to form the cool ( 10,000 K) prominence material. We find
that both the data and model show characteristic heating and cooling
signatures that are significantly different from those seen in active
regions. Supported by NASA's Living with a Star program.
---------------------------------------------------------
Title: The Dynamics of Open-Field Corridors
Authors: Viall, N. M.; Antiochos, S. K.; Higginson, A. K.; DeVore,
C. R.
2016AGUFMSH54A..06V Altcode:
The source of the slow solar wind and the origins of its dynamics
have long been major problems in solar/heliospheric physics. Due to
its observed location in the heliosphere, its plasma composition, and
its variability, the slow wind is widely believed to be due to the
release of closed-field plasma onto open field lines. In the S-Web
model the slow wind is postulated to result from the driving of the
open-closed boundary in the corona by the quasi-random photospheric
convective motions. A key feature of the model is the topological
complexity of the open field regions at the Sun, in other words,
the distribution and geometry of coronal holes. In particular, narrow
corridors of open field and even singular topologies are required in
order to account for the observed angular extent of the slow wind in
the heliosphere. We present the first calculations of the dynamics of
an open-field corridor driven by photospheric flows. The calculations
use our high-resolution MHD code and an isothermal approximation for
the coronal and solar wind plasma. We show that the corridor dynamics
do, in fact, result in the release of closed field plasma far from
the heliospheric current sheet, in agreement with observations and
as predicted by the S-Web model. The implications of our results for
understanding the corona-heliosphere connection and especially for
interpreting observations from the upcoming Solar Orbiter and Solar
Probe Plus missions will be discussed. This research was supported by
the NASA LWS programs.
---------------------------------------------------------
Title: Using SDO/AIA to Understand the Thermal Evolution of Solar
Prominence Formation
Authors: Viall, Nicholeen; M.; Kucera, Therese T.; Karpen, Judith
2016usc..confE..49V Altcode:
In this study, we investigate prominence formation using time series
analysis of Solar Dynamics Observatory's Atmospheric Imaging Assembly
(SDO/AIA) data. We investigate the thermal properties of forming
prominences by analyzing observed light curves using the same technique
that we have already successfully applied to active regions to diagnose
heating and cooling cycles. This technique tracks the thermal evolution
using emission formed at different temperatures, made possible by
AIA's different wavebands and high time resolution. We also compute the
predicted light curves in the same SDO/AIA channels of a hydrodynamic
model of thermal nonequilibrium formation of prominence material,
an evaporation-condensation model. In these models of prominence
formation, heating at the foot-points of sheared coronal flux-tubes
results in evaporation of material of a few MK into the corona followed
by catastrophic cooling of the hot material to form cool ( 10,000 K)
prominence material. We demonstrate that the SDO/AIA light curves
for flux tubes undergoing thermal nonequilibrium vary at different
locations along the flux tube, especially in the region where the
condensate forms, and we compare the predicted light curves with those
observed. Supported by NASA's Living with a Star program.
---------------------------------------------------------
Title: Signatures of Steady Heating in Time Lag Analysis of Coronal
Emission
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2016ApJ...828...76V Altcode: 2016arXiv160702008V
Among the multitude of methods used to investigate coronal heating,
the time lag method of Viall & Klimchuk is becoming increasingly
prevalent as an analysis technique that is complementary to those
that are traditionally used. The time lag method cross correlates
light curves at a given spatial location obtained in spectral bands
that sample different temperature plasmas. It has been used most
extensively with data from the Atmospheric Imaging Assembly on the
Solar Dynamics Observatory. We have previously applied the time lag
method to entire active regions and surrounding the quiet Sun and
created maps of the results. We find that the majority of time lags
are consistent with the cooling of coronal plasma that has been
impulsively heated. Additionally, a significant fraction of the
map area has a time lag of zero. This does not indicate a lack of
variability. Rather, strong variability must be present, and it must
occur in phase between the different channels. We have previously
shown that these zero time lags are consistent with the transition
region response to coronal nanoflares, although other explanations are
possible. A common misconception is that the zero time lag indicates
steady emission resulting from steady heating. Using simulated and
observed light curves, we demonstrate here that highly correlated
light curves at zero time lag are not compatible with equilibrium
solutions. Such light curves can only be created by evolution.
---------------------------------------------------------
Title: Fading Coronal Structure and the Onset of Turbulence in the
Young Solar Wind
Authors: DeForest, C. E.; Matthaeus, W. H.; Viall, N. M.; Cranmer,
S. R.
2016ApJ...828...66D Altcode: 2016arXiv160607718D
Above the top of the solar corona, the young, slow solar wind
transitions from low-β, magnetically structured flow dominated
by radial structures to high-β, less structured flow dominated by
hydrodynamics. This transition, long inferred via theory, is readily
apparent in the sky region close to 10° from the Sun in processed,
background-subtracted solar wind images. We present image sequences
collected by the inner Heliospheric Imager instrument on board the
Solar-Terrestrial Relations Observatory (STEREO/HI1) in 2008 December,
covering apparent distances from approximately 4° to 24° from the
center of the Sun and spanning this transition in the large-scale
morphology of the wind. We describe the observation and novel techniques
to extract evolving image structure from the images, and we use those
data and techniques to present and quantify the clear textural shift in
the apparent structure of the corona and solar wind in this altitude
range. We demonstrate that the change in apparent texture is due both
to anomalous fading of the radial striae that characterize the corona
and to anomalous relative brightening of locally dense puffs of solar
wind that we term “flocculae.” We show that these phenomena are
inconsistent with smooth radial flow, but consistent with the onset
of hydrodynamic or magnetohydrodynamic instabilities leading to a
turbulent cascade in the young solar wind.
---------------------------------------------------------
Title: Using Periodic Density Structures to Understand the Origin
of the Slow Solar Wind
Authors: Viall, Nicholeen
2016shin.confE..81V Altcode:
Variability in the slow solar wind has many sources. One source of
variability in the slow solar wind involves its formation and/or its
origin. Understanding the origin of the slow solar wind has challenged
scientists for many years. The challenge understanding slow solar wind
formation and how that creates variability is that the data are very
disparate, with global-scale, lower resolution remote sensing of the
corona and high resolution, in situ point measurements near Earth. In
this presentation, we show how a particular kind of solar wind density
variations can be used to understand slow solar wind formation. We
present new analysis of STEREO coronagraph and heliospheric imager
observations and of in situ ACE measurements that reveal an important
connection between the dynamics of the corona and of the solar wind. In
particular, we have discovered quasi-periodic density variations in the
slow wind that propagates along the so-called streamers that connect
back to the large-scale closed field regions at the Sun. Furthermore,
we have discovered similar quasi-periodic variations in the plasma
properties of the slow wind near Earth. We discuss the extension of
this study to the upcoming Solar Orbiter and Solar Probe Plus missions.
---------------------------------------------------------
Title: The Transition Region Response to a Coronal Nanoflare:
Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, James A.
2016SPD....4720202V Altcode:
The corona and transition region (TR) are fundamentally coupled
through the processes of thermal conduction and mass exchange. Yet
the temperature-dependent emissions from the two locations behave
quite differently in the aftermath of an impulsive heating event such
as a coronal nanoflare. In this presentation, we use results from the
EBTEL hydrodynamics code to demonstrate that after a coronal nanoflare,
the TR is multithermal and the emission at all temperatures responds
in unison. This is in contrast to the coronal plasma, which cools
sequentially, emitting first at higher temperatures and then at lower
temperatures. We apply the time lag technique of Viall & Klimchuk
(2012) to the simulated Atmospheric Imaging Assembly (AIA) on the
Solar Dynamics Observatory emission and show that coronal plasma light
curves exhibit post-nanoflare cooling time lags, while TR light curves
show time lags of zero, as observed. We further demonstrate that time
lags of zero, regardless of physical cause, do not indicate a lack of
variability. Rather, strong variability must be present, and it must
occur in unison in the different channels. Lastly, we show that the
'coronal' channels in AIA can be dominated by bright TR emission. When
defined in a physically meaningful way, the TR reaches a temperature
of roughly 60% the peak temperature in a flux tube. The TR resulting
from impulsive heating can extend to 3 MK and higher, well within the
range of the 'coronal' AIA channels.
---------------------------------------------------------
Title: Implications of L1 observations for slow solar wind formation
by solar reconnection
Authors: Kepko, L.; Viall, N. M.; Antiochos, S. K.; Lepri, S. T.;
Kasper, J. C.; Weberg, M.
2016GeoRL..43.4089K Altcode:
While the source of the fast solar wind is known to be coronal holes,
the source of the slow solar wind has remained a mystery. Long
time scale trends in the composition and charge states show strong
correlations between solar wind velocity and plasma parameters, yet
these correlations have proved ineffective in determining the slow
wind source. We take advantage of new high time resolution (12 min)
measurements of solar wind composition and charge state abundances
at L1 and previously identified 90 min quasiperiodic structures
to probe the fundamental timescales of slow wind variability. The
combination of new high temporal resolution composition measurements
and the clearly identified boundaries of the periodic structures
allows us to utilize these distinct solar wind parcels as tracers of
slow wind origin and acceleration. We find that each 90 min (2000 Mm)
parcel of slow wind has near-constant speed yet exhibits repeatable,
systematic charge state and composition variations that span the entire
range of statistically determined slow solar wind values. The classic
composition-velocity correlations do not hold on short, approximately
hourlong, time scales. Furthermore, the data demonstrate that these
structures were created by magnetic reconnection. Our results impose
severe new constraints on slow solar wind origin and provide new,
compelling evidence that the slow wind results from the sporadic
release of closed field plasma via magnetic reconnection at the boundary
between open and closed flux in the Sun's atmosphere.
---------------------------------------------------------
Title: Patterns of Activity Revealed by a Time Lag Analysis of a
Model Active Region
Authors: Bradshaw, Stephen; Viall, Nicholeen
2016SPD....4720201B Altcode:
We investigate the global activity patterns predicted from a model
active region heated by distributions of nanoflares that have a
range of average frequencies. The activity patterns are manifested in
time lag maps of narrow-band instrument channel pairs. We combine an
extrapolated magnetic skeleton with hydrodynamic and forward modeling
codes to create a model active region, and apply the time lag method to
synthetic observations. Our aim is to recover some typical properties
and patterns of activity observed in active regions. Our key findings
are: 1. Cooling dominates the time lag signature and the time lags
between the channel pairs are generally consistent with observed
values. 2. Shorter coronal loops in the core cool more quickly than
longer loops at the periphery. 3. All channel pairs show zero time lag
when the line-of-sight passes through coronal loop foot-points. 4. There
is strong evidence that plasma must be re-energized on a time scale
comparable to the cooling timescale to reproduce the observed coronal
activity, but it is likely that a relatively broad spectrum of heating
frequencies operates across active regions. 5. Due to their highly
dynamic nature, we find nanoflare trains produce zero time lags along
entire flux tubes in our model active region that are seen between
the same channel pairs in observed active regions.
---------------------------------------------------------
Title: Patterns of Activity in a Global Model of a Solar Active Region
Authors: Bradshaw, S. J.; Viall, N. M.
2016ApJ...821...63B Altcode: 2016arXiv160306670B
In this work we investigate the global activity patterns predicted from
a model active region heated by distributions of nanoflares that have
a range of frequencies. What differs is the average frequency of the
distributions. The activity patterns are manifested in time lag maps
of narrow-band instrument channel pairs. We combine hydrodynamic
and forward modeling codes with a magnetic field extrapolation
to create a model active region and apply the time lag method to
synthetic observations. Our aim is not to reproduce a particular set
of observations in detail, but to recover some typical properties
and patterns observed in active regions. Our key findings are the
following. (1) Cooling dominates the time lag signature and the time
lags between the channel pairs are generally consistent with observed
values. (2) Shorter coronal loops in the core cool more quickly than
longer loops at the periphery. (3) All channel pairs show zero time
lag when the line of sight passes through coronal loop footpoints. (4)
There is strong evidence that plasma must be re-energized on a timescale
comparable to the cooling timescale to reproduce the observed coronal
activity, but it is likely that a relatively broad spectrum of heating
frequencies are operating across active regions. (5) Due to their
highly dynamic nature, we find nanoflare trains produce zero time
lags along entire flux tubes in our model active region that are seen
between the same channel pairs in observed active regions.
---------------------------------------------------------
Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, N. M.; Klimchuk, J. A.
2015AGUFMSH31D..05V Altcode:
How the solar corona is heated to temperatures of over 1 MK, while
the photosphere below is only ~ 6000 K remains one of the outstanding
problems in all of space science. Solving this problem is crucial for
understanding Sun-Earth connections, and will provide new insight into
universal processes such as magnetic reconnection and wave-particle
interactions. We use a systematic technique to analyze the properties
of coronal heating throughout the solar corona using data taken
with the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our technique computes cooling times of the coronal plasma
on a pixel-by-pixel basis and has the advantage that it analyzes all
of the coronal emission, including the diffuse emission surrounding
distinguishable coronal features. We have already applied this technique
to 15 different active regions, and find clear evidence for dynamic
heating and cooling cycles that are consistent with the 'impulsive
nanoflare' scenario. What about the rest of the Solar corona? Whether
the quiet Sun is heated in a similar or distinct manner from active
regions is a matter of great debate. Here we apply our coronal heating
analysis technique to quiet Sun locations. We find areas of quiet
Sun locations that also undergo dynamic heating and cooling cycles,
consistent with impulsive nanoflares. However, there are important
characteristics that are distinct from those of active regions.
---------------------------------------------------------
Title: Periodic Density Structures and the Origin of the Slow
Solar Wind
Authors: Viall, Nicholeen M.; Vourlidas, Angelos
2015ApJ...807..176V Altcode:
The source of the slow solar wind has challenged scientists for
years. Periodic density structures (PDSs), observed regularly in the
solar wind at 1 AU, can be used to address this challenge. These
structures have length scales of hundreds to several thousands of
megameters and frequencies of tens to hundreds of minutes. Two lines
of evidence indicate that PDSs are formed in the solar corona as part
of the slow solar wind release and/or acceleration processes. The first
is corresponding changes in compositional data in situ, and the second
is PDSs observed in the inner Heliospheric Imaging data on board the
Solar Terrestrial Relations Observatory (STEREO)/Sun Earth Connection
Coronal and Heliospheric Investigation (SECCHI) suite. The periodic
nature of these density structures is both a useful identifier as well
as an important physical constraint on their origin. In this paper,
we present the results of tracking periodic structures identified
in the inner Heliospheric Imager in SECCHI back in time through the
corresponding outer coronagraph (COR2) images. We demonstrate that
the PDSs are formed around or below 2.5 solar radii—the inner edge
of the COR2 field of view. We compute the occurrence rates of PDSs
in 10 days of COR2 images both as a function of their periodicity
and location in the solar corona, and we find that this set of PDSs
occurs preferentially with a periodicity of ∼90 minutes and occurs
near streamers. Lastly, we show that their acceleration and expansion
through COR2 is self-similar, thus their frequency is constant at
distances beyond 2.5 solar radii.
---------------------------------------------------------
Title: Using the fingerprints of solar magnetic reconnection to
identify the elemental building blocks of the slow solar wind
Authors: Kepko, Larry; Viall, Nicholeen M.; Kasper, Justin; Lepri, Sue
2015TESS....110802K Altcode:
While the source of the fast solar wind is well understood to be linked
to coronal holes, the source of the slow solar wind has remained
elusive. Many previous studies of the slow solar wind have examined
trends in the composition and charge states over long time scales and
found strong relationships between the solar wind velocity and these
plasma parameters. These relationships have been used to constrain
models of solar wind source and acceleration. In this study, we take
advantage of high time resolution (12 min) measurements of solar wind
composition and charge-state abundances recently reprocessed by the ACE
Solar Wind Ion Composition Spectrometer (SWICS) science team to probe
the timescales of solar wind variability at relatively small scales. We
study an interval of slow solar wind containing quasi-periodic 90
minute structures and show that they are remnants of solar magnetic
reconnection. Each 90-minute parcel of slow solar wind, though the
speed remains steady, exhibits the complete range of charge state and
composition variations expected for the entire range of slow solar
wind, which is repeated again in the next 90-minute interval. These
observations show that previous statistical results break down on
these shorter timescales, and impose new and important constraints on
models of slow solar wind creation. We conclude by suggesting these
structures were created through interchange magnetic reconnection and
form elemental building blocks of the slow solar wind. We also discuss
the necessity of decoupling separately the process(es) responsible
for the release and acceleration.
---------------------------------------------------------
Title: Nanoflare Heating of the Quiet Sun
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2015TESS....121303V Altcode:
How the solar corona is heated to temperatures of over 1 MK, while
the photosphere below is only ~ 6000 K remains one of the outstanding
problems in all of space science. Solving this problem is crucial for
understanding Sun-Earth connections, and will provide new insight into
universal processes such as magnetic reconnection and wave-particle
interactions. We use a new systematic technique to analyze the
properties of coronal heating throughout the solar corona using data
taken with the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our technique computes cooling times of the coronal plasma
on a pixel-by-pixel basis and has the advantage that it analyzes all
of the coronal emission, including the diffuse emission surrounding
distinguishable coronal features. We have already applied this technique
to 15 different active regions, and find clear evidence for dynamic
heating and cooling cycles that are consistent with the 'impulsive
nanoflare' scenario. What about the rest of the Solar corona? Whether
the quiet Sun is heated in a similar or distinct manner from active
regions is a matter of great debate. In this paper, we apply our coronal
heating analysis technique to quiet Sun locations. We find that the
majority of the analyzed quiet Sun locations do undergo dynamic heating
and cooling cycles, consistent with impulsive nanoflares. However, there
are important characteristics that are distinct from those of active
regions.This research was supported by a NASA Guest Investigator grant.
---------------------------------------------------------
Title: Synthetic 3D modeling of active regions and simulation of
their multi-wavelength emission
Authors: Nita, Gelu M.; Fleishman, Gregory; Kuznetsov, Alexey A.;
Loukitcheva, Maria A.; Viall, Nicholeen M.; Klimchuk, James A.; Gary,
Dale E.
2015TESS....131204N Altcode:
To facilitate the study of solar active regions, we have created a
synthetic modeling framework that combines 3D magnetic structures
obtained from magnetic extrapolations with simplified 1D thermal
models of the chromosphere, transition region, and corona. To handle,
visualize, and use such synthetic data cubes to compute multi-wavelength
emission maps and compare them with observations, we have
undertaken a major enhancement of our simulation tools, GX_Simulator
(ftp://sohoftp.nascom.nasa.gov/solarsoft/packages/gx_simulator/),
developed earlier for modeling emission from flaring loops. The greatly
enhanced, object-based architecture, which now runs on Windows, Mac,
and UNIX platform, offers important new capabilities that include the
ability to either import 3D density and temperature distribution models,
or to assign to each individual voxel numerically defined coronal
or chromospheric temperature and densities, or coronal Differential
Emission Measure distributions. Due to these new capabilities, the
GX_Simulator can now apply parametric heating models involving average
properties of the magnetic field lines crossing a given voxel volume,
as well as compute and investigate the spatial and spectral properties
of radio (to be compared with VLA or EOVSA data), (sub-)millimeter
(ALMA), EUV (AIA/SDO), and X-ray (RHESSI) emission calculated from the
model. The application integrates shared-object libraries containing
fast free-free, gyrosynchrotron, and gyroresonance emission codes
developed in FORTRAN and C++, and soft and hard X-ray and EUV codes
developed in IDL. We use this tool to model and analyze an active
region and compare the synthetic emission maps obtained in different
wavelengths with observations.This work was partially supported
by NSF grants AGS-1250374, AGS-1262772, NASA grant NNX14AC87G, the
Marie Curie International Research Staff Exchange Scheme "Radiosun"
(PEOPLE-2011-IRSES-295272), RFBR grants 14-02-91157, 15-02-01089,
15-02-03717, 15-02-03835, 15-02-08028.
---------------------------------------------------------
Title: Investigating the Thermal Evolution of Solar Prominence
Formation
Authors: Kucera, Therese A.; Viall, Nicholeen M.; Karpen, Judith T.
2015TESS....120315K Altcode:
We present a study of prominence formation using time series analysis of
Solar Dynamics Observatory’s Atmospheric Imaging Assembly (SDO/AIA)
data. In evaporation-condensation models of prominence formation,
heating at the foot-points of sheared coronal flux-tubes results in
evaporation of hot (a few MK) material into the corona and subsequent
catastrophic cooling of the hot material to form the cool (~10,000 K)
prominence material. We present the results of a time-lag analysis
that tracks the thermal evolution using emission formed at different
temperatures. This analysis is made possible by AIA's many wavebands
and high time resolution, and it allows us to look for signs of the
evaporation-condensation process and to study the heating time scales
involved. Supported by NASA’s Living with a Star program.
---------------------------------------------------------
Title: The Transition Region Response to a Coronal Nanoflare:
Forward Modeling and Observations in SDO/AIA
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2015ApJ...799...58V Altcode:
The corona and transition region (TR) are fundamentally coupled
through the processes of thermal conduction and mass exchange. It
is not possible to understand one without the other. Yet the
temperature-dependent emissions from the two locations behave quite
differently in the aftermath of an impulsive heating event such as a
coronal nanoflare. Whereas the corona cools sequentially, emitting
first at higher temperatures and then at lower temperatures, the
TR is multithermal and the emission at all temperatures responds in
unison. We have previously applied the automated time lag technique
of Viall & Klimchuk to disk observations of an active region
(AR) made by the Atmospheric Imaging Assembly (AIA) on the Solar
Dynamics Observatory. Lines of sight passing through coronal plasma
show clear evidence for post-nanoflare cooling, while lines of sight
intersecting the TR footpoints of coronal strands show zero time lag. In
this paper, we use the EBTEL hydrodynamics code to demonstrate that
this is precisely the expected behavior when the corona is heated by
nanoflares. We also apply the time lag technique for the first time
to off-limb observations of an AR. Since TR emission is not present
above the limb, the occurrence of zero time lags is greatly diminished,
supporting the conclusion that zero time lags measured on the disk are
due to TR plasma. Lastly, we show that the ”coronal” channels in AIA
can be dominated by bright TR emission. When defined in a physically
meaningful way, the TR reaches a temperature of roughly 60% the peak
temperature in a flux tube. The TR resulting from impulsive heating
can extend to 3 MK and higher, well within the range of the ”coronal”
AIA channels.
---------------------------------------------------------
Title: Elemental building blocks of the slow solar wind
Authors: Kepko, L.; Viall, N. M.; Lepri, S. T.
2014AGUFMSH33A4126K Altcode:
While the source of the fast solar wind is well understood to be linked
to coronal holes, the source of the slow solar wind has remained
elusive. A distinguishing characteristic of the slow solar wind is
the high variability of the plasma parameters, such as magnetic field,
velocity, density, composition, and charge state. Many previous studies
of the slow solar wind have examined trends in the composition and
charge states over long time scales and using data with comparatively
low temporal resolution. In this study, we take advantage of high time
resolution (12 min) measurements of the charge-state abundances recently
reprocessed by the ACE SWICS science team to probe the timescales of
solar wind variability of coherent structures at relatively small
scales (<2000 Mm, or ~ 90 minutes at slow wind speeds). We use
an interval of slow solar wind containing quasi pressure-balanced,
periodic number density structures previously studied by Kepko et al
and shown to be important in solar wind-magnetospheric coupling. The
combination of high temporal resolution composition measurements and
the clearly identified boundaries of the periodic structures allows us
to probe the elemental slow solar wind flux tubes/structures. We use
this train of 2000Mm periodic density structures as tracers of solar
wind origin and/or acceleration. We find that each 2000 Mm parcel of
slow solar wind, though its speed is steady, exhibits the complete
range of charge state and composition variations expected for the
entire range of slow solar wind, in a repeated sequence. Each parcel
cycles through three states: 1) 'normal' slow wind, 2) compositionally
slow wind with very high density, and 3) compositionally fast but
typical slow solar wind density. We conclude by suggesting these
structures form elemental building blocks of the slow solar wind, and
discuss whether it is necessary to decouple separately the process(es)
responsible for the release and acceleration.
---------------------------------------------------------
Title: Periodic Density Structures and the Origin of the Slow
Solar Wind
Authors: Viall, N. M.; Vourlidas, A.
2014AGUFMSH21B4114V Altcode:
Periodic density structures with length-scales of hundreds to several
thousands of Mm and frequencies of tens to hundreds of minutes are
observed regularly in the solar wind at 1 AU. These structures coexist
with, but are not due to, fluctuations in the plasma resulting from the
turbulent cascade. Two lines of evidence - one identifying corresponding
changes in compositional data in situ, and another identifying periodic
density structures in the inner Heliospheric Imaging data onboard the
Solar Terrestrial Relations Observatory (STEREO)/ Sun Earth Connection
Coronal and Heliospheric Investigation (SECCHI) suite - indicate that
periodic density structures are formed in the solar corona as part
of the slow solar wind release and/or acceleration processes. The
periodic nature of these density structures is an important physical
constraint on their origin. In this presentation, we present the results
of tracking periodic structures identified in the SECCHI/HI1 images
down through the corresponding SECCHI/COR2 images. We demonstrate that
the periodic density structures are formed around or below 2.5 solar
radii - the inner edge of the COR2 field of view. Further, we compute
the occurrence rate of periodic density structures in 10 days of COR2
images as a function of location in the solar corona. We find that this
set of periodic density structures occurs preferentially in relation
to coronal streamers. Periodic density structures are tracers of solar
wind origin and/or acceleration; this study is a pilot for the kinds
of investigations that we can carry out with the better temporal and
spatial resolution of the heliospheric imagers on Solar Orbiter and
Solar Probe Plus.
---------------------------------------------------------
Title: The effect of magnetopause motion on fast mode resonance
Authors: Hartinger, M. D.; Welling, D.; Viall, N. M.; Moldwin, M. B.;
Ridley, A.
2014JGRA..119.8212H Altcode:
The Earth's magnetosphere supports several types of ultralow frequency
(ULF) waves. These include fast mode resonance (FMR): cavity
modes, waveguide modes, and tunneling modes/virtual resonance. The
magnetopause, often treated as the outer boundary for cavity/waveguide
modes in the dayside magnetosphere, is not stationary. A rapidly
changing outer boundary condition—e.g., due to rapid magnetopause
motion—is not favorable for FMR generation and may explain the
sparseness of FMR observations in the outer magnetosphere. We examine
how magnetopause motion affects the dayside magnetosphere's ability
to sustain FMR with idealized Space Weather Modeling Framework
(SWMF) simulations using the BATS-R-US global magnetohydrodynamic
(MHD) code coupled with the Ridley Ionosphere Model (RIM). We present
observations of FMR in BATS-R-US, reproducing results from other global
MHD codes. We further show that FMR is present for a wide range of solar
wind conditions, even during periods with large and rapid magnetopause
displacements. We compare our simulation results to FMR observations
in the dayside magnetosphere, finding that FMR occurrence does not
depend on solar wind dynamic pressure, which can be used as a proxy
for dynamic pressure fluctuations and magnetopause perturbations. Our
results demonstrate that other explanations besides a nonstationary
magnetopause—such as the inability to detect FMR in the presence of
other ULF wave modes with large amplitudes—are required to explain
the rarity of FMR observations in the outer magnetosphere.
---------------------------------------------------------
Title: Periodic Density Structures and the Source of the Slow
Solar Wind
Authors: Viall, Nicholeen; Vourlidas, Angelos
2014AAS...22440202V Altcode:
Periodic density structures with length-scales of hundreds to several
thousands of megameters, and frequencies of tens to hundreds of minutes,
are observed regularly in the solar wind at 1 AU. These structures
coexist with, but are not due to, fluctuations in the plasma resulting
from the turbulent cascade. Two lines of evidence suggest that periodic
density structures are formed in the solar corona as part of the slow
solar wind release and/or acceleration processes. The first is the
identification of corresponding changes in compositional data in situ,
and the other is the identification of periodic density structures in
the inner Heliospheric Imaging data onboard the STEREO/SECCHI suite. In
this presentation, we show the results of tracking periodic structures
identified in the SECCHI/Hi1 images down through the corresponding
SECCHI/Cor2 images. We demonstrate that the periodic density structures
are formed around or below 2.5 Rs - the inner edge of the Cor2 field
of view. Further, we compute the occurrence rate of periodic density
structures in 10 days of Cor2 images as a function of location in the
solar corona. We find that periodic density structures do not occur
throughout the entire space-filling volume of the solar wind; rather,
there are particular places where they occur preferentially, suggesting
source locations for periodic density structures in the slow solar wind.
---------------------------------------------------------
Title: A Survey of Coronal Heating Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, James A.
2014AAS...22432315V Altcode:
We investigate the properties of coronal heating in solar active
regions (AR) by systematically analyzing coronal light curves
observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics
Observatory. Our automated technique computes time-lags (cooling times)
on a pixel-by-pixel basis, and has the advantage that it allows us
to analyze all of the coronal AR emission, including the so-called
diffuse emission between coronal loops. We recently presented results
using this time-lag analysis on NOAA AR 11082 (Viall & Klimchuk
2012) and found that the majority of the pixels contained cooling
plasma along their line of sight. This result is consistent with
impulsive coronal nanoflare heating of both coronal loops and the
surrounding diffuse emission in the AR. Here we present the results
of our time-lag technique applied to a survey of 15 AR of different
magnetic complexity, total unsigned magnetic flux, size and age. We
show that the post-nanoflare cooling patterns identified in NOAA AR
11082 are identified throughout all of the active regions in this
survey, indicating that nanoflare heating is ubiquitous in solar
active regions. However, some details of the nanoflare properties,
such as the nanoflare energy, are different across these different
active regions.We thank the SDO/AIA team for the use of these data,
and the Coronal Heating ISSI team for helpful discussion of these
topics. This research was supported by a NASA Heliophysics GI.
---------------------------------------------------------
Title: Measuring Temperature-dependent Propagating Disturbances in
Coronal Fan Loops Using Multiple SDO/AIA Channels and the Surfing
Transform Technique
Authors: Uritsky, Vadim M.; Davila, Joseph M.; Viall, Nicholeen M.;
Ofman, Leon
2013ApJ...778...26U Altcode: 2013arXiv1308.6195U
A set of co-aligned high-resolution images from the Atmospheric
Imaging Assembly (AIA) on board the Solar Dynamics Observatory is
used to investigate propagating disturbances (PDs) in warm fan loops
at the periphery of a non-flaring active region NOAA AR 11082. To
measure PD speeds at multiple coronal temperatures, a new data
analysis methodology is proposed enabling a quantitative description
of subvisual coronal motions with low signal-to-noise ratios of the
order of 0.1%. The technique operates with a set of one-dimensional
"surfing" signals extracted from position-time plots of several AIA
channels through a modified version of Radon transform. The signals are
used to evaluate a two-dimensional power spectral density distribution
in the frequency-velocity space that exhibits a resonance in the
presence of quasi-periodic PDs. By applying this analysis to the same
fan loop structures observed in several AIA channels, we found that
the traveling velocity of PDs increases with the temperature of the
coronal plasma following the square-root dependence predicted for slow
mode magneto-acoustic waves which seem to be the dominating wave mode in
the loop structures studied. This result extends recent observations by
Kiddie et al. to a more general class of fan loop system not associated
with sunspots and demonstrating consistent slow mode activity in up
to four AIA channels.
---------------------------------------------------------
Title: Modeling the Line-of-sight Integrated Emission in the Corona:
Implications for Coronal Heating
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2013ApJ...771..115V Altcode: 2013arXiv1304.5439V
One of the outstanding problems in all of space science is uncovering
how the solar corona is heated to temperatures greater than 1 MK. Though
studied for decades, one of the major difficulties in solving this
problem has been unraveling the line-of-sight (LOS) effects in the
observations. The corona is optically thin, so a single pixel measures
counts from an indeterminate number (perhaps tens of thousands)
of independently heated flux tubes, all along that pixel's LOS. In
this paper we model the emission in individual pixels imaging the
active region corona in the extreme ultraviolet. If LOS effects are
not properly taken into account, erroneous conclusions regarding both
coronal heating and coronal dynamics may be reached. We model the corona
as an LOS integration of many thousands of completely independently
heated flux tubes. We demonstrate that despite the superposition of
randomly heated flux tubes, nanoflares leave distinct signatures in
light curves observed with multi-wavelength and high time cadence
data, such as those data taken with the Atmospheric Imaging Assembly
on board the Solar Dynamics Observatory. These signatures are readily
detected with the time-lag analysis technique of Viall & Klimchuk
in 2012. Steady coronal heating leaves a different and equally distinct
signature that is also revealed by the technique.
---------------------------------------------------------
Title: A Survey of Nanoflare Properties in Solar Active Regions
Authors: Viall, Nicholeen; Klimchuk, J. A.
2013SPD....44...16V Altcode:
We investigate the characteristics of coronal heating using a systematic
technique that analyzes the properties of nanoflares in active regions
(AR). Our technique computes cooling times, or time lags, using SDO/AIA
light curves of all of the coronal AR emission, including the so-called
diffuse emission. We recently presented results using this time-lag
analysis on NOAA AR 11082 (Viall & Klimchuk 2012). We found that
the majority of the pixels had cooling plasma along their line of sight,
consistent with impulsive coronal nanoflare heating. Additionally, our
results using the AIA 94 channel data showed that the nanoflare energy
is stronger in the AR core and weaker in the AR periphery. Are these
results representative of the nanoflare characteristics exhibited in
the majority of active regions, or is AR 11082 unique? Here we present
the time-lag results for a survey of active regions and determine
whether these nanoflare patterns are born out in other active regions
as well. This research was supported by the NASA Heliophysics Guest
Investigator program.
---------------------------------------------------------
Title: Slow mode waves and quasi-periodic upflows in the
multi-temperature solar corona as seen by the SDO
Authors: Uritsky, Vadim; Davila, J. M.; Viall, N.; Ofman, L.
2013SPD....4410405U Altcode:
We report results the analysis of coronal fan loops in a non-flaring
solar active region exhibiting temperature-dependent propagating optical
disturbances. A 6-hour set of high resolution coronal observations
provided by the Atmospheric Imaging Assembly (AIA) on board the Solar
Dynamics Observatory (SDO) has been used for characterizing apparent
propagating patterns at multiple coronal temperatures (131A, 171A,
193A and 211A). A new data analysis methodology has been developed
enabling an identification of subvisual motions with low signal-to-noise
ratios not previously examined in this context. The technique involves
spatiotemporal tracking of fan loop filaments containing propagating
disturbances, construction of position - time plots for different
temperature channels, obtaining the waveforms of the propagating optical
features, and evaluation of Fourier spectral power of the waveforms
as a function of phase speed and frequency. Using this methodology,
we identified the parameters of propagating optical disturbances in
different magnetic geometries, and classified these events as waves
and/or plasma jets. We explored coronal conditions favoring wave-like
and jet-like traveling plasma density enhancements in fan loops and
the mechanisms of their generation, damping and interaction. The
results obtained are compared with the behavior of a resistive MHD
model exhibiting both types of propagating disturbances.
---------------------------------------------------------
Title: Understanding Coronal Heating by Comparing SDO/AIA Observations
with Modeled Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2013enss.confE..18V Altcode:
An important signature of nanoflare heated coronal plasma is the
sudden appearance of the plasma at hot temperatures, followed by
a comparatively slow cooling and draining phase. This is due to
the impulsive nature of nanoflare heating and the heat conduction
and mass exchange between the corona and chromosphere. Identifying
such nanoflare signatures is complicated by the fact that the solar
corona is optically thin: many thousands of flux tubes which are
heated completely independently are contributing to the total emission
along a given line of sight. One approach has been to analyze isolated
features such as coronal loops; however the diffuse emission between
and around isolated features contribute as much, if not more to the
EUV coronal emission, and therefore is crucial to the understanding of
coronal heating. In this study we move beyond isolated features and
analyze all of the emission in an entire active region and quiet Sun
area. We investigate SDO/AIA light curves, systematically identifying
nanoflare signatures. We compare the observations with a model of the
corona as a line-of-sight integration of many thousands of completely
independently heated flux tubes. We consider that the emission from
these flux tubes may be due exclusively to impulsive nanoflare bursts,
quasi-steady heating, or a mix of both, depending on the cadence of
heat release. We demonstrate that despite the superposition of randomly
heated flux tubes, different distributions of nanoflare cadences produce
distinct signatures in light curves observed with multi-wavelength and
high time cadence data, such as those from SDO/AIA. We find that much
of the solar corona is heated through impulsive nanoflares.
---------------------------------------------------------
Title: Nanoflare Heating of the Solar Corona: Comparing SDO/AIA
Observations with Modeled Light Curves
Authors: Viall, N. M.; Klimchuk, J. A.
2012AGUFMSH42A..03V Altcode:
A significant outstanding issue in current solar and astrophysical
research is that of the heating of the solar corona. Coronal plasma is
typically measured to be at temperatures near ~1-3 MK. Is the majority
of the coronal plasma maintained at these temperatures through a
form of quasi-steady heating, or is this simply a measure of the
average temperature of widely varying, impulsively heated coronal
plasma? Addressing even this basic question is complicated by the
fact that the corona is optically thin: many thousands of flux tubes
which are heated completely independently are contributing to the
total emission along a given line of sight. There is a large body of
work focused on the heating of isolated features - coronal loops in
active regions- which are impulsively heated, however understanding
of the diffuse emission between loops and the emission from the quiet
Sun are also crucial. Therefore in this study we move beyond isolated
features and analyze all of the emission in an entire active region and
quiet Sun area from all contributing flux tubes. We investigate light
curves systematically using SDO/AIA observations. We also model the
corona as a line-of-sight integration of many thousands of completely
independently heated flux tubes. The emission from these flux tubes
may be time dependent, quasi-steady, or a mix of both, depending on the
cadence of heat release. We demonstrate that despite the superposition
of randomly heated flux tubes, different distributions of nanoflare
cadences produce distinct signatures in light curves observed with
multi-wavelength and high time cadence data, such as those from
SDO/AIA. We discuss the quiet Sun and active region emission in the
context of these predicted nanoflare signatures.
---------------------------------------------------------
Title: Evidence for Widespread Cooling in an Active Region Observed
with the SDO Atmospheric Imaging Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2012ApJ...753...35V Altcode: 2012arXiv1202.4001V
A well-known behavior of EUV light curves of discrete coronal loops
is that the peak intensities of cooler channels or spectral lines
are reached at progressively later times than hotter channels. This
time lag is understood to be the result of hot coronal loop plasma
cooling through these lower respective temperatures. However, loops
typically comprise only a minority of the total emission in active
regions (ARs). Is this cooling pattern a common property of AR coronal
plasma, or does it only occur in unique circumstances, locations, and
times? The new Solar Dynamics Observatory/Atmospheric Imaging Assembly
(SDO/AIA) data provide a wonderful opportunity to answer this question
systematically for an entire AR. We measure the time lag between pairs
of SDO/AIA EUV channels using 24 hr of images of AR 11082 observed
on 2010 June 19. We find that there is a time-lag signal consistent
with cooling plasma, just as is usually found for loops, throughout
the AR including the diffuse emission between loops for the entire 24
hr duration. The pattern persists consistently for all channel pairs
and choice of window length within the 24 hr time period, giving us
confidence that the plasma is cooling from temperatures of greater
than 3 MK, and sometimes exceeding 7 MK, down to temperatures lower
than ~0.8 MK. This suggests that the bulk of the emitting coronal
plasma in this AR is not steady; rather, it is dynamic and constantly
evolving. These measurements provide crucial constraints on any model
which seeks to describe coronal heating.
---------------------------------------------------------
Title: SDO / AIA Observations of Slow Mode Waves in Coronal Fan Loops
Authors: Uritsky, Vadim; Davila, J. M.; Viall, N. M.
2012AAS...22032205U Altcode:
We investigate slow mode waves in fan coronal loops observed by
the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics
Observatory spacecraft ( 12 sec cadence, 0.6 arcsec pixels). The warm
fan structure studied here was located at the periphery of quiescent
active region NOAA AR 11082. A specialized software package has been
developed for extracting subvisual modulations of SDO AIA intensity
propagating along dynamic loop segments changing their shape and
position during the observation. The processing steps include manual
tracking of the segment location in time sequences of co-aligned
multispectral AIA images, extracting wave-carrying filamentary
segments from the surrounding background, constructing position -
time diagrams representing temporal evolution of optical brightness
along the filaments, and analysis of the obtained wave signatures. The
results reveal a persistent wave activity in many fan loop segments
characterized by a well-defined frequency and phase speed, with the best
signal-to-noise ratio in 171Å and 193Å channels. For some filamentary
segments, the wave parameters remained almost constant over the entire
observing interval ( 6 hours). The wave parameters varied across
the studied structures. The fastest wave fronts exhibited strictly
outward propagation while the slower waves could travel both inward
and outward. The estimated phase velocity (80-100 km/s) and period
(3-4 min) of the most stable outward wave mode are in a good agreement
with earlier SOHO EIT and TRACE observations of slow magnetosonic waves
in fan loops. The newly observed features include (1) the remarkable
coherency of the wave pattern over a course of several hours, and (2)
the detailed wave form of the process enabling quantitative analysis
of nonlinear propagation and damping effects. No consistent dependence
of the wave speed on the distance from the hot core region of AR 11082
was identified, which challenges the traditional picture of traveling
magnetoacoustic oscillations in fan loops.
---------------------------------------------------------
Title: Nanoflare Properties throughout Active Regions: Comparing
SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen; Klimchuk, J.
2012AAS...22030904V Altcode:
Coronal plasma in active regions is typically measured to be at
temperatures near 1-3 MK. Is the majority of the coronal plasma in
hydrostatic equilibrium, maintained at these temperatures through
a form of quasi-steady heating, or is this simply a measure of the
average temperature of widely varying, impulsively heated coronal
plasma? Addressing this question is complicated by the fact that
the corona is optically thin: many thousands of flux tubes which
are heated completely independently are contributing to the total
emission along a given line of sight. There is a large body of work
focused on the heating of isolated features - coronal loops - which are
impulsively heated, however it is the diffuse emission between loops
which often comprises the majority of active region emission. Therefore
in this study we move beyond isolated features and analyze all of the
emission in an entire active region from all contributing flux tubes. We
investigate light curves systematically using SDO/AIA observations. We
also model the active region corona as a line-of-sight integration
of many thousands of completely independently heated flux tubes. The
emission from these flux tubes may be time dependent, quasi-steady, or
a mix of both, depending on the cadence of heat release. We demonstrate
that despite the superposition of randomly heated flux tubes, different
distributions of nanoflare cadences produce distinct signatures in
light curves observed with multi-wavelength and high time cadence data,
such as those from SDO/AIA. We conclude that the majority of the active
region plasma is not maintained in hydrostatic equilibrium, rather
it is undergoing dynamic heating and cooling cycles. The observed
emission is consistent with heating through impulsive nanoflares,
whose energy is a function of location within the active region. <P
/>This research was supported by an appointment to the NASA Postdoctoral
Program at GSFC/NASA.
---------------------------------------------------------
Title: Determining the Typical Nanoflare Cadence in Active Regions:
Comparing SDO/AIA Observations with Modeled Active Region Light Curves
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2012decs.confE..40V Altcode:
Coronal plasma in active regions is typically measured to be at
temperatures near 1-3 MK. Is the majority of the coronal plasma in
hydrostatic equilibrium, maintained at these temperatures through a
form of quasi-steady heating, or is this simply a measure of the average
temperature of widely varying, impulsively heated coronal plasma which
is continually undergoing heating and cooling cycles? Addressing this
question is complicated by the fact that the corona is optically thin:
many thousands of strands which are heated completely independently are
contributing to the total emission along a given line of sight. There
is a large body of work focused on the heating of coronal loops,
which are impulsively heated, however it is the diffuse emission
between loops which often comprises the majority of active region
emission. Therefore, a different and necessary approach to analyzing
active region heating is to analyze all of the emission in an active
region, and account for emission along the line of sight from all of
the contributing strands. We investigate light curves systematically in
an entire active region using SDO/AIA observations. We also model the
active region corona as a line-of-sight integration of many thousands
of completely independently heated strands. The emission from these
flux tubes may be time dependent, quasi-steady, or a mix of both,
depending on the cadence of heat release on each strand. We examine a
full range of heat cadences from effectively steady (heat pulse repeat
time << plasma cooling time) to fully impulsive (heat pulse repeat
time >> plasma cooling time) and model the resulting emission
when superposing strands undergoing these differing heat cycles. We
demonstrate that despite the superposition of randomly heated strands,
different distributions of heat cadences produce distinct signatures
in light curves observed with multi-wavelength and high time cadence
data, such as those from the AIA telescopes on SDO. Using these model
predictions in conjunction with SDO/AIA observations, we evaluate the
typical cadence of heat release in different active regions and patterns
therein, which is a crucial constraint on coronal heating mechanisms.
---------------------------------------------------------
Title: Determining the Typical Nanoflare Cadence in Active Regions:
Modeling Light Curves of Active Regions
Authors: Viall, N. M.; Klimchuk, J. A.
2011AGUFMSH33B2057V Altcode:
Active region coronal loops visible at 1MK are likely composed of many
unresolved strands, heated by storms of impulsive nanoflares. Though
well-studied, these loops often contribute only a fraction of the total
emission in an active region; the degree to which the entire active
region is heated in the same manner as loops are is highly debated. Is
the majority of coronal active region plasma heated impulsively, or
is the majority of the heating quasi-steady? Addressing this question
is complicated by the fact that the corona is optically thin: many
thousands of strands which are heated completely independently
are contributing to the total emission along a given line of
sight. Furthermore, certain geometries preclude even the best background
subtraction methods from fully isolating the emission from even a
single coronal loop. Therefore, a different and necessary approach
to analyzing active region heating is to account for emission along
the line of sight from all of the contributing strands. We model the
active region corona as a line-of-sight integration of many thousands
of completely independently heated strands. The emission from these
flux tubes may be time dependent, quasi-steady, or a mix of both,
depending on the cadence of heat release on each strand. We examine a
full range of heat cadences from effectively steady (heat pulse repeat
time << plasma cooling time) to fully impulsive (heat pulse repeat
time >> plasma cooling time) and model the resulting emission
when superposing strands undergoing these differing heat cycles. We
demonstrate that despite the superposition of randomly heated strands,
different distributions of heat cadences produce distinct signatures
in light curves observed with multi-wavelength and high time cadence
data, such as those from the AIA telescopes on SDO. For this reason,
high time cadence spectral information for lines sensitive to the 1-10
MK range will be especially useful in future missions. Using these
model predictions, we evaluate the typical cadence of heat release
in different active regions and patterns therein, which is a crucial
constraint on coronal heating mechanisms.
---------------------------------------------------------
Title: Patterns of Nanoflare Storm Heating Exhibited by an Active
Region Observed with Solar Dynamics Observatory/Atmospheric Imaging
Assembly
Authors: Viall, Nicholeen M.; Klimchuk, James A.
2011ApJ...738...24V Altcode: 2011arXiv1106.4196V
It is largely agreed that many coronal loops—those observed at a
temperature of about 1 MK—are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Are these regions also
heated impulsively, or is the heating quasi-steady? The spectacular
new data from the Atmospheric Imaging Assembly (AIA) telescopes on the
Solar Dynamics Observatory offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in six different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function the nanoflare storm properties. We show that
while some sets of light curves exhibit clear evidence of cooling
after nanoflare storms, other cases are less straightforward to
interpret. Complications arise because of line-of-sight integration
through many different structures, the broadband nature of the AIA
channels, and because physical properties can change substantially
depending on the magnitude of the energy release. Nevertheless, the
light curves exhibit predictable and understandable patterns consistent
with impulsive nanoflare heating.
---------------------------------------------------------
Title: Heating of Active Regions by Impulsive Nanoflares
Authors: Viall, Nicholeen Mary; Klimchuk, James A.
2011shin.confE..57V Altcode:
It has been proposed that plasma on active regions may be contributing
plasma to the slow solar wind. If this is the case, then understanding
the heating and dynamics of active regions adds vital knowledge to
our understanding of the heating and acceleration of the slow solar
wind. It seems largely agreed that many coronal loops--those observed
at a temperature of about 1 MK--are bundles of unresolved strands that
are heated by storms of impulsive nanoflares. The nature of coronal
heating in hotter loops and in the very important but largely ignored
diffuse component of active regions is much less clear. Are these
regions also heated impulsively, or is the heating quasi steady? The
spectacular new data from the Atmospheric Imaging Assembly (AIA)
telescopes on the Solar Dynamics Observatory (SDO) offer an excellent
opportunity to address this question. We analyze the light curves
of coronal loops and the diffuse corona in 6 different AIA channels
and compare them with the predicted light curves from theoretical
models. Light curves in the different AIA channels reach their peak
intensities with predictable orderings as a function of the nanoflare
storm properties. These orderings, or time lags, are clearly exhibited
in loop observations in all channels. What is especially exciting
is that we identify these time lag patterns in observations of the
seemingly steady diffuse corona as well. We model the diffuse corona as
a line-of-sight integration of many thousands of completely independent,
impulsively heated strands. The time lags of the simulated and actual
observations are in excellent agreement. Our results suggest that
impulsive nanoflare heating is ubiquitous within active regions.
---------------------------------------------------------
Title: Pulsed Flows Along a Cusp Structure Observed with SDO/AIA
Authors: Thompson, Barbara; Démoulin, P.; Mandrini, C.; Mays, M.;
Ofman, L.; Van Driel-Gesztelyi, L.; Viall, N.
2011SPD....42.2117T Altcode: 2011BAAS..43S.2117T
We present observations of a cusp-shaped structure that formed after
a flare and coronal mass ejection on 14 February 2011. Throughout
the evolution of the cusp structure, blob features up to a few Mm in
size were observed flowing along the legs and stalk of the cusp at
projected speeds ranging from 50 to 150 km/sec. Around two dozen blob
features, on order of 1 - 3 minutes apart, were tracked in multiple
AIA EUV wavelengths. The blobs flowed outward (away from the Sun)
along the cusp stalk, and most of the observed speeds were either
constant or decelerating. We attempt to reconstruct the 3-D magnetic
field of the evolving structure, discuss the possible drivers of the
flows (including pulsed reconnection and tearing mode instability),
and compare the observations to studies of pulsed reconnection and
blob flows in the solar wind and the Earth's magnetosphere.
---------------------------------------------------------
Title: Patterns of Nanoflare Heating Exhibited by Active Regions
Observed with SDO/AIA
Authors: Viall, Nicholeen; Klimchuk, J.
2011SPD....42.2103V Altcode: 2011BAAS..43S.2103V
It seems largely agreed that many coronal loops---those observed at a
temperature of about 1 MK---are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Are these regions also
heated impulsively, or is the heating quasi steady? The spectacular new
data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
Dynamics Observatory (SDO) offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in 6 different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function of the nanoflare storm properties. These
orderings, or time lags, are clearly exhibited in loop observations in
all channels. What is especially exciting is that we identify these time
lag patterns in observations of the seemingly steady diffuse corona
as well. We model the diffuse corona as a line-of-sight integration
of many thousands of completely independent, impulsively heated
strands. The time lags of the simulated and actual observations are
in excellent agreement. Our results suggest that impulsive nanoflare
heating is ubiquitous within active regions. <P />This research was
supported through an appointment to the NASA Postdoctoral Program at
the Goddard Space Flight Center, administered by Oak Ridge Associated
Universities through a contract with NASA.
---------------------------------------------------------
Title: SDO/AIA Light Curves and Implications for Coronal Heating:
Observations
Authors: Viall, N. M.; Klimchuk, J. A.
2010AGUFMSH41E..02V Altcode:
It seems largely agreed that many coronal loops---those observed at a
temperature of about 1 MK---are bundles of unresolved strands that are
heated by storms of impulsive nanoflares. The nature of coronal heating
in hotter loops and in the very important but largely ignored diffuse
component of active regions is much less clear. Is it also impulsive
or is it quasi steady? The spectacular new data from the Atmospheric
Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory
(SDO) offer an excellent opportunity to address this question. We
analyze the light curves of coronal loops and the diffuse corona in
6 different AIA channels and compare them with the predicted light
curves from theoretical models. Light curves in the different AIA
channels reach their peak intensities with predictable orderings as a
function the nanoflare storm properties. We show that while some sets of
light curves exhibit clear evidence of cooling after nanoflare storms,
other cases are less straightforward to interpret. Complications arise
because of line-of-sight integration through many different structures,
the broadband nature of the AIA channels, and because physical
properties can change substantially depending on the magnitude of the
energy release. Nevertheless, the light curves exhibit predictable and
understandable patterns. This presentation emphasizes the observational
aspects of our study. A companion presentation emphasizes the models.
---------------------------------------------------------
Title: SDO/AIA Light Curves and Implications for Coronal Heating:
Model Predictions
Authors: Klimchuk, J. A.; Viall, N. M.
2010AGUFMSH41E..03K Altcode:
It seems largely agreed that many coronal loops---those observed
at a temperature of about 1 MK---are bundles of unresolved strands
that are heated by storms of impulsive nanoflares. The nature of
coronal heating in hotter loops and in the very important but largely
ignored diffuse component of active regions is much less clear. Is
it also impulsive or is it quasi steady? The spectacular new data
from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar
Dynamics Observatory (SDO) offer an excellent opportunity to address
this question. We analyze the light curves of coronal loops and the
diffuse corona in 6 different AIA channels and compare them with the
predicted light curves from theoretical models. Light curves in the
different AIA channels reach their peak intensities with predictable
orderings as a function the nanoflare storm properties. We show that
while some sets of light curves exhibit clear evidence of cooling
after nanoflare storms, other cases are less straightforward to
interpret. Complications arise because of line-of-sight integration
through many different structures, the broadband nature of the AIA
channels, and because physical properties can change substantially
depending on the magnitude of the energy release. Nevertheless, the
light curves exhibit predictable and understandable patterns. This
presentation emphasizes the modeling aspects of our study. A companion
presentation emphasizes the observations.
---------------------------------------------------------
Title: Examining Periodic Solar-Wind Density Structures Observed in
the SECCHI Heliospheric Imagers
Authors: Viall, Nicholeen M.; Spence, Harlan E.; Vourlidas, Angelos;
Howard, Russell
2010SoPh..267..175V Altcode: 2010arXiv1009.5885V; 2010SoPh..tmp..174V
We present an analysis of small-scale, periodic, solar-wind density
enhancements (length scales as small as ≈ 1000 Mm) observed
in images from the Heliospheric Imager (HI) aboard STEREO-A. We
discuss their possible relationship to periodic fluctuations
of the proton density that have been identified at 1 AU using
in-situ plasma measurements. Specifically, Viall, Kepko, and Spence
(J. Geophys. Res.113, A07101, 2008) examined 11 years of in-situ
solar-wind density measurements at 1 AU and demonstrated that not
only turbulent structures, but also nonturbulent, periodic density
structures exist in the solar wind with scale sizes of hundreds to
one thousand Mm. In a subsequent paper, Viall, Spence, and Kasper
(Geophys. Res. Lett.36, L23102, 2009) analyzed the α-to-proton
solar-wind abundance ratio measured during one such event of periodic
density structures, demonstrating that the plasma behavior was highly
suggestive that either temporally or spatially varying coronal source
plasma created those density structures. Large periodic density
structures observed at 1 AU, which were generated in the corona, can
be observable in coronal and heliospheric white-light images if they
possess sufficiently high density contrast. Indeed, we identify such
periodic density structures as they enter the HI field of view and
follow them as they advect with the solar wind through the images. The
smaller, periodic density structures that we identify in the images
are comparable in size to the larger structures analyzed in-situ at 1
AU, yielding further evidence that periodic density enhancements are
a consequence of coronal activity as the solar wind is formed.
---------------------------------------------------------
Title: Examining Periodic Solar Wind Density Structures in SECCHI HI1A
Authors: Viall, Nicholeen; Vourlidas, A.; Spence, H.; Howard, R.
2010AAS...21630303V Altcode:
We present an analysis of small-scale periodic solar wind density
enhancements observed in SECCHI HI1. We discuss their possible
relationship to periodic fluctuations of the proton density observed
in-situ with the Wind SWE data. Viall et al. [2008] used 11 years
of solar wind density measurements at 1 AU and demonstrated that in
addition to turbulent fluctuations, non-turbulent periodic density
structures with length scales of tens to hundreds of megameters exist in
the solar wind. Event studies of the periodic density structures reveal
instances in which the density structures have alpha/proton abundance
ratio changes associated with the density structures. Specifically,
the alpha density varies with the same periodicity as the protons,
but in antiphase. For those events, this strongly suggests either time
varying or spatially varying coronal source plasma that created the
density structures. If such periodic density structures observed at 1
AU are generated in the corona, then they may be observable in SECCHI
HI1 data. We identify periodic density structures as they convect
with the solar wind into the field of view of SECCHI HI and follow
the train of structures as a function of time. The periodic density
structures we analyze are comparable in size to the larger structures
identified in-situ at 1 AU. <P />This research was supported through
NASA Grant No. NNG05GK65G and an appointment to the NASA Postdoctoral
Program at the Goddard Space Flight Center, administered by Oak Ridge
Associated Universities through a contract with NASA.
---------------------------------------------------------
Title: Periodic solar wind density structures
Authors: Viall, Nicholeen Mary
2010PhDT.........1V Altcode:
This dissertation addresses a specific aspect of the Sun-Earth
connection: we show that coronal activity creates periodic density
structures in the solar wind which convect radially outward and interact
with Earth's magnetosphere. First, we analyze 11 years (1995-2005)
of in situ solar wind density observations from the Wind spacecraft
and find that periodic density structures occur at particular sets
of radial length-scales more often than others. This indicates that
these density fluctuations, which have radial length-scales of hundreds
of megameters, cannot be attributed entirely to turbulence. Next, we
analyze their effect on Earth's magnetosphere. Though these structures
are not waves in the solar wind rest frame, they appear at discrete
frequencies in Earth's reference frame. They compress the magnetosphere
as they convect past, driving global magnetospheric oscillations at
the same discrete frequencies as the periodic density structures. Last,
we investigate source regions and mechanisms of the periodic solar wind
density structures. We analyze the alpha particle to proton abundance
ratio during events of periodic density structures. In many events,
the proton and alpha density fluctuations are anti- correlated, which
strongly argues for either temporally or spatially varying coronal
source plasma. We examine white light images of the solar wind taken
with SECCHI HI1 on the STEREO spacecraft and find periodic density
structures as near to the Sun as 15 solar radii. The smallest resolvable
periodic structures that we identify are of comparable length to those
found at 1 AU, providing further evidence that at least some periodic
density structures are generated in the solar corona as the solar wind
is formed. Guided by the properties observed during previous studies
and the characteristics established through the work presented here,
we examine possible candidate mechanisms in the solar corona that can
form periodic density structures. We conclude that: coronal activity
creates coherent structures in the solar wind at smaller size scales
than previously thought; corona-formed coherent structures persist
to 1 AU largely intact; finally, a significant amount of discrete
frequency wave power in Earth's magnetosphere is directly driven by
these structures once they reach Earth.
---------------------------------------------------------
Title: Are periodic solar wind number density structures formed in
the solar corona?
Authors: Viall, Nicholeen M.; Spence, Harlan E.; Kasper, Justin
2009GeoRL..3623102V Altcode:
We present an analysis of the alpha to proton solar wind abundance
ratio (A<SUB>He</SUB>) during a period characterized by significant
large size scale density fluctuations, focusing on an event in which
the proton and alpha enhancements are anti-correlated. In a recent
study using 11 years (1995-2005) of solar wind observations from the
Wind spacecraft, N. M. Viall et al. [2008] showed that periodic proton
density structures occurred at particular radial length-scales more
often than others. The source of these periodic density structures
is a significant and outstanding question. Are they generated in
the interplanetary medium, or are they a relic of coronal activity
as the solar wind was formed? We use A<SUB>He</SUB> to answer this
question, as solar wind elemental abundance ratios are not expected
to change during transit. For this event, the anti-phase nature of
the A<SUB>He</SUB> variations strongly suggests that periodic solar
wind density structures originate in the solar corona.
---------------------------------------------------------
Title: Examining Solar Wind Number Density Structures Observed in
SECCHI HI 1
Authors: Viall, N. M.; Spence, H. E.; Vourlidas, A.; Howard, R. A.
2009AGUFMSH13B1516V Altcode:
We present an analysis of small-scale periodic solar wind density
enhancements observed in SECCHI HI 1. We discuss their possible
relationship to periodic fluctuations of the proton density observed
in-situ with the Wind SWE data. Viall et al. [2008] used 11 years
of solar wind density measurements at 1 AU and demonstrated that in
addition to turbulent fluctuations, non-turbulent, periodic density
structures exist in the solar wind. In the slow wind, periodic
density structures occurred most often with radial length-scales of
approximately 73, 120, 136 and 180 Mm. In the fast wind, periodic
density structures occurred most often with radial length-scales
of approximately 187, 270 and 400 Mm. Event studies of the periodic
density structures reveal instances in which the density structures
have alpha/proton abundance ratio changes associated with the density
structures. Specifically, the alpha density varies with the same
periodicity as the protons, but in antiphase. For those events, this
strongly suggests either time varying or spatially varying coronal
source plasma that created the density structures. If such periodic
density structures observed at 1 AU are generated in the corona,
then they may be observable in SECCHI HI1 data. For instance, larger
scale plasmoids have been observed in the corona [e.g. Sheeley et al.,
2009] and it is plausible that smaller, periodic structures may exist
as well. We identify periodic density structures as they convect with
the solar wind into the field of view of SECCHI HI and follow the train
of structures as a function of time. The periodic density structures
we analyze are comparable in size to the larger structures identified
in-situ at 1 AU.
---------------------------------------------------------
Title: Examining Solar Wind Number Density Structures Observed in
SECCHI HI 1
Authors: Viall, Nicholeen Mary; Spence, Harlan E.; Vourlidas, Angelos;
Howard, Russ
2009shin.confE.133V Altcode:
We explore small-scale quasi-periodic solar wind density fluctuations
observed in SECCHI HI 1. We discuss their possible relationship to
periodic fluctuations of the proton density observed in-situ with the
Wind SWE data. Viall et al. [2008] used 11 years of solar wind density
measurements at 1 AU and demonstrated that in addition to turbulent
fluctuations, non-turbulent, periodic density structures exist in the
solar wind. In the slow wind, periodic density structures occurred most
often with radial length-scales of approximately 73, 120, 136 and 180
Mm. In the fast wind, periodic density structures occurred most often
with radial length-scales of approximately 187, 270 and 400 Mm. Event
studies of the periodic density structures reveal instances in which the
density structure has alpha/proton abundance ratio changes associated
with the density structures. Specifically, the alpha density varies
with the same periodicity as the protons, but in antiphase. This
strongly suggests either time varying or spatially varying coronal
source plasma that created the density structures. If such periodic
density structures observed at 1AU are generated in the corona, then
they may be observable in SECCHI HI1 data. For instance, larger scale
plasmoids have been observed in the corona [e.g. Sheeley et al., 2009]
and it is plausible that smaller, quasi-periodic structures may exist
as well. We identify quasi-periodic density structures in the SECCHI HI1
images that are comparable in size to those identified in-situ at 1AU.
---------------------------------------------------------
Title: Multipoint Analysis of Meso-scale Structures in the Ambient
Solar Wind: STEREO-A, -B, and L1 Observations
Authors: Spence, H. E.; Viall, N. M.; Vourlidas, A.; Howard, R. A.;
Simunac, K.; Kistler, L. M.; Galvin, A. B.; Kasper, J. C.; Lazarus,
A. J.
2008AGUFMSH12A..06S Altcode:
We explore sources of apparent time-dependence of meso-scale structures
(those lasting two to three days and less) in the ambient solar wind
through analysis of measurements from STEREO-A, -B, and L1 spacecraft
(WIND, ACE, and SOHO). In early 2008, stable corotating interaction
regions and high-speed streams provided excellent boundaries and
features for co-registering the large-scale, corotating solar wind
observed by several heliospheric spacecraft separated in solar orbital
phase near 1 AU. During this period, STEREO-B (located 23 degrees behind
the Earth in heliographic longitude) first observed the large-scale
corotating stream structures, followed by the WIND, ACE, and SOHO
spacecraft at Earth, then finally by STEREO-A (located 22 degrees ahead
of the Earth in heliographic longitude). Conspicuous similarities
in the macro-scale solar wind flow dominate the comparison between
spacecraft observations and permit us to time-adjust the observed flow
features reasonably well by assuming a simple corotating solar wind
source. While the co-registered, large-scale solar wind structure agrees
well, mesoscale flow features can exhibit large measured differences
at the various spacecraft. We focus on one such interesting feature
which exhibits apparent time dependence. Though this few-day-long,
significant flow speed event is observed by the PLASTIC experiments
on both STEREO-A and STEREO-B, it is not seen at the L1 spacecraft
which the STEREO spacecraft bracket in space and time. We explore
potential sources of the apparent time dependence of this meso-scale
feature. Latitudinal differences in the multipoint measurements is one
source that could account for the apparent mesoscale flow structure
variability. We also explore explicit time variation of the solar wind's
source, by analyzing relevant coronal holes observed simultaneously
by the STEREO spacecraft imagers. This event and analysis underscores
that multipoint heliospheric observations and analysis reveals the
existence of mesoscale structure in the solar wind and can be used to
constrain its possible source(s).
---------------------------------------------------------
Title: On the Source of Periodic Solar Wind Number Density Structures
Using the Alpha to Proton Abundance Ratio
Authors: Viall, N. M.; Spence, H. E.; Kasper, J.
2008AGUFMSH21A1573V Altcode:
Solar wind elemental abundance ratios provide information regarding
the source region of the solar wind plasma because they are not
modified by propagation. We present an analysis of the alpha to proton
abundance ratio during periods in the solar wind with significant long
wavelength number density fluctuations. We discuss the implications
of the abundance ratio variations on possible source regions of those
periodic solar wind number density structures. In a recent study using
11 years (1995-2005) of solar wind number density observations taken on
the Wind spacecraft, we analyzed the radial length-scales of periodic
number density structures. We performed Fourier analysis on short
(approximately equal to six hours) data windows and determined the
wavelength (length-scale) of the number density structures. We found
that particular length-scales occurred more often than others in the
solar wind number density during those eleven years. For the slow
wind, the most significant length-scales are approximately 73, 120,
136 and 180 Mm. For the fast wind, the most significant length-scales
are approximately 187, 270 and 400 Mm. The outstanding question is the
source of these periodic density structures. Using the alpha to proton
abundance ratio during periodic solar wind number density events,
we address whether periodic density structures are generated in the
local interplanetary medium, or if they are more probably generated
somewhere near the solar surface or solar corona, frozen into the
solar wind, and convected out to Earth.
---------------------------------------------------------
Title: Inherent length-scales of periodic solar wind number density
structures
Authors: Viall, N. M.; Kepko, L.; Spence, H. E.
2008JGRA..113.7101V Altcode:
We present an analysis of the radial length-scales of periodic solar
wind number density structures. We converted 11 years (1995-2005)
of solar wind number density data into radial length series segments
and Fourier analyzed them to identify all spectral peaks with radial
wavelengths between 72 (116) and 900 (900) Mm for slow (fast) wind
intervals. Our window length for the spectral analysis was 9072 Mm,
approximately equivalent to 7 (4) h of data for the slow (fast) solar
wind. We required that spectral peaks pass both an amplitude test and
a harmonic F-test at the 95% confidence level simultaneously. From
the occurrence distributions of these spectral peaks for slow and
fast wind, we find that periodic number density structures occur
more often at certain radial length-scales than at others, and are
consistently observed within each speed range over most of the 11-year
interval. For the slow wind, those length-scales are L ∼ 73, 120,
136, and 180 Mm. For the fast wind, those length-scales are L ∼ 187,
270 and 400 Mm. The results argue for the existence of inherent radial
length-scales in the solar wind number density.
---------------------------------------------------------
Title: Shock and discontinuity associated periodicities in the solar
wind and magnetosphere
Authors: Kepko, L.; Viall, N. M.; Spence, H. E.
2006AGUFMSH31A0378K Altcode:
Solar wind shocks and discontinuities are often cited as sources
of broadband compressional power that couple to magnetospheric
eigenmodes. Some recent event studies have indicated that the
post-shock solar wind sometimes contains discrete periodicities in
the number density. These periodicities lasted 10s of minutes to hours
and matched oscillations observed in the magnetosphere that otherwise
may have been attributed to global cavity modes. In addition, it has
been shown recently that the ambient solar wind contains intervals
with significant power at multiple discrete frequencies, and that
these density variations drive multiple, discrete magnetospheric
oscillations. We present the results of a statistical analysis of
solar wind shocks and discontinuities and seek answers to the following
questions: 1) How often and under what conditions are number density
periodicities observed following shocks and discontinuities? 2)
To what degree are the periodicities 'discrete' vs random? 3) Are
the shock-associated periodicities related somehow to the multiple,
discrete periodicities observed in the ambient solar wind?
---------------------------------------------------------
Title: Magnetospheric Oscillations and Their Relation to Periodic
Multiple Discrete Solar Wind Number Density Variations
Authors: Viall, N. M.; Kepko, L.; Spence, H.
2006AGUFMSH31A0384V Altcode:
Sets of discrete magnetospheric oscillations in the mHz range have
been observed in satellite measurements, HF radar measurements, and
ground magnetometer data for the last decade. Kepko et al. [2002]
and Kepko and Spence [2003] presented case studies showing that,
in at least some instances, the apparent frequencies of periodic
solar wind number density structures in Earth's reference frame are
correlated with observations of magnetospheric oscillations. This
strongly suggests that the solar wind is sometimes a driver of
multiple discrete magnetospheric oscillations. We recently completed
a statistical analysis of 11 years of solar wind data (1995-2005)
investigating the occurrence of periodic solar wind number density
structures and showed that they occur at preferred frequencies. We
defined multiple discrete periodicity events in the solar wind by
imposing a criterion that an event contained at least three discrete
frequencies, and found stronger evidence for preferred multiple discrete
apparent frequencies. In this paper, we extend this analysis to the
magnetosphere. We present the results of an analysis of statistically
robust magnetospheric peaks in power spectra observed at geosynchronous
satellites and ground magnetometers. We compare the occurrence rate
of statistically significant spectral peaks in the magnetosphere to
our results from the upstream solar wind number density.
---------------------------------------------------------
Title: Discrete Frequency Magnetospheric Oscillations and Their
Relation to Periodic Solar Wind Number Density Structures
Authors: Viall, N. M.; Kepko, L.; Spence, H.
2006AGUSMSM42A..02V Altcode:
Discrete magnetospheric oscillations near f = 1.3, 1.9, 2.6 and 3.4 mHz
have been observed in satellite measurements, HF radar measurements,
and ground magnetometer data for the last decade. Case studies have
shown instances in which the apparent frequencies of periodic solar wind
number density structures in Earth's reference frame correlate well with
magnetospheric oscillations. This strongly suggests that the solar wind
is sometimes a direct driver of discrete magnetospheric oscillations. We
have recently completed a statistical analysis of periodic solar wind
number density structures that indicated a preference for the solar
wind to contain periodicities near the f = 1.3, 1.9, 2.6 and 3.4 mHz
oscillations. In this paper we extend this analysis to magnetospheric
data and determine the occurrence rate of statistically robust
magnetospheric peaks in power spectra using data from GOES, ground
magnetometers, Geotail, and LANL EP. We compare the occurrence rate
of statistically significant spectral peaks in the magnetosphere to
those observed upstream in the solar wind number density.
---------------------------------------------------------
Title: The Occurrence Rate of Solar Wind Periodic Number Density
Structures
Authors: Viall, N. M.; Kepko, L.; Spence, H.
2005AGUFMSH11B0264V Altcode:
The solar wind is generally described as a fully turbulent plasma, with
a featureless power-law spectrum that does not contain excess power
at discrete frequencies. However, recent observations have indicated
that the solar wind sometimes contains highly periodic number density
variations in the mHz range in the Earth's reference frame. These
periodic structures remained coherent over several hours and were highly
geoeffective, driving global magnetospheric oscillations at the same
discrete frequencies. These previous studies examined only a few events,
and did not determine the occurrence rate of these structures. In
this paper we determine the occurrence rate of discrete oscillations
in the solar wind number density. We perform a multi-tapered analysis
for spectral estimation and robust significance testing using several
years of solar wind data. In addition we attempt to reconcile the
turbulent picture of the solar wind with the new observations.
