Author name code: pariat
ADS astronomy entries on 2022-09-14
author:"Pariat, Etienne" 
------------------------------------------------------------------------  

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<SUP>∘</SUP> ambiguity
  in the orientation of the transverse component. The successful
  launch and operation of Solar Orbiter have made the removal of
  the 180<SUP>∘</SUP> 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 &amp; 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR
  /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <BR
  /> 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. <BR />
  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}<SUB>{{J</SUB>}}| /| {H}<SUB>{ \mathcal V </SUB>}| , shows a strong
  ability to indicate the eruptive potential of a solar AR. However, |
  {H}<SUB>{{J</SUB>}}| /| {H}<SUB>{ \mathcal V </SUB>}| 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}<SUB>{ \mathcal V </SUB>}/{φ }<SUP>2</SUP>, 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. <BR /> 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. <BR />
  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. <BR /> 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 &amp; 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. <BR /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
  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. <P />A movie is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201629654/olm">http://www.aanda.org</A>

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 L<SUB>ss</SUB>, WL<SUB>ss</SUB>, L<SUB>sg</SUB>,
  and WL<SUB>sg</SUB>, as well as the new current integral WL<SUB>sc</SUB>
  and length L<SUB>sc</SUB> 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<SUP>-1</SUP>,
  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. <P />The movie associated to Fig. 2 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201629703/olm">http://www.aanda.org</A>

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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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). <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P />The movie associated to Fig. 1 is available at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628984/olm">http://www.aanda.org</A>

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 L<SUB>ssm</SUB> and L<SUB>sgm</SUB> and WL<SUB>sg</SUB> 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. <BR /> 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. <BR />
  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. <BR /> Results: The parametric study
  validates our model of jets for plasma β ranging from 10<SUP>-3</SUP>
  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. <BR
  /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <P
  />A movie associated to Fig. 1 is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201628406/olm">http://www.aanda.org</A>

Title: Erratum: "Hooked Flare Ribbons and Flux-rope Related QSL
    Footprints"<A href="/abs/2016ApJ...823...62Z">(2016, ApJ, 823, 62)</A>
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. <BR /> 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. <BR /> 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. <BR /> 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 (&lt;2.2%), in particular compared to the relative dissipation
  of magnetic energy (&gt;30 times higher). We finally illustrate
  how the helicity-flux terms involving velocity components are gauge
  dependent, which limits their physical meaning. <BR /> 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. <P />Appendix A is available in electronic form at <A
  href="http://www.aanda.org/10.1051/0004-6361/201525811/olm">http://www.aanda.org</A>

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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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 (&lt;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 (10<SUP>10</SUP> cm<SUP>-3</SUP>) 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) ×
  10<SUP>40</SUP> Mx<SUP>2</SUP> hr<SUP>-1</SUP>. 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<SUP>-1</SUP>. 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. <BR /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR />
  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. <BR /> 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. <P />Appendices are available in electronic form at
  <A href="http://www.aanda.org">http://www.aanda.org</A>

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. <BR /> Aims: The largest solar flares observed over the
  past few decades have reached energies of a few times 10<SUP>32</SUP>
  erg, possibly up to 10<SUP>33</SUP> erg. Flares in active Sun-like
  stars reach up to about 10<SUP>36</SUP> 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. <BR /> 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. <BR />
  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 × 10<SUP>33</SUP> erg. <BR /> 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 &lt; 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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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<SUB>⊙</SUB>, 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. <P />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. <BR /> 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. <BR /> 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. <BR /> 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. <BR /> 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 &gt;1 to &lt;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 &amp; 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. <P />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. <P />This work was supported
  by the NASA Theory and SR&amp;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. <P />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. <P />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<SUP>-1</SUP> and then, after an
  apparent "jump" takes place, it accelerates impulsively to velocities
  exceeding 300 km s<SUP>-1</SUP> 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. <BR />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. <BR />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. <BR />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. <BR />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. <BR /> Aims: . Pariat et al. (2005) recently showed
  that G<SUB>A</SUB> is only a proxy of the helicity flux density,
  which tends to create spurious polarities. They proposed a better
  proxy, G<SUB>θ</SUB>. We investigate here the implications of this
  new approach on observed active regions.<BR /> Methods: . The magnetic
  data are from MDI/SoHO instrument and the photospheric velocities are
  computed by local correlation tracking. Maps and temporal evolution of
  G<SUB>A</SUB> and G<SUB>θ</SUB> are compared using the same data set
  for 5 active regions.<BR /> Results: . Unlike the usual G<SUB>A</SUB>
  maps, most of our G<SUB>θ</SUB> maps show almost unipolar spatial
  structures because the nondominant helicity flux densities are
  significantly suppressed. In a few cases, the G<SUB>θ</SUB> 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 G<SUB>A</SUB> and G<SUB>θ</SUB>
  show small differences. However, unlike G_A, with G<SUB>θ</SUB>
  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. <BR /> 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 <SUB>α</SUB> 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 G<SUB>A</SUB> (=-2
  ( A\cdot {u}) B_n) which involves the vector potential A of the
  magnetic field. In all the studied active regions, the density
  G<SUB>A</SUB> has strong polarities of both signs with comparable
  magnitude. Unfortunately, the density G<SUB>A</SUB> 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, G<SUB>θ</SUB>, 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, G<SUB>Φ</SUB>. We show that it is a field-weighted average
  of G<SUB>θ</SUB> at both photospheric feet of coronal connections. We
  compare these three densities (G_A, G<SUB>θ</SUB>, G<SUB>Φ</SUB>)
  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 G<SUB>θ</SUB> is a much better proxy of the
  magnetic helicity flux density than G<SUB>A</SUB> because most fake
  polarities are removed. Indeed G<SUB>θ</SUB> gives results close to
  G<SUB>Φ</SUB> 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. <P />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.

