Author name code: pariat
ADS astronomy entries on 2022-09-14
author:"Pariat, Etienne"
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Title: The Importance of Method Redundancy in Studying Pre-Eruption
Evolution in Solar Active Regions
Authors: Georgoulis, Manolis K.; Pariat, Etienne; Liu, Yang; Thalmann,
Julia K.
Bibcode: 2022cosp...44.1358G
Altcode:
In a recent synergistic work stemming from a prior International
Space Science Institute (ISSI) Working Group, the evolution of
magnetic helicity in an intensely eruptive solar active region was
studied using several different helicity calculation methods. This
was the first time all these methods were tested on real solar data,
without the possibility of a ground truth. Focusing on the pre-eruption
evolution prior to an eruptive X-class flare (SOL2006-12-13T02:14X3.4)
in NOAA active region (AR) 10930, we reveal a more complex picture than
what any single method might convey. Through imperfect but overall
converging calculations from different methods, we find artifacts
that could mislead conclusions. More importantly, we find evidence of
competing physical tendencies in the active region whose omission could
lead to counterintuitive, hence misleading, again, conclusions. While
for the Sun we have the capability to use different data and methods
for related purposes, this is not the case for other eruptive stars,
which is a fact calling for robust modeling approaches, relying
on scarce and indirect observations of stellar magnetic fields and
CME properties. Confluence of any data available and modeling could
offer the redundancy needed to critically assess partial findings
and reconcile them into a physically consistent picture of stellar
eruptions, quite possibly with qualitative / quantitative similarities
and differences from the eruptions of our own Sun.
Title: Disambiguation of Vector Magnetograms by Stereoscopic
Observations from the Solar Orbiter (SO)/Polarimetric and Helioseismic
Imager (PHI) and the Solar Dynamic Observatory (SDO)/Helioseismic
and Magnetic Imager (HMI)
Authors: Valori, Gherardo; Löschl, Philipp; Stansby, David; Pariat,
Etienne; Hirzberger, Johann; Chen, Feng
Bibcode: 2022SoPh..297...12V
Altcode: 2021arXiv211210650V
Spectropolarimetric reconstructions of the photospheric vector magnetic
field are intrinsically limited by the 180∘ ambiguity
in the orientation of the transverse component. The successful
launch and operation of Solar Orbiter have made the removal of
the 180∘ ambiguity possible using solely observations
obtained from two different vantage points. While the exploitation
of such a possibility is straightforward in principle, it is less so
in practice, and it is therefore important to assess the accuracy
and limitations as a function of both the spacecrafts' orbits and
measurement principles. In this work, we present a stereoscopic
disambiguation method (SDM) and discuss thorough testing of its
accuracy in applications to modeled active regions and quiet-Sun
observations. In the first series of tests, we employ magnetograms
extracted from three different numerical simulations as test fields
and model observations of the magnetograms from different angles and
distances. In these more idealized tests, SDM is proven to reach a 100%
disambiguation accuracy when applied to moderately-to-well resolved
fields. In such favorable conditions, the accuracy is almost independent
of the relative position of the spacecraft with the obvious exceptions
of configurations where the spacecraft are within a few degrees of
co-alignment or quadrature. Even in the case of disambiguation of
quiet-Sun magnetograms with significant under-resolved spatial scales,
SDM provides an accuracy between 82% and 98%, depending on the field
strength. The accuracy of SDM is found to be mostly sensitive to the
variable spatial resolution of Solar Orbiter in its highly elliptic
orbit, as well as to the intrinsic spatial scale of the observed
field. Additionally, we provide an example of the expected accuracy as
a function of time that can be used to optimally place remote-sensing
observing windows during Solar Orbiter observation planning. Finally,
as a more realistic test, we consider magnetograms that are obtained
using a radiative-transfer inversion code and the SO/PHI Software
siMulator (SOPHISM) applied to a 3D-simulation of a pore, and we
present a preliminary discussion of the effect of the viewing angle
on the observed field. In this more realistic test of the application
of SDM, the method is able to successfully remove the ambiguity in
strong-field areas.
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. IV. Application to Solar Observations
Authors: Thalmann, J. K.; Georgoulis, M. K.; Liu, Y.; Pariat, E.;
Valori, G.; Anfinogentov, S.; Chen, F.; Guo, Y.; Moraitis, K.; Yang,
S.; Mastrano, Alpha; ISSI Team on Magnetic Helicity
Bibcode: 2021ApJ...922...41T
Altcode: 2021arXiv210808525T
In this ISSI-supported series of studies on magnetic helicity in the
Sun, we systematically implement different magnetic helicity calculation
methods on high-quality solar magnetogram observations. We apply
finite-volume, discrete flux tube (in particular, connectivity-based)
and flux-integration methods to data from Hinode's Solar Optical
Telescope. The target is NOAA Active Region 10930 during a 1.5-day
interval in 2006 December that included a major eruptive flare
(SOL2006-12-13T02:14X3.4). Finite-volume and connectivity-based methods
yield instantaneous budgets of the coronal magnetic helicity, while
the flux-integration methods allow an estimate of the accumulated
helicity injected through the photosphere. The objectives of our work
are twofold: a cross-validation of methods, as well as an interpretation
of the complex events leading to the eruption. To the first objective,
we find (i) strong agreement among the finite-volume methods, (ii)
a moderate agreement between the connectivity-based and finite-volume
methods, (iii) an excellent agreement between the flux-integration
methods, and (iv) an overall agreement between finite-volume- and
flux-integration-based estimates regarding the predominant sign and
magnitude of the helicity. To the second objective, we are confident
that the photospheric helicity flux significantly contributed to the
coronal helicity budget and that a right-handed structure erupted from
a predominantly left-handed corona during the X-class flare. Overall,
we find that the use of different methods to estimate the (accumulated)
coronal helicity may be necessary in order to draw a complete picture
of an active region corona, given the careful handling of identified
data (preparation) issues, which otherwise would mislead the event
analysis and interpretation.
Title: Switch-on Shock and Nonlinear Kink Alfvén Waves in Solar
Coronal-Hole Jets
Authors: DeVore, C. R.; Karpen, J. T.; Antiochos, S. K.; Uritsky,
V. M.; Roberts, M. A.; Pariat, E.
Bibcode: 2021AAS...23821322D
Altcode:
It is generally accepted that solar coronal-hole jets are generated by
fast magnetic reconnection in the low corona, whether driven directly by
flux emergence from below or indirectly by instability onset above the
photosphere. In either case, twisted flux on closed magnetic field lines
reconnects with untwisted flux on neighboring open field lines. Some
of that twist is inherited by the newly reconnected open flux, which
rapidly relaxes due to magnetic tension forces that transmit the twist
impulsively into the outer corona and heliosphere. We suggest that the
transfer of twist launches switch-on MHD shock waves, which propagate
parallel to the ambient coronal magnetic field ahead of the shock
and convect a perpendicular component of magnetic field behind the
shock. In the frame moving with the shock front, the post-shock flow
is precisely Alfvénic in all three directions, whereas the pre-shock
flow is super-Alfvénic along the ambient magnetic field. Consequently,
there is a density enhancement across the shock front. Nonlinear kink
Alfvén waves are exact solutions of the time-dependent MHD equations
in the post-shock region when the ambient corona is uniform and the
magnetic field is straight. We report 3D spherical simulations of
coronal-hole jets driven by instability onset in the corona. The results
are consistent with the generation of MHD switch-on shocks trailed
predominantly by incompressible, irrotational, kink Alfvén waves. We
will discuss the implications of our results for understanding solar
jets and interpreting their heliospheric signatures in light of the
new data on S-bends (a.k.a. switchbacks) from Parker Solar Probe. Our
research is supported by NASA's H-ISFM program.
Title: The flare likelihood and region eruption forecasting
(FLARECAST) project: flare forecasting in the big data & machine
learning era
Authors: Georgoulis, Manolis K.; Bloomfield, D. Shaun; Piana,
Michele; Massone, Anna Maria; Soldati, Marco; Gallagher, Peter T.;
Pariat, Etienne; Vilmer, Nicole; Buchlin, Eric; Baudin, Frederic;
Csillaghy, Andre; Sathiapal, Hanna; Jackson, David R.; Alingery,
Pablo; Benvenuto, Federico; Campi, Cristina; Florios, Konstantinos;
Gontikakis, Constantinos; Guennou, Chloe; Guerra, Jordan A.;
Kontogiannis, Ioannis; Latorre, Vittorio; Murray, Sophie A.; Park,
Sung-Hong; von Stachelski, Samuelvon; Torbica, Aleksandar; Vischi,
Dario; Worsfold, Mark
Bibcode: 2021JSWSC..11...39G
Altcode: 2021arXiv210505993G
The European Union funded the FLARECAST project, that ran from January
2015 until February 2018. FLARECAST had a research-to-operations
(R2O) focus, and accordingly introduced several innovations into the
discipline of solar flare forecasting. FLARECAST innovations were:
first, the treatment of hundreds of physical properties viewed as
promising flare predictors on equal footing, extending multiple
previous works; second, the use of fourteen (14) different machine
learning techniques, also on equal footing, to optimize the immense
Big Data parameter space created by these many predictors; third,
the establishment of a robust, three-pronged communication effort
oriented toward policy makers, space-weather stakeholders and the wider
public. FLARECAST pledged to make all its data, codes and infrastructure
openly available worldwide. The combined use of 170+ properties (a
total of 209 predictors are now available) in multiple machine-learning
algorithms, some of which were designed exclusively for the project,
gave rise to changing sets of best-performing predictors for the
forecasting of different flaring levels, at least for major flares. At
the same time, FLARECAST reaffirmed the importance of rigorous training
and testing practices to avoid overly optimistic pre-operational
prediction performance. In addition, the project has (a) tested new
and revisited physically intuitive flare predictors and (b) provided
meaningful clues toward the transition from flares to eruptive flares,
namely, events associated with coronal mass ejections (CMEs). These
leads, along with the FLARECAST data, algorithms and infrastructure,
could help facilitate integrated space-weather forecasting efforts
that take steps to avoid effort duplication. In spite of being
one of the most intensive and systematic flare forecasting efforts
to-date, FLARECAST has not managed to convincingly lift the barrier of
stochasticity in solar flare occurrence and forecasting: solar flare
prediction thus remains inherently probabilistic.
Title: Additivity of relative magnetic helicity in finite volumes
Authors: Valori, Gherardo; Démoulin, Pascal; Pariat, Etienne; Yeates,
Anthony; Moraitis, Kostas; Linan, Luis
Bibcode: 2020A&A...643A..26V
Altcode: 2020arXiv200800968V
Context. Relative magnetic helicity is conserved by magneto-hydrodynamic
evolution even in the presence of moderate resistivity. For that reason,
it is often invoked as the most relevant constraint on the dynamical
evolution of plasmas in complex systems, such as solar and stellar
dynamos, photospheric flux emergence, solar eruptions, and relaxation
processes in laboratory plasmas. However, such studies often indirectly
imply that relative magnetic helicity in a given spatial domain can be
algebraically split into the helicity contributions of the composing
subvolumes, in other words that it is an additive quantity. A limited
number of very specific applications have shown that this is not the
case.
Aims: Progress in understanding the nonadditivity of
relative magnetic helicity requires removal of restrictive assumptions
in favor of a general formalism that can be used in both theoretical
investigations and numerical applications.
Methods: We derive the
analytical gauge-invariant expression for the partition of relative
magnetic helicity between contiguous finite volumes, without any
assumptions on either the shape of the volumes and interface, or the
employed gauge.
Results: We prove the nonadditivity of relative
magnetic helicity in finite volumes in the most general, gauge-invariant
formalism, and verify this numerically. We adopt more restrictive
assumptions to derive known specific approximations, which yields a
unified view of the additivity issue. As an example, the case of a
flux rope embedded in a potential field shows that the nonadditivity
term in the partition equation is, in general, non-negligible.
Conclusions: The nonadditivity of relative magnetic helicity can
potentially be a serious impediment to the application of relative
helicity conservation as a constraint on the complex dynamics of
magnetized plasmas. The relative helicity partition formula can be
applied to numerical simulations to precisely quantify the effect of
nonadditivity on global helicity budgets of complex physical processes.
Title: Erratum: "On the Reliability of Magnetic Energy and Helicity
Computations Based on Nonlinear Force-free Coronal Magnetic Field
Models" (2019, ApJL, 880, L6)
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
Bibcode: 2020ApJ...902L..48T
Altcode:
No abstract at ADS
Title: Models and data analysis tools for the Solar Orbiter mission
Authors: Rouillard, A. P.; Pinto, R. F.; Vourlidas, A.; De Groof, A.;
Thompson, W. T.; Bemporad, A.; Dolei, S.; Indurain, M.; Buchlin, E.;
Sasso, C.; Spadaro, D.; Dalmasse, K.; Hirzberger, J.; Zouganelis, I.;
Strugarek, A.; Brun, A. S.; Alexandre, M.; Berghmans, D.; Raouafi,
N. E.; Wiegelmann, T.; Pagano, P.; Arge, C. N.; Nieves-Chinchilla,
T.; Lavarra, M.; Poirier, N.; Amari, T.; Aran, A.; Andretta, V.;
Antonucci, E.; Anastasiadis, A.; Auchère, F.; Bellot Rubio, L.;
Nicula, B.; Bonnin, X.; Bouchemit, M.; Budnik, E.; Caminade, S.;
Cecconi, B.; Carlyle, J.; Cernuda, I.; Davila, J. M.; Etesi, L.;
Espinosa Lara, F.; Fedorov, A.; Fineschi, S.; Fludra, A.; Génot,
V.; Georgoulis, M. K.; Gilbert, H. R.; Giunta, A.; Gomez-Herrero, R.;
Guest, S.; Haberreiter, M.; Hassler, D.; Henney, C. J.; Howard, R. A.;
Horbury, T. S.; Janvier, M.; Jones, S. I.; Kozarev, K.; Kraaikamp,
E.; Kouloumvakos, A.; Krucker, S.; Lagg, A.; Linker, J.; Lavraud,
B.; Louarn, P.; Maksimovic, M.; Maloney, S.; Mann, G.; Masson, A.;
Müller, D.; Önel, H.; Osuna, P.; Orozco Suarez, D.; Owen, C. J.;
Papaioannou, A.; Pérez-Suárez, D.; Rodriguez-Pacheco, J.; Parenti,
S.; Pariat, E.; Peter, H.; Plunkett, S.; Pomoell, J.; Raines, J. M.;
Riethmüller, T. L.; Rich, N.; Rodriguez, L.; Romoli, M.; Sanchez,
L.; Solanki, S. K.; St Cyr, O. C.; Straus, T.; Susino, R.; Teriaca,
L.; del Toro Iniesta, J. C.; Ventura, R.; Verbeeck, C.; Vilmer, N.;
Warmuth, A.; Walsh, A. P.; Watson, C.; Williams, D.; Wu, Y.; Zhukov,
A. N.
Bibcode: 2020A&A...642A...2R
Altcode:
Context. The Solar Orbiter spacecraft will be equipped with a wide
range of remote-sensing (RS) and in situ (IS) instruments to record
novel and unprecedented measurements of the solar atmosphere and
the inner heliosphere. To take full advantage of these new datasets,
tools and techniques must be developed to ease multi-instrument and
multi-spacecraft studies. In particular the currently inaccessible
low solar corona below two solar radii can only be observed
remotely. Furthermore techniques must be used to retrieve coronal
plasma properties in time and in three dimensional (3D) space. Solar
Orbiter will run complex observation campaigns that provide interesting
opportunities to maximise the likelihood of linking IS data to their
source region near the Sun. Several RS instruments can be directed
to specific targets situated on the solar disk just days before
data acquisition. To compare IS and RS, data we must improve our
understanding of how heliospheric probes magnetically connect to the
solar disk.
Aims: The aim of the present paper is to briefly
review how the current modelling of the Sun and its atmosphere
can support Solar Orbiter science. We describe the results of a
community-led effort by European Space Agency's Modelling and Data
Analysis Working Group (MADAWG) to develop different models, tools,
and techniques deemed necessary to test different theories for the
physical processes that may occur in the solar plasma. The focus here
is on the large scales and little is described with regards to kinetic
processes. To exploit future IS and RS data fully, many techniques have
been adapted to model the evolving 3D solar magneto-plasma from the
solar interior to the solar wind. A particular focus in the paper is
placed on techniques that can estimate how Solar Orbiter will connect
magnetically through the complex coronal magnetic fields to various
photospheric and coronal features in support of spacecraft operations
and future scientific studies.
Methods: Recent missions such as
STEREO, provided great opportunities for RS, IS, and multi-spacecraft
studies. We summarise the achievements and highlight the challenges
faced during these investigations, many of which motivated the Solar
Orbiter mission. We present the new tools and techniques developed
by the MADAWG to support the science operations and the analysis of
the data from the many instruments on Solar Orbiter.
Results:
This article reviews current modelling and tool developments that ease
the comparison of model results with RS and IS data made available
by current and upcoming missions. It also describes the modelling
strategy to support the science operations and subsequent exploitation
of Solar Orbiter data in order to maximise the scientific output
of the mission.
Conclusions: The on-going community effort
presented in this paper has provided new models and tools necessary
to support mission operations as well as the science exploitation of
the Solar Orbiter data. The tools and techniques will no doubt evolve
significantly as we refine our procedure and methodology during the
first year of operations of this highly promising mission.
Title: The Solar Orbiter Science Activity Plan. Translating solar
and heliospheric physics questions into action
Authors: Zouganelis, I.; De Groof, A.; Walsh, A. P.; Williams, D. R.;
Müller, D.; St Cyr, O. C.; Auchère, F.; Berghmans, D.; Fludra,
A.; Horbury, T. S.; Howard, R. A.; Krucker, S.; Maksimovic, M.;
Owen, C. J.; Rodríguez-Pacheco, J.; Romoli, M.; Solanki, S. K.;
Watson, C.; Sanchez, L.; Lefort, J.; Osuna, P.; Gilbert, H. R.;
Nieves-Chinchilla, T.; Abbo, L.; Alexandrova, O.; Anastasiadis, A.;
Andretta, V.; Antonucci, E.; Appourchaux, T.; Aran, A.; Arge, C. N.;
Aulanier, G.; Baker, D.; Bale, S. D.; Battaglia, M.; Bellot Rubio,
L.; Bemporad, A.; Berthomier, M.; Bocchialini, K.; Bonnin, X.; Brun,
A. S.; Bruno, R.; Buchlin, E.; Büchner, J.; Bucik, R.; Carcaboso,
F.; Carr, R.; Carrasco-Blázquez, I.; Cecconi, B.; Cernuda Cangas, I.;
Chen, C. H. K.; Chitta, L. P.; Chust, T.; Dalmasse, K.; D'Amicis, R.;
Da Deppo, V.; De Marco, R.; Dolei, S.; Dolla, L.; Dudok de Wit, T.;
van Driel-Gesztelyi, L.; Eastwood, J. P.; Espinosa Lara, F.; Etesi,
L.; Fedorov, A.; Félix-Redondo, F.; Fineschi, S.; Fleck, B.; Fontaine,
D.; Fox, N. J.; Gandorfer, A.; Génot, V.; Georgoulis, M. K.; Gissot,
S.; Giunta, A.; Gizon, L.; Gómez-Herrero, R.; Gontikakis, C.; Graham,
G.; Green, L.; Grundy, T.; Haberreiter, M.; Harra, L. K.; Hassler,
D. M.; Hirzberger, J.; Ho, G. C.; Hurford, G.; Innes, D.; Issautier,
K.; James, A. W.; Janitzek, N.; Janvier, M.; Jeffrey, N.; Jenkins,
J.; Khotyaintsev, Y.; Klein, K. -L.; Kontar, E. P.; Kontogiannis,
I.; Krafft, C.; Krasnoselskikh, V.; Kretzschmar, M.; Labrosse, N.;
Lagg, A.; Landini, F.; Lavraud, B.; Leon, I.; Lepri, S. T.; Lewis,
G. R.; Liewer, P.; Linker, J.; Livi, S.; Long, D. M.; Louarn, P.;
Malandraki, O.; Maloney, S.; Martinez-Pillet, V.; Martinovic, M.;
Masson, A.; Matthews, S.; Matteini, L.; Meyer-Vernet, N.; Moraitis,
K.; Morton, R. J.; Musset, S.; Nicolaou, G.; Nindos, A.; O'Brien,
H.; Orozco Suarez, D.; Owens, M.; Pancrazzi, M.; Papaioannou, A.;
Parenti, S.; Pariat, E.; Patsourakos, S.; Perrone, D.; Peter, H.;
Pinto, R. F.; Plainaki, C.; Plettemeier, D.; Plunkett, S. P.; Raines,
J. M.; Raouafi, N.; Reid, H.; Retino, A.; Rezeau, L.; Rochus, P.;
Rodriguez, L.; Rodriguez-Garcia, L.; Roth, M.; Rouillard, A. P.;
Sahraoui, F.; Sasso, C.; Schou, J.; Schühle, U.; Sorriso-Valvo, L.;
Soucek, J.; Spadaro, D.; Stangalini, M.; Stansby, D.; Steller, M.;
Strugarek, A.; Štverák, Š.; Susino, R.; Telloni, D.; Terasa, C.;
Teriaca, L.; Toledo-Redondo, S.; del Toro Iniesta, J. C.; Tsiropoula,
G.; Tsounis, A.; Tziotziou, K.; Valentini, F.; Vaivads, A.; Vecchio,
A.; Velli, M.; Verbeeck, C.; Verdini, A.; Verscharen, D.; Vilmer, N.;
Vourlidas, A.; Wicks, R.; Wimmer-Schweingruber, R. F.; Wiegelmann,
T.; Young, P. R.; Zhukov, A. N.
Bibcode: 2020A&A...642A...3Z
Altcode: 2020arXiv200910772Z
Solar Orbiter is the first space mission observing the solar plasma
both in situ and remotely, from a close distance, in and out of the
ecliptic. The ultimate goal is to understand how the Sun produces
and controls the heliosphere, filling the Solar System and driving
the planetary environments. With six remote-sensing and four in-situ
instrument suites, the coordination and planning of the operations are
essential to address the following four top-level science questions:
(1) What drives the solar wind and where does the coronal magnetic field
originate?; (2) How do solar transients drive heliospheric variability?;
(3) How do solar eruptions produce energetic particle radiation that
fills the heliosphere?; (4) How does the solar dynamo work and drive
connections between the Sun and the heliosphere? Maximising the
mission's science return requires considering the characteristics
of each orbit, including the relative position of the spacecraft
to Earth (affecting downlink rates), trajectory events (such
as gravitational assist manoeuvres), and the phase of the solar
activity cycle. Furthermore, since each orbit's science telemetry
will be downloaded over the course of the following orbit, science
operations must be planned at mission level, rather than at the level
of individual orbits. It is important to explore the way in which those
science questions are translated into an actual plan of observations
that fits into the mission, thus ensuring that no opportunities are
missed. First, the overarching goals are broken down into specific,
answerable questions along with the required observations and the
so-called Science Activity Plan (SAP) is developed to achieve this. The
SAP groups objectives that require similar observations into Solar
Orbiter Observing Plans, resulting in a strategic, top-level view of
the optimal opportunities for science observations during the mission
lifetime. This allows for all four mission goals to be addressed. In
this paper, we introduce Solar Orbiter's SAP through a series of
examples and the strategy being followed.
Title: Energy and helicity fluxes in line-tied eruptive simulations
Authors: Linan, L.; Pariat, É.; Aulanier, G.; Moraitis, K.; Valori, G.
Bibcode: 2020A&A...636A..41L
Altcode: 2020arXiv200301698L
Context. Conservation properties of magnetic helicity and energy in
the quasi-ideal and low-β solar corona make these two quantities
relevant for the study of solar active regions and eruptions.
Aims: Based on a decomposition of the magnetic field into potential
and nonpotential components, magnetic energy and relative helicity
can both also be decomposed into two quantities: potential and free
energies, and volume-threading and current-carrying helicities. In
this study, we perform a coupled analysis of their behaviors in a set
of parametric 3D magnetohydrodynamic (MHD) simulations of solar-like
eruptions.
Methods: We present the general formulations for
the time-varying components of energy and helicity in resistive
MHD. We calculated them numerically with a specific gauge, and
compared their behaviors in the numerical simulations, which differ
from one another by their imposed boundary-driving motions. Thus,
we investigated the impact of different active regions surface flows
on the development of the energy and helicity-related quantities.
Results: Despite general similarities in their overall behaviors,
helicities and energies display different evolutions that cannot be
explained in a unique framework. While the energy fluxes are similar
in all simulations, the physical mechanisms that govern the evolution
of the helicities are markedly distinct from one simulation to another:
the evolution of volume-threading helicity can be governed by boundary
fluxes or helicity transfer, depending on the simulation.
Conclusions: The eruption takes place for the same value of the
ratio of the current-carrying helicity to the total helicity in all
simulations. However, our study highlights that this threshold can be
reached in different ways, with different helicity-related processes
dominating for different photospheric flows. This means that the
details of the pre-eruptive dynamics do not influence the eruption-onset
helicity-related threshold. Nevertheless, the helicity-flux dynamics
may be more or less efficient in changing the time required to reach
the onset of the eruption.
Title: Magnetic Helicity Budget of Solar Active Regions Prolific of
Eruptive and Confined Flares
Authors: Thalmann, Julia K.; Moraitis, K.; Linan, L.; Pariat, E.;
Valori, G.; Dalmasse, K.
Bibcode: 2019ApJ...887...64T
Altcode: 2019arXiv191006563T
We compare the coronal magnetic energy and helicity of two solar active
regions (ARs), prolific in major eruptive (AR 11158) and confined
(AR 12192) flaring, and analyze the potential of deduced proxies
to forecast upcoming flares. Based on nonlinear force-free (NLFF)
coronal magnetic field models with a high degree of solenoidality,
and applying three different computational methods to investigate
the coronal magnetic helicity, we are able to draw conclusions
with a high level of confidence. Based on real observations of two
solar ARs we checked trends regarding the potential eruptivity of
the active-region corona, as suggested earlier in works that were
based on numerical simulations, or solar observations. Our results
support that the ratio of current-carrying to total helicity, |
{H}{{J}}| /| {H}{ \mathcal V }| , shows a strong
ability to indicate the eruptive potential of a solar AR. However, |
{H}{{J}}| /| {H}{ \mathcal V }| does not seem to
be indicative for the magnitude or type of an upcoming flare (confined
or eruptive). Interpreted in the context of earlier observational
studies, our findings furthermore support that the total relative
helicity normalized to the magnetic flux at the NLFF model’s lower
boundary, {H}{ \mathcal V }/{φ }2, represents
no indicator for the eruptivity.
Title: Magnetic helicity and eruptivity in active region 12673
Authors: Moraitis, K.; Sun, X.; Pariat, É.; Linan, L.
Bibcode: 2019A&A...628A..50M
Altcode: 2019arXiv190706365M
Context. In September 2017, the largest X-class flare of solar cycle 24
occurred from the most active region (AR) of this cycle, AR 12673. This
AR attracted much interest because of its unique morphological and
evolution characteristics. Among the parameters that were examined
in the AR was magnetic helicity, but either only approximately, or
intermittently, or both.
Aims: We here study the evolution
of the relative magnetic helicity and of the two components of its
decomposition, the non-potential, and the volume-threading one, in the
time interval around the highest activity of AR 12673. We especially
focus on the ratio of the non-potential to total helicity, which
has recently been proposed as an indicator of AR eruptivity.
Methods: We first approximated the coronal magnetic field of the AR
with two different optimization-based extrapolation procedures, and
chose the method that produced the most reliable helicity value at each
instant. Moreover, in one of these methods, we weighted the optimization
by the uncertainty estimates derived from the Helioseismic and Magnetic
Imager (HMI) instrument for the first time. We then followed an accurate
method to compute all quantities of interest.
Results: The first
observational determination of the evolution of the non-potential to
total helicity ratio seems to confirm the quality it has in indicating
eruptivity. This ratio increased before the major flares of AR 12673
and afterwards relaxed to lower values. Additionally, we discuss the
evolution patterns of the various helicity and energy budgets of AR
12673 and compare them with results from other works.
Title: On the Reliability of Magnetic Energy and Helicity Computations
Based on Nonlinear Force-free Coronal Magnetic Field Models
Authors: Thalmann, Julia K.; Linan, L.; Pariat, E.; Valori, G.
Bibcode: 2019ApJ...880L...6T
Altcode: 2019arXiv190701179T
We demonstrate the sensitivity of magnetic energy and helicity
computations regarding the quality of the underlying coronal magnetic
field model. We apply the method of Wiegelmann & Inhester to a
series of Solar Dynamics Observatory/Helioseismic and Magnetic Imager
vector magnetograms, and discuss nonlinear force-free (NLFF) solutions
based on two different sets of the free model parameters. The two
time series differ from each other concerning their force-free and
solenoidal quality. Both force- and divergence-freeness are required
for a consistent NLFF solution. Full satisfaction of the solenoidal
property is inherent in the definition of relative magnetic helicity
in order to ensure gauge independence. We apply two different magnetic
helicity computation methods to both NLFF time series and find that
the output is highly dependent on the level to which the NLFF magnetic
fields satisfy the divergence-free condition, with the computed magnetic
energy being less sensitive than the relative helicity. Proxies for
the nonpotentiality and eruptivity derived from both quantities are
also shown to depend strongly on the solenoidal property of the NLFF
fields. As a reference for future applications, we provide quantitative
thresholds for the force- and divergence-freeness, for the assurance
of reliable computation of magnetic energy and helicity, and of their
related eruptivity proxies.
Title: Relative magnetic field line helicity
Authors: Moraitis, K.; Pariat, E.; Valori, G.; Dalmasse, K.
Bibcode: 2019A&A...624A..51M
Altcode: 2019arXiv190210410M
Context. Magnetic helicity is an important quantity in studies
of magnetized plasmas as it provides a measure of the geometrical
complexity of the magnetic field in a given volume. A more detailed
description of the spatial distribution of magnetic helicity is given
by the field line helicity, which expresses the amount of helicity
associated to individual field lines rather than in the full analysed
volume.
Aims: Magnetic helicity is not a gauge-invariant quantity
in general, unless it is computed with respect to a reference field,
yielding the so-called relative magnetic helicity. The field line
helicity corresponding to the relative magnetic helicity has only been
examined under specific conditions so far. This work aims to define
the field line helicity corresponding to relative magnetic helicity
in the most general way. In addition to its general form, we provide
the expression for the relative magnetic field line helicity in a few
commonly used gauges, and reproduce known results as a limit of our
general formulation.
Methods: By starting from the definition
of relative magnetic helicity, we derived the corresponding field line
helicity, and we noted the assumptions on which it is based.
Results: We checked that the developed quantity reproduces relative
magnetic helicity by using three different numerical simulations. For
these cases we also show the morphology of field line helicity in
the volume, and on the photospheric plane. As an application to solar
situations, we compared the morphology of field line helicity on the
photosphere with that of the connectivity-based helicity flux density
in two reconstructions of an active region's magnetic field. We
discuss how the derived relative magnetic field line helicity has
a wide range of applications, notably in solar physics and magnetic
reconnection studies.
Title: Time Variations of the Nonpotential and Volume-threading
Magnetic Helicities
Authors: Linan, L.; Pariat, É.; Moraitis, K.; Valori, G.; Leake, J.
Bibcode: 2018ApJ...865...52L
Altcode: 2018arXiv180903765L
Relative magnetic helicity is a gauge-invariant quantity suitable
for the study of the magnetic helicity content of heliospheric
plasmas. Relative magnetic helicity can be decomposed uniquely
into two gauge-invariant quantities, the magnetic helicity
of the nonpotential component of the field and a complementary
volume-threading helicity. Recent analysis of numerical experiments
simulating the generation of solar eruptions have shown that the
ratio of the nonpotential helicity to the total relative helicity is
a clear marker of the eruptivity of the magnetic system, and that the
high value of that quantity could be a sufficient condition for the
onset of the instability generating the eruptions. The present study
introduces the first analytical examination of the time variations
of these nonpotential and volume-threading helicities. The validity
of the analytical formulae derived are confirmed with analysis of 3D
magnetohydrodynamics (MHD) simulations of solar coronal dynamics. Both
the analytical investigation and the numerical application show that,
unlike magnetic helicity, the nonpotential and the volume-threading
helicities are not conserved quantities, even in the ideal MHD regime. A
term corresponding to the transformation between the nonpotential and
volume-threading helicities frequently dominates their dynamics. This
finding has an important consequence for their estimation in the
solar corona: unlike with relative helicity, their volume coronal
evolution cannot be ascertained by the flux of these quantities
through the volume’s boundaries. Only techniques extrapolating the
3D coronal field will enable both the proper study of the nonpotential
and volume-threading helicities and the observational analysis of
helicity-based solar-eruptivity proxies.
Title: Threshold of Non-potential Magnetic Helicity Ratios at the
Onset of Solar Eruptions
Authors: Zuccarello, F. P.; Pariat, E.; Valori, G.; Linan, L.
Bibcode: 2018ApJ...863...41Z
Altcode: 2018arXiv180700532Z
The relative magnetic helicity is a quantity that is often used to
describe the level of entanglement of non-isolated magnetic fields,
such as the magnetic field of solar active regions. The aim of this
paper is to investigate how different kinds of photospheric boundary
flows accumulate relative magnetic helicity in the corona and if and
how well magnetic-helicity-related quantities identify the onset
of an eruption. We use a series of three-dimensional, parametric
magnetohydrodynamic simulations of the formation and eruption of
magnetic flux ropes. All the simulations are performed on the same
grid, using the same parameters, but they are characterized by different
driving photospheric flows, i.e., shearing, convergence, stretching, and
peripheral- and central- dispersion flows. For each of the simulations,
the instant of the onset of the eruption is carefully identified
by using a series of relaxation runs. We find that magnetic energy
and total relative helicity are mostly injected when shearing flows
are applied at the boundary, while the magnetic energy and helicity
associated with the coronal electric currents increase regardless of
the kind of photospheric flows. We also find that, at the onset of
the eruptions, the ratio between the non-potential magnetic helicity
and the total relative magnetic helicity has the same value for all
the simulations, suggesting the existence of a threshold in this
quantity. Such a threshold is not observed for other quantities as,
for example, those related to the magnetic energy.
Title: Computation of Relative Magnetic Helicity in Spherical
Coordinates
Authors: Moraitis, Kostas; Pariat, Étienne; Savcheva, Antonia;
Valori, Gherardo
Bibcode: 2018SoPh..293...92M
Altcode: 2018arXiv180603011M
Magnetic helicity is a quantity of great importance in solar studies
because it is conserved in ideal magnetohydrodynamics. While many
methods for computing magnetic helicity in Cartesian finite volumes
exist, in spherical coordinates, the natural coordinate system
for solar applications, helicity is only treated approximately. We
present here a method for properly computing the relative magnetic
helicity in spherical geometry. The volumes considered are finite,
of shell or wedge shape, and the three-dimensional magnetic field is
considered to be fully known throughout the studied domain. Testing of
the method with well-known, semi-analytic, force-free magnetic-field
models reveals that it has excellent accuracy. Further application to
a set of nonlinear force-free reconstructions of the magnetic field of
solar active regions and comparison with an approximate method used
in the past indicates that the proposed method can be significantly
more accurate, thus making our method a promising tool in helicity
studies that employ spherical geometry. Additionally, we determine
and discuss the applicability range of the approximate method.
Title: Studying the Transfer of Magnetic Helicity in Solar Active
Regions with the Connectivity-based Helicity Flux Density Method
Authors: Dalmasse, K.; Pariat, É.; Valori, G.; Jing, J.; Démoulin, P.
Bibcode: 2018ApJ...852..141D
Altcode: 2017arXiv171204691D
In the solar corona, magnetic helicity slowly and continuously
accumulates in response to plasma flows tangential to the photosphere
and magnetic flux emergence through it. Analyzing this transfer of
magnetic helicity is key for identifying its role in the dynamics of
active regions (ARs). The connectivity-based helicity flux density
method was recently developed for studying the 2D and 3D transfer
of magnetic helicity in ARs. The method takes into account the 3D
nature of magnetic helicity by explicitly using knowledge of the
magnetic field connectivity, which allows it to faithfully track the
photospheric flux of magnetic helicity. Because the magnetic field is
not measured in the solar corona, modeled 3D solutions obtained from
force-free magnetic field extrapolations must be used to derive the
magnetic connectivity. Different extrapolation methods can lead to
markedly different 3D magnetic field connectivities, thus questioning
the reliability of the connectivity-based approach in observational
applications. We address these concerns by applying this method to the
isolated and internally complex AR 11158 with different magnetic field
extrapolation models. We show that the connectivity-based calculations
are robust to different extrapolation methods, in particular with
regard to identifying regions of opposite magnetic helicity flux. We
conclude that the connectivity-based approach can be reliably used in
observational analyses and is a promising tool for studying the transfer
of magnetic helicity in ARs and relating it to their flaring activity.
Title: The Next Level in Automated Solar Flare Forecasting: the EU
FLARECAST Project
Authors: Georgoulis, M. K.; Bloomfield, D.; Piana, M.; Massone,
A. M.; Gallagher, P.; Vilmer, N.; Pariat, E.; Buchlin, E.; Baudin,
F.; Csillaghy, A.; Soldati, M.; Sathiapal, H.; Jackson, D.; Alingery,
P.; Argoudelis, V.; Benvenuto, F.; Campi, C.; Florios, K.; Gontikakis,
C.; Guennou, C.; Guerra, J. A.; Kontogiannis, I.; Latorre, V.; Murray,
S.; Park, S. H.; Perasso, A.; Sciacchitano, F.; von Stachelski, S.;
Torbica, A.; Vischi, D.
Bibcode: 2017AGUFMSA21C..07G
Altcode:
We attempt an informative description of the Flare Likelihood And
Region Eruption Forecasting (FLARECAST) project, European Commission's
first large-scale investment to explore the limits of reliability
and accuracy achieved for the forecasting of major solar flares. We
outline the consortium, top-level objectives and first results of
the project, highlighting the diversity and fusion of expertise
needed to deliver what was promised. The project's final product,
featuring an openly accessible, fully modular and free to download
flare forecasting facility will be delivered in early 2018. The
project's three objectives, namely, science, research-to-operations and
dissemination / communication, are also discussed: in terms of science,
we encapsulate our close-to-final assessment on how close (or far)
are we from a practically exploitable solar flare forecasting. In
terms of R2O, we briefly describe the architecture of the FLARECAST
infrastructure that includes rigorous validation for each forecasting
step. From the three different communication levers of the project we
finally focus on lessons learned from the two-way interaction with the
community of stakeholders and governmental organizations. The FLARECAST
project has received funding from the European Union's Horizon 2020
research and innovation programme under grant agreement No. 640216.
Title: Flux rope, hyperbolic flux tube, and late extreme ultraviolet
phases in a non-eruptive circular-ribbon flare
Authors: Masson, Sophie; Pariat, Étienne; Valori, Gherardo; Deng,
Na; Liu, Chang; Wang, Haimin; Reid, Hamish
Bibcode: 2017A&A...604A..76M
Altcode: 2017arXiv170401450M
Context. The dynamics of ultraviolet (UV) emissions during solar flares
provides constraints on the physical mechanisms involved in the trigger
and the evolution of flares. In particular it provides some information
on the location of the reconnection sites and the associated magnetic
fluxes. In this respect, confined flares are far less understood
than eruptive flares generating coronal mass ejections.
Aims:
We present a detailed study of a confined circular flare dynamics
associated with three UV late phases in order to understand more
precisely which topological elements are present and how they constrain
the dynamics of the flare.
Methods: We perform a non-linear
force-free field extrapolation of the confined flare observed with the
Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly
(AIA) instruments on board Solar Dynamics Observatory (SDO). From the
3D magnetic field we compute the squashing factor and we analyse its
distribution. Conjointly, we analyse the AIA extreme ultraviolet (EUV)
light curves and images in order to identify the post-flare loops,
and their temporal and thermal evolution. By combining the two analyses
we are able to propose a detailed scenario that explains the dynamics
of the flare.
Results: Our topological analysis shows that in
addition to a null-point topology with the fan separatrix, the spine
lines and its surrounding quasi-separatix layer (QSL) halo (typical
for a circular flare), a flux rope and its hyperbolic flux tube (HFT)
are enclosed below the null. By comparing the magnetic field topology
and the EUV post-flare loops we obtain an almost perfect match between
the footpoints of the separatrices and the EUV 1600 Å ribbons and
between the HFT field line footpoints and bright spots observed inside
the circular ribbons. We show, for the first time in a confined flare,
that magnetic reconnection occurred initially at the HFT below the flux
rope. Reconnection at the null point between the flux rope and the
overlying field is only initiated in a second phase. In addition, we
showed that the EUV late phase observed after the main flare episode
is caused by the cooling loops of different length which have all
reconnected at the null point during the impulsive phase.
Conclusions: Our analysis shows in one example that flux ropes are
present in null-point topology not only for eruptive and jet events,
but also for confined flares. This allows us to conjecture on the
analogies between conditions that govern the generation of jets,
confined flares or eruptive flares. A movie is available at http://www.aanda.org
Title: Studying the transfer of magnetic helicity in solar active
regions
Authors: Dalmasse, Kevin; Valori, Gherardo; Jing, Ju; Pariat, Etienne;
Demoulin, Pascal
Bibcode: 2017SPD....4811206D
Altcode:
Analyzing the transfer of magnetic helicity in active regions is a
key component for understanding the nature of its coronal storage
and release and for identifying its role in the coronal dynamics
of active regions. We recently developed a method for studying the
photospheric flux of magnetic helicity in both 2D and 3D. The method
takes into account the 3D nature of magnetic helicity by explicitly
using knowledge of the magnetic field connectivity. Since the coronal
magnetic field in active regions is not measured, we rely on the
non-unique 3D solution obtained from force-free coronal magnetic
field extrapolations to derive the magnetic field connectivity. In
this poster, we apply the method to the complex and highly-flaring
active region NOAA 11158 using the magnetic field connectivity derived
from different force-free extrapolation models and implementations. We
show that the calculations of photospheric flux of magnetic helicity
are robust to different extrapolation methods and assumptions, in
particular with regards to identifying regions of opposite magnetic
helicity flux. Finally, we discuss the implications of our results
for tracking the transfer of magnetic helicity in active regions and
relate it to their flaring activity.
Title: Testing predictors of eruptivity using parametric flux
emergence simulations
Authors: Guennou, Chloé; Pariat, Etienne; Leake, James E.; Vilmer,
Nicole
Bibcode: 2017JSWSC...7A..17G
Altcode: 2017arXiv170604915G
Solar flares and coronal mass ejections (CMEs) are among the most
energetic events in the solar system, impacting the near-Earth
environment. Flare productivity is empirically known to be correlated
with the size and complexity of active regions. Several indicators,
based on magnetic field data from active regions, have been tested
for flare forecasting in recent years. None of these indicators, or
combinations thereof, have yet demonstrated an unambiguous eruption
or flare criterion. Furthermore, numerical simulations have been
only barely used to test the predictability of these parameters. In
this context, we used the 3D parametric magnetohydrodynamic (MHD)
numerical simulations of the self-consistent formation of the flux
emergence of a twisted flux tube, inducing the formation of stable and
unstable magnetic flux ropes of Leake et al. (2013, 2014). We use these
numerical simulations to investigate the eruptive signatures observable
in various magnetic scalar parameters and provide highlights on data
analysis processing. Time series of 2D photospheric-like magnetograms
are used from parametric simulations of stable and unstable flux
emergence, to compute a list of about 100 different indicators. This
list includes parameters previously used for operational forecasting,
physical parameters used for the first time, as well as new quantities
specifically developed for this purpose. Our results indicate that only
parameters measuring the total non-potentiality of active regions
associated with magnetic inversion line properties, such as the
Falconer parameters Lss, WLss, Lsg,
and WLsg, as well as the new current integral WLsc
and length Lsc parameters, present a significant ability
to distinguish the eruptive cases of the model from the non-eruptive
cases, possibly indicating that they are promising flare and eruption
predictors. A preliminary study about the effect of noise on the
detection of the eruptive signatures is also proposed.
Title: Studying the transfer of magnetic helicity in solar active
regions
Authors: Dalmasse, Kévin; Jing, J.; Pariat, E.; Valori, G.;
Démoulin, P.
Bibcode: 2017shin.confE.160D
Altcode:
Analyzing the transfer of magnetic helicity in active regions is a
key component for understanding the nature of its coronal storage
and release and for identifying its role in the coronal dynamics
of active regions. We recently developed a method for studying the
photospheric flux of magnetic helicity in both 2D and 3D. The method
takes into account the 3D nature of magnetic helicity by explicitly
using knowledge of the magnetic field connectivity. Since the coronal
magnetic field in active regions is not measured, we rely on the
approximate 3D solution obtained from force-free coronal magnetic
field extrapolations to derive the magnetic field connectivity. In
this poster, we apply the method to the complex and highly-flaring
active region NOAA 11158 using the magnetic field connectivity derived
from different force-free extrapolation models and implementations. We
show that the calculations of photospheric flux of magnetic helicity
are robust to different extrapolation methods and assumptions, in
particular with regards to identifying regions of opposite magnetic
helicity flux. Finally, we discuss the implications of our results
for tracking the transfer of magnetic helicity in active regions and
relate it to their flaring activity.
Title: Analysis and modelling of recurrent solar flares observed
with Hinode/EIS on March 9, 2012
Authors: Polito, V.; Del Zanna, G.; Valori, G.; Pariat, E.; Mason,
H. E.; Dudík, J.; Janvier, M.
Bibcode: 2017A&A...601A..39P
Altcode: 2016arXiv161203504P
Three homologous C-class flares and one last M-class flare were observed
by both the Solar Dynamics Observatory (SDO) and the Hinode EUV Imaging
Spectrometer (EIS) in the AR 11429 on March 9, 2012. All the recurrent
flares occurred within a short interval of time (less than 4 h),
showed very similar plasma morphology and were all confined, until the
last one when a large-scale eruption occurred. The C-class flares are
characterized by the appearance, at approximatively the same locations,
of two bright and compact footpoint sources of ≈3-10 MK evaporating
plasma, and a semi-circular ribbon. During all the flares, the
continuous brightening of a spine-like hot plasma (≈10 MK) structure
is also observed. Spectroscopic observations with Hinode/EIS are used to
measure and compare the blueshift velocities in the Fe xxiii emission
line and the electron number density at the flare footpoints for each
flare. Similar velocities, of the order of 150-200 km s-1,
are observed during the C2.0 and C4.7 confined flares, in agreement
with the values reported by other authors in the study of the last M1.8
class flare. On the other hand, lower electron number densities and
temperatures tend to be observed in flares with lower peak soft X-ray
flux. In order to investigate the homologous nature of the flares, we
performed a non-linear force-free field (NLFFF) extrapolation of the 3D
magnetic field configuration in the corona. The NLFFF extrapolation and
the Quasi-Separatrix Layers (QSLs) provide the magnetic field context
which explains the location of the kernels, spine-like hot plasma and
semi-circular brightenings observed in the (non-eruptive) flares. Given
the absence of a coronal null point, we argue that the homologous
flares were all generated by the continuous recurrence of bald patch
reconnection. The movie associated to Fig. 2 is available at http://www.aanda.org
Title: Relative magnetic helicity as a diagnostic of solar eruptivity
Authors: Pariat, E.; Leake, J. E.; Valori, G.; Linton, M. G.;
Zuccarello, F. P.; Dalmasse, K.
Bibcode: 2017A&A...601A.125P
Altcode: 2017arXiv170310562P
Context. The discovery of clear criteria that can deterministically
describe the eruptive state of a solar active region would lead
to major improvements on space weather predictions.
Aims:
Using series of numerical simulations of the emergence of a magnetic
flux rope in a magnetized coronal, leading either to eruptions or to
stable configurations, we test several global scalar quantities for
the ability to discriminate between the eruptive and the non-eruptive
simulations.
Methods: From the magnetic field generated by the
three-dimensional magnetohydrodynamical simulations, we compute and
analyze the evolution of the magnetic flux, of the magnetic energy
and its decomposition into potential and free energies, and of the
relative magnetic helicity and its decomposition.
Results:
Unlike the magnetic flux and magnetic energies, magnetic helicities
are able to markedly distinguish the eruptive from the non-eruptive
simulations. We find that the ratio of the magnetic helicity of the
current-carrying magnetic field to the total relative helicity presents
the highest values for the eruptive simulations, in the pre-eruptive
phase only. We observe that the eruptive simulations do not possess the
highest value of total magnetic helicity.
Conclusions: In the
framework of our numerical study, the magnetic energies and the total
relative helicity do not correspond to good eruptivity proxies. Our
study highlights that the ratio of magnetic helicities diagnoses very
clearly the eruptive potential of our parametric simulations. Our study
shows that magnetic-helicity-based quantities may be very efficient
for the prediction of solar eruptions.
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. III. Twist Number Method
Authors: Guo, Y.; Pariat, E.; Valori, G.; Anfinogentov, S.; Chen,
F.; Georgoulis, M. K.; Liu, Y.; Moraitis, K.; Thalmann, J. K.; Yang, S.
Bibcode: 2017ApJ...840...40G
Altcode: 2017arXiv170402096G
We study the writhe, twist, and magnetic helicity of different
magnetic flux ropes, based on models of the solar coronal magnetic
field structure. These include an analytical force-free Titov-Démoulin
equilibrium solution, non-force-free magnetohydrodynamic simulations,
and nonlinear force-free magnetic field models. The geometrical
boundary of the magnetic flux rope is determined by the quasi-separatrix
layer and the bottom surface, and the axis curve of the flux rope is
determined by its overall orientation. The twist is computed by the
Berger-Prior formula, which is suitable for arbitrary geometry and
both force-free and non-force-free models. The magnetic helicity is
estimated by the twist multiplied by the square of the axial magnetic
flux. We compare the obtained values with those derived by a finite
volume helicity estimation method. We find that the magnetic helicity
obtained with the twist method agrees with the helicity carried by the
purely current-carrying part of the field within uncertainties for
most test cases. It is also found that the current-carrying part of
the model field is relatively significant at the very location of the
magnetic flux rope. This qualitatively explains the agreement between
the magnetic helicity computed by the twist method and the helicity
contributed purely by the current-carrying magnetic field.
Title: Magnetic helicity estimations in models and observations of
the solar magnetic field
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
Chen, Feng; Georgoulis, Manolis; Guo, Yang; Liu, Yang; Moraitis,
Kostas; Thalmann, Julia K.; Yang, Shangbin
Bibcode: 2017EGUGA..19.3692V
Altcode:
Magnetic helicity, as one of the few conserved quantities
in magneto-hydrodynamics, is often invoked as the principle
driving the generation and structuring of magnetic fields in a
variety of environments, from dynamo models in stars and planets,
to post-disruption reconfigurations of tokamak's plasmas. Most
particularly magnetic helicity has raised the interest of solar
physicists, since helicity is suspected to represent a key quantity for
the understanding of solar flares and the generation of coronal mass
ejections. In recent years, several methods of estimation of magnetic
helicity have been proposed and already applied to observations and
numerical simulations. However, no systematic comparison of accuracy,
mutual consistency, and reliability of such methods has ever been
performed. We present the results of the first benchmark of several
finite-volume methods in estimating magnetic helicity in 3D test
models. In addition to finite volume methods, two additional methods
are also included that estimate magnetic helicity based either on the
field line's twist, or on the field's values on one boundary and an
inferred minimal volume connectivity. The employed model tests range
from solutions of the force-free equations to 3D magneto-hydrodynamical
numerical simulations. Almost all methods are found to produce the same
value of magnetic helicity within few percent in all tests. However,
methods show differences in the sensitivity to numerical resolution and
to errors in the solenoidal property of input fields. Our benchmark of
finite volume methods allows to determine the reliability and precision
of estimations of magnetic helicity in practical cases. As a next step,
finite volume methods are used to test estimation methods that are
based on the flux of helicity through one boundary, in particular
for applications to observation-based models of coronal magnetic
fields. The ultimate goal is to assess if and how can helicity be
meaningfully used as a diagnostic of the evolution of magnetic fields
in the solar atmosphere.
Title: Blowout jets and impulsive eruptive flares in a bald-patch
topology
Authors: Chandra, R.; Mandrini, C. H.; Schmieder, B.; Joshi, B.;
Cristiani, G. D.; Cremades, H.; Pariat, E.; Nuevo, F. A.; Srivastava,
A. K.; Uddin, W.
Bibcode: 2017A&A...598A..41C
Altcode: 2016arXiv161001918C
Context. A subclass of broad extreme ultraviolet (EUV) and X-ray jets,
called blowout jets, have become a topic of research since they could
be the link between standard collimated jets and coronal mass ejections
(CMEs).
Aims: Our aim is to understand the origin of a series of
broad jets, some of which are accompanied by flares and associated with
narrow and jet-like CMEs.
Methods: We analyze observations of
a series of recurrent broad jets observed in AR 10484 on 21-24 October
2003. In particular, one of them occurred simultaneously with an M2.4
flare on 23 October at 02:41 UT (SOLA2003-10-23). Both events were
observed by the ARIES Hα Solar Tower-Telescope, TRACE, SOHO, and RHESSI
instruments. The flare was very impulsive and followed by a narrow
CME. A local force-free model of AR 10484 is the basis to compute its
topology. We find bald patches (BPs) at the flare site. This BP topology
is present for at least two days before to events. Large-scale field
lines, associated with the BPs, represent open loops. This is confirmed
by a global potential free source surface (PFSS) model. Following
the brightest leading edge of the Hα and EUV jet emission, we can
temporarily associate these emissions with a narrow CME.
Results:
Considering their characteristics, the observed broad jets appear to
be of the blowout class. As the most plausible scenario, we propose
that magnetic reconnection could occur at the BP separatrices forced
by the destabilization of a continuously reformed flux rope underlying
them. The reconnection process could bring the cool flux-rope material
into the reconnected open field lines driving the series of recurrent
blowout jets and accompanying CMEs.
Conclusions: Based on
a model of the coronal field, we compute the AR 10484 topology at
the location where flaring and blowout jets occurred from 21 to 24
October 2003. This topology can consistently explain the origin of
these events. The movie associated to Fig. 1 is available at http://www.aanda.org
Title: Observational Evidence of Magnetic Reconnection for
Brightenings and Transition Region Arcades in IRIS Observations
Authors: Zhao, Jie; Schmieder, Brigitte; Li, Hui; Pariat, Etienne;
Zhu, Xiaoshuai; Feng, Li; Grubecka, Michalina
Bibcode: 2017ApJ...836...52Z
Altcode: 2017arXiv170108356Z
By using a new method of forced-field extrapolation, we study the
emerging flux region AR11850 observed by the Interface Region Imaging
Spectrograph and Solar Dynamical Observatory. Our results suggest
that the bright points (BPs) in this emerging region exhibit responses
in lines formed from the upper photosphere to the transition region,
which have relatively similar morphologies. They have an oscillation
of several minutes according to the Atmospheric Imaging Assembly
data at 1600 and 1700 Å. The ratio between the BP intensities
measured in 1600 and 1700 Å filtergrams reveals that these BPs are
heated differently. Our analysis of the Helioseismic and Magnetic
Imager vector magnetic field and the corresponding topology in AR11850
indicates that the BPs are located at the polarity inversion line and
most of them are related to magnetic reconnection or cancelation. The
heating of the BPs might be different due to different magnetic
topology. We find that the heating due to the magnetic cancelation
would be stronger than the case of bald patch reconnection. The
plasma density rather than the magnetic field strength could play a
dominant role in this process. Based on physical conditions in the
lower atmosphere, our forced-field extrapolation shows consistent
results between the bright arcades visible in slit-jaw image 1400 Å
and the extrapolated field lines that pass through the bald patches. It
provides reliable observational evidence for testing the mechanism
of magnetic reconnection for the BPs and arcades in the emerging flux
region, as proposed in simulation studies.
Title: Reconnection-Driven Coronal-Hole Jets with Gravity and
Solar Wind
Authors: Karpen, J. T.; DeVore, C. R.; Antiochos, S. K.; Pariat, E.
Bibcode: 2017ApJ...834...62K
Altcode: 2016arXiv160609201K
Coronal-hole jets occur ubiquitously in the Sun's coronal holes, at
EUV and X-ray bright points associated with intrusions of minority
magnetic polarity. The embedded-bipole model for these jets posits
that they are driven by explosive, fast reconnection between the
stressed closed field of the embedded bipole and the open field of
the surrounding coronal hole. Previous numerical studies in Cartesian
geometry, assuming uniform ambient magnetic field and plasma while
neglecting gravity and solar wind, demonstrated that the model is
robust and can produce jet-like events in simple configurations. We
have extended these investigations by including spherical geometry,
gravity, and solar wind in a nonuniform, coronal hole-like ambient
atmosphere. Our simulations confirm that the jet is initiated by the
onset of a kink-like instability of the internal closed field, which
induces a burst of reconnection between the closed and external open
field, launching a helical jet. Our new results demonstrate that the
jet propagation is sustained through the outer corona, in the form
of a traveling nonlinear Alfvén wave front trailed by slower-moving
plasma density enhancements that are compressed and accelerated by
the wave. This finding agrees well with observations of white-light
coronal-hole jets, and can explain microstreams and torsional Alfvén
waves detected in situ in the solar wind. We also use our numerical
results to deduce scaling relationships between properties of the
coronal source region and the characteristics of the resulting jet,
which can be tested against observations.
Title: Investigating The Reliability Of Solar Photospheric Eruptivity
Proxies.
Authors: Guennou, C.; Pariat, E.; Vilmer, N.
Bibcode: 2016AGUFMSH11C2236G
Altcode:
Solar flares and coronal mass ejections (CMEs) are among the most
energetic events in the solar system, impacting the near-Earth
environment and thus our technologies. The European H2020 research
project FLARECAST (Flare Likelihood and Region Eruption Forecasting)
aims to develop a fully automated solar flare forecasting system with
unmatched accuracy compared to existing facilities. FLARECAST will
automatically extract magnetic-field parameters of solar active regions
from solar magnetogram and white-light images to produce accurate
predictions using the state-of-the-art forecasting techniques based on
data-mining and machine learning. Flare productivity is empirically
known to be correlated with the size and complexity of active
regions. Several parameters, based on magnetic-field data from active
regions have been tested in recent years. None of these parameters, or
combination of thereof, have yet demonstrated an unambiguous eruption
criterion. However, the predictability of these parameters has so far
only been tested on observational data and never on controlled-cases,
e.g., originating from numerical datasets. In the framework of
the FLARECAST explorative research component, we use MHD numerical
simulations of the formation of stable and unstable magnetic flux
ropes (Leake et al. 2013, 2014) to evaluate the predictive potential
of different magnetic parameters. Time series of magnetograms are used
from parametric simulations of stable and unstable flux emergence, to
compute a list of about 111 different parameters. This list includes
parameters previously used for forecasting, as well as parameters used
for the first time for this purpose. Our results indicate that only
parameters measuring the total non-potentiality of active regions,
such as Lssm and Lsgm and WLsg and
the total length of the inversion line present significant preflare
signatures, probably making them successful flare predictors.
Title: Space-weather assets developed by the French space-physics
community
Authors: Rouillard, A. P.; Pinto, R. F.; Brun, A. S.; Briand, C.;
Bourdarie, S.; Dudok De Wit, T.; Amari, T.; Blelly, P. -L.; Buchlin,
E.; Chambodut, A.; Claret, A.; Corbard, T.; Génot, V.; Guennou, C.;
Klein, K. L.; Koechlin, L.; Lavarra, M.; Lavraud, B.; Leblanc, F.;
Lemorton, J.; Lilensten, J.; Lopez-Ariste, A.; Marchaudon, A.; Masson,
S.; Pariat, E.; Reville, V.; Turc, L.; Vilmer, N.; Zucarello, F. P.
Bibcode: 2016sf2a.conf..297R
Altcode:
We present a short review of space-weather tools and services developed
and maintained by the French space-physics community. They include
unique data from ground-based observatories, advanced numerical
models, automated identification and tracking tools, a range of space
instrumentation and interconnected virtual observatories. The aim of
the article is to highlight some advances achieved in this field of
research at the national level over the last decade and how certain
assets could be combined to produce better space-weather tools
exploitable by space-weather centres and customers worldwide. This
review illustrates the wide range of expertise developed nationally
but is not a systematic review of all assets developed in France.
Title: Solar Coronal Jets: Observations, Theory, and Modeling
Authors: Raouafi, N. E.; Patsourakos, S.; Pariat, E.; Young, P. R.;
Sterling, A. C.; Savcheva, A.; Shimojo, M.; Moreno-Insertis, F.;
DeVore, C. R.; Archontis, V.; Török, T.; Mason, H.; Curdt, W.;
Meyer, K.; Dalmasse, K.; Matsui, Y.
Bibcode: 2016SSRv..201....1R
Altcode: 2016arXiv160702108R; 2016SSRv..tmp...31R
Coronal jets represent important manifestations of ubiquitous solar
transients, which may be the source of significant mass and energy
input to the upper solar atmosphere and the solar wind. While
the energy involved in a jet-like event is smaller than that of
"nominal" solar flares and coronal mass ejections (CMEs), jets
share many common properties with these phenomena, in particular,
the explosive magnetically driven dynamics. Studies of jets could,
therefore, provide critical insight for understanding the larger,
more complex drivers of the solar activity. On the other side of the
size-spectrum, the study of jets could also supply important clues on
the physics of transients close or at the limit of the current spatial
resolution such as spicules. Furthermore, jet phenomena may hint to
basic process for heating the corona and accelerating the solar wind;
consequently their study gives us the opportunity to attack a broad
range of solar-heliospheric problems.
Title: Magnetic Helicity Estimations in Models and Observations of
the Solar Magnetic Field. Part I: Finite Volume Methods
Authors: Valori, Gherardo; Pariat, Etienne; Anfinogentov, Sergey;
Chen, Feng; Georgoulis, Manolis K.; Guo, Yang; Liu, Yang; Moraitis,
Kostas; Thalmann, Julia K.; Yang, Shangbin
Bibcode: 2016SSRv..201..147V
Altcode: 2016SSRv..tmp...68V; 2016arXiv161002193V
Magnetic helicity is a conserved quantity of ideal magneto-hydrodynamics
characterized by an inverse turbulent cascade. Accordingly, it
is often invoked as one of the basic physical quantities driving
the generation and structuring of magnetic fields in a variety of
astrophysical and laboratory plasmas. We provide here the first
systematic comparison of six existing methods for the estimation of
the helicity of magnetic fields known in a finite volume. All such
methods are reviewed, benchmarked, and compared with each other,
and specifically tested for accuracy and sensitivity to errors. To
that purpose, we consider four groups of numerical tests, ranging
from solutions of the three-dimensional, force-free equilibrium, to
magneto-hydrodynamical numerical simulations. Almost all methods are
found to produce the same value of magnetic helicity within few percent
in all tests. In the more solar-relevant and realistic of the tests
employed here, the simulation of an eruptive flux rope, the spread
in the computed values obtained by all but one method is only 3 %,
indicating the reliability and mutual consistency of such methods in
appropriate parameter ranges. However, methods show differences in the
sensitivity to numerical resolution and to errors in the solenoidal
property of the input fields. In addition to finite volume methods,
we also briefly discuss a method that estimates helicity from the
field lines' twist, and one that exploits the field's value at one
boundary and a coronal minimal connectivity instead of a pre-defined
three-dimensional magnetic-field solution.
Title: A model for straight and helical solar jets. II. Parametric
study of the plasma beta
Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.;
Karpen, J. T.
Bibcode: 2016A&A...596A..36P
Altcode: 2016arXiv160908825P
Context. Jets are dynamic, impulsive, well-collimated plasma events
that develop at many different scales and in different layers of
the solar atmosphere.
Aims: Jets are believed to be induced
by magnetic reconnection, a process central to many astrophysical
phenomena. Within the solar atmosphere, jet-like events develop in many
different environments, e.g., in the vicinity of active regions, as well
as in coronal holes, and at various scales, from small photospheric
spicules to large coronal jets. In all these events, signatures of
helical structure and/or twisting/rotating motions are regularly
observed. We aim to establish that a single model can generally
reproduce the observed properties of these jet-like events.
Methods: Using our state-of-the-art numerical solver ARMS, we present
a parametric study of a numerical tridimensional magnetohydrodynamic
(MHD) model of solar jet-like events. Within the MHD paradigm, we study
the impact of varying the atmospheric plasma β on the generation and
properties of solar-like jets.
Results: The parametric study
validates our model of jets for plasma β ranging from 10-3
to 1, typical of the different layers and magnetic environments of
the solar atmosphere. Our model of jets can robustly explain the
generation of helical solar jet-like events at various β ≤ 1. We
introduces the new result that the plasma β modifies the morphology of
the helical jet, explaining the different observed shapes of jets at
different scales and in different layers of the solar atmosphere.
Conclusions: Our results enable us to understand the energisation,
triggering, and driving processes of jet-like events. Our model enables
us to make predictions of the impulsiveness and energetics of jets as
determined by the surrounding environment, as well as the morphological
properties of the resulting jets.
Title: Enabling Solar Flare Forecasting at an Unprecedented Level:
the FLARECAST Project
Authors: Georgoulis, Manolis K.; Pariat, Etienne; Massone, Anna
Maria; Vilmer, Nicole; Jackson, David; Buchlin, Eric; Csillaghy,
Andre; Bommier, Veronique; Kontogiannis, Ioannis; Gallagher, Peter;
Gontikakis, Costis; Guennou, Chloé; Murray, Sophie; Bloomfield,
D. Shaun; Alingery, Pablo; Baudin, Frederic; Benvenuto, Federico;
Bruggisser, Florian; Florios, Konstantinos; Guerra, Jordan; Park,
Sung-Hong; Perasso, Annalisa; Piana, Michele; Sathiapal, Hanna;
Soldati, Marco; Von Stachelski, Samuel; Argoudelis, Vangelis;
Caminade, Stephane
Bibcode: 2016cosp...41E.657G
Altcode:
We attempt a brief but informative description of the Flare
Likelihood And Region Eruption Forecasting (FLARECAST) project,
European Commission's first large-scale investment to explore the
limits of reliability and accuracy for the forecasting of major solar
flares. The consortium, objectives, and first results of the project
- featuring an openly accessible, interactive flare forecasting
facility by the end of 2017 - will be outlined. In addition, we will
refer to the so-called "explorative research" element of project,
aiming to connect solar flares with coronal mass ejections (CMEs)
and possibly pave the way for CME, or eruptive flare, prediction. We
will also emphasize the FLARECAST modus operandi, namely the diversity
of expertise within the consortium that independently aims to science,
infrastructure development and dissemination, both to stakeholders and
to the general public. Concluding, we will underline that the FLARECAST
project responds squarely to the joint COSPAR - ILWS Global Roadmap
to shield society from the adversities of space weather, addressing
its primary goal and, in particular, its Research Recommendations
1, 2 and 4, Teaming Recommendations II and III, and Collaboration
Recommendations A, B, and D. The FLARECAST project has received funding
from the European Union's Horizon 2020 research and innovation programme
under grant agreement No. 640216.
Title: Evolution of flare ribbons, electric currents, and
quasi-separatrix layers during an X-class flare
Authors: Janvier, M.; Savcheva, A.; Pariat, E.; Tassev, S.;
Millholland, S.; Bommier, V.; McCauley, P.; McKillop, S.; Dougan, F.
Bibcode: 2016A&A...591A.141J
Altcode: 2016arXiv160407241J
Context. The standard model for eruptive flares has been extended
to three dimensions (3D) in the past few years. This model predicts
typical J-shaped photospheric footprints of the coronal current
layer, forming at similar locations as the quasi-separatrix layers
(QSLs). Such a morphology is also found for flare ribbons observed in
the extreme ultraviolet (EUV) band, and in nonlinear force-free field
(NLFFF) magnetic field extrapolations and models.
Aims: We
study the evolution of the photospheric traces of the current density
and flare ribbons, both obtained with the Solar Dynamics Observatory
instruments. We aim to compare their morphology and their time
evolution, before and during the flare, with the topological features
found in a NLFFF model.
Methods: We investigated the photospheric
current evolution during the 06 September 2011 X-class flare
(SOL2011-09-06T22:20) occurring in NOAA AR 11283 from observational data
of the magnetic field obtained with the Helioseismic and Magnetic Imager
aboard the Solar Dynamics Observatory. We compared this evolution with
that of the flare ribbons observed in the EUV filters of the Atmospheric
Imager Assembly. We also compared the observed electric current density
and the flare ribbon morphology with that of the QSLs computed from
the flux rope insertion method-NLFFF model.
Results: The NLFFF
model shows the presence of a fan-spine configuration of overlying
field lines, due to the presence of a parasitic polarity, embedding
an elongated flux rope that appears in the observations as two parts
of a filament. The QSL signatures of the fan configuration appear as
a circular flare ribbon that encircles the J-shaped ribbons related
to the filament ejection. The QSLs, evolved via a magnetofrictional
method, also show similar morphology and evolution as both the current
ribbons and the EUV flare ribbons obtained several times during the
flare.
Conclusions: For the first time, we propose a combined
analysis of the photospheric traces of an eruptive flare, in a complex
topology, with direct measurements of electric currents and QSLs
from observational data and a magnetic field model. The results,
obtained by two different and independent approaches 1) confirm
previous results of current increase during the impulsive phase of the
flare and 2) show how NLFFF models can capture the essential physical
signatures of flares even in a complex magnetic field topology. A movie associated to Fig. 1 is available in electronic form at http://www.aanda.org
Title: Erratum: "Hooked Flare Ribbons and Flux-rope Related QSL
Footprints"(2016, ApJ, 823, 62)
Authors: Zhao, Jie; Gilchrist, Stuart A.; Aulanier, Guillaume;
Schmieder, Brigitte; Pariat, Etienne; Li, Hui
Bibcode: 2016ApJ...825...80Z
Altcode:
No abstract at ADS
Title: Evolution of the Topology, Electric Currents, and Ribbons
during an X-class Flare
Authors: Savcheva, Antonia; Janvier, M.; Pariat, E.; Tassev, S.
Bibcode: 2016shin.confE.126S
Altcode:
The standard model for eruptive flares has in the past few years been
extended to 3D. It predicts typical J-shaped photospheric footprints
of the coronal current layer, forming at similar locations as the
Quasi-Separatrix Layers (QSLs). Such a morphology is also found for
flare ribbons observed in EUV, as well as in non-linear force-free
field (NLFFF) magnetic field extrapolations and models. We study the
evolution of the photospheric traces of the current density and the
flare ribbons, both obtained with SDO instruments. We aim at comparing
their morphology and their time evolution, before and during the flare,
with the topological features found in a NLFFF and an unstable magnetic
field model. For this purpose we investigate the photospheric current
evolution during the 06 September 2011 X-class flare occurring in NOAA
AR11283 from observational data of the magnetic field obtained with
HMI. This evolution is compared with that of the flare ribbons observed
with AIA. We also compare the observed electric current density and the
flare ribbon morphology with that of the QSLs computed from magnetic
field models obtained from the the flux rope insertion method. Both
the NLFFF and the unstable (eruptive) model show the presence of a
fan-spine configuration of overlying field lines, due to the presence
of a parasitic polarity, embedding an elongated flux rope that appears
in the observations as two parts of a filament. The magnetofrictional
evolution of the unstable model tell a consistent story of the filament
eruption in which topology plays an important role. The photospheric
QSL traces of the fan configuration appear as an elongated flare
ribbon that encircles the J-shaped ribbons related to the filament
ejection. The QSLs, evolved via a magnetofrictional method, also show
similar morphology and evolution as both the current ribbons and the
EUV flare ribbons obtained at several times during the flare. For the
first time, we propose a combined analysis of the photospheric traces
of an eruptive flare, in a complex topology, with direct measurements
of electric currents and QSLs from observational data and a magnetic
field model. The results, obtained by two different and independent
approaches, 1) confirm previous results of current increase during
the impulsive phase of the flare, 2) show how NLFFF extrapolations can
capture the essential physical signatures of flares even in a complex
magnetic field topology.
Title: Hooked Flare Ribbons and Flux-rope-related QSL Footprints
Authors: Zhao, Jie; Gilchrist, Stuart A.; Aulanier, Guillaume;
Schmieder, Brigitte; Pariat, Etienne; Li, Hui
Bibcode: 2016ApJ...823...62Z
Altcode: 2016arXiv160307563Z
We studied the magnetic topology of active region 12158 on 2014
September 10 and compared it with the observations before and early in
the flare that begins at 17:21 UT (SOL2014-09-10T17:45:00). Our results
show that the sigmoidal structure and flare ribbons of this active
region observed by the Solar Dynamics Observatory/Atmospheric Imaging
Assembly can be well reproduced from a Grad-Rubin nonlinear force-free
field extrapolation method. Various inverse-S- and inverse-J-shaped
magnetic field lines, which surround a coronal flux rope, coincide with
the sigmoid as observed in different extreme-ultraviolet wavelengths,
including its multithreaded curved ends. Also, the observed distribution
of surface currents in the magnetic polarity where it was not prescribed
is well reproduced. This validates our numerical implementation and
setup of the Grad-Rubin method. The modeled double inverse-J-shaped
quasi-separatrix layer (QSL) footprints match the observed flare
ribbons during the rising phase of the flare, including their hooked
parts. The spiral-like shape of the latter may be related to a complex
pre-eruptive flux rope with more than one turn of twist, as obtained
in the model. These ribbon-associated flux-rope QSL footprints are
consistent with the new standard flare model in 3D, with the presence
of a hyperbolic flux tube located below an inverse-teardrop-shaped
coronal QSL. This is a new step forward forecasting the locations of
reconnection and ribbons in solar flares and the geometrical properties
of eruptive flux ropes.
Title: Evolution of the Topology, Electric Currents, and Ribbons
during an X-class Flare
Authors: Savcheva, Antonia; Janvier, Miho; Pariat, Etienne
Bibcode: 2016SPD....4740101S
Altcode:
The standard model for eruptive flares has in the past few years
been extended to 3D. It predicts typical J-shaped photospheric
footprints of the coronal current layer, forming at similar locations
as the Quasi-Separatrix Layers (QSLs). We study the evolution of
the photospheric traces of the current density and the flare ribbons
observed with SDO. We aim at comparing their morphology and their time
evolution, before and during the flare, with the topological features
found in a magnetic field model. For this purpose we investigate the
photospheric current evolution during the 6 Sep 2011 X-class flare
occurring in AR11283 from observational data of the magnetic field
obtained with HMI. This evolution is compared with that of the flare
ribbons observed with AIA. We also compare the observed electric current
density and the flare ribbon morphology with that of the QSLs computed
from magnetic field models obtained from the the flux rope insertion
method. Both the NLFFF and the unstable (eruptive) model show the
presence of a fan-spine configuration of overlying field lines, due
to the presence of a parasitic polarity, embedding in elongated flux
rope that appears in the observations as two parts of a filament. The
magnetofrictional evolution of the unstable model tells a consistent
story of the filament eruption in which topology plays an important
role. The photospheric QSL traces of the fan configuration appear as
an elongated flare ribbon that encircles the J-shaped ribbons related
to the filament ejection. The QSLs, evolved via a magnetofrictional
method, also show similar morphology and evolution as both the current
ribbons and the EUV flare ribbons obtained at several times during
the flare. For the first time, we propose a combined analysis of the
photospheric traces of an eruptive flare, in a complex topology, with
direct measurements of electric currents and QSLs from observational
data and a magnetic field model. The results obtained by two independent
approaches confirm previous results and show how NLFFF models can
capture the essential physical signatures of flares even in a complex
magnetic field topology.
Title: The Relation between Solar Eruption Topologies and Observed
Flare Features. II. Dynamical Evolution
Authors: Savcheva, A.; Pariat, E.; McKillop, S.; McCauley, P.; Hanson,
E.; Su, Y.; DeLuca, E. E.
Bibcode: 2016ApJ...817...43S
Altcode:
A long-established goal of solar physics is to build understanding
of solar eruptions and develop flare and coronal mass ejection (CME)
forecasting models. In this paper, we continue our investigation of
nonlinear forces free field (NLFFF) models by comparing topological
properties of the solutions to the evolution of the flare ribbons. In
particular, we show that data-constrained NLFFF models of three erupting
sigmoid regions (SOL2010-04-08, SOL2010-08-07, and SOL2012-05-12) built
to reproduce the active region magnetic field in the pre-flare state can
be rendered unstable and the subsequent sequence of unstable solutions
produces quasi-separatrix layers that match the flare ribbon evolution
as observed by SDO/AIA. We begin with a best-fit equilibrium model for
the pre-flare active region. We then add axial flux to the flux rope
in the model to move it across the stability boundary. At this point,
the magnetofrictional code no longer converges to an equilibrium
solution. The flux rope rises as the solutions are iterated. We
interpret the sequence of magnetofrictional steps as an evolution of
the active region as the flare/CME begins. The magnetic field solutions
at different steps are compared with the flare ribbons. The results are
fully consistent with the three-dimensional extension of the standard
flare/CME model. Our ability to capture essential topological features
of flaring active regions with a non-dynamic magnetofrictional code
strongly suggests that the pre-flare, large-scale topological structures
are preserved as the flux rope becomes unstable and lifts off.
Title: A Circular-ribbon Solar Flare Following an Asymmetric Filament
Eruption
Authors: Liu, Chang; Deng, Na; Liu, Rui; Lee, Jeongwoo; Pariat,
Étienne; Wiegelmann, Thomas; Liu, Yang; Kleint, Lucia; Wang, Haimin
Bibcode: 2015ApJ...812L..19L
Altcode: 2015arXiv150908414L
The dynamic properties of flare ribbons and the often associated
filament eruptions can provide crucial information on the flaring
coronal magnetic field. This Letter analyzes the GOES-class X1.0 flare
on 2014 March 29 (SOL2014-03-29T17:48), in which we found an asymmetric
eruption of a sigmoidal filament and an ensuing circular flare
ribbon. Initially both EUV images and a preflare nonlinear force-free
field model show that the filament is embedded in magnetic fields with
a fan-spine-like structure. In the first phase, which is defined by a
weak but still increasing X-ray emission, the western portion of the
sigmoidal filament arches upward and then remains quasi-static for
about five minutes. The western fan-like and the outer spine-like
fields display an ascending motion, and several associated ribbons
begin to brighten. Also found is a bright EUV flow that streams down
along the eastern fan-like field. In the second phase that includes the
main peak of hard X-ray (HXR) emission, the filament erupts, leaving
behind two major HXR sources formed around its central dip portion
and a circular ribbon brightened sequentially. The expanding western
fan-like field interacts intensively with the outer spine-like field,
as clearly seen in running difference EUV images. We discuss these
observations in favor of a scenario where the asymmetric eruption of
the sigmoidal filament is initiated due to an MHD instability and
further facilitated by reconnection at a quasi-null in corona; the
latter is in turn enhanced by the filament eruption and subsequently
produces the circular flare ribbon.
Title: The Relation between Solar Eruption Topologies and Observed
Flare Features. I. Flare Ribbons
Authors: Savcheva, A.; Pariat, E.; McKillop, S.; McCauley, P.; Hanson,
E.; Su, Y.; Werner, E.; DeLuca, E. E.
Bibcode: 2015ApJ...810...96S
Altcode: 2015arXiv150603452S
In this paper we present a topological magnetic field investigation
of seven two-ribbon flares in sigmoidal active regions observed with
Hinode, STEREO, and Solar Dynamics Observatory. We first derive the
3D coronal magnetic field structure of all regions using marginally
unstable 3D coronal magnetic field models created with the flux rope
insertion method. The unstable models have been shown to be a good
model of the flaring magnetic field configurations. Regions are selected
based on their pre-flare configurations along with the appearance and
observational coverage of flare ribbons, and the model is constrained
using pre-flare features observed in extreme ultraviolet and X-ray
passbands. We perform a topology analysis of the models by computing the
squashing factor, Q, in order to determine the locations of prominent
quasi-separatrix layers (QSLs). QSLs from these maps are compared to
flare ribbons at their full extents. We show that in all cases the
straight segments of the two J-shaped ribbons are matched very well by
the flux-rope-related QSLs, and the matches to the hooked segments are
less consistent but still good for most cases. In addition, we show that
these QSLs overlay ridges in the electric current density maps. This
study is the largest sample of regions with QSLs derived from 3D coronal
magnetic field models, and it shows that the magnetofrictional modeling
technique that we employ gives a very good representation of flaring
regions, with the power to predict flare ribbon locations in the event
of a flare following the time of the model.
Title: The Origin of Net Electric Currents in Solar Active Regions
Authors: Dalmasse, K.; Aulanier, G.; Démoulin, P.; Kliem, B.; Török,
T.; Pariat, E.
Bibcode: 2015ApJ...810...17D
Altcode: 2015arXiv150705060D
There is a recurring question in solar physics regarding whether or not
electric currents are neutralized in active regions (ARs). This question
was recently revisited using three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulations of magnetic flux emergence into the solar
atmosphere. Such simulations showed that flux emergence can generate
a substantial net current in ARs. Other sources of AR currents are
photospheric horizontal flows. Our aim is to determine the conditions
for the occurrence of net versus neutralized currents with this second
mechanism. Using 3D MHD simulations, we systematically impose line-tied,
quasi-static, photospheric twisting and shearing motions to a bipolar
potential magnetic field. We find that such flows: (1) produce
both direct and return currents, (2) induce very weak compression
currents—not observed in 2.5D—in the ambient field present in the
close vicinity of the current-carrying field, and (3) can generate
force-free magnetic fields with a net current. We demonstrate that
neutralized currents are in general produced only in the absence of
magnetic shear at the photospheric polarity inversion line—a special
condition that is rarely observed. We conclude that photospheric flows,
as magnetic flux emergence, can build up net currents in the solar
atmosphere, in agreement with recent observations. These results thus
provide support for eruption models based on pre-eruption magnetic
fields that possess a net coronal current.
Title: Testing magnetic helicity conservation in a solar-like
active event
Authors: Pariat, E.; Valori, G.; Démoulin, P.; Dalmasse, K.
Bibcode: 2015A&A...580A.128P
Altcode: 2015arXiv150609013P
Context. Magnetic helicity has the remarkable property of being a
conserved quantity of ideal magnetohydrodynamics (MHD). Therefore, it
could be used as an effective tracer of the magnetic field evolution
of magnetized plasmas.
Aims: Theoretical estimations indicate
that magnetic helicity is also essentially conserved with non-ideal MHD
processes, for example, magnetic reconnection. This conjecture has been
barely tested, however, either experimentally or numerically. Thanks
to recent advances in magnetic helicity estimation methods, it is
now possible to numerically test its dissipation level in general
three-dimensional datasets.
Methods: We first revisit the
general formulation of the temporal variation of relative magnetic
helicity on a fully bounded volume when no hypothesis on the gauge is
made. We introduce a method for precisely estimating its dissipation
independently of which type of non-ideal MHD processes occurs. For
a solar-like eruptive-event simulation, using different gauges, we
compare an estimate of the relative magnetic helicity computed in a
finite volume with its time-integrated flux through the boundaries. We
thus test the conservation and dissipation of helicity.
Results:
We provide an upper bound of the real dissipation of magnetic helicity:
It is quasi-null during the quasi-ideal MHD phase. Even with magnetic
reconnection, the relative dissipation of magnetic helicity is also very
low (<2.2%), in particular compared to the relative dissipation
of magnetic energy (>30 times higher). We finally illustrate
how the helicity-flux terms involving velocity components are gauge
dependent, which limits their physical meaning.
Conclusions:
Our study paves the way for more extended and diverse tests of the
magnetic helicity conservation properties. Our study confirms the
central role of helicity in the study of MHD plasmas. For instance,
the conservation of helicity can be used to track the evolution of
solar magnetic fields from when they form in the solar interior
until their detection as magnetic clouds in the interplanetary
space. Appendix A is available in electronic form at http://www.aanda.org
Title: The Relation between CME Topologies and Observed Flare Features
Authors: Savcheva, Antonia Stefanova; Pariat, E.; MaKillop, S.;
McCauley, P.; Hanson, E.; Werner, E.; Su, Y.; DeLuca, E.
Bibcode: 2015shin.confE...6S
Altcode:
A long established goal of solar physics is to build physics-based
flare and CME forecasting models. This study, building on the recent
successes in non-linear forces free field (NLFFF) modeling and detailed
numerical simulations, brings us closer to that goal. We show that
data-constrained NLFFF models built to reproduce the active region
magnetic field in the pre-flare state can be rendered unstable and
the sequence of unstable solutions produce quasi-separatrix layers
(QSLs) that reproduce the observed flare ribbons. The results are fully
consistant with the 3D extension of the standard flare/CME model. Our
ability to capture essential topological features of flaring active
regions with non-dynamic magneto-frictional code strongly suggests
that the pre-flare, large scale topological structures are preserved
as the flux rope becomes unstable and lifts off.
Title: Electric current neutralization in solar active regions
Authors: Dalmasse, Kévin; Aulanier, Guillaume; Török, Tibor;
Démoulin, Pascal; Pariat, Etienne; Kliem, Bernhard
Bibcode: 2015TESS....111303D
Altcode:
There is a recurring question in solar physics of whether or not
photospheric vertical electric currents are neutralized in solar active
regions, i.e., whether or not the total electric current integrated
over a single magnetic polarity of an active region vanishes. While
different arguments have been proposed in favor of, or against, the
neutralization of electric currents, both theory and observations are
still not fully conclusive. Providing the answer to this question is
crucial for theoretical models of solar eruptions. Indeed, if currents
are neutralized in active regions, then any eruption model based on net
- i.e., non-zero - electric currents, such as the torus instability,
requires further consideration. We address the question of electric
current neutralization in active regions using 3D zero-beta MHD
simulations of line-tied, slow photospheric driving motions imposed
on an initially potential magnetic field. We compare our results to a
recent study of the build-up of coronal electric currents in an MHD
simulation of the emergence of a current-neutralized twisted flux
tube into the solar atmosphere. Our parametric study shows that, in
accordance with the flux emergence simulation, photospheric motions are
associated with the formation of both direct and return currents. It
further shows that both processes (flux emergence and photospheric
flows) can lead to the formation of strong net currents in the solar
corona, and that the non-neutralization of electric currents is related
to the presence of magnetic shear at the polarity inversion line. We
discuss the implications of our results for the observations and for
theoretical models of solar eruptions.
Title: Magnetic Flux Emergence Along the Solar Cycle
Authors: Schmieder, B.; Archontis, V.; Pariat, E.
Bibcode: 2015sac..book..227S
Altcode:
No abstract at ADS
Title: Model for straight and helical solar jets. I. Parametric
studies of the magnetic field geometry
Authors: Pariat, E.; Dalmasse, K.; DeVore, C. R.; Antiochos, S. K.;
Karpen, J. T.
Bibcode: 2015A&A...573A.130P
Altcode:
Context. Jets are dynamic, impulsive, well-collimated plasma events
developing at many different scales and in different layers of
the solar atmosphere.
Aims: Jets are believed to be induced
by magnetic reconnection, a process central to many astrophysical
phenomena. Studying their dynamics can help us to better understand the
processes acting in larger eruptive events (e.g., flares and coronal
mass ejections) as well as mass, magnetic helicity, and energy transfer
at all scales in the solar atmosphere. The relative simplicity of
their magnetic geometry and topology, compared with larger solar active
events, makes jets ideal candidates for studying the fundamental role
of reconnection in energetic events.
Methods: In this study,
using our recently developed numerical solver ARMS, we present several
parametric studies of a 3D numerical magneto-hydrodynamic model of
solar-jet-like events. We studied the impact of the magnetic field
inclination and photospheric field distribution on the generation
and properties of two morphologically different types of solar jets,
straight and helical, which can account for the observed so-called
standard and blowout jets.
Results: Our parametric studies
validate our model of jets for different geometric properties of the
magnetic configuration. We find that a helical jet is always triggered
for the range of parameters we tested. This demonstrates that the 3D
magnetic null-point configuration is a very robust structure for the
energy storage and impulsive release characteristic of helical jets. In
certain regimes determined by magnetic geometry, a straight jet precedes
the onset of a helical jet. We show that the reconnection occurring
during the straight-jet phase influences the triggering of the helical
jet.
Conclusions: Our results allow us to better understand
the energization, triggering, and driving processes of straight and
helical jets. Our model predicts the impulsiveness and energetics of
jets in terms of the surrounding magnetic field configuration. Finally,
we discuss the interpretation of the observationally defined standard
and blowout jets in the context of our model, as well as the physical
factors that determine which type of jet will occur.
Title: Magnetic Flux Emergence Along the Solar Cycle
Authors: Schmieder, B.; Archontis, V.; Pariat, E.
Bibcode: 2014SSRv..186..227S
Altcode: 2014SSRv..tmp...47S
Flux emergence plays an important role along the solar cycle. Magnetic
flux emergence builds sunspot groups and solar activity. The sunspot
groups contribute to the large scale behaviour of the magnetic field
over the 11 year cycle and the reversal of the North and South magnetic
polarity every 22 years. The leading polarity of sunspot groups is
opposite in the North and South hemispheres and reverses for each
new solar cycle. However the hemispheric rule shows the conservation
of sign of the magnetic helicity with positive and negative magnetic
helicity in the South and North hemispheres, respectively. MHD models
of emerging flux have been developed over the past twenty years but
have not yet succeeded to reproduce solar observations. The emergence
of flux occurs through plasma layers of very high gradients of pressure
and changing of modes from a large β to a low β plasma (<1). With
the new armada of high spatial and temporal resolution instruments
on the ground and in space, emergence of magnetic flux is observed
in tremendous detail and followed during their transit through the
upper atmosphere. Signatures of flux emergence in the corona depend
on the pre-existing magnetic configuration and on the strength of the
emerging flux. We review in this paper new and established models as
well as the recent observations.
Title: A Topological View at Observed Flare Features: An Extension
of the Standard Flare Model to 3D
Authors: Savcheva, Antonia; Pariat, Etienne; McKillop, Sean; Hanson,
Elizabeth; Su, Yingna; DeLuca, Edward E.
Bibcode: 2014AAS...22430301S
Altcode:
We conduct topology analysis of erupting non-linear force-free field
(NLFFF) configurations of eight sigmoidal active regions observed
with Hinode/XRT and SDO/AIA. The NLFFF models are computed using
the flux rope insertion method and unstable models are utilized to
represent the erupting configurations. Topology analysis shows that the
quasi-separatrix layers (QSLs) in the chromosphere match well the flare
ribbons observed in these regions. In addition, we show that low-lying
QSLs associated with the rising flux rope change shape and extent to
match the separating flare ribbons as observed by AIA. Post-flare loops
are fit well by field lines lying under the generalized X-line at the
bottom of the flux rope. We show a correspondence in the evolution
of the post-flare loops from a strong-to-weak sheared state and the
behavior of the field lines as the flux rope expands in the corona. We
show that transient corona holes are associated with the footprints
of the flux rope in the low atmosphere. In addition, we compute the
reconnected flux in one of the regions and using information from
the models constrain how much energy has been released during the
event. We use this kind of topology analysis to extend the standard
CME/flare model to full 3D and find implications to reconnection in 3D.
Title: Electric Currents in Flare Ribbons: Observations and
Three-dimensional Standard Model
Authors: Janvier, M.; Aulanier, G.; Bommier, V.; Schmieder, B.;
Démoulin, P.; Pariat, E.
Bibcode: 2014ApJ...788...60J
Altcode: 2014arXiv1402.2010J
We present for the first time the evolution of the photospheric electric
currents during an eruptive X-class flare, accurately predicted by the
standard three-dimensional (3D) flare model. We analyze this evolution
for the 2011 February 15 flare using Helioseismic and Magnetic
Imager/Solar Dynamics Observatory magnetic observations and find
that localized currents in J-shaped ribbons increase to double their
pre-flare intensity. Our 3D flare model, developed with the OHM code,
suggests that these current ribbons, which develop at the location of
extreme ultraviolet brightenings seen with Atmospheric Imaging Assembly
imagery, are driven by the collapse of the flare's coronal current
layer. These findings of increased currents restricted in localized
ribbons are consistent with the overall free energy decrease during a
flare, and the shapes of these ribbons also give an indication of how
twisted the erupting flux rope is. Finally, this study further enhances
the close correspondence obtained between the theoretical predictions
of the standard 3D model and flare observations, indicating that the
main key physical elements are incorporated in the model.
Title: Coronal Magnetic Reconnection Driven by CME Expansion—the
2011 June 7 Event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Kliem, B.; Long, D. M.; Matthews, S. A.; Malherbe, J. -M.
Bibcode: 2014ApJ...788...85V
Altcode: 2014arXiv1406.3153V
Coronal mass ejections (CMEs) erupt and expand in a magnetically
structured solar corona. Various indirect observational pieces of
evidence have shown that the magnetic field of CMEs reconnects with
surrounding magnetic fields, forming, e.g., dimming regions distant
from the CME source regions. Analyzing Solar Dynamics Observatory
(SDO) observations of the eruption from AR 11226 on 2011 June 7, we
present the first direct evidence of coronal magnetic reconnection
between the fields of two adjacent active regions during a CME. The
observations are presented jointly with a data-constrained numerical
simulation, demonstrating the formation/intensification of current
sheets along a hyperbolic flux tube at the interface between the CME
and the neighboring AR 11227. Reconnection resulted in the formation of
new magnetic connections between the erupting magnetic structure from
AR 11226 and the neighboring active region AR 11227 about 200 Mm from
the eruption site. The onset of reconnection first becomes apparent
in the SDO/AIA images when filament plasma, originally contained
within the erupting flux rope, is redirected toward remote areas in
AR 11227, tracing the change of large-scale magnetic connectivity. The
location of the coronal reconnection region becomes bright and directly
observable at SDO/AIA wavelengths, owing to the presence of down-flowing
cool, dense (1010 cm-3) filament plasma in its
vicinity. The high-density plasma around the reconnection region is
heated to coronal temperatures, presumably by slow-mode shocks and
Coulomb collisions. These results provide the first direct observational
evidence that CMEs reconnect with surrounding magnetic structures,
leading to a large-scale reconfiguration of the coronal magnetic field.
Title: Temporal Evolution of the Magnetic Topology of the NOAA Active
Region 11158
Authors: Zhao, Jie; Li, Hui; Pariat, Etienne; Schmieder, Brigitte;
Guo, Yang; Wiegelmann, Thomas
Bibcode: 2014ApJ...787...88Z
Altcode: 2014arXiv1404.5004Z
We studied the temporal evolution of the magnetic topology of the active
region (AR) 11158 based on the reconstructed three-dimensional magnetic
fields in the corona. The non-linear force-free field extrapolation
method was applied to the 12 minute cadence data obtained with the
Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory
during 5 days. By calculating the squashing degree factor Q in the
volume, the derived quasi-separatrix layers (QSLs) show that this AR has
an overall topology, resulting from a magnetic quadrupole, including a
hyperbolic flux tube (HFT) configuration that is relatively stable at
the timescale of the flare (~1-2 hr). A strong QSL, which corresponds
to some highly sheared arcades that might be related to the formation
of a flux rope, is prominent just before the M6.6 and X2.2 flares,
respectively. These facts indicate the close relationship between the
strong QSL and the high flare productivity of AR 11158. In addition,
with a close inspection of the topology, we found a small-scale HFT
that has an inverse tear-drop structure above the aforementioned
QSL before the X2.2 flare. It indicates the existence of magnetic
flux rope at this place. Even though a global configuration (HFT)
is recognized in this AR, it turns out that the large-scale HFT only
plays a secondary role during the eruption. In conclusion, we dismiss
a trigger based on the breakout model and highlight the central role
of the flux rope in the related eruption.
Title: Magnetic topology of emerging flux regions
Authors: Pariat, Etienne
Bibcode: 2014cosp...40E2444P
Altcode:
Coronal magnetic fields structure and governs the dynamics of the
solar atmosphere. These magnetic fields are often complex, composed
of multiples domains of magnetic-field-lines connectivity. The
topology of the magnetic field allows a synthetic description
of these complex magnetic field by highlighting the structural
elements that are important for the dynamic and the activity of the
corona. Topology identifies the key elements where magnetic reconnection
will preferentially occurs, and allows to explain and predict the
evolution of the coronal plasma. However the topological elements -
such as null points, separatrices, separators - do not appear out of
thin air. Along with energy, and helicity, the magnetic topology of an
active region is build up as the consequence of flux emergence. Some
topological elements, such as bald-patches, are even fully part of the
mechanism of flux emergence mechanism and drive the evolution and the
structuration of the coronal magnetic field as it crosses the lower
layer of the solar atmosphere. In the present talk I will therefore
review our current understanding of the formation of active region
in terms of magnetic topology. I will speak on how the topological
structures which are key to solar activity are formed. Meanwhile I'll
also discus the topological properties of emerging active region and
how topology influences the very process of flux emergence.
Title: Constraining magnetic flux emergence from a timeseries of
helicitigrams
Authors: Dalmasse, Kévin; Pariat, Etienne; Green, Lucie M.; Aulanier,
Guillaume; Demoulin, Pascal; Valori, Gherardo
Bibcode: 2014cosp...40E.612D
Altcode:
Magnetic helicity quantifies how globally twisted and/or sheared is
the magnetic field in a volume. Observational studies have reported
the injection of large amounts of magnetic helicity associated with
the emergence of magnetic flux into the solar atmosphere. Because
magnetic helicity is conserved in the convection zone, the injection of
magnetic helicity into the solar corona reflects the helicity content
of emerging magnetic flux tubes. Mapping the photospheric injection
of magnetic helicity thus seems to be a key tool for constraining the
parameters of the emerging flux tubes in numerical case-studies of
observed active regions. We recently developed a method to compute the
distribution of magnetic helicity flux. Contrary to previous proxies,
this method takes into account the 3D nature of magnetic helicity, and
is thus, better-suited to study the distribution of helicity flux. After
introducing this method, we will present the results of its application
to the NOAA AR 11158. We will show that, the distribution of helicity
flux is complex, with patterns of real mixed signals of helicity flux
related to the specific topology of the active region's magnetic
field. Finally, we will discuss the implications of our results on
the evolution and dynamics of this active region.
Title: Electric currents in solar active regions
Authors: Dalmasse, Kévin; Pariat, Etienne; Kliem, Bernhard; Aulanier,
Guillaume; Demoulin, Pascal; Torok, Tibor
Bibcode: 2014cosp...40E.613D
Altcode:
There is a recurring question in solar physics about whether or not
photospheric vertical electric currents are neutralized in solar active
regions, i.e. if the total electric current integrated over a single
photospheric magnetic polarity of an active region vanishes. Different
arguments have been proposed in favor of, or against, the neutralization
of electric currents, but both theory and observations are still not
fully conclusive. The answer to this question has implications for
eruption models. Indeed, if currents are neutralized in active regions,
then any eruption model based on non-neutralized electric currents,
such as the torus instability, would need to be further analyzed. We
addressed the question of electric currents neutralization in active
regions using 3D zero-beta, line-tied, slow driving motions of an
initially potential magnetic field. We compared our results to a recent
study of electric currents build-up in a MHD numerical simulation of the
emergence of a current-neutralized twisted flux tube. Our parametric
analyses show that, as for the emergence, photospheric motions are
associated with the formation of both direct and return currents. It
further shows that both processes can lead to the formation of strong
net currents in the solar corona, and that the non-neutralization of
electric currents is related to the presence of magnetic shear at the
polarity inversion line. We will discuss the implications of our results
for the observations and for the different solar eruption models.
Title: Topological study of active region 11158
Authors: Zhao, Jie; Li, Hui; Pariat, Etienne; Schmieder, Brigitte;
Guo, Yang; Wiegelmann, Thomas
Bibcode: 2014IAUS..300..479Z
Altcode:
With the cylindrical equal area (CEA) projection data from the
Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics
Observatory (SDO), we reconstructed the three-dimensional (3D)
magnetic fields in the corona, using a non-linear force-free field
(NLFFF) extrapolation method every 12 minutes during five days, to
calculate the squashing degree factor Q in the volume. The results
show that this AR has an hyperbolic flux tube (HFT) configuration,
a typical topology of quadrupole, which is stable even during the two
large flares (M6.6 and X2.2 class flares).
Title: Photospheric Injection of Magnetic Helicity: Connectivity-Based
Flux Density Method
Authors: Dalmasse, K.; Pariat, E.; Démoulin, P.; Aulanier, G.
Bibcode: 2014SoPh..289..107D
Altcode: 2013arXiv1307.2829D
Magnetic helicity quantifies the degree to which the magnetic field
in a volume is globally sheared and/or twisted. This quantity is
believed to play a key role in solar activity due to its conservation
property. Helicity is continuously injected into the corona during
the evolution of active regions (ARs). To better understand and
quantify the role of magnetic helicity in solar activity, the
distribution of magnetic helicity flux in ARs needs to be studied. The
helicity distribution can be computed from the temporal evolution of
photospheric magnetograms of ARs such as the ones provided by SDO/HMI
and Hinode/SOT. Most recent analyses of photospheric helicity flux
derived a proxy to the helicity-flux density based on the relative
rotation rate of photospheric magnetic footpoints. Although this
proxy allows a good estimate of the photospheric helicity flux, it is
still not a true helicity flux density because it does not take into
account the connectivity of the magnetic field lines. For the first
time, we implement a helicity density that takes this connectivity
into account. To use it for future observational studies, we tested
the method and its precision on several types of models involving
different patterns of helicity injection. We also tested it on more
complex configurations - from magnetohydrodynamics (MHD) simulations
- containing quasi-separatrix layers. We demonstrate that this
connectivity-based proxy is best-suited to map the true distribution
of photospheric helicity injection.
Title: Magnetic reconnection driven by filament eruption in the 7
June 2011 event
Authors: van Driel-Gesztelyi, L.; Baker, D.; Török, T.; Pariat, E.;
Green, L. M.; Williams, D. R.; Carlyle, J.; Valori, G.; Démoulin,
P.; Matthews, S. A.; Kliem, B.; Malherbe, J. -M.
Bibcode: 2014IAUS..300..502V
Altcode:
During an unusually massive filament eruption on 7 June 2011,
SDO/AIA imaged for the first time significant EUV emission around a
magnetic reconnection region in the solar corona. The reconnection
occurred between magnetic fields of the laterally expanding CME
and a neighbouring active region. A pre-existing quasi-separatrix
layer was activated in the process. This scenario is supported by
data-constrained numerical simulations of the eruption. Observations
show that dense cool filament plasma was re-directed and heated in
situ, producing coronal-temperature emission around the reconnection
region. These results provide the first direct observational evidence,
supported by MHD simulations and magnetic modelling, that a large-scale
re-configuration of the coronal magnetic field takes place during
solar eruptions via the process of magnetic reconnection.
Title: Twist Accumulation and Topology Structure of a Solar Magnetic
Flux Rope
Authors: Guo, Y.; Ding, M. D.; Cheng, X.; Zhao, J.; Pariat, E.
Bibcode: 2013ApJ...779..157G
Altcode: 2013arXiv1311.1883G
To study the buildup of a magnetic flux rope before a major flare and
coronal mass ejection (CME), we compute the magnetic helicity injection,
twist accumulation, and topology structure of the three-dimensional
(3D) magnetic field, which is derived by the nonlinear force-free field
model. The Extreme-ultraviolet Imaging Telescope on board the Solar and
Heliospheric Observatory observed a series of confined flares without
any CME before a major flare with a CME at 23:02 UT on 2005 January 15
in active region NOAA 10720. We derive the vector velocity at eight
time points from 18:27 UT to 22:20 UT with the differential affine
velocity estimator for vector magnetic fields, which were observed by
the Digital Vector Magnetograph at Big Bear Solar Observatory. The
injected magnetic helicity is computed with the vector magnetic and
velocity fields. The helicity injection rate was (- 16.47 ± 3.52) ×
1040 Mx2 hr-1. We find that only about
1.8% of the injected magnetic helicity became the internal helicity of
the magnetic flux rope, whose twist increasing rate was -0.18 ± 0.08
Turns hr-1. The quasi-separatrix layers (QSLs) of the 3D
magnetic field are computed by evaluating the squashing degree, Q. We
find that the flux rope was wrapped by QSLs with large Q values, where
the magnetic reconnection induced by the continuously injected magnetic
helicity further produced the confined flares. We suggest that the flux
rope was built up and heated by the magnetic reconnection in the QSLs.
Title: The standard flare model in three dimensions. III. Slip-running
reconnection properties
Authors: Janvier, M.; Aulanier, G.; Pariat, E.; Démoulin, P.
Bibcode: 2013A&A...555A..77J
Altcode: 2013arXiv1305.4053J
Context. A standard model for eruptive flares aims at describing
observational 3D features of the reconnecting coronal magnetic
field. Extensions to the 2D model require the physical understanding of
3D reconnection processes at the origin of the magnetic configuration
evolution. However, the properties of 3D reconnection without null point
and separatrices still need to be analyzed.
Aims: We focus on
magnetic reconnection associated with the growth and evolution of a
flux rope and associated flare loops during an eruptive flare. We aim
at understanding the intrinsic characteristics of 3D reconnection in
the presence of quasi-separatrix layers (QSLs), how QSL properties are
related to the slip-running reconnection mode in general, and how this
applies to eruptive flares in particular.
Methods: We studied
the slip-running reconnection of field lines in a magnetohydrodynamic
simulation of an eruptive flare associated with a torus-unstable flux
rope. The squashing degree and the mapping norm are two parameters
related to the QSLs. We computed them to investigate their relation
with the slip-running reconnection speed of selected field lines.
Results: Field lines associated with the flux rope and the flare loops
undergo a continuous series of magnetic reconnection, which results
in their super-Alfvénic slipping motion. The time profile of their
slippage speed and the space distribution of the mapping norm are shown
to be strongly correlated. We find that the motion speed is proportional
to the mapping norm. Moreover, this slip-running motion becomes faster
as the flux rope expands, since the 3D current layer evolves toward a
current sheet, and QSLs to separatrices.
Conclusions: The present
analysis extends our understanding of the 3D slip-running reconnection
regime. We identified a controlling parameter of the apparent velocity
of field lines while they slip-reconnect, enabling the interpretation
of the evolution of post flare loops. This work completes the standard
model for flares and eruptions by giving its 3D properties.
Title: First observational application of a connectivity-based
helicity flux density
Authors: Dalmasse, K.; Pariat, E.; Valori, G.; Démoulin, P.; Green,
L. M.
Bibcode: 2013A&A...555L...6D
Altcode: 2013arXiv1307.2838D
Context. Measuring the magnetic helicity distribution in the solar
corona can help in understanding the trigger of solar eruptive
events because magnetic helicity is believed to play a key role in
solar activity due to its conservation property.
Aims: A new
method for computing the photospheric distribution of the helicity
flux was recently developed. This method takes into account the
magnetic field connectivity whereas previous methods were based
on photospheric signatures only. This novel method maps the true
injection of magnetic helicity in active regions. We applied this
method for the first time to an observed active region, NOAA 11158,
which was the source of intense flaring activity.
Methods: We
used high-resolution vector magnetograms from the SDO/HMI instrument
to compute the photospheric flux transport velocities and to perform
a nonlinear force-free magnetic field extrapolation. We determined
and compared the magnetic helicity flux distribution using a purely
photospheric as well as a connectivity-based method.
Results:
While the new connectivity-based method confirms the mixed pattern
of the helicity flux in NOAA 11158, it also reveals a different, and
more correct, distribution of the helicity injection. This distribution
can be important for explaining the likelihood of an eruption from the
active region.
Conclusions: The connectivity-based approach is
a robust method for computing the magnetic helicity flux, which can
be used to study the link between magnetic helicity and eruptivity of
observed active regions.
Title: A Topological View at CME/flare Features with Application to
3D Reconnection
Authors: Savcheva, Antonia Stefanova; Pariat, E.; van Ballegooijen,
A.; Mckillop, S.; Hanson, E.; DeLuca, Y. Su E.
Bibcode: 2013shin.confE.143S
Altcode:
We conduct topology analysis of erupting non-linear force-free
configurations of five sigmoidal active regions observed with Hinode/XRT
and SDO/AIA. The models are computed using the flux rope insertion
method and unstable models are utilized to represent the erupting
configurations. Topology analysis shows that the quasi-separatrix layers
(QSLs) in the chromosphere match well the flare ribbons observed in
these regions. Post-flare loops are also matched well by field lines
lying under the X-line in the models. In addition, we show that
low-lying QSLs associated with the rising flux rope change shape
and extent to match the separating flare ribbons in the images. We
use this kind of topology analysis to extend the standard CME/flare
model to full 3D in observed configurations and find implications to
reconnection in 3D.
Title: Study of magnetic flux emergence and related activity in
active region NOAA 10314
Authors: Poisson, Mariano; López Fuentes, Marcelo; Mandrini, Cristina
H.; Démoulin, Pascal; Pariat, Etienne
Bibcode: 2013AdSpR..51.1834P
Altcode:
We study the extremely complex active region (AR) NOAA 10314, that
was observed from March 13-19, 2003. This AR was the source of several
energetic events, among them two major (X class) flares, along a few
days. We follow the evolution of this AR since the very first stages
of its emergence. From the photospheric evolution of the magnetic
polarities observed with SOHO/MDI we infer the morphology of the
flux tube that originates the AR. Using a computation technique that
combines Local Correlation Tracking with magnetic induction constrains,
we compute the rate of magnetic helicity injection at the photosphere
during the observed evolution. From our results we conclude that the
AR originated by the emergence of a severely deformed magnetic flux
tube having a dominantly positive magnetic helicity.
Title: Accuracy of magnetic energy computations
Authors: Valori, G.; Démoulin, P.; Pariat, E.; Masson, S.
Bibcode: 2013A&A...553A..38V
Altcode: 2013arXiv1303.6773V
Context. For magnetically driven events, the magnetic energy of
the system is the prime energy reservoir that fuels the dynamical
evolution. In the solar context, the free energy (i.e., the energy in
excess of the potential field energy) is one of the main indicators
used in space weather forecasts to predict the eruptivity of active
regions. A trustworthy estimation of the magnetic energy is therefore
needed in three-dimensional (3D) models of the solar atmosphere, e.g.,
in coronal fields reconstructions or numerical simulations.
Aims:
The expression of the energy of a system as the sum of its potential
energy and its free energy (Thomson's theorem) is strictly valid when
the magnetic field is exactly solenoidal. For numerical realizations on
a discrete grid, this property may be only approximately fulfilled. We
show that the imperfect solenoidality induces terms in the energy that
can lead to misinterpreting the amount of free energy present in a
magnetic configuration.
Methods: We consider a decomposition
of the energy in solenoidal and nonsolenoidal parts which allows
the unambiguous estimation of the nonsolenoidal contribution to the
energy. We apply this decomposition to six typical cases broadly used
in solar physics. We quantify to what extent the Thomson theorem is
not satisfied when approximately solenoidal fields are used.
Results: The quantified errors on energy vary from negligible to
significant errors, depending on the extent of the nonsolenoidal
component of the field. We identify the main source of errors and
analyze the implications of adding a variable amount of divergence to
various solenoidal fields. Finally, we present pathological unphysical
situations where the estimated free energy would appear to be negative,
as found in some previous works, and we identify the source of this
error to be the presence of a finite divergence.
Conclusions:
We provide a method of quantifying the effect of a finite divergence in
numerical fields, together with detailed diagnostics of its sources. We
also compare the efficiency of two divergence-cleaning techniques. These
results are applicable to a broad range of numerical realizations of
magnetic fields. Appendices are available in electronic form at
http://www.aanda.org
Title: The standard flare model in three dimensions. II. Upper limit
on solar flare energy
Authors: Aulanier, G.; Démoulin, P.; Schrijver, C. J.; Janvier, M.;
Pariat, E.; Schmieder, B.
Bibcode: 2013A&A...549A..66A
Altcode: 2012arXiv1212.2086A
Context. Solar flares strongly affect the Sun's atmosphere as well as
the Earth's environment. Quantifying the maximum possible energy of
solar flares of the present-day Sun, if any, is thus a key question in
heliophysics.
Aims: The largest solar flares observed over the
past few decades have reached energies of a few times 1032
erg, possibly up to 1033 erg. Flares in active Sun-like
stars reach up to about 1036 erg. In the absence of direct
observations of solar flares within this range, complementary methods
of investigation are needed to assess the probability of solar flares
beyond those in the observational record.
Methods: Using
historical reports for sunspot and solar active region properties
in the photosphere, we scaled to observed solar values a realistic
dimensionless 3D MHD simulation for eruptive flares, which originate
from a highly sheared bipole. This enabled us to calculate the magnetic
fluxes and flare energies in the model in a wide paramater space.
Results: Firstly, commonly observed solar conditions lead to modeled
magnetic fluxes and flare energies that are comparable to those
estimated from observations. Secondly, we evaluate from observations
that 30% of the area of sunspot groups are typically involved in
flares. This is related to the strong fragmentation of these groups,
which naturally results from sub-photospheric convection. When the
model is scaled to 30% of the area of the largest sunspot group ever
reported, with its peak magnetic field being set to the strongest value
ever measured in a sunspot, it produces a flare with a maximum energy of
~6 × 1033 erg.
Conclusions: The results of the model
suggest that the Sun is able to produce flares up to about six times as
energetic in total solar irradiance fluence as the strongest directly
observed flare of Nov. 4, 2003. Sunspot groups larger than historically
reported would yield superflares for spot pairs that would exceed tens
of degrees in extent. We thus conjecture that superflare-productive
Sun-like stars should have a much stronger dynamo than in the Sun.
Title: On the kinematics and trigger mechanism of a twisting solar
jet as observed by SDO/AIA
Authors: Kumar Srivastava, Abhishek; Pariat, E.; Chandra, R.; Kayshap,
P.; Murawski, K.
Bibcode: 2013ASInC...9...70K
Altcode:
Using the SDO/AIA data of 304 Å and 211 Å, we observe a twisting
solar jet on 26 January 2011. This jet presents an episodic brightening
at its base in the 304 Å and 211 Å lines that we interpret as a
sign of localized heating. We also observe the conversion of writhe
to the twisting motion during upward propagation of the jet and
vice-versa. This is a rare observational evidence of the helicity
conservation and its backward transfer in the polar corona. The jet
rises with a speed of ∼300 km/s, while it rotates at its central part
with an angular speed of ∼0.002 rad/s. The injected helical twist
in the jet may subject to the kink instability that probably affects
the stability and dynamics of the jet. We conjecture that the initial
heating at the base of the jet may be due to the reconnection between
emerging flux and the pre-existing field lines, and allowed the transfer
of helicity that most likely triggered the jet motion in the corona.
Title: X-ray and ultraviolet investigation into the magnetic
connectivity of a solar flare
Authors: Reid, H. A. S.; Vilmer, N.; Aulanier, G.; Pariat, E.
Bibcode: 2012A&A...547A..52R
Altcode: 2012arXiv1210.2916R
We investigate the X-ray and UV emission detected by RHESSI and TRACE
in the context of a solar flare on the 16th November 2002 with the goal
of better understanding the evolution of the flare. We analysed the
characteristics of the X-ray emission in the 12-25 and 25-50 keV energy
range while we looked at the UV emission at 1600 Å . The flare appears
to have two distinct phases of emission separated by a 25-s time delay,
with the first phase being energetically more important. We found good
temporal and spatial agreement between the 25-50 keV X-rays and the
most intense areas of the 1600 Å UV emission. We also observed an
extended 100-arcsec < 25 keV source that appears coronal in nature
and connects two separated UV ribbons later in the flare. Using the
observational properties in X-ray and UV wavelengths, we propose two
explanations for the flare evolution in relation to the spine/fan
magnetic field topology and the accelerated electrons. We find that
a combination of quasi separatrix layer reconnection and null-point
reconnection is required to account for the observed properties of
the X-ray and UV emission.
Title: Comparison of a Magnetohydrodynamical Simulation and a
Non-Linear Force-Free Field Model of a Sigmoidal Active Region.
Authors: Pariat, Etienne; DeLuca, Edward; Van Ballegooijen, Adriaan;
Aulanier, Guillaume; Savcheva, Antonia
Bibcode: 2012cosp...39.1448P
Altcode: 2012cosp.meet.1448P
Sigmoids are solar magnetic structures where highly non-potential
fields (strong shear/twist) are believed to be present. Thanks to
the high level of free magnetic energy, active regions with sigmoids
possess a higher eruptivity. In the present study, we will present
a comparive topological analysis between a Non-Linear Force Free
Field (NLFFF) model of sigmoid region, and a three-dimensional (3D)
magnetohydrodynamics numerical simulation of the formation and eruption
of such a structure. The MHD simulation is based on an idealized
magnetic field distribution and the sigmoidal flux rope is built by
means of shearing motions and magnetic polarity diffusion. The NLFFF
model is based on the flux rope insertion method which utilizes line of
sight magnetograms and X-ray observations of the region to constrain the
models. We compare the geometrical and topological properties of the 3D
magnetic fields given by both methods in their pre-eruptive phases. We
arrive at a consistent picture for the evolution and eruption of the
sigmoid by using the idealized MHD simulation as a context for the more
specific observationally-constrained NLFFF models and data. Although,
the two models are very different in their setups, we identify strong
similarities between the two models and understandable differences. By
computing the squashing factor in different horizontal maps at various
heights above the photosphere and in vertical cuts in the domains,
we demonstrate the existence of key Quasi-Separatrix Layers (QSL)
eventually involved in the dynamic of the structure. We also show that
there are electric current concentrations coinciding with the main
QSLs. Finally, we perform torus instability analysis and show that
a combination between reconnection at the main QSL and the resulting
expansion of the flux rope into the torus instability domain is the
cause of the CME in both models. This study finally highlights the
interest of the use of in-depth topological tools to study highly
non-potential magnetic fields.
Title: Existence of two MHD reconnection modes in a solar 3D magnetic
null point topology
Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard;
Dalmasse, Kévin
Bibcode: 2012cosp...39.1450P
Altcode: 2012cosp.meet.1450P
Magnetic topologies with a 3D magnetic null point are common in
the solar atmosphere and occur at different spatial scales: such
structures can be associated with some solar eruptions, with the
so-called pseudo-streamers, and with numerous coronal jets. We have
recently developed a series of numerical experiments that model
magnetic reconnection in such configurations in order to study and
explain the properties of jet-like features. Our model uses our
state-of-the-art adaptive-mesh MHD solver ARMS. Energy is injected
in the system by line-tied motion of the magnetic field lines in a
corona-like configuration. We observe that, in the MHD framework, two
reconnection modes eventually appear in the course of the evolution of
the system. A very impulsive one, associated with a highly dynamic and
fully 3D current sheet, is associated with the energetic generation
of a jet. Before and after the generation of the jet, a quasi-steady
reconnection mode, more similar to the standard 2D Sweet-Parker model,
presents a lower global reconnection rate. We show that the geometry of
the magnetic configuration influences the trigger of one or the other
mode. We argue that this result carries important implications for
the observed link between observational features such as solar jets,
solar plumes, and the emission of coronal bright points.
Title: Evolution of a very complex active region during the decay
phase of Cycle 23
Authors: Poisson, Mariano; Fuentes, Marcelo López; Mandrini, Cristina
H.; Démoulin, Pascal; Pariat, Etienne
Bibcode: 2012IAUS..286..246P
Altcode:
We study the emergence and evolution of AR NOAA 10314, observed on
the solar disk during March 13-19, 2003. This extremely complex AR is
of particular interest due to its unusual magnetic flux distribution
and the clear rotation of the polarities of a δ-spot within the
AR. Using SOHO/MDI magnetograms we follow the evolution of the
photospheric magnetic flux to infer the morphology of the structure
that originates the AR. We determine the tilt angle variation for
the δ-spot and find a counter-clockwise rotation corresponding to a
positive writhed flux tube. We compute the magnetic helicity injection
and the total accumulated helicity in the AR and find a correlation
with the observed rotation.
Title: Photospheric injection of magnetic helicity: implementation
of a new density estimate
Authors: Dalmasse, Kévin; Pariat, Etienne; Demoulin, Pascal
Bibcode: 2012cosp...39..393D
Altcode: 2012cosp.meet..393D
Magnetic helicity quantifies how globally sheared and/or twisted is
the magnetic field, and thus, is a tracer of the non--potentiality
of the magnetic field in a volume. In the conditions of the solar
corona, magnetic helicity is a conserved quantity, and thus, imposes
a high constraint on the evolution of the magnetic field. Helicity
is continuously injected into the corona during the evolution of
active regions (ARs), and CMEs are possibly the manifestation of the
ejection of helicity excess. To better understand and quantify the role
of magnetic helicity in solar activity, the distribution of magnetic
helicity flux in ARs needs to be studied. The helicity distribution can
be computed from the temporal evolution of photospheric magnetograms
of ARs such as the ones provided by SDO/HMI and Hinode/SOT. Most of
previous analyses of photospheric helicity flux derive helicity flux
density proxies such as the so-called G_{A}, and recently G_{θ}. The
proxy G_{θ} had been developed in order to reduce the fake signals
observed using G_{A}. Although G_{θ} allows a better estimate of the
photospheric helicity flux, it is still not a true helicity flux density
because it does not account for the connectivity of the magnetic field
lines. For the first time, we implement the helicity density G_{Φ}
which takes into account such connectivity. In order to use G_{Φ}
for future observational studies, we test the method and its precision
on several models involving different types of helicity injection (by
rotation and relative motions of two opposite magnetic polarities). We
show that G_{Φ} is a much better proxy to estimate the photospheric
helicity flux than G_{A} and G_{θ}. We discuss how it could be
implemented from the dataset provided by SDO/HMI.
Title: Generation of plasma flows and waves during the development
of coronal jets
Authors: Pariat, Etienne; Antiochos, Spiro; DeVore, C. Richard
Bibcode: 2012cosp...39.1449P
Altcode: 2012cosp.meet.1449P
No abstract at ADS
Title: Slip-running reconnection and evolution of shear in post-flare
loops
Authors: Janvier, Miho; Schmieder, Brigitte; Pariat, Etienne;
Aulanier, Guillaume
Bibcode: 2012cosp...39..816J
Altcode: 2012cosp.meet..816J
We analyze the physical mechanisms of an eruptive flare via 3D
magnetohydrodynamic simulations of a flux rope. We focus on the
relaxation process associated with the reconnection of magnetic field
lines driven by the free expansion of the magnetic field. First, the
origin of the shearing of post-flare magnetic loops is investigated
in relation to the pre-flare geometry of the magnetic field. Indeed,
space-borne satellites can observe the temporal changes of post-flare
structures that are important observational manifestations of the
solar flare phenomenon. As such, understanding the evolution of
post-flare loops can reveal the characteristics of the pre-flare
magnetic field. Here, we introduce different proxies to quantify
the shear angle. We show that strong geometrical similarities exist
between the initial magnetic field and the post-flare loops. Analysis
of the eruption dynamics shows that magnetic reconnection at the origin
of the post-flare field lines forms less and less sheared magnetic
loops on top of one another. We confirm this tendency by direct
measurements of the shear angle seen in flare events such as that
of May 9, 2011 recorded by STEREO-B/EUVI. Our results also highlight
that vertical stretching of the magnetic field lines may play a role
in the shear angle evolution of post-flare loops. The analysis of
the eruptive flare evolution is followed by a detailed investigation
of the flux rope growth and of the post-flare loops formation due to
coronal slip-running reconnection. For that, we study the dynamics of
different regions around two ribbons of opposite current. We find that
these ribbons correspond to quasi-separatrix layers (QSLs), associated
with J-shaped pre-flare magnetic field lines, reconnected S-shaped
flux rope lines and post-flare loops. Simulations with very small time
steps are required so as to show the detailed time evolution of those
QSLs as well as the time variations of the slip-running velocities. Our
results provide a fully 3D extension of the standard 2D flare model.
Title: Resistive magnetic flux emergence and formation of solar
active regions
Authors: Pariat, E.; Schmieder, B.; Masson, S.; Aulanier, G.
Bibcode: 2012EAS....55..115P
Altcode:
Magnetic flux emergence as the mechanism leading to the formation of
magnetized structures in the solar atmosphere plays a key role in the
dynamic of the Sun. Observed as a whole, emerging flux regions show
clear signs of twisted structure, bearing the magnetic free energy
necessary to power active events. The high resolution observations
of the recent solar observatories (e.g. Hinode, SDO) have revealed
how intermittent the magnetic field appears and how various active
events induced by flux emergence are. Magnetic field reconstructions
methods show that the topology of the field in interspot regions
presents a serpentine structure, i.e. field lines having successive U
and Ω parts. Associated with the appearance of magnetic polarities,
a tremendous number of brief small scale brightening are observed
in different photospheric and chromospheric lines, e.g. Ellerman
Bombs, along with small scale jet-like structures. These events are
believed to be the observational signatures of the multiple magnetic
reconnections which enable the magnetic field to emerge further up
and magnetically structure the corona above active region. Meanwhile
a world-wide effort to numerically model the emergence of magnetic
field forming solar active region is been carried on. Using different
types of physical paradigm - e.g. idealized magnetohydrodynamic model,
advanced treatment of the physical equations, data-driven simulations
- these numerical experiments highlight how electric currents can
build-up during flux emergence, lead to reconnection and thus explain
the formation of the different observed transients.
Title: Coronal jets in an inclined coronal magnetic field : a
parametric 3D MHD study
Authors: Dalmasse, K.; Pariat, E.; Antiochos, S. K.; DeVore, C. R.
Bibcode: 2012EAS....55..201D
Altcode:
X-ray solar coronal jets are short-duration, fast, well collimated
plasma brightenings occurring in the solar corona. To explain and
understand the processes driving the jets, one must be able to model an
explosive release of free energy. Magnetic reconnection is believed
to play a key role in the generation of these energetic bursting
events. The model of jets that we have been developing is based on
a magnetic field constructed by embedding a vertical magnetic dipole
in a uniform open magnetic field. In this study, we investigate the
influence of the inclination of the open field on the properties of the
jet using numerical simulations. We will show that the inclination of
the open field is of critical importance for the properties of the jet
such as the energy released. We conclude that the characteristics of
the open field at the time of observations are a central criterion that
must be taken into account and reported on in observational studies.
Title: Comparing Values of the Relative Magnetic Helicity in Finite
Volumes
Authors: Valori, G.; Démoulin, P.; Pariat, E.
Bibcode: 2012SoPh..278..347V
Altcode: 2012SoPh..tmp..271G
Relative magnetic helicity, as a conserved quantity of ideal
magnetohydrodynamics, has been highlighted as an important quantity to
study in plasma physics. Due to its nonlocal nature, its estimation
is not straightforward in both observational and numerical data. In
this study we derive expressions for the practical computation of the
gauge-independent relative magnetic helicity in three-dimensional
finite domains. The derived expressions are easy to implement and
rapid to compute. They are derived in Cartesian coordinates, but can
be easily written in other coordinate systems. We apply our method to
a numerical model of a force-free equilibrium containing a flux rope,
and compare the results with those obtained employing known half-space
equations. We find that our method requires a much smaller volume
than half-space expressions to derive the full helicity content. We
also prove that values of relative magnetic helicity of different
magnetic fields can be compared with each other in the same sense as
free-energy values can. Therefore, relative magnetic helicity can
be meaningfully and directly compared between different datasets,
such as those from different active regions, but also within the
same dataset at different times. Typical applications of our formulae
include the helicity computation in three-dimensional models of the
solar atmosphere, e.g., coronal-field reconstructions by force-free
extrapolation and discretized magnetic fields of numerical simulations.
Title: Sigmoidal Active Region on the Sun: Comparison of a
Magnetohydrodynamical Simulation and a Nonlinear Force-free Field
Model
Authors: Savcheva, A.; Pariat, E.; van Ballegooijen, A.; Aulanier,
G.; DeLuca, E.
Bibcode: 2012ApJ...750...15S
Altcode:
In this paper we show that when accurate nonlinear force-free
field (NLFFF) models are analyzed together with high-resolution
magnetohydrodynamic (MHD) simulations, we can determine the physical
causes for the coronal mass ejection (CME) eruption on 2007 February
12. We compare the geometrical and topological properties of the
three-dimensional magnetic fields given by both methods in their
pre-eruptive phases. We arrive at a consistent picture for the
evolution and eruption of the sigmoid. Both the MHD simulation and
the observed magnetic field evolution show that flux cancellation
plays an important role in building the flux rope. We compute the
squashing factor, Q, in different horizontal maps in the domains. The
main shape of the quasi-separatrix layers (QSLs) is very similar
between the NLFFF and MHD models. The main QSLs lie on the edge of
the flux rope. While the QSLs in the NLFFF model are more complex due
to the intrinsic large complexity in the field, the QSLs in the MHD
model are smooth and possess lower maximum value of Q. In addition,
we demonstrate the existence of hyperbolic flux tubes (HFTs) in both
models in vertical cross sections of Q. The main HFT, located under the
twisted flux rope in both models, is identified as the most probable
site for reconnection. We also show that there are electric current
concentrations coinciding with the main QSLs. Finally, we perform torus
instability analysis and show that a combination between reconnection
at the HFT and the resulting expansion of the flux rope into the torus
instability domain is the cause of the CME in both models.
Title: Topological Tools For The Analysis Of Active Region Filament
Stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
Pariat, E.; Aulanier, G.; Su, Y.
Bibcode: 2012AAS...22020207D
Altcode:
The combination of accurate NLFFF models and high resolution MHD
simulations allows us to study the changes in stability of an active
region filament before a CME. Our analysis strongly supports the
following sequence of events leading up to the CME: first there is a
build up of magnetic flux in the filament through flux cancellation
beneath a developing flux rope; as the flux rope develops a hyperbolic
flux tube (HFT) forms beneath the flux rope; reconnection across
the HFT raises the flux rope while adding addition flux to it; the
eruption is triggered when the flux rope becomes torus-unstable. The
work applies topological analysis tools that have been developed over
the past decade and points the way for future work on the critical
problem of CME initiation in solar active regions. We will present
the uses of this approach, current limitations and future prospects.
Title: Estimation of the squashing degree within a three-dimensional
domain
Authors: Pariat, E.; Démoulin, P.
Bibcode: 2012A&A...541A..78P
Altcode:
Context. The study of the magnetic topology of magnetic
fields aims at determining the key sites for the development of
magnetic reconnection. Quasi-separatrix layers (QSLs), regions of
strong connectivity gradients, are topological structures where
intense-electric currents preferentially build-up, and where,
later on, magnetic reconnection occurs.
Aims: QSLs are
volumes of intense squashing degree, Q; the field-line invariant
quantifying the deformation of elementary flux tubes. QSL are complex
and thin three-dimensional (3D) structures difficult to visualize
directly. Therefore Q maps, i.e. 2D cuts of the 3D magnetic domain, are
a more and more common features used to study QSLs.
Methods:
We analyze several methods to derive 2D Q maps and discuss their
analytical and numerical properties. These methods can also be used to
compute Q within the 3D domain.
Results: We demonstrate that
while analytically equivalent, the numerical implementation of these
methods can be significantly different. We derive the analytical formula
and the best numerical methodology that should be used to compute
Q inside the 3D domain. We illustrate this method with two twisted
magnetic configurations: a theoretical case and a non-linear force
free configuration derived from observations.
Conclusions: The
representation of QSL through 2D planar cuts is an efficient procedure
to derive the geometry of these structures and to relate them with other
quantities, e.g. electric currents and plasma flows. It will enforce
a more direct comparison of the role of QSL in magnetic reconnection.
Title: 3D MHD Simulation of Current Intensification along Serpentine
Emerging Magnetic Fields
Authors: Pariat, E.; Masson, S.; Aulanier, G.
Bibcode: 2012ASPC..455..177P
Altcode:
The high resolution observations of the Hinode instruments have
revealed many important features of the magnetic flux evolution and its
interaction with the solar plasma in emerging flux regions. The high
intermittency of the magnetic field distribution in interspot regions
confirms the serpentine topology adopted by the magnetic field as it
cross the solar photosphere. Precise information about the evolution
of localized brightenings, usually called Ellerman bombs (EBs), typical
events of emerging flux regions, have been gathered by Hinode: the link
between EBs and the magnetic topology, the EBs detailed spectral time
evolution and their relation with other dynamic events such as small
scale jets, etc. Ellerman bombs are believed to be the observational
signatures of the multiple magnetic reconnections which enable
the magnetic field to emerge further up and magnetically structure
the corona above active regions. This work is part of a world-wide
effort to model the emergence of magnetic field forming solar active
regions. Using a data-driven, three-dimensional (3D) magnetohydrodynamic
(MHD) numerical simulation of a flux emergence region, we study the
development of 3D electric current sheets. We show that these currents
buildup along the 3D serpentine magnetic-field structure as a result
of photospheric diverging horizontal line-tied motions that emulate
the observed photospheric evolution. We study which types of motion
and magnetic topology lead to the highest current intensification
and therefore to the highest reconnection probability. We discuss how
these currents can explain the formation of Ellerman bombs, facilitate
the flux emergence, and account for some observed pattern of emerging
flux regions.
Title: Topological tools for the analysis of active region filament
stability
Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.;
Pariat, E.; Aulanier, G.; Su, Y.
Bibcode: 2012decs.confE..64D
Altcode:
The combination of accurate NLFFF models and high resolution MHD
simulations allows us to study the changes in stability of an active
region filament before a CME. Our analysis strongly supports the
following sequence of events leading up to the CME: first there is a
build up of magnetic flux in the filament through flux cancellation
beneath a developing flux rope; as the flux rope develops a hyperbolic
flux tube (HFT) forms beneath the flux rope; reconnection across
the HFT raises the flux rope while adding addition flux to it; the
eruption is triggered when the flux rope becomes torus-unstable. The
work applies topological analysis tools that have been developed over
the past decade and points the way for future work on the critical
problem of CME initiation in solar active regions. We will discuss
the uses of this approach, current limitations and future prospects.
Title: Interchange Slip-Running Reconnection and Sweeping SEP Beams
Authors: Masson, S.; Aulanier, G.; Pariat, E.; Klein, K. -L.
Bibcode: 2012SoPh..276..199M
Altcode: 2011arXiv1109.5678M
We present a new model to explain how particles (solar energetic
particles; SEPs), accelerated at a reconnection site that is not
magnetically connected to the Earth, could eventually propagate
along the well-connected open flux tube. Our model is based on the
results of a low-β resistive magnetohydrodynamics simulation of a
three-dimensional line-tied and initially current-free bipole, which
is embedded in a non-uniform open potential field. The topology of
this configuration is that of an asymmetric coronal null point, with
a closed fan surface and an open outer spine. When driven by slow
photospheric shearing motions, field lines, initially fully anchored
below the fan dome, reconnect at the null point, and jump to the open
magnetic domain. This is the standard interchange mode as sketched and
calculated in 2D. The key result in 3D is that reconnected open field
lines located in the vicinity of the outer spine keep reconnecting
continuously, across an open quasi-separatrix layer, as previously
identified for non-open-null-point reconnection. The apparent slipping
motion of these field lines leads to formation of an extended narrow
magnetic flux tube at high altitude. Because of the slip-running
reconnection, we conjecture that if energetic particles would be
traveling through, or be accelerated inside, the diffusion region,
they would be successively injected along continuously reconnecting
field lines that are connected farther and farther from the spine. At
the scale of the full Sun, owing to the super-radial expansion of
field lines below 3 R⊙, such energetic particles could
easily be injected in field lines slipping over significant distances,
and could eventually reach the distant flux tube that is well-connected
to the Earth.
Title: The 2011 February 15 X2 Flare, Ribbons, Coronal Front, and
Mass Ejection: Interpreting the Three-dimensional Views from the
Solar Dynamics Observatory and STEREO Guided by Magnetohydrodynamic
Flux-rope Modeling
Authors: Schrijver, Carolus J.; Aulanier, Guillaume; Title, Alan M.;
Pariat, Etienne; Delannée, Cecile
Bibcode: 2011ApJ...738..167S
Altcode:
The 2011 February 15 X2.2 flare and associated Earth-directed halo
coronal mass ejection were observed in unprecedented detail with
high resolution in spatial, temporal, and thermal dimensions by the
Atmospheric Imaging Assembly (AIA) on the Solar Dynamics Observatory,
as well as by instruments on the two STEREO spacecraft, then at
near-quadrature relative to the Sun-Earth line. These observations
enable us to see expanding loops from a flux-rope-like structure over
the shearing polarity-inversion line between the central δ-spot groups
of AR 11158, developing a propagating coronal front ("EIT wave"),
and eventually forming the coronal mass ejection moving into the inner
heliosphere. The observations support the interpretation that all of
these features, including the "EIT wave," are signatures of an expanding
volume traced by loops (much larger than the flux rope only), surrounded
by a moving front rather than predominantly wave-like perturbations;
this interpretation is supported by previously published MHD models
for active-region and global scales. The lateral expansion of the
eruption is limited to the local helmet-streamer structure and halts
at the edges of a large-scale domain of connectivity (in the process
exciting loop oscillations at the edge of the southern polar coronal
hole). The AIA observations reveal that plasma warming occurs within
the expansion front as it propagates over quiet Sun areas. This warming
causes dimming in the 171 Å (Fe IX and Fe X) channel and brightening
in the 193 and 211 Å (Fe XII-XIV) channels along the entire front,
while there is weak 131 Å (Fe VIII and Fe XXI) emission in some
directions. An analysis of the AIA response functions shows that
sections of the front running over the quiet Sun are consistent with
adiabatic warming; other sections may require additional heating which
MHD modeling suggests could be caused by Joule dissipation. Although
for the events studied here the effects of volumetric expansion are
much more obvious than true wave phenomena, we discuss how different
magnetic environments within and around the erupting region can lead
to the signatures of either or both of these aspects.
Title: Solar activity due to magnetic complexity of active regions
Authors: Schmieder, Brigitte; Mandrini, Cristina; Chandra, Ramesh;
Démoulin, Pascal; Török, Tibor; Pariat, Etienne; Uddin, Wahab
Bibcode: 2011IAUS..273..164S
Altcode:
Active regions (ARs), involved in the Halloween events during
October-November 2003, were the source of unusual activity during
the following solar rotation. The flares on 18-20 November 2003 that
occur in the AR NOAA10501 were accompanied by coronal mass ejections
associated to some particularly geoeffective magnetic clouds. Our
analysis of the magnetic flux and helicity injection revealed that
a new emerging bipole and consequent shearing motions continuously
energized the region during its disk passage. The stored energy was
eventually released through the interaction of the various systems
of magnetic loops by several magnetic reconnection events. Active
events on November 18 (filament eruptions and CMEs) were originated by
shearing motions along a section of the filament channel that injected
magnetic helicity with sign opposite to that of the AR. Two homologous
flares, that occurred on November 20, were apparently triggered by
different mechanisms as inferred from the flare ribbons evolution
(filament eruption and CMEs). We studied in detail the behaviour of
two North-South oriented filaments on November 20 2003. They merged
and split following a process suggestive of `sling-shot' reconnection
between two coronal flux ropes. We successfully tested this scenario
in a 3D MHD simulation that is presented in this paper.
Title: Interchange slip-running reconnection and sweeping SEP beams
Authors: Masson, Sophie; Aulanier, G.; Pariat, E.; Klein, K. -L.
Bibcode: 2011shin.confE..38M
Altcode:
We present a new model to explain how particles, accelerated at a
reconnection site that is not magnetically connected to the Earth,
could eventually propagate along the well-connected open flux tube. Our
model is based on the results of a low beta resistive MHD simulation of
a 3D line-tied and initially current-free bipole, that is embedded in
a non-uniform open potential field. The topology of this configuration
is that of an asymmetric coronal null-point, with a closed fan surface
and an open outer spine. When driven by slow photospheric shearing
motions, field lines initially anchored at both feet below the fan dome
reconnect at the null point, and jump to the open magnetic domain. This
is the standard interchange mode as sketched and calculated in 2D. The
key result in 3D is that, after the interchange, and just as found
earlier in non-open null-point reconnection, reconnected open field
lines located in the vicinity of the outer spine keep reconnecting
continuously, across an open quasi-separatrix layer. The apparent
slipping motion of these field lines leads to forming an extended
narrow magnetic flux tube at high altitude. Because of the slip-running
reconnection, we conjecture that if energetic particles would be
traveling through, or be accelerated inside, the diffusion region,
they would be successively injected along continuously reconnecting
field lines, that are connected farther and farther from the spine. At
the scale of the full Sun, owing to the super-radial expansion of field
lines below 3 Rs, such energetic particles could easily be injected in
field lines slipping over significant distances, and could eventually
reach the distant flux tube that is well connected to the Earth.
Title: Actors of the main activity in large complex centres during
the 23 solar cycle maximum
Authors: Schmieder, B.; Démoulin, P.; Pariat, E.; Török, T.;
Molodij, G.; Mandrini, C. H.; Dasso, S.; Chandra, R.; Uddin, W.;
Kumar, P.; Manoharan, P. K.; Venkatakrishnan, P.; Srivastava, N.
Bibcode: 2011AdSpR..47.2081S
Altcode:
During the maximum of Solar Cycle 23, large active regions had a long
life, spanning several solar rotations, and produced large numbers of
X-class flares and CMEs, some of them associated to magnetic clouds
(MCs). This is the case for the Halloween active regions in 2003. The
most geoeffective MC of the cycle (Dst = -457) had its source during
the disk passage of one of these active regions (NOAA 10501) on
18 November 2003. Such an activity was presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
different magnetic helicities. The complexity was observed to reach
such unprecedented levels that a detailed multi-wavelength analysis
is necessary to precisely identify the solar sources of CMEs and
MCs. Magnetic clouds are identified using in situ measurements and
interplanetary scintillation (IPS) data. Results from these two
different sets of data are also compared.
Title: Interchange Slip-running Reconnection and Sweeping SEP Beams
Authors: Masson, Sophie; Aulanier, G.; Pariat, E.; Klein, K.
Bibcode: 2011SPD....42.1403M
Altcode: 2011BAAS..43S.1403M
We present a new model to explain how particles, accelerated at a
reconnection site that is not magnetically connected to the Earth,
could eventually propagate along the well-connected open flux tube. Our
model is based on the results of a low beta resistive MHD simulation of
a 3D line-tied and initially current-free bipole, that is embedded in
a non-uniform open potential field. The topology of this configuration
is that of an asymmetric coronal null-point, with a closed fan surface
and an open outer spine. When driven by slow photospheric shearing
motions, field lines initially anchored at both feet below the fan dome
reconnect at the null point, and jump to the open magnetic domain. This
is the standard interchange mode as sketched and calculated in 2D. The
key result in 3D is that, after the interchange, and just as found
earlier in non-open null-point reconnection, reconnected open field
lines located in the vicinity of the outer spine keep reconnecting
continuously, across an open quasi-separatrix layer. The apparent
slipping motion of these field lines leads to forming an extended
narrow magnetic flux tube at high altitude. Because of the slip-running
reconnection, we conjecture that if energetic particles would be
traveling through, or be accelerated inside, the diffusion region,
they would be successively injected along continuously reconnecting
field lines, that are connected farther and farther from the spine. At
the scale of the full Sun, owing to the super-radial expansion of field
lines below 3 Rs, such energetic particles could easily be injected in
field lines slipping over significant distances, and could eventually
reach the distant flux tube that is well connected to the Earth.
Title: Homologous Flares and Magnetic Field Topology in Active Region
NOAA 10501 on 20 November 2003
Authors: Chandra, R.; Schmieder, B.; Mandrini, C. H.; Démoulin, P.;
Pariat, E.; Török, T.; Uddin, W.
Bibcode: 2011SoPh..269...83C
Altcode: 2010arXiv1011.1187C; 2010SoPh..tmp..249C
We present and interpret observations of two morphologically homologous
flares that occurred in active region (AR) NOAA 10501 on 20 November
2003. Both flares displayed four homologous Hα ribbons and were
both accompanied by coronal mass ejections (CMEs). The central flare
ribbons were located at the site of an emerging bipole in the centre
of the active region. The negative polarity of this bipole fragmented
in two main pieces, one rotating around the positive polarity by
≈ 110° within 32 hours. We model the coronal magnetic field and
compute its topology, using as boundary condition the magnetogram
closest in time to each flare. In particular, we calculate the
location of quasi-separatrix layers (QSLs) in order to understand the
connectivity between the flare ribbons. Though several polarities were
present in AR 10501, the global magnetic field topology corresponds
to a quadrupolar magnetic field distribution without magnetic null
points. For both flares, the photospheric traces of QSLs are similar
and match well the locations of the four Hα ribbons. This globally
unchanged topology and the continuous shearing by the rotating bipole
are two key factors responsible for the flare homology. However, our
analyses also indicate that different magnetic connectivity domains
of the quadrupolar configuration become unstable during each flare,
so that magnetic reconnection proceeds differently in both events.
Title: Filament Interaction Modeled by Flux Rope Reconnection
Authors: Török, T.; Chandra, R.; Pariat, E.; Démoulin, P.;
Schmieder, B.; Aulanier, G.; Linton, M. G.; Mandrini, C. H.
Bibcode: 2011ApJ...728...65T
Altcode:
Hα observations of solar active region NOAA 10501 on 2003 November
20 revealed a very uncommon dynamic process: during the development
of a nearby flare, two adjacent elongated filaments approached each
other, merged at their middle sections, and separated again, thereby
forming stable configurations with new footpoint connections. The
observed dynamic pattern is indicative of "slingshot" reconnection
between two magnetic flux ropes. We test this scenario by means
of a three-dimensional zero β magnetohydrodynamic simulation,
using a modified version of the coronal flux rope model by Titov
and Démoulin as the initial condition for the magnetic field. To
this end, a configuration is constructed that contains two flux
ropes which are oriented side-by-side and are embedded in an ambient
potential field. The choice of the magnetic orientation of the flux
ropes and of the topology of the potential field is guided by the
observations. Quasi-static boundary flows are then imposed to bring
the middle sections of the flux ropes into contact. After sufficient
driving, the ropes reconnect and two new flux ropes are formed,
which now connect the former adjacent flux rope footpoints of opposite
polarity. The corresponding evolution of filament material is modeled
by calculating the positions of field line dips at all times. The dips
follow the morphological evolution of the flux ropes, in qualitative
agreement with the observed filaments.
Title: A solar eruption triggered by the interaction between two
magnetic flux systems with opposite magnetic helicity
Authors: Romano, P.; Pariat, E.; Sicari, M.; Zuccarello, F.
Bibcode: 2011A&A...525A..13R
Altcode:
Context. In recent years the accumulation of magnetic helicity via
emergence of new magnetic flux and/or shearing photospheric motions
has been considered to play an important role in the destabilization
processes that lead to eruptive phenomena occurring in the solar
atmosphere.
Aims: In this paper we want to highlight a specific
aspect of magnetic helicity accumulation, providing new observational
evidence of the role played by the interaction of magnetic fields
characterized by opposite magnetic helicity signs in triggering solar
eruption.
Methods: We used 171 Å TRACE data to describe a
filament eruption on 2001 Nov. 1 in active region NOAA 9682 and MDI full
disk line-of-sight magnetograms to measure the accumulation of magnetic
helicity in corona before the event. We used the local correlation
tracking (LCT) and the differential affine velocity estimator (DAVE)
techniques to determine the horizontal velocities and two methods for
estimating the magnetic helicity flux.
Results: The chirality
signatures of the filament involved in the eruption were ambiguous,
and the overlying arcade visible during the main phase of the event
was characterized by a mixing of helicity signs. However, the measures
of the magnetic helicity flux allowed us to deduce that the magnetic
helicity was positive in the whole active region where the event
took place, while it was negative near the magnetic inversion line
where the filament footpoints were located.
Conclusions: These
results suggest that the filament eruption may be caused by magnetic
reconnection between two magnetic field systems characterized by
opposite signs of magnetic helicity. We also find that only the DAVE
method allowed us to obtain the crucial information on the horizontal
velocity field near the magnetic inversion line.
Title: Study of solar flares and filament interaction in NOAA 10501
on 20 November, 2003
Authors: Chandra, R.; Schmieder, B.; Mandrini, C. H.; Démoulin, P.;
Pariat, E.; Török, T.; Aulanier, G.; Uddin, W.; Linton, M. G.
Bibcode: 2011ASInC...2..323C
Altcode:
We analyze the observations of two flares from NOAA AR 10501 on 20
November, 2003. The flares are homologous, exhibit four ribbons and
are located in a quadrupolar magnetic configuration. The evolution
of the ribbons suggests that the first eruption is triggered by
"tether cutting" (with subsequent quadrupolar reconnection as in the
"magnetic breakout" model), whereas the second one is consistent
with the "magnetic breakout" model. Another interesting feature of
our observations is the interaction of two filaments elongated in the
north-south direction. The filaments merge at their central parts and
afterwards change their orientation to the east-west direction. This
merging and splitting is closely related to the evolution found in an
MHD simulation as a result of reconnection between two flux ropes.
Title: Symmetric Coronal Jets: A Reconnection-controlled Study
Authors: Rachmeler, L. A.; Pariat, E.; DeForest, C. E.; Antiochos,
S.; Török, T.
Bibcode: 2010ApJ...715.1556R
Altcode:
Current models and observations imply that reconnection is a key
mechanism for destabilization and initiation of coronal jets. We evolve
a system described by the theoretical symmetric jet formation model
using two different numerical codes with the goal of studying the
role of reconnection in this system. One of the codes is the Eulerian
adaptive mesh code ARMS, which simulates magnetic reconnection through
numerical diffusion. The quasi-Lagrangian FLUX code, on the other hand,
is ideal and able to evolve the system without reconnection. The ideal
nature of FLUX allows us to provide a control case of evolution without
reconnection. We find that during the initial symmetric and ideal phase
of evolution, both codes produce very similar morphologies and energy
growth. The symmetry is then broken by a kink-like motion of the axis
of rotation, after which the two systems diverge. In ARMS, current
sheets formed and reconnection rapidly released the stored magnetic
energy. In FLUX, the closed field remained approximately constant
in height while expanding in width and did not release any magnetic
energy. We find that the symmetry threshold is an ideal property of the
system, but the lack of energy release implies that the observed kink is
not an instability. Because of the confined nature of the FLUX system,
we conclude that reconnection is indeed necessary for jet formation
in symmetric jet models in a uniform coronal background field.
Title: Three-dimensional Modeling of Quasi-homologous Solar Jets
Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R.
Bibcode: 2010ApJ...714.1762P
Altcode:
Recent solar observations (e.g., obtained with Hinode and STEREO)
have revealed that coronal jets are a more frequent phenomenon than
previously believed. This higher frequency results, in part, from the
fact that jets exhibit a homologous behavior: successive jets recur at
the same location with similar morphological features. We present the
results of three-dimensional (3D) numerical simulations of our model
for coronal jets. This study demonstrates the ability of the model to
generate recurrent 3D untwisting quasi-homologous jets when a stress is
constantly applied at the photospheric boundary. The homology results
from the property of the 3D null-point system to relax to a state
topologically similar to its initial configuration. In addition, we find
two distinct regimes of reconnection in the simulations: an impulsive
3D mode involving a helical rotating current sheet that generates the
jet and a quasi-steady mode that occurs in a 2D-like current sheet
located along the fan between the sheared spines. We argue that these
different regimes can explain the observed link between jets and plumes.
Title: How Can a Negative Magnetic Helicity Active Region Generate
a Positive Helicity Magnetic Cloud?
Authors: Chandra, R.; Pariat, E.; Schmieder, B.; Mandrini, C. H.;
Uddin, W.
Bibcode: 2010SoPh..261..127C
Altcode: 2009arXiv0910.0968C
The geoeffective magnetic cloud (MC) of 20 November 2003 was associated
with the 18 November 2003 solar active events in previous studies. In
some of these, it was estimated that the magnetic helicity carried by
the MC had a positive sign, as did its solar source, active region (AR)
NOAA 10501. In this article we show that the large-scale magnetic field
of AR 10501 has a negative helicity sign. Since coronal mass ejections
(CMEs) are one of the means by which the Sun ejects magnetic helicity
excess into interplanetary space, the signs of magnetic helicity in
the AR and MC must agree. Therefore, this finding contradicts what is
expected from magnetic helicity conservation. However, using, for the
first time, correct helicity density maps to determine the spatial
distribution of magnetic helicity injections, we show the existence
of a localized flux of positive helicity in the southern part of AR
10501. We conclude that positive helicity was ejected from this portion
of the AR leading to the observed positive helicity MC.
Title: Vector Magnetic Field in Emerging Flux Regions
Authors: Schmieder, B.; Pariat, E.
Bibcode: 2010ASSP...19..505S
Altcode: 2010mcia.conf..505S
A crucial phase in magnetic flux emergence is the rise of magnetic
flux tubes through the solar photosphere, which represents a severe
transition between the very different environments of the solar interior
and corona. Multi-wavelength observations with Flare Genesis, TRACE,
SoHO, and more recently with the vector magnetographs at THEMIS and Hida
(DST) led to the following conclusions. The fragmented magnetic field
in the emergence region - with dipped field lines or bald patches -
is directly related with Ellerman bombs, arch filament systems, and
overlying coronal loops. Measurements of vector magnetic fields have
given evidence that undulating "serpentine" fields are present while
magnetic flux tubes cross the photosphere. See the sketch below, and
for more detail see Pariat et al. (2004, 2007); Watanabe et al. (2008):
Title: Ejective events from a complex active region
Authors: Mandrini, Cristina H.; Chandra, Ramesh; Pariat, Etienne;
Schmieder, Brigitte; Demoulin, Pascal; Toeroek, Tibor; Uddin, Wahab
Bibcode: 2010cosp...38.1886M
Altcode: 2010cosp.meet.1886M
On 18 and 20 November 2003 active region (AR) 10501 produced a series of
M flares all of them associated with coronal mass ejections (CMEs). The
particularity of this AR is that while observational tracers of the
magnetic helicity sign indicate that the large scale field in the
region had a negative magnetic helicity sign, the MC associated
to the most intense flare/CME on November 18 showed the opposite
sign. Furthermore, the filaments observed on November 20 present
morphological characteristics that correspond to a negative magnetic
helicity sign, the rotation of the polarities of an emerging bipole
indicate negative magnetic helicity sign injection; however, the flare
ribbons observed after two homologous events can be connected either
by field lines computed using a positive or a negative helicity sign
magnetic field. We combine Hα, EUV, hard X-rays, and magnetic field
data analysis with magnetic field modelling, and magnetic helicity
injection computations to understand the origin of the helicity
sign discrepancies discussed above. On November 20 magnetic field
modeling and topology computations (in particular, the location of
quasi-separatrix layers in relation to flare ribbons and evolution)
give us clues about the CME initiation process.
Title: Generation of solar coronal jets by 3D MHD simulations
Authors: Pariat, Etienne
Bibcode: 2010cosp...38.2935P
Altcode: 2010cosp.meet.2935P
In the solar atmosphere, jet-like features are observed over a
broad range of spatial and temporal scales. Recent solar missions
(e.g. STEREO, Hinode, ...) have recently provided ground breaking
observations of these active events. The high spatial and temporal
resolution of the Hinode observations yields new insights into the
origins of coronal jets, and provides detailed data that can be used
to test and refine models. Despite their name, jet structure may not
be fully formed by bulk flows. Physical process such as conduction and
compressive wave may also explain many observed features. Numerical
models have therefore appears as important tools to understand the
underlying physics of these dynamic events. In this review, I will
detail the results of several recent numerical experiments of the
trigger of coronal jets. I will show that magnetic reconnection is
the key process and I will discuss how 3D simulations are modifying
our previous understanding of the driving process of solar jets.
Title: Actors of the main activity of large complex centres during
the 23 Solar Cycle maximum
Authors: Schmieder, Brigitte; Chandra, Ramesh; Demoulin, Pascal;
Mandrini, Cristina H.; Venkatakrishnan, P.; Manoharan, P. K.; Uddin,
Wahab; Pariat, Etienne; Toeroek, Tibor; Molodij, Guillaume; Kumar, P.
Bibcode: 2010cosp...38.1861S
Altcode: 2010cosp.meet.1861S
During the maximum of the last Solar Cycle solar cycle 23, large
active regions had a long life spanning several solar rotations and
produced a large number of X-ray class flares, CMEs and Magnetic
clouds (MC). This was the case for the Halloween active regions in
2003. The most geoeffective magnetic cloud of the cycle (Dst=-457)
has its source in one passage of the active region (NOAA 10501) on
November 18, 2003. Such an activity is presumably due to continuous
emerging magnetic flux that was observed during this passage. Moreover,
the region exhibited a complex topology with multiple domains of
distinct magnetic helicities. The complexity is observed to reach
such unprecedented levels that a detailed multi wavelength analysis
is necessary to precisely identify the sources of CMEs and MCs.
Title: Current Buildup in Emerging Serpentine Flux Tubes
Authors: Pariat, E.; Masson, S.; Aulanier, G.
Bibcode: 2009ApJ...701.1911P
Altcode:
The increase of magnetic flux in the solar atmosphere during
active-region formation involves the transport of the magnetic field
from the solar convection zone through the lowest layers of the solar
atmosphere, through which the plasma β changes from >1 to <1 with
altitude. The crossing of this magnetic transition zone requires the
magnetic field to adopt a serpentine shape also known as the sea-serpent
topology. In the frame of the resistive flux-emergence model, the
rising of the magnetic flux is believed to be dynamically driven by
a succession of magnetic reconnections which are commonly observed
in emerging flux regions as Ellerman bombs. Using a data-driven,
three-dimensional (3D) magnetohydrodynamic numerical simulation of flux
emergence occurring in active region 10191 on 2002 November 16-17,
we study the development of 3D electric current sheets. We show
that these currents buildup along the 3D serpentine magnetic-field
structure as a result of photospheric diverging horizontal line-tied
motions that emulate the observed photospheric evolution. We observe
that reconnection can not only develop following a pinching evolution
of the serpentine field line, as usually assumed in two-dimensional
geometry, but can also result from 3D shearing deformation of the
magnetic structure. In addition, we report for the first time on the
observation in the UV domain with the Transition Region and Coronal
Explorer (TRACE) of extremely transient loop-like features, appearing
within the emerging flux domain, which link several Ellermam bombs
with one another. We argue that these loop transients can be explained
as a consequence of the currents that build up along the serpentine
magnetic field.
Title: The Nature of Flare Ribbons in Coronal Null-Point Topology
Authors: Masson, S.; Pariat, E.; Aulanier, G.; Schrijver, C. J.
Bibcode: 2009ApJ...700..559M
Altcode:
Flare ribbons are commonly attributed to the low-altitude impact, along
the footprints of separatrices or quasi-separatrix layers (QSLs),
of particle beams accelerated through magnetic reconnection. If
reconnection occurs at a three-dimensional coronal magnetic null
point, the footprint of the dome-shaped fan surface would map a closed
circular ribbon. This paper addresses the following issues: does the
entire circular ribbon brighten simultaneously, as expected because
all fan field lines pass through the null point? And since the spine
separatrices are singular field lines, do spine-related ribbons look
like compact kernels? What can we learn from these observations about
current sheet formation and magnetic reconnection in a null-point
topology? The present study addresses these questions by analyzing
Transition Region and Coronal Explorer and Solar and Heliospheric
Observatory/Michelson Doppler Imager observations of a confined flare
presenting a circular ribbon. Using a potential field extrapolation,
we linked the circular shape of the ribbon with the photospheric
mapping of the fan field lines originating from a coronal null
point. Observations show that the flare ribbon outlining the fan lines
brightens sequentially along the counterclockwise direction and that
the spine-related ribbons are elongated. Using the potential field
extrapolation as initial condition, we conduct a low-β resistive
magnetohydrodynamics simulation of this observed event. We drive the
coronal evolution by line-tied diverging boundary motions, so as to
emulate the observed photospheric flow pattern associated with some
magnetic flux emergence. The numerical analysis allows us to explain
several observed features of the confined flare. The vorticity induced
in the fan by the prescribed motions causes the spines to tear apart
along the fan. This leads to formation of a thin current sheet and
induces null-point reconnection. We also find that the null point
and its associated topological structure is embedded within QSLs,
already present in the asymmetric potential field configuration. We
find that the QSL footprints correspond to the observed elongated
spine ribbons. Finally, we observe that before and after reconnecting
at the null point, all field lines undergo slipping and slip-running
reconnection within the QSLs. Field lines, and therefore particle
impacts, slip or slip-run according to their distance from the spine,
in directions and over distances that are compatible with the observed
dynamics of the ribbons.
Title: Coronal loops, flare ribbons and aurora during slip-running
Authors: Schmieder, Brigitte; Aulanier, Guillaume; Démoulin, Pascal;
Pariat, Etienne
Bibcode: 2009EP&S...61..565S
Altcode: 2009EP&S...61L.565S
Solar two ribbon flares are commonly explained by magnetic field
reconnections in the low corona. During the reconnection energetic
particles (electrons and protons) are accelerated from the reconnection
site. These particles are following the magnetic field lines down
to the chromosphere. As the plasma density is higher in these lower
layers, there are collisions and emission of radiation. Thus bright
ribbons are observed at both ends of flare loops. These ribbons are
typically observed in Hα and in EUV with SoHO and TRACE. As the
time is going, these ribbons are expanding away of each other. In
most studied models, the reconnection site is a separator line,
where two magnetic separatrices intersect. They define four distinct
connectivity domains, across which the magnetic connectivity changes
discontinuously. In this paper, we present a generalization of this
model to 3D complex magnetic topologies where there are no null points,
but quasi-separatrices layers instead. In that case, while the ribbons
spread away during reconnection, we show that magnetic field lines
can quickly slip along them. We propose that this new phenomenon could
explain fast extension of Hα and TRACE 1600 Å ribbons, fast moving
HXR footpoints along the ribbons as observed by RHESSI, and that it
is observed in soft X rays with Hinode/XRT.
Title: Generation of Homologous Coronal Jets
Authors: Pariat, Etienne; Antiochos, S. K.; DeVore, C. R.
Bibcode: 2009SPD....40.3201P
Altcode:
Recent solar observations (e.g. Hinode & STEREO) have revealed
that coronal jets are a more frequent phenomenon than previously
believed. This higher frequency results, in part, from the fact that
jets exhibit a homologous behavior; successive jets re-occur at the
same location. We present the results of 3D numerical simulations
of our model for coronal jets. The simulations were performed with our
state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model
is that a jet is due to the release of twist as a closed field region
undergoes interchange reconnection with surrounding open field. If a
stress is constantly applied at the photospheric boundary we demonstrate
that our model of jets is able to reproduce the observed homologous
property. In addition, we find that two regimes of reconnection can
occur in the simulations. This result has important implications for
the observed link between jets and plumes. This work was supported
by the NASA Theory and SR&T Programs.
Title: Modelling and observations of photospheric magnetic helicity
Authors: Démoulin, P.; Pariat, E.
Bibcode: 2009AdSpR..43.1013D
Altcode:
Mounting observational evidence of the emergence of twisted magnetic
flux tubes through the photosphere have now been published. Such flux
tubes, formed by the solar dynamo and transported through the convection
zone, eventually reach the solar atmosphere. Their accumulation in
the solar corona leads to flares and coronal mass ejections. Since
reconnections occur during the evolution of the flux tubes, the concepts
of twist and magnetic stress become inappropriate. Magnetic helicity,
as a well preserved quantity, in particular in plasma with high magnetic
Reynolds number, is a more suitable physical quantity to use, even
if reconnection is involved. Only recently, it has been realized
that the flux of magnetic helicity can be derived from magnetogram time
series. This paper reviews the advances made in measuring the helicity
injection rate at the photospheric level, mostly in active regions. It
relates the observations to our present theoretical understanding of
the emergence process. Most of the helicity injection is found during
magnetic flux emergence, whereas the effect of differential rotation
is small, and the long-term evolution of active regions is still
puzzling. The photospheric maps of the injection of magnetic helicity
provide new spatial information about the basic properties of the link
between the solar activity and its sub-photospheric roots. Finally,
the newest techniques to measure photospheric flows are reviewed.
Title: A Model for Solar Polar Jets
Authors: Pariat, E.; Antiochos, S. K.; DeVore, C. R.
Bibcode: 2009ApJ...691...61P
Altcode:
We propose a model for the jetting activity that is commonly observed
in the Sun's corona, especially in the open-field regions of polar
coronal holes. Magnetic reconnection is the process driving the jets
and a relevant magnetic configuration is the well known null-point
and fan-separatrix topology. The primary challenge in explaining
the observations is that reconnection must occur in a short-duration
energetic burst, rather than quasi-continuously as is implied by the
observations of long-lived structures in coronal holes, such as polar
plumes. The key idea underlying our model for jets is that reconnection
is forbidden for an axisymmetrical null-point topology. Consequently,
by imposing a twisting motion that maintains the axisymmetry, magnetic
stress can be built up to high levels until an ideal instability
breaks the symmetry and leads to an explosive release of energy via
reconnection. Using three-dimensional magnetohydrodynamic simulations,
we demonstrate that this mechanism does produce massive, high-speed
jets driven by nonlinear Alfvén waves. We discuss the implications
of our results for observations of the solar corona.
Title: Magnetic reconnection and particle accelerationinitiated by
flux emergence
Authors: Masson, S.; Aulanier, G.; Pariat, E.; Klein, K. -L.;
Schrijver, C. J.
Bibcode: 2008sf2a.conf..555M
Altcode:
So as to perform an MHD simulation of the evolution of the corona driven
by the evolution of the photosphere, a key aspect is the definition of
the boundary conditions for reaching a good compromise between physical
conditions and numerical constraints. In this work, we focused on the
simulation of a confined flare observed on Nov 16, 2002. As initial
configuration, we considered a uniform temperature corona, with a
magnetic field resulting from a 3D potential field extrapolation
from a SOHO/MDI magnetogram. We prescribed a velocity field at the
photospheric boundary of the domain, so as to mimic the observed flow
pattern associated to a flux emergence. This resulted in a combination
of ``slipping reconnection'' in a halo of QSLs surrounding a 3D null
point, through which a ``fan reconnection'' regime took place. This
simplified approach of flux emergence has successfully reproduced
the main characteristics of the observed flare: the flare ribbons
observed in the EUV with TRACE being due to the chromospheric impact
of particles accelerated along reconnecting field lines, this bimodal
regime could explain both the shapes and dynamics of these ribbons. We
foresee that this kind of modeling should be able to simulate the
evolution of slipping magnetic flux tubes in open configurations,
allowing to predict the spatio-temporal evolution of particle beams
injected into the heliosphere.
Title: 3D Numerical Simulation of a New Model for Coronal Jets
Authors: Pariat, E.; Antiochos, S.; DeVore, C. R.; Patsourakos, S.
Bibcode: 2008ESPM...12.3.28P
Altcode:
Recent solar observations with STEREO and HINODE have revealed evidence
of twisting motions during the evolution of coronal jets. Furthermore,
the observations indicate that some jets achieve near-Alfvenic
velocities. Most models of jet are not capable of explaining these
new observational features. In addition, the impulsiveness of jets,
manifested as a brief, violent energy release phase in contrast to a
slow, quasi-static energy storage phase storage, is an issue not easily
addressed. We will present the results of 3D numerical simulations
of our model for coronal jets. The simulations were performed with our
state-of-art adaptive mesh MHD solver ARMS. The basic idea of the model
is that a jet is due to the release of magnetic twist when a closed
field region undergoes interchange reconnection with surrounding open
field. The fast reconnection between open and closed field results
in the generation of nonlinear Alfven waves that propagate along
the open field, accelerating plasma upward. We will show how the new
stereoscopically-observed features of jets can be explained by the
results of our numerical simulations
Title: STEREO SECCHI Stereoscopic Observations Constraining the
Initiation of Polar Coronal Jets
Authors: Patsourakos, S.; Pariat, E.; Vourlidas, A.; Antiochos, S. K.;
Wuelser, J. P.
Bibcode: 2008ApJ...680L..73P
Altcode: 2008arXiv0804.4862P
We report on the first stereoscopic observations of polar coronal jets
made by the EUVI/SECCHI imagers on board the twin STEREO spacecraft. The
significantly separated viewpoints (~11°) allowed us to infer the 3D
dynamics and morphology of a well-defined EUV coronal jet for the first
time. Triangulations of the jet's location in simultaneous image pairs
led to the true 3D position and thereby its kinematics. Initially the
jet ascends slowly at ≈10-20 km s-1 and then, after an
apparent "jump" takes place, it accelerates impulsively to velocities
exceeding 300 km s-1 with accelerations exceeding the solar
gravity. Helical structure is the most important geometrical feature
of the jet which shows evidence of untwisting. The jet structure
appears strikingly different from each of the two STEREO viewpoints:
face-on in one viewpoint and edge-on in the other. This provides
conclusive evidence that the observed helical structure is real and
does not result from possible projection effects of single-viewpoint
observations. The clear demonstration of twisted structure in polar jets
compares favorably with synthetic images from a recent MHD simulation of
jets invoking magnetic untwisting as their driving mechanism. Therefore,
the latter can be considered as a viable mechanism for the initiation
of polar jets.
Title: 3D Numerical Simulation and Stereoscopic Observations of
Coronal Jets
Authors: Pariat, E.; Antiochos, S. K.; Patsourakos, S.; DeVore, C. R.
Bibcode: 2008AGUSMSP53A..05P
Altcode:
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted
that magnetic reconnection is the fundamental mechanism that gives
rise to the jets. The improved spatial and temporal resolution of
the STEREO observations in combination with stereoscopy yields new
insights into the origins of coronal jets, and provides detailed data
that can be used to test and refine models. We present the results
of 3D numerical simulations of our model for coronal jets. The
simulations were performed with our state-of-art adaptive mesh MHD
solver ARMS. The basic idea of the model is that a jet is due to
the release of twist as a closed field region undergoes interchange
reconnection with surrounding open field. The photospheric driven
evolution of the structure results in the generation of a non linear
Alfven wave along the open fields. Using stereoscopic EUVI images,
we reveal the presence of such twisted structure in a coronal jet
event. This work was supported, in part, by NASA and ONR.
Title: Understanding the Initiation of Polar Coronal Jets with
STEREO/SECCHI Stereoscopic Observations
Authors: Vourlidas, A.; Patsourakos, S.; Pariat, E.; Antiochos, S.
Bibcode: 2008AGUSMSH23A..02V
Altcode:
Polar coronal jets are collimated transient ejections of plasma
occurring in polar coronal holes. The kinematics and mostly the 3D
morphology of jets place strong constraints on the physical mechanism(s)
responsible for their initiation, and were not accessible before
the STEREO mission. We report on the first stereoscopic observations
of polar coronal jets made by the EUVI/SECCHI imagers on-board the
twin STEREO spacecraft at spacecraft separations of ~ 11° and ~
45°. Triangulations of the jet locations in simultaneous image pairs
led to the true 3D position and thereby their kinematics. The most
important geometrical feature of the observed jets is helical structures
showing evidence of untwisting. The jet structure appear strikingly
different from each of the two STEREO viewpoints: face-on in the
one viewpoint and edge-on in the other. This provides solid evidence
that the observed helical structure is real and not resulting from
possible projection effects of single viewpoint observations. The clear
demonstration of twisted structure in polar jets compares favorably
with synthetic images from a recent MHD simulation of jets invoking
magnetic untwisting as their driving mechanism.
Title: The Build-up of Current Sheets in Complex Topologies by
Photospheric Driving.
Authors: Pariat, E.
Bibcode: 2008AGUSMSH51C..02P
Altcode:
The most violent solar coronal phenomena all involve magnetic
reconnection which allows the release of stored magnetic energy into
other forms of energy. The triggering of solar reconnection in a
low resistivity environment requires the build-up of intense electric
current sheets, which are also the cornerstone of particle acceleration
mechanisms. Magnetic configurations with a complex topology, i.e., with
separatrices, are the most obvious configurations where current sheets
can form, and therefore where reconnection can efficiently occur. As
I will show through several examples, motions of the field lines at
the photospheric level, even if regular, slow and spatially smooth, can
lead to the formation of current sheets along the separatrices. However,
with such topology the formation of the current sheet is extremely fast,
so there is little time for energy to build up before reconnection
sets in. How can large amounts of magnetic energy be stored before
reconnection is triggered? "Quasi-Separatrix Layers" (QSLs), which are
regions where there is a drastic yet continuous change in field-line
linkage, generalizing the definition of separatrices, offer a natural
solution to this storage problem. Based on observational and numerical
examples, I will compare the energy build-up problem in separatrices
and QSLs topologies and discuss the implications on the observable
properties of reconnection.
Title: Modeling Coronal Jets with FLUX
Authors: Rachmeler, L. A.; Pariat, E.; Antiochos, S. K.; Deforest,
C. E.
Bibcode: 2008AGUSMSP43B..01R
Altcode:
We report on a comparative study of coronal jet formation with and
without reconnection using two different simulation strategies. Coronal
jets are features on the solar surface that appear to have some
properties in common with coronal mass ejections, but are less
energetic, massive, and broad. Magnetic free energy is built up over
time and then suddenly released, which accelerates plasma outward in
the form of a coronal jet. We compare results from the ARMS adaptive
mesh and FLUX reconnection-less codes to study the role of reconnection
in this system. This is the first direct comparison between FLUX and
a numerical model with a 3D spatial grid.
Title: 3D numerical simulation and stereoscopic observations of
coronal jets.
Authors: Pariat, Etienne; Antiochos, Spiro; Patsourakos, Spiro;
DeVore, C. R.
Bibcode: 2008cosp...37.2354P
Altcode: 2008cosp.meet.2354P
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted that
magnetic reconnection is the fundamental mechanism that gives rise to
the jets. The improved spatial and temporal resolution of the STEREO
observations in combination with stereoscopy yields new insights into
the origins of coronal jets, and provides detailed data that can be
used to test and refine models. We present the results of a 3D numerical
simulation of our model for coronal jets. The simulations were performed
with our state-of-art adaptive mesh MHD solver ARMS. The basic idea
of the model is that a jet is due to the release of twist as a closed
field region undergoes interchange reconnection with surrounding open
field. The photospheric driven evolution of the structure results in
the generation of nonlinear Alfven waves propagating along the open
field, which drive the jet flows. Using stereoscopic EUVI images,
we reveal the presence of such twisted structure in a coronal jet
event. This work was supported, in part, by NASA and ONR.
Title: Comparison of 3D Numerical Simulations with STEREO Observations
of Coronal Jets
Authors: Pariat, E.; Patsourakos, S.; Antiochos, S. K.; DeVore, C. R.
Bibcode: 2007AGUFMSH41B..03P
Altcode:
Recent solar observations have revealed that coronal jets are a more
frequent phenomenon than previously believed. It is widely accepted that
magnetic reconnection is the fundamental mechanism that gives rise to
the jets. The improved spatial and temporal resolution of the STEREO
observations in combination with stereoscopy yields new insights into
the origins of coronal jets, and provides detailed data that can be
used to test and refine models. We present the results of a 3D numerical
simulation of our model for coronal jets. The simulations were performed
with our state-of-art adaptive mesh MHD solver ARMS. The basic idea
of the model is that a jet is due to the release of twist as a closed
field region undergoes interchange reconnection with surrounding open
field. We compare the structure and dynamics of the simulated jet with
actual EUVI observations, focusing on how the reconfiguration of the
3D magnetic field explains observed properties of the jet. We also
discuss possible signatures for STEREO of twisted structures within
jets. Finally, we discuss the implications of our simulations for
future stereoscopic observations with STEREO. This work was supported,
in part, by NASA and ONR.
Title: A Model for Coronal Hole Jets
Authors: Antiochos, S. K.; Pariat, E.; DeVore, C.
Bibcode: 2007AGUFMSH21B..01A
Altcode:
The recent observations from XRT on Hinode show dramatically that
coronal hole are populated with intense X-ray jets that can reach
heights of solar radii. These jets appear to originate from closed
magnetic-field regions inside the holes; consequently, a natural
explanation for these jets is that they are due to interchange
reconnection between the open field of the hole and the closed field
of an embedded bipole. This type of interchange reconnection has long
been postulated as the driver, not only of coronal jets, but also
for the solar wind itself. We argue, however, that the explosive
nature of the jets imposes severe requirements on the reconnection
that are not easily satisfied by realistic 3D models. In particular,
the reconnection must have a "switch-on" nature in that it stays off
until a substantial store of free energy has been accumulated, but
then turns on abruptly and stays on until much of this free energy is
released. We discuss the possible magnetic topologies of an embedded
bipole in an open field region and present recent 3D simulations of a
model in which interchange reconnection does, indeed, yield a large
burst of energy release. We also discuss the implications of these
results for the Hinode observations. This work was supported, in part,
by NASA, ONR, and the NSF.
Title: Spectrophotometric analysis of Ellerman bombs in the Ca II,
Hα, and UV range
Authors: Pariat, E.; Schmieder, B.; Berlicki, A.; Deng, Y.; Mein,
N.; López Ariste, A.; Wang, S.
Bibcode: 2007A&A...473..279P
Altcode:
Context: Even if Ellerman bombs have been observed in the Hα line
within emerging magnetic flux regions since the early 20th century,
their origin and the mechanisms that lead to their formation have been
strongly debated. Recently, new arguments in favor of chromospheric
magnetic reconnection have been advanced. Ellerman bombs seem to be
the signature of reconnections that take place during the emergence
of the magnetic field.
Aims: We have observed an active region
presenting emergence of magnetic flux. We detected and studied
Ellerman bombs in two chromospheric lines: Ca ii 8542 Å and Hα. We
investigated the link between Ellerman bombs and other structures and
phenomena appearing in an emerging active region: UV bright points,
arch filament systems, and magnetic topology.
Methods: On August
3, 2004, we performed multi-wavelength observations of the active
region NOAA 10655. This active region was the target of SoHO Joint
Observation Program 157. Both SoHO/MDI and TRACE (195 Å and 1600 Å)
were used. Simultaneously, we observed in the Ca ii and Na D1 lines
with the spectro-imager MSDP mode of THEMIS. Alternately to the MSDP,
we used the MTR spectropolarimeter on THEMIS to observe in Hα and
in the Fe i doublet at 6302 Å. We derived the magnetic field vectors
around some Ellerman bombs.
Results: We present the first images
of EBs in the Ca ii line and confirm that Ellerman bombs can indeed
be observed in the Ca ii line, presenting the same “moustache”
geometry profiles as in the Hα line, but with a narrower central
absorption in the Ca ii line, in which the peaks of emission are
around ±0.35 Å. We noticed that the Ellerman bombs observed in the
wings of Ca ii line have an elongated shape - the length about 50%
greater than the width. We derived mean semi-axis lengths of 1.4''
× 2.0''. In the UV time profiles of the Ellerman bombs, we noticed
successive enhanced emissions. The distribution of lifetimes of these
individual impulses presents a strong mode around 210 s. Study of the
magnetic topology shows that 9 out of the 13 EBs are located on the
inversion line of the longitudinal field and that some typical examples
might be associated with a bald patch topology.
Conclusions: We
provide new arguments in favor of the reconnection origin of Ellerman
bombs. The different individual impulses observed in UV may be related
to a bursty mode of reconnection. We also show that this Ca ii 8542
Å chromospheric line is a good indicator of Ellerman bombs and can
bring new information about these phenomena.
Title: Spectrophotometry of Ellerman Bombs with THEMIS
Authors: Pariat, E.; Schmieder, B.; Berlicki, A.; López Ariste, A.
Bibcode: 2007ASPC..368..253P
Altcode:
During coordinated campaigns with THEMIS and space missions (TRACE,
SOHO) emerging flux was observed in multi-wavelengths. Ellerman bombs
(EBs) have been identified in TRACE 1600 Å and in chromospheric
lines. The Hα and Ca II 8542 Å lines present two enhanced emission
peaks respectively at 1 Å and at 0.35 Å which are signatures of
EBs. Vector magnetic field measured in the photosphere are consistent
with previous results indicating the presence of bald-patches under
the EBs and consequently the emergence of horizontal flux tubes from
below the photosphere.
Title: Slip running reconnection in the Sun's atmosphere observed
by RHESSI, SOHO, TRACE and Hinode
Authors: Schmieder, B.; Aulanier, G.; Démoulin, P.; Pariat, E.;
Golub, L.
Bibcode: 2007AGUSMSH22A..01S
Altcode:
Solar double ribbon flares are commonly explained by magnetic field
reconnections in the high corona. The bright ribbons, typically
observed in Halpha, in EUV with SoHO, TRACE correspond to the ends
of the reconnected loops. In most studied cases, the reconnection
site is an X-point, where two magnetic separatrices intersect. In
this presentation, we show a generalization of this model to 3D
complex magnetic topologies where there are no null points, but
quasi-separatrices layers instead. In that case, while the ribbons
spread away during reconnection, we show that magnetic field lines
can quickly slip along them. We propose that this new phenomenon
could explain also fast moving HXR footpoints as observed by RHESSI,
and that it may be observed in soft X rays with XRT.
Title: Computing magnetic energy and helicity fluxes from series of
magnetograms .
Authors: Démoulin, P.; Pariat, E.
Bibcode: 2007MmSAI..78..136D
Altcode:
Magnetic energy and helicity fluxes can now be derived from measurements
of the photospheric magnetic and velocity fields. We show that
only photospheric flux-tube motions are needed to estimate the full
fluxes. The derived maps of flux densities permit to localize where
energy and helicity input occurs in active regions (ARs). The precision
of the energy flux density is dominantly limited by the precision
obtained on the transverse component of the magnetic field. On the
contrary, the helicity flux density requires only the measurement of
the vertical component of the magnetic field. Previously, the magnetic
helicity maps were strongly affected by a false definition of the
helicity flux density involving the magnetic vector potential. Applied
to observations, this approach introduces important fake polarities. We
define a better helicity flux density; it reduces the fake polarities by
more than an order of magnitude. The spatial distribution of helicity
injected into the studied ARs is much more coherent than previously
thought, and presents a dominant sign in each AR. Finally, the correct
helicity flux density could be derived from magnetograms if coronal
connectivities are known.
Title: How to improve the maps of magnetic helicity injection in
active regions?
Authors: Pariat, Etienne; Démoulin, Pascal; Nindos, Alexander
Bibcode: 2007AdSpR..39.1706P
Altcode:
Magnetic helicity, a topological quantity which measures the twist,
the writhe and the shear of a magnetic field, has recently appeared
as a key quantity to understand some mechanisms of the solar activity
such as Coronal Mass Ejections and flare onset. It is thus becoming
of major importance to be able to compute magnetic helicity in active
regions. Computing photospheric maps of the injection of magnetic
helicity provides new spatial information that helps us to understand
basic properties of solar activity, such as where and how magnetic
helicity is injected. Several helicity flux density maps have been
published for different active regions. Unfortunately, the classical
helicity flux density is not a correct physical quantity and it does
induce spurious signals (fake polarities) which mask the real injection
of helicity. To map the real helicity injection, the knowledge of
the complete connectivity of the field lines is fundamental. Even
without the connectivity, improved helicity flux density maps can
be derived. They have fake polarities which are lower by more than a
factor 10 than the previous incorrect maps. Rather than a mixture of
negative and positive injection patterns, they show almost unipolar
injection on the active region scale. This leads to a completely new
way of understanding the dynamics of active regions, in the frame of
magnetic helicity studies.
Title: Companion Event and Precursor of the X17 Flare on 28 October
2003
Authors: Mandrini, C. H.; Demoulin, P.; Schmieder, B.; Deluca, E. E.;
Pariat, E.; Uddin, W.
Bibcode: 2006SoPh..238..293M
Altcode: 2006SoPh..tmp...79M
A major two-ribbon X17 flare occurred on 28 October 2003, starting
at 11:01 UT in active region NOAA 10486. This flare was accompanied
by the eruption of a filament and by one of the fastest halo coronal
mass ejections registered during the October-November 2003 strong
activity period. We focus on the analysis of magnetic field (SOHO/MDI),
chromospheric (NainiTal observatory and TRACE), and coronal (TRACE) data
obtained before and during the 28 October event. By combining our data
analysis with a model of the coronal magnetic field, we concentrate
on the study of two events starting before the main flare. One
of these events, evident in TRACE images around one hour prior to
the main flare, involves a localized magnetic reconnection process
associated with the presence of a coronal magnetic null point. This
event extends as long as the major flare and we conclude that it is
independent from it. A second event, visible in Hα and TRACE images,
simultaneous with the previous one, involves a large-scale quadrupolar
reconnection process that contributes to decrease the magnetic field
tension in the overlaying field configuration; this allows the filament
to erupt in a way similar to that proposed by the breakout model,
but with magnetic reconnection occurring at Quasi-Separatrix Layers
(QSLs) rather than at a magnetic null point.
Title: Slip-Running Reconnection in Quasi-Separatrix Layers
Authors: Aulanier, G.; Pariat, E.; Démoulin, P.; Devore, C. R.
Bibcode: 2006SoPh..238..347A
Altcode: 2006SoPh..tmp...62A; 2006SoPh..tmp...81A
Using time dependent MHD simulations, we study the nature of
three-dimensional magnetic reconnection in thin quasi-separatrix layers
(QSLs), in the absence of null points. This process is believed to
take place in the solar atmosphere, in many solar flares and possibly
in coronal heating. We consider magnetic field configurations which
have previously been weakly stressed by asymmetric line-tied twisting
motions and whose potential fields already possessed thin QSLs. When the
line-tied driving is suppressed, magnetic reconnection is solely due to
the self-pinching and dissipation of narrow current layers previously
formed along the QSLs. A generic property of this reconnection process
is the continuous slippage of magnetic field lines along each other,
while they pass through the current layers. This is contrary to standard
null point reconnection, in which field lines clearly reconnect by
pair and abruptly exchange their connectivities. For sufficiently
thin QSLs and high resistivities, the field line footpoints slip-run
at super-Alfvénic speeds along the intersection of the QSLs with the
line-tied boundary, even though the plasma velocity and resistivity
are there fixed to zero. The slip-running velocities of a given
footpoint have a well-defined maximum when the field line crosses the
thinnest regions of the QSLs. QSLs can then physically behave as true
separatrices on MHD time scales, since magnetic field lines can change
their connections on time scales far shorter than the travel-time of
Alfvén waves along them. Since particles accelerated in the diffusive
regions travel along the field much faster than the Alfvén speed,
slip-running reconnection may also naturally account for the fast
motion of hard X-ray sources along chromospheric ribbons, as observed
during solar flares.
Title: Injection of magnetic flux and helicity in the solar atmosphere
Authors: Pariat, E.
Bibcode: 2006PhDT.........7P
Altcode:
This thesis is related to the mechanisms of emergence into the solar
atmosphere, of two quantities playing key roles in solar activity:
magnetic flux and magnetic helicity. Helicity, which is a topological
measure of twist and shear, is believed to be a conserved quantity
for solar conditions, in the frame of magnetohydrodynamics (MHD). A
crucial phase in the emergence process of these quantities, which are
generated and amplified in the solar interior, are their injection
through the solar photosphere, the transition region between the solar
interior and atmosphere. The first part of my work provided new answers
to questions unsolved by the classical scenario of emergence. I have
analyzed multi-wavelength observations (FGE, TRACE, SoHO, THEMIS) of
an emerging active region. I demonstrated that magnetic flux tubes
emerge with a flat undulated shape and that small scale magnetic
reconnection events, are necessary to this emergence process. Then,
using a 3D MHD numerical simulation, I studied the mechanism of magnetic
reconnection and in particular the natural formation of current layers
where regions of strong variations of magnetic connectivity, called
quasi-separatrix layers, are present. Finally, I demonstrated that the
classical definition of helicity flux density is biased and proposed a
more accurate definition. I applied my new definition to observations
of active regions and showed that the photospheric injection pattern
of magnetic helicity is unipolar and homogenous. This study allows to
link the generation of helicity in the solar atmosphere, its injection
and its distribution in the solar corona and its ejection in the
interplanetary medium.
Title: A new concept for magnetic reconnection : slip-running
reconnection
Authors: Pariat, E.; Aulanier, G.; Démoulin, P.
Bibcode: 2006sf2a.conf..559P
Altcode:
In magnetohydrodynamics (MHD), most models of magnetic reconnection
suppose that this mechanism takes places when the magnetic field
configuration contains separatrices. Separatrices are surfaces
through which the magnetic field connectivity is discontinuous. But
such topological structures are not always present when solar flares
takes place. Quasi-separatrix layers (QSLs), which are regions of
strong variations of magnetic connectivity, are a generalisation
of separatrices. Using a 3D MHD simulation of several solar-like
magnetic configurations containing QSLs, we investigated the link
between the build-up of current layers and the location of QSLs. Thin
current sheets are naturally formed along QSLs whatever the line-tied
boundary driven motions are. When the line-tied driving is suppressed,
magnetic reconnection is solely due to the self-pinching and dissipation
of narrow current layers. In this reconnection process, field line
continuously slip along each other while they pass through the current
layers. This slip-running reconnection may naturally account for
the fast motion of hard X-ray sources along chromospheric ribbons,
as observed during solar flares.
Title: What is the spatial distribution of magnetic helicity injected
in a solar active region?
Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M. A.
Bibcode: 2006A&A...452..623P
Altcode:
Context: .Magnetic helicity is suspected to play a key role in
solar phenomena such as flares and coronal mass ejections. Several
investigations have recently computed the photospheric flux of
magnetic helicity in active regions. The derived spatial maps of the
helicity flux density, called G_A, have an intrinsic mixed-sign patchy
distribution.
Aims: . Pariat et al. (2005) recently showed
that GA is only a proxy of the helicity flux density,
which tends to create spurious polarities. They proposed a better
proxy, Gθ. We investigate here the implications of this
new approach on observed active regions.
Methods: . The magnetic
data are from MDI/SoHO instrument and the photospheric velocities are
computed by local correlation tracking. Maps and temporal evolution of
GA and Gθ are compared using the same data set
for 5 active regions.
Results: . Unlike the usual GA
maps, most of our Gθ maps show almost unipolar spatial
structures because the nondominant helicity flux densities are
significantly suppressed. In a few cases, the Gθ maps still
contain spurious bipolar signals. With further modelling we infer that
the real helicity flux density is again unipolar. On time-scales larger
than their transient temporal variations, the time evolution of the
total helicity fluxes derived from GA and Gθ
show small differences. However, unlike G_A, with Gθ
the time evolution of the total flux is determined primarily by the
predominant-signed flux while the nondominant-signed flux is roughly
stable and probably mostly due to noise.
Conclusions: .Our
results strongly support the conclusion that the spatial distribution
of helicity injected into active regions is much more coherent than
previously thought: on the active region scale the sign of the injected
helicity is predominantly uniform. These results have implications for
the generation of the magnetic field (dynamo) and for the physics of
both flares and coronal mass ejections.
Title: Basic Properties of Mutual Magnetic Helicity
Authors: Demoulin, P.; Pariat, E.; Berger, M. A.
Bibcode: 2006SoPh..233....3D
Altcode:
We derive the magnetic helicity for configurations formed by flux tubes
contained fully or only partially in the spatial domain considered
(called closed and open configurations, respectively). In both cases,
magnetic helicity is computed as the sum of mutual helicity over
all possible pairs of magnetic flux tubes weighted by their magnetic
fluxes. We emphasize that these mutual helicities have properties which
are not those of mutual inductances in classical circuit theory. For
closed configurations, the mutual helicity of two closed flux tubes is
their relative winding around each other (known as the Gauss linkage
number). For open configurations, the magnetic helicity is derived
directly from the geometry of the interlaced flux tubes so it can
be computed without reference to a ground state (such as a potential
field). We derive the explicit expression in the case of a planar and
spherical boundary. The magnetic helicity has two parts. The first
one is given only by the relative positions of the flux tubes on the
boundary. It is the only part if all flux tubes are arch-shaped. The
second part counts the integer number of turns each pair of flux tubes
wind about each other. This provides a general method to compute
the magnetic helicity with discrete or continuous distributions of
magnetic field. The method sets closed and open configurations on an
equal level within the same theoretical framework.
Title: Magnetic reconfiguration before the X 17 Solar flare of
October 28 2003
Authors: Schmieder, B.; Mandrini, C. H.; Démoulin, P.; Pariat, E.;
Berlicki, A.; Deluca, E.
Bibcode: 2006AdSpR..37.1313S
Altcode:
An active region (AR) NOAA 10486, which produced a large number of
X-ray flares during October November 2003, was observed during a
multi-wavelength campaign with ground based and space instruments. We
focus our analysis on the observations of October 28, 2003. The
magnetic field was observed with THEMIS (Na D1) and MDI (Ni I), the
chromosphere with THEMIS (Ca II 8542 Å) and with the Meudon heliograph
in Hα, the EUV images with SOHO/EIT and TRACE. Two pre-events started
just before the major X 17 flare. One was related to localized flux
emergence and lasted until the decay phase of the X flare; while the
second one involved a large scale quadrupolar reconnection, that we
infer by modeling the AR magnetic field. Extended dimming areas across
the equator (EIT), large arcades of post-flare loops (TRACE 195 Å)
and a halo CME (LASCO) were observed consequently after the flare. We
perform an extrapolation of the magnetic field above the photosphere
using a linear force-free-field approximation that allows us to find
the connectivity among the four polarities that would be involved
in the quadrupolar reconnection event. The X 17 flare is plausibly
due to the destabilisation of a twisted flux tube, the bottom part
of this magnetic structure can be visualized by the presence of a
filament. The destabilization is caused by converging and shearing
photospheric motions towards the main magnetic inversion line. The
large scale quadrupolar reconnection related to the second pre-event
would favour the opening of the field above the twisted flux tube and,
consequently, the coronal mass ejection.
Title: On the origin of the 28 October 2003 X17 event and its
companion event
Authors: Mandrini, C. H.; Demoulin, P.; Schmieder, B.; de Luca, E. E.;
Pariat, E.; Uddin, W.
Bibcode: 2006BAAA...49..109M
Altcode:
An X17 flare started at 11:01 UT on 28 October, 2003, in active region
(AR) NOAA 10486. This event was accompanied by a filament eruption
and one of the fastest coronal mass ejections (CMEs) observed during
the extreme activity period of October-November 2003. Combining
chromospheric, coronal and magnetic field data with modeling, we
concentrate in the study of two events that started before the X17
flare. One of them, which appears in UV images one hour before the major
event, is associated with localized magnetic reconnection occurring at
a magnetic mull point. T his event lasts as long as the X17 flare and
our analysis indicates that it is independent of it. The other one,
visible in Hα and UV images and simultaneous with the previous one,
is related to a large scale quadrupolar reconnection process. This
process is similar to the one proposed by the breakout model for the
initiation of CMEs, but it takes place at quasiseparatrices and not
in null points. These results will be published in Solar Physics.
Title: Emergence of undulatory magnetic flux tubes by small scale
reconnections
Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2006AdSpR..38..902P
Altcode:
With Flare Genesis Experiment (FGE), a balloon borne observatory
launched in Antarctica on January 2000, series of high spatial
resolution vector magnetograms, Dopplergrams, and Hα filtergrams
have been obtained in an emerging active region (AR 8844). Previous
analyses of this data revealed the occurence of many short-lived
and small-scale H α brightenings called 'Ellerman bombs'
(EBs) within the AR. We performed an extrapolation of the field above
the photosphere using the linear force-free field approximation. The
analysis of the magnetic topology reveals a close connexion between
the loci of EBs and the existence of "Bald patches" (BP) regions
(BPs are regions where the vector magnetic field is tangential to
the photosphere). Some of these EBs/BPs are magnetically connected
by low-lying field lines, presenting a serpentine shape. This results
leads us to conjecture that arch filament systems and active regions
coronal loops do not result from the smooth emergence of large scale
Ω-loops, but rather from the rise of flat undulatory flux tubes which
get released from their photospheric anchorage by reconnection at BPs,
which observational signature is Ellerman bombs.
Title: How to derive the real pattern of magnetic helicity injection
in an active region?
Authors: Pariat, E.; Nindos, A.; Démoulin, P.; Berger, M.
Bibcode: 2006cosp...36..851P
Altcode: 2006cosp.meet..851P
Magnetic helicity a topological quantity which measures the twist the
writhe and the shear of a magnetic field has recently appeared as
a key quantity to understand some mechanisms of the solar activity
such as Coronal Mass Ejections and flare onset It is thus becoming
of major importance to be able to compute magnetic helicity in active
regions Looking at the pattern of the photospheric injection of magnetic
helicity may provide new useful pieces of information to understand the
basic properties of solar activity If several helicity flux density
maps were published no one yet wondered if helicity flux density is
a correct physical quantity Unfortunately the classical helicity flux
density do induce spurious signal fake polarities which mask the real
injection of helicity To map the real helicity injection the knowledge
of the complete connectivity of the field lines is fundamental Even
without the connectivity improved helicity flux density maps can be
derived which present strong differences with the previous incorrect
maps This leads to a complete new way of understanding the dynamics
of the active region in the frame of the magnetic helicity study
Title: Current sheet formation in quasi-separatrix layers and
hyperbolic flux tubes
Authors: Aulanier, G.; Pariat, E.; Démoulin, P.
Bibcode: 2005A&A...444..961A
Altcode:
In 3D magnetic field configurations, quasi-separatrix layers (QSLs) are
defined as volumes in which field lines locally display strong gradients
of connectivity. Considering QSLs both as the preferential locations for
current sheet development and magnetic reconnection, in general, and as
a natural model for solar flares and coronal heating, in particular,
has been strongly debated issues over the past decade. In this paper,
we perform zero-β resistive MHD simulations of the development of
electric currents in smooth magnetic configurations which are, strictly
speaking, bipolar though they are formed by four flux concentrations,
and whose potential fields contain QSLs. The configurations are driven
by smooth and large-scale sub-Alfvénic footpoint motions. Extended
electric currents form naturally in the configurations, which evolve
through a sequence of quasi non-linear force-free equilibria. Narrow
current layers also develop. They spontaneously form at small scales
all around the QSLs, whatever the footpoint motions are. For long
enough motions, the strongest currents develop where the QSLs are the
thinnest, namely at the Hyperbolic Flux Tube (HFT), which generalizes
the concept of separator. These currents progressively take the shape
of an elongated sheet, whose formation is associated with a gradual
steepening of the magnetic field gradients over tens of Alfvén times,
due to the different motions applied to the field lines which pass
on each side of the HFT. Our model then self-consistently accounts
for the long-duration energy storage prior to a flare, followed by a
switch-on of reconnection when the currents reach the dissipative scale
at the HFT. In configurations whose potential fields contain broader
QSLs, when the magnetic field gradients reach the dissipative scale,
the currents at the HFT reach higher magnitudes. This implies that
major solar flares which are not related to an early large-scale ideal
instability, must occur in regions whose corresponding potential fields
have broader QSLs. Our results lead us to conjecture that physically,
current layers must always form on the scale of the QSLs. This implies
that electric currents around QSLs may be gradually amplified in time
only if the QSLs are broader than the dissipative length-scale. We
also discuss the potential role of QSLs in coronal heating in bipolar
configurations made of a continuous distribution of flux concentrations.
Title: Observation of Small Scale Reconnection Role in Undulated
Flux Tube Emergence
Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2005ESASP.596E..34P
Altcode: 2005ccmf.confE..34P
No abstract at ADS
Title: Erratum: Photospheric flux density of magnetic helicity
Authors: Pariat, E.; Démoulin, P.; Berger, M. A.
Bibcode: 2005A&A...442.1105P
Altcode:
No abstract at ADS
Title: Photospheric flux density of magnetic helicity
Authors: Pariat, E.; Démoulin, P.; Berger, M. A.
Bibcode: 2005A&A...439.1191P
Altcode:
Several recent studies have developed the measurement of magnetic
helicity flux from the time evolution of photospheric magnetograms. The
total flux is computed by summing the flux density over the analyzed
region. All previous analyses used the density GA (=-2
( A\cdot {u}) B_n) which involves the vector potential A of the
magnetic field. In all the studied active regions, the density
GA has strong polarities of both signs with comparable
magnitude. Unfortunately, the density GA can exhibit spurious
signals which do not provide a true helicity flux density. The main
objective of this study is to resolve the above problem by defining the
flux of magnetic helicity per unit surface. In a first step, we define a
new density, Gθ, which reduces the fake polarities by more
than an order of magnitude in most cases (using the same photospheric
data as G_A). In a second step, we show that the coronal linkage needs
to be provided in order to define the true helicity flux density. It
represents how all the elementary flux tubes move relatively to a
given elementary flux tube, and the helicity flux density is defined
per elementary flux tube. From this we define a helicity flux per unit
surface, GΦ. We show that it is a field-weighted average
of Gθ at both photospheric feet of coronal connections. We
compare these three densities (G_A, Gθ, GΦ)
using theoretical examples representing the main cases found in
magnetograms (moving magnetic polarities, separating polarities, one
polarity rotating around another one and emergence of a twisted flux
tube). We conclude that Gθ is a much better proxy of the
magnetic helicity flux density than GA because most fake
polarities are removed. Indeed Gθ gives results close to
GΦ and should be used to monitor the photospheric injection
of helicity (when coronal linkages are not well known). These results
are applicable to the results of any method determining the photospheric
velocities. They can provide separately the flux density coming from
shearing and advection motions if plasma motions are known.
Title: Flux tube emergence, from photosphere to corona
Authors: Pariat, E.; Schmieder, B.; Aulanier, G.
Bibcode: 2004sf2a.conf..103P
Altcode: 2004sf2a.confE.339P
From a campaign of multi-wavelength observations of an emerging active
region, we have studied the dynamics of the solar atmosphere due to
this emergence and the magnetic field topology of the active region. In
addition with the observations obtained with Yohkoh, SOHO and TRACE,
a balloon borne 80 cm telescope (Flare Genesis Experiment) provided us
a series of high spatial resolution vector magnetograms. For the first
time we highlight that magnetic flux tubes do not directly emerge with
a large Omega-loop shape, as suggest the TRACE observations of the
corona, but rather within an undulatory shape. We demonstrated that
the resistive Parker instability allows the flux tube to go through
the low atmosphere. This result has been obtained by performing
an extrapolation of the field above the active region.
Title: Resistive Emergence of Undulatory Flux Tubes
Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2004ApJ...614.1099P
Altcode:
During its 2000 January flight, the Flare Genesis Experiment observed
the gradual emergence of a bipolar active region, by recording a series
of high-resolution photospheric vector magnetograms and images in the
blue wing of the Hα line. Previous analyses of these data revealed the
occurrence of many small-scale, transient Hα brightenings identified
as Ellerman bombs (EBs). They occur during the flux emergence,
and many of them are located near moving magnetic dipoles in which
the vector magnetic field is nearly tangential to the photosphere. A
linear force-free field extrapolation of one of the magnetograms was
performed to study the magnetic topology of small-scale EBs and their
possible role in the flux emergence process. We found that 23 out of 47
EBs are cospatial with bald patches (BPs), while 15 are located at the
footpoints of very flat separatrix field lines passing through distant
BPs. We conclude that EBs can be due to magnetic reconnection, not only
at BP locations, but also along their separatrices, occurring in the
low chromosphere. The topological analysis reveals, for the first time,
that many EBs and BPs are linked by a hierarchy of elongated flux tubes
showing aperiodic spatial undulations, whose wavelengths are typically
above the threshold of the Parker instability. These findings suggest
that arch filament systems and coronal loops do not result from the
smooth emergence of large-scale Ω-loops from below the photosphere,
but rather from the rise of undulatory flux tubes whose upper parts
emerge because of the Parker instability and whose dipped lower parts
emerge because of magnetic reconnection. EBs are then the signature
of this resistive emergence of undulatory flux tubes.
Title: Emergence of undulatory magnetic flux tubes by small scale
reconnections
Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2004cosp...35.1482P
Altcode: 2004cosp.meet.1482P
With Flare Genesis Experiment (FGE), a balloon borne observatory
launched in Antarctica on January 2000, series of high spatial
resolution vector magnetograms, Dopplergrams, and Hα filtergrams
have been obtained in an emerging active region (AR 8844). Previous
analyses of this data revealed the occurence of many short-lived and
small-scale Hα brightenings called 'Ellerman bombs' (EBs) within the
AR. We performed an extrapolation of the field above the photosphere
using the linear force-free field approximation. The analysis of the
magnetic topology reveals a close connexion between the loci of EBs
and the existence of ``Bald patches'' regions (BPs are regions where
the vector magnetic field is tangential to the photosphere). Among
47 identified EBs, we found that 23 are co-spatial with a BP, while
19 are located at the footpoint of very flat separatrix field lines
passing throught a distant BP. We reveal for the first time that
some of these EBs/BPs are magneticaly connected by low-lying lines,
presenting a 'sea-serpent' shape. This results leads us to conjecture
that arch filament systems and active regions coronal loops do not
result from the smooth emergence of large scale Ω loops, but rather
from the rise of flat undulatory flux tubes which get released from
their photospheric anchorage by reconnection at BPs, whose observational
signature is Ellerman bombs.
Title: Flare Genesis Experiment: magnetic topology of Ellerman bombs
Authors: Schmieder, B.; Pariat, E.; Aulanier, G.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2002ESASP.506..911S
Altcode: 2002svco.conf..911S; 2002ESPM...10..911S
Flare Genesis Experiment (FGE), a balloon borne Observatory was launched
in Antarctica on January 10, 2000 and flew during 17 days. FGE consists
of an 80 cm Cassegrain telescope with an F/1.5 ultra-low-expansion
glass primary mirror and a crystalline silicon secondary mirror. A
helium-filled balloon carried the FGE to an altitude of 37 km
(Bernasconi et al. 2000, 2001). We select among all the observations a
set of high spatial and temporal resolution observations of an emerging
active region with numerous Ellerman bombs (EBs). Statistical and
morphology analysis have been performed. We demonstrate that Ellerman
bombs are the result of magnetic reconnection in the low chromosphere
by a magnetic topology analysis. The loci of EBs coincide with "bald
patches" (BPs). BPs are regions where the vector field is tangential to
the boundary (photosphere) along an inversion line. We conclude that
emerging flux through the photosphere is achieved through resistive
emergence of U loops connecting small Ω loops before rising in the
chromosphere and forming Arch Filament System (AFS).
Title: Vector magnetic field observations of flux tube emergence
Authors: Schmieder, B.; Aulanier, G.; Pariat, E.; Georgoulis, M. K.;
Rust, D. M.; Bernasconi, P. N.
Bibcode: 2002ESASP.505..575S
Altcode: 2002IAUCo.188..575S; 2002solm.conf..575S
With Flare Genesis Experiment (FGE), a balloon borne Observatory high
spatial and temporal resolution vector magnetograms have been obtained
in an emerging active region. The comparison of the observations
(FGE and TRACE) with a linear force-free field analysis of the region
shows where the region is non-force-free. An analysis of the magnetic
topology furnishes insights into the existence of "bald patches"
regions (BPs are regions where the vector field is tangential to the
boundary (photosphere) along an inversion line). Magnetic reconnection
is possible and local heating of the chromopshere is predicted near the
BPs. Ellerman bombs (EBs) were found to coincide with few BPs computed
from a linear force-free extrapolation of the observed longitudinal
field. But when the actual observations of transverse fields were used
to identify BPs, then the correspondence with EB positions improved
significantly. We conclude that linear force-free extrapolations must
be done with the true observed vertical fields, which require the
measurement of the three components of the magnetic field.