Author name code: aulanier ADS astronomy entries on 2022-09-14 author:"Aulanier, Guillaume" ------------------------------------------------------------------------ Title: Filament Leg--Leg Reconnection as a Source of Prominent Supra-Arcade Downflows Authors: Dudik, Jaroslav; Aulanier, Guillaume; Kasparova, Jana; Karlicky, Marian; Zemanova, Alena; Lorincik, Juraj; Druckmuller, Miloslav Bibcode: 2022arXiv220900306D Altcode: We report on interaction of the legs of the erupting filament of 2012 August 31 and associated prominent supra-arcade downflows (P-SADs) as observed by the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory. We employ a number of image processing techniques to enhance weak interacting features. As the filament erupts, both legs stretch outwards. The positive-polarity leg also untwists and splits into two parts. The first part runs into the conjugate (negative-polarity) leg, tearing it apart. The second part then converges into the remnant of the conjugate leg, after which both weaken and finally disappear. All these episodes of interaction of oppositely-oriented filament legs are followed by appearance of P-SADs, seen in the on-disk projection to be shaped as loop-tops, along with many weaker SADs. All SADs are preceded by hot supra-arcade downflowing loops. This observed evolution is consistent with the three-dimensional rr-rf (leg-leg) reconnection, where the erupting flux rope reconnects with itself. In our observations, as well as in some models, the reconnection in this geometry is found to be long-lasting. It plays a substantial role in the evolution of the flux rope of the erupting filament and leads to prominent supra-arcade downflows. Title: The European Solar Telescope Authors: Quintero Noda, C.; Schlichenmaier, R.; Bellot Rubio, L. R.; Löfdahl, M. G.; Khomenko, E.; Jurcak, J.; Leenaarts, J.; Kuckein, C.; González Manrique, S. J.; Gunar, S.; Nelson, C. J.; de la Cruz Rodríguez, J.; Tziotziou, K.; Tsiropoula, G.; Aulanier, G.; Collados, M.; the EST team Bibcode: 2022arXiv220710905Q Altcode: The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope (SST), the German Vacuum Tower Telescope (VTT) and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THÉMIS), and the Dutch Open Telescope (DOT). With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems. Title: Mini solar flare and jet due to small scale surface motions Authors: Joshi, Reetika; Vilmer, Nicole; Chandra, Ramesh; Heinzel, Petr; Bommier, Veronique; Schmieder, Brigitte; Aulanier, Guillaume; Tomin, James Bibcode: 2022cosp...44.2535J Altcode: Here we present the study of the fine structure and dynamics of the plasma at a jet base forming a mini-flare between two emerging magnetic fluxes (EMFs) observed with Interface Region Imaging Spectrograph (IRIS) and the Solar Dynamics Observatory instruments. This active region is an outcome of the collapse of two EMFs overlaid by arch filament systems. We observed that, before the jet an extension of the flux rope was present and a part of it was detached and formed a small bipole with a bald patch region, which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time in the site of reconnection, the IRIS C II, Si IV, and Mg II line profiles present bi directional flows in a tiny region. These types of spectra are typically associated with twist, rotation, or the presence of plasma in helical structures. The tilt observed in our spectra can be explained by the presence of a helical structure at the jet base during the reconnection process due to a transfer of the twist from a flux rope in the vicinity of the jet. We also combined the observations of the Balmer continuum obtained with the IRIS (spectra and SJIs 2832 Å). The calibrated Balmer continuum was compared to non-local thermodynamic equilibrium radiative transfer flare models and the radiated energy was estimated. Assuming thick target HXR emission, we calculated the energy of the non thermal electrons detected by the Fermi/GBM and compared it to the radiated energy. The electron-beam flux estimated from Fermi/GBM between 10$ ^{9}$ and 10$ ^{10}$ erg s$ ^{-1}$ cm$ ^{-2}$ is consistent with the beam flux required in non-LTE radiative transfer models to obtain the excess of Balmer continuum emission observed in this IRIS spectra. We conclude that the bombardment of electrons could be in a region smaller than the IRIS spatial resolution. Title: Empirical atmosphere model in a mini flare during magnetic reconnection Authors: Schmieder, Brigitte; Joshi, Reetika; Chandra, Ramesh; Aulanier, Guillaume; Tei, Akiko; Heinzel, Petr; Tomin, James; Vilmer, Nicole; Bommier, Veronique Bibcode: 2021arXiv211206790S Altcode: A spatio-temporal analysis of IRIS spectra of MgII, CII, and SiIV ions allows us to study the dynamics and the stratification of the flare atmosphere along the line of sight during the magnetic reconnection phase at the jet base. Strong asymmetric MgII and CII line profiles with extended blue wings observed at the reconnection site are interpreted by the presence of two chromospheric temperature clouds: one explosive cloud with blueshifts at 290 km/s and one cloud with smaller Doppler shift (around 36 km/s). Simultaneously at the same location a mini flare was observed with strong emission in multi temperatures (AIA), in several spectral IRIS lines (e.g. Oiv and Siiv, Mgii), absorption of identified chromospheric lines in Siiv line profile, enhancement of the Balmer continuum and X-ray emission by FERMI/GBM. With the standard thick-target flare model we calculate the energy of non thermal electrons observed by FERMI and compare it to the energy radiated by the Balmer continuum emission. We show that the low energy input by non thermal electrons above 20 keV was still sufficient to produce the excess of Balmer continuum. Title: The return of the jet Authors: Aulanier, Guillaume Bibcode: 2021NatAs...5.1096A Altcode: 2021NatAs.tmp..129A Turmoil has engulfed the solar community for decades about which physical mechanisms are sufficient to trigger and drive solar eruptions. New high-resolution numerical magnetohydrodynamic simulations bring an old idea back into the light: the reconnection jet from the tether-cutting model. Title: Saddle-shaped Solar Flare Arcades Authors: Lörinčík, Juraj; Dudík, Jaroslav; Aulanier, Guillaume Bibcode: 2021ApJ...909L...4L Altcode: 2021arXiv210210858L Arcades of flare loops form as a consequence of magnetic reconnection powering solar flares and eruptions. We analyze the morphology and evolution of flare arcades that formed during five well-known eruptive flares. We show that the arcades have a common saddle-like shape. The saddles occur despite the fact that the flares were of different classes (C to X), occurred in different magnetic environments, and were observed in various projections. The saddles are related to the presence of longer, relatively higher, and inclined flare loops, consistently observed at the ends of the arcades, which we term "cantles." Our observations indicate that cantles typically join straight portions of flare ribbons with hooked extensions of the conjugate ribbons. The origin of the cantles is investigated in stereoscopic observations of the 2011 May 9 eruptive flare carried out by the Atmospheric Imaging Assembly and Extreme Ultraviolet Imager. The mutual separation of the instruments led to ideal observational conditions allowing for simultaneous analysis of the evolving cantle and the underlying ribbon hook. Based on our analysis we suggest that the formation of one of the cantles can be explained by magnetic reconnection between the erupting structure and its overlying arcades. We propose that the morphology of flare arcades can provide information about the reconnection geometries in which the individual flare loops originate. Title: Multi-thermal atmosphere of a mini-solar flare during magnetic reconnection observed with IRIS Authors: Joshi, Reetika; Schmieder, Brigitte; Tei, Akiko; Aulanier, Guillaume; Lörinčík, Juraj; Chandra, Ramesh; Heinzel, Petr Bibcode: 2021A&A...645A..80J Altcode: 2020arXiv201015401J Context. The Interface Region Imaging Spectrograph (IRIS) with its high spatial and temporal resolution facilitates exceptional plasma diagnostics of solar chromospheric and coronal activity during magnetic reconnection.
Aims: The aim of this work is to study the fine structure and dynamics of the plasma at a jet base forming a mini-flare between two emerging magnetic fluxes (EMFs) observed with IRIS and the Solar Dynamics Observatory instruments.
Methods: We proceed to a spatio-temporal analysis of IRIS spectra observed in the spectral ranges of Mg II, C II, and Si IV ions. Doppler velocities from Mg II lines were computed using a cloud model technique.
Results: Strong asymmetric Mg II and C II line profiles with extended blue wings observed at the reconnection site (jet base) are interpreted by the presence of two chromospheric temperature clouds: one explosive cloud with blueshifts at 290 km s-1 and one cloud with smaller Doppler shift (around 36 km s-1). Simultaneously at the same location (jet base), strong emission of several transition region lines (e.g. O IV and Si IV), emission of the Mg II triplet lines, and absorption of identified chromospheric lines in Si IV broad profiles have been observed and analysed.
Conclusions: Such observations of IRIS line and continuum emissions allow us to propose a stratification model for the white light, mini-flare atmosphere with multiple layers of different temperatures along the line of sight in a reconnection current sheet. It is the first time that we could quantify the fast speed (possibly Alfvénic flows) of cool clouds ejected perpendicularly to the jet direction via the cloud model technique. We conjecture that the ejected clouds come from plasma which was trapped between the two EMFs before reconnection or be caused by chromospheric-temperature (cool) upflow material similar to a surge during reconnection.

Movies are available at https://www.aanda.org Title: Magnetic support of the solar filaments Authors: Schmieder, Brigitte; Aulanier, Guillaume; Gunár, Stanislav; Dudik, Jaroslav; Heinzel, Petr Bibcode: 2021cosp...43E1766S Altcode: The scale height in prominence is 500 km at maximum. However, non activated prominences can reach an altitude of 50000 km or more. Hanging filaments observed over the limb give the impression of quasi vertical structures. It was already the view in the past but reactivated by the movies of Hinode in Ca II line showing fuzzing vertical structures. In the corona it is not possible to measure the magnetic field supporting prominences. Only extrapolations of the photospheric magnetic field explain how cool plasma embedded in the dips of the magnetic field lines can be supported in the hot corona. I will review a few papers showing the distribution of the cool plasma dips and the observations of filaments on the disk visible in H$\alpha$ and in UV. A recent paper shows how a 3D extrapolation model and a radiative-transfer based H$\alpha$ visualization method leads to H$\alpha$ prominence fine structures. Finally, I will discuss the perspective effects on the perceived morphology of observed and modeled prominences. Title: Twist transfer to a solar jet from a big flux rope detected in the HMI magnetogram Authors: Joshi, Reetika; Schmieder, Brigitte; Aulanier, Guillaume; Chandra, Ramesh; Bommier, Veronique Bibcode: 2021cosp...43E1752J Altcode: Solar jets often have a helical structure containing both hot and cool ejected plasma. Different mechanisms are proposed to trigger jets by magnetic reconnection between the emergence of magnetic flux and environment, or induced by twisted photospheric motions bringing the system to instability. Multi-wavelength observations of a twisted jet observed with the AIA and IRIS is presented to understand how the twist was injected in the jet from a flux rope, fortunately, IRIS spectrographic slit was just crossing the reconnection site. This active region is a result of the collapse of two emerging magnetic fluxes (EMFs) overlaid by arch filament systems. In the magnetic field maps, we evidenced the pattern of a long sigmoidal flux rope along the polarity inversion line between the two EMFs which is the site of the reconnection. Before the jet, there was an extension of the flux rope, and a part of it was detached and formed a small bipole with a bald patch region which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time of the reconnection, the Mg II spectra exhibited a strong extension of the blue wing which is decreasing over a distance of 10 Mm (from -300 km/s to a few km/s). This is the signature of the transfer of the twist to the jet. Comparison with numerical magnetohydrodynamics simulations confirmed the existence of the long flux rope in the neighborhood of the jet. We conjecture that there is a transfer of twist to the jet during the extension of the flux rope to the reconnection site without the flux rope eruption. The reconnection would start in the low atmosphere in the bald patch reconnection region and extend at an X-point along the current sheet formed above. Title: Signature of the expansion of eruptive flux ropes measured by electric currents Authors: Schmieder, Brigitte; Aulanier, Guillaume; Janvier, Miho; Masson, Sophie; Barczynski, Krzysztof Bibcode: 2021cosp...43E1758S Altcode: MHD models demonstrate that hooks of flare ribbons are the footprints of eruptive flux ropes and that a decrease of the electric currents could be the signature of the evolution of the coronal magnetic field, e.g. the expansion of a line-tied flux rope with constant end-to-end external twist during the eruption. However in circuit models the surface electric current has a subsurface fixed source and therefore the currents should be constant . We analyze 19 X-class flares observed by Solar Dynamics Observatory (SDO) from 2011 to 2016, where flare ribbons with hooks are identifiable. For the first time fine measurements of time-evolution of electric currents inside the hooks in the observations as well as in the OHM 3D MHD simulation are performed. Our analysis shows a decrease of the electric current in the area surrounded by the ribbon hooks during and after the eruption. In the simulation the rate of current deceasing is similar to that of the field line elongation. So we interpret the decrease of the electric currents as due to the expansion of the flux rope in the corona during the eruption. Our analysis brings a new stone to the standard flare model in 3D. Title: Imaging Evidence for Solar Wind Outflows Originating from a Coronal Mass Ejection Footpoint Authors: Lörinčík, Juraj; Dudík, Jaroslav; Aulanier, Guillaume; Schmieder, Brigitte; Golub, Leon Bibcode: 2021ApJ...906...62L Altcode: 2020arXiv201004250L We report on Atmospheric Imaging Assembly observations of plasma outflows originating in a coronal dimming during a 2015 April 28 filament eruption. After the filament started to erupt, two flare ribbons formed, one of which had a well-visible hook enclosing a core (twin) dimming region. Along multiple funnels located in this dimming, a motion of plasma directed outward started to be visible in the 171 and 193 Å filter channels of the instrument. In time-distance diagrams, this motion generated a strip-like pattern, which lasted for more than 5 hr and whose characteristics did not change along the funnel. We therefore suggest the motion is a signature of outflows corresponding to velocities ranging between ≍70 and 140 km s-1. Interestingly, the pattern of the outflows and their velocities were found to be similar to those we observed in a neighboring ordinary coronal hole. Therefore, the outflows were most likely a signature of a coronal mass ejection-induced solar wind flowing along the open-field structures rooted in the dimming region. Further, the evolution of the hook encircling the dimming region was examined in the context of the latest predictions imposed for 3D magnetic reconnection. The observations indicate that the filament's footpoints were, during their transformation to the dimming region, reconnecting with surrounding canopies. To our knowledge, our observations present the first imaging evidence for outflows of plasma from a dimming region. Title: Links between prominence/filament magnetic field and plasma: What can 3D WPFS models teach us? Authors: Gunár, Stanislav; Schmieder, Brigitte; Aulanier, Guillaume; Heinzel, Petr; Mackay, Duncan; Dudik, Jaroslav Bibcode: 2021cosp...43E1769G Altcode: The magnetic field constitutes the skeleton and the driving force of prominences/filaments. It supports the dense prominence plasma against gravity and insulates it from the hot, coronal environment. The magnetic field is also responsible for the prominence stability, evolution and eruptions which affect the heliosphere and ultimately the Earth. However, a strong imbalance exists between the numerous efforts in detailed modelling of prominence magnetic field and its understanding from observations. That is due to the complex nature of the direct (and indirect) observations of solar magnetic fields which are challenging at the best of times and even more so in prominences or filaments. The direct observations of the prominence magnetic field require high-precision spectro-polarimetric measurements and realistic assumptions about the plasma structure which allow us to infer the field configuration from its effect on the polarized light emergent from the observed structures. The indirect observations rely on the perceived location, shape and dynamics of the prominence or filament plasma, often using moving small-scale plasma structures as tracers guided by the field lines. Both methods thus rely on the presence of observable plasma in the magnetic field configuration, and on the radiation which carries the information about the in-situ conditions to the observer. No prominence/filament magnetic field measurements are made without these two additional components, which are sometimes an afterthought in the magnetic field models. We have developed 3D Whole-Prominence Fine Structure (WPFS) models to illuminate the links between the prominence magnetic field, its plasma distributed among numerous fine structures and the radiation which carries the information about the prominence physical conditions to the observer. What can we learn from these 3D models? For example, we can see that a small change of the magnetic field configuration can have a large effect on the perceived structure of prominences and filaments visible in the H-alpha line. Consequently, this means that significant changes observed in prominences or filaments do not need to suggest that equally large changes in the underlying magnetic field configuration had to occur. In another example, we see that seemingly incomparable differences in the morphological look of prominences (long horizontal fine structures versus small blobs of plasma arranged into more-less vertical features) may not need to imply the existence of radically different magnetic field configurations. Rather, they might simply be manifestations of projection effects that can differ greatly depending on the viewing angle under which we observe the naturally three-dimensional prominences/filaments. Title: Decoding the Pre-Eruptive Magnetic Field Configurations of Coronal Mass Ejections Authors: Patsourakos, S.; Vourlidas, A.; Török, T.; Kliem, B.; Antiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Leake, J. E.; Moore, R.; Nindos, A.; Syntelis, P.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2020SSRv..216..131P Altcode: 2020arXiv201010186P A clear understanding of the nature of the pre-eruptive magnetic field configurations of Coronal Mass Ejections (CMEs) is required for understanding and eventually predicting solar eruptions. Only two, but seemingly disparate, magnetic configurations are considered viable; namely, sheared magnetic arcades (SMA) and magnetic flux ropes (MFR). They can form via three physical mechanisms (flux emergence, flux cancellation, helicity condensation). Whether the CME culprit is an SMA or an MFR, however, has been strongly debated for thirty years. We formed an International Space Science Institute (ISSI) team to address and resolve this issue and report the outcome here. We review the status of the field across modeling and observations, identify the open and closed issues, compile lists of SMA and MFR observables to be tested against observations and outline research activities to close the gaps in our current understanding. We propose that the combination of multi-viewpoint multi-thermal coronal observations and multi-height vector magnetic field measurements is the optimal approach for resolving the issue conclusively. We demonstrate the approach using MHD simulations and synthetic coronal images. Title: The role of small-scale surface motions in the transfer of twist to a solar jet from a remote stable flux rope Authors: Joshi, Reetika; Schmieder, Brigitte; Aulanier, Guillaume; Bommier, Véronique; Chandra, Ramesh Bibcode: 2020A&A...642A.169J Altcode: 2020arXiv200806887J Context. Jets often have a helical structure containing ejected plasma that is both hot and also cooler and denser than the corona. Various mechanisms have been proposed to explain how jets are triggered, primarily attributed to a magnetic reconnection between the emergence of magnetic flux and environment or that of twisted photospheric motions that bring the system into a state of instability.
Aims: Multi-wavelength observations of a twisted jet observed with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory and the Interface Region Imaging Spectrograph (IRIS) were used to understand how the twist was injected into the jet, thanks to the IRIS spectrographic slit fortuitously crossing the reconnection site at that time.
Methods: We followed the magnetic history of the active region based on the analysis of the Helioseismic and Magnetic Imager vector magnetic field computed with the UNNOFIT code. The nature and dynamics of the jet reconnection site are characterised by the IRIS spectra.
Results: This region is the result of the collapse of two emerging magnetic fluxes (EMFs) overlaid by arch filament systems that have been well-observed with AIA, IRIS, and the New Vacuum Solar Telescope in Hα. In the magnetic field maps, we found evidence of the pattern of a long sigmoidal flux rope (FR) along the polarity inversion line between the two EMFs, which is the site of the reconnection. Before the jet, an extension of the FR was present and a part of it was detached and formed a small bipole with a bald patch (BP) region, which dynamically became an X-current sheet over the dome of one EMF where the reconnection took place. At the time of the reconnection, the Mg II spectra exhibited a strong extension of the blue wing that is decreasing over a distance of 10 Mm (from -300 km s-1 to a few km s-1). This is the signature of the transfer of the twist to the jet.
Conclusions: A comparison with numerical magnetohydrodynamics simulations confirms the existence of the long FR. We conjecture that there is a transfer of twist to the jet during the extension of the FR to the reconnection site without FR eruption. The reconnection would start in the low atmosphere in the BP reconnection region and extend at an X-point along the current sheet formed above.

Movies attached to Figs. 1, 3, 4, and 7 are available at https://www.aanda.org 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: Case study of multi-temperature coronal jets for emerging flux MHD models Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte; Moreno-Insertis, Fernando; Aulanier, Guillaume; Nóbrega-Siverio, Daniel; Devi, Pooja Bibcode: 2020A&A...639A..22J Altcode: 2020arXiv200506064J Context. Hot coronal jets are a basic observed feature of the solar atmosphere whose physical origin is still actively debated.
Aims: We study six recurrent jets that occurred in active region NOAA 12644 on April 4, 2017. They are observed in all the hot filters of AIA as well as cool surges in IRIS slit-jaw high spatial and temporal resolution images.
Methods: The AIA filters allow us to study the temperature and the emission measure of the jets using the filter ratio method. We studied the pre-jet phases by analysing the intensity oscillations at the base of the jets with the wavelet technique.
Results: A fine co-alignment of the AIA and IRIS data shows that the jets are initiated at the top of a canopy-like double-chambered structure with cool emission on one and hot emission on the other side. The hot jets are collimated in the hot temperature filters, have high velocities (around 250 km s-1) and are accompanied by cool surges and ejected kernels that both move at about 45 km s-1. In the pre-phase of the jets, we find quasi-periodic intensity oscillations at their base that are in phase with small ejections; they have a period of between 2 and 6 min, and are reminiscent of acoustic or magnetohydrodynamic waves.
Conclusions: This series of jets and surges provides a good case study for testing the 2D and 3D magnetohydrodynamic emerging flux models. The double-chambered structure that is found in the observations corresponds to the regions with cold and hot loops that are in the models below the current sheet that contains the reconnection site. The cool surge with kernels is comparable with the cool ejection and plasmoids that naturally appears in the models.

Movies are available at https://www.aanda.org Title: Quasi Periodic Oscillations in the Pre Phases of Recurrent Jets Highlighting Plasmoids in Current Sheet Authors: Joshi, Reetika; Chandra, Ramesh; Schmieder, Brigitte; Aulanier, Guillaume; Devi, Pooja; Moreno-Insertis, Fernando; Nóbrega-Siverio, Daniel Bibcode: 2020EGUGA..2222351J Altcode: Solar jets observed at the limb are important to determine the location of reconnection sites in the corona. In this study, we investigate six recurrent hot and cool jets occurring in the active region NOAA 12644 as it is crossing the west limb on April 04, 2017. These jets are observed in all the UV/EUV filters of SDO/AIA and in cooler temperature formation lines in IRIS slit jaw images. The jets are initiated at the top of a double chamber vault with cool loops on one side and hot loops on the other side. The existence of such double chamber vaults suggests the presence of emerging flux with cool loops, the hot loops being the reconnected loops similarly as in the models of Moreno-Insertiset al. 2008, 2013 and Nóbrega-Siverio et al. 2016. In the preliminary phase of the main jets, quasi periodic intensity oscillations accompanied by smaller jets are detected in the bright current sheet between the vault and the preexisting magnetic field. Individual kernels and plasmoids are ejected in open field lines along the jets. Plasmoids may launch torsional Alfven waves and the kernels would be the result of the untwist of the plasmoids in open magnetic field as proposed in the model of Wyper et al. 2016. Title: Electric Current Evolution at the Footpoints of Solar Eruptions Authors: Barczynski, Krzysztof; Aulanier, Guillaume; Janvier, Miho; Schmieder, Brigitte; Masson, Sophie Bibcode: 2020ApJ...895...18B Altcode: 2020arXiv200407990B Electric currents play a critical role in the triggering of solar flares and their evolution. The aim of the present paper is to test whether the surface electric current has a surface or subsurface fixed source as predicted by the circuit approach of flare physics, or is the response of the surface magnetic field to the evolution of the coronal magnetic field as the MHD approach proposes? Out of all 19 X-class flares observed by SDO from 2011 to 2016 near the disk center, we analyzed the only nine eruptive flares for which clear ribbon hooks were identifiable. Flare ribbons with hooks are considered to be the footprints of eruptive flux ropes in MHD flare models. For the first time, fine measurements of the time evolution of electric currents inside the hooks in the observations as well as in the OHM 3D MHD simulation are performed. Our analysis shows a decrease of the electric current in the area surrounded by the ribbon hooks during and after the eruption. We interpret the decrease of the electric currents as due to the expansion of the flux rope in the corona during the eruption. Our analysis brings a new contribution to the standard flare model in 3D. Title: Energy and helicity fluxes in line-tied eruptive simulations Authors: Linan, L.; Pariat, É.; Aulanier, G.; Moraitis, K.; Valori, G. Bibcode: 2020A&A...636A..41L Altcode: 2020arXiv200301698L Context. Conservation properties of magnetic helicity and energy in the quasi-ideal and low-β solar corona make these two quantities relevant for the study of solar active regions and eruptions.
Aims: Based on a decomposition of the magnetic field into potential and nonpotential components, magnetic energy and relative helicity can both also be decomposed into two quantities: potential and free energies, and volume-threading and current-carrying helicities. In this study, we perform a coupled analysis of their behaviors in a set of parametric 3D magnetohydrodynamic (MHD) simulations of solar-like eruptions.
Methods: We present the general formulations for the time-varying components of energy and helicity in resistive MHD. We calculated them numerically with a specific gauge, and compared their behaviors in the numerical simulations, which differ from one another by their imposed boundary-driving motions. Thus, we investigated the impact of different active regions surface flows on the development of the energy and helicity-related quantities.
Results: Despite general similarities in their overall behaviors, helicities and energies display different evolutions that cannot be explained in a unique framework. While the energy fluxes are similar in all simulations, the physical mechanisms that govern the evolution of the helicities are markedly distinct from one simulation to another: the evolution of volume-threading helicity can be governed by boundary fluxes or helicity transfer, depending on the simulation.
Conclusions: The eruption takes place for the same value of the ratio of the current-carrying helicity to the total helicity in all simulations. However, our study highlights that this threshold can be reached in different ways, with different helicity-related processes dominating for different photospheric flows. This means that the details of the pre-eruptive dynamics do not influence the eruption-onset helicity-related threshold. Nevertheless, the helicity-flux dynamics may be more or less efficient in changing the time required to reach the onset of the eruption. Title: Observation of All Pre- and Post-reconnection Structures Involved in Three-dimensional Reconnection Geometries in Solar Eruptions Authors: Dudík, Jaroslav; Lörinčík, Juraj; Aulanier, Guillaume; Zemanová, Alena; Schmieder, Brigitte Bibcode: 2019ApJ...887...71D Altcode: 2019arXiv191008620D We report on observations of the two newly identified reconnection geometries involving erupting flux ropes. In 3D, a flux rope can reconnect either with a surrounding coronal arcade (recently named “ar-rf” reconnection) or with itself (“rr-rf” reconnection), and both kinds of reconnection create a new flux-rope field line and a flare loop. For the first time, we identify all four constituents of both reconnections in a solar eruptive event, the filament eruption of 2011 June 7 observed by Solar Dynamics Observatory/Atmospheric Imaging Assembly. The ar-rf reconnection manifests itself as shift of one leg of the filament by more than 25″ northward. At its previous location, a flare arcade is formed, while the new location of the filament leg previously corresponded to a footpoint of a coronal loop in 171 Å. In addition, the evolution of the flare ribbon hooks is also consistent with the occurrence of ar-rf reconnection as predicted by MHD simulations. Specifically, the growing hook sweeps footpoints of preeruptive coronal arcades, and these locations become inside the hook. Furthermore, the rr-rf reconnection occurs during the peak phase above the flare arcade, in an apparently X-type geometry involving a pair of converging bright filament strands in the erupting filament. A new flare loop forms near the leg of one of the strands, while a bright blob, representing a remnant of the same strand, is seen ascending into the erupting filament. All together, these observations vindicate recent predictions of the 3D standard solar-flare model. Title: Manifestations of Three-dimensional Magnetic Reconnection in an Eruption of a Quiescent Filament: Filament Strands Turning to Flare Loops Authors: Lörinčík, Juraj; Dudík, Jaroslav; Aulanier, Guillaume Bibcode: 2019ApJ...885...83L Altcode: 2019arXiv190903825L We report on observations of the conversion of bright filament strands into flare loops during the 2012 August 31 filament eruption. Prior to the eruption, individual bright strands composing one of the legs of the filament were observed in the 171 Å filter channel data of the Atmospheric Imaging Assembly. After the onset of the eruption, one of the hooked ribbons started to propagate and contract, sweeping footpoints of the bright filament strands as well as coronal loops located close by. Later on, hot flare loops appeared in regions swept by the hook, where the filament strands were rooted. Timing and localization of these phenomena suggest that they are caused by reconnection of field lines composing the filament at the hook, which, to our knowledge, has not been observed before. This process is not included in the standard flare model (CSHKP), as it does not address footpoints of erupting flux ropes and ribbon hooks. It has, however, been predicted using the recent three-dimensional extensions to the standard flare model. There, the erupting flux rope can reconnect with surrounding coronal arcades as the hooked extensions of current ribbons sweep its footpoints. This process results in formation of flare loops rooted in previous footpoints of the flux rope. Our observations of sweeping of filament footpoints are well described by this scenario. In all observed cases, all of the footpoints of the erupting filament became footpoints of flare loops. This process was observed to last for about 150 minutes, throughout the whole eruption. Title: Bidirectional Reconnection Outflows in an Active Region Authors: Ruan, Guiping; Schmieder, Brigitte; Masson, Sophie; Mein, Pierre; Mein, Nicole; Aulanier, Guillaume; Chen, Yao Bibcode: 2019ApJ...883...52R Altcode: We report on bidirectional coronal reconnection outflows reaching ±200 km s-1 as observed in an active region with the Si IV and C II spectra of the Interface Region Imaging Spectrograph (IRIS). The evolution of the active region with an emerging flux, a failed filament eruption, and a jet is followed in Solar Dynamical Observatory (SDO)/Atmospheric Imaging Assembly (AIA) filters from 304 to 94 Å, IRIS slit jaw images, and SDO/Helioseismic and Magnetic Imager movies. The bidirectional outflow reconnection is located at a bright point visible in multiwavelength AIA filters above an arch filament system. This suggests that the reconnection occurs between rising loops above the emergence of magnetic bipoles and the longer, twisted magnetic field lines remnant of the failed filament eruption one hour before. The reconnection occurs continuously in the corona between quasi-parallel magnetic field lines, which is possible in a 3D configuration. The reconnection also triggers a jet with transverse velocities around 60 km s-1. Blueshifts and redshifts along its axis confirm the existence of a twist along the jet, which could have been transferred from the filament flux rope. The jet finally blows up the material of the filament before coming back during the second phase. In the Hα Dopplergrams provided by the MSDP spectrograph, we see more redshift than blueshift, indicating the return of the jet and filament plasma. Title: Observations of a Footpoint Drift of an Erupting Flux Rope Authors: Zemanová, Alena; Dudík, Jaroslav; Aulanier, Guillaume; Thalmann, Julia K.; Gömöry, Peter Bibcode: 2019ApJ...883...96Z Altcode: 2019arXiv190802082Z We analyze the imaging observations of an M-class eruptive flare of 2015 November 4. The pre-eruptive Hα filament was modeled by the nonlinear force-free field model, which showed that it consisted of two helical systems. Tether-cutting reconnection involving these two systems led to the formation of a hot sigmoidal loop structure rooted in a small hook that formed at the end of the flare ribbon. Subsequently, the hot loops started to slip away from the small hook until it disappeared. The loops continued slipping and the ribbon elongated itself by several tens of arcseconds. A new and larger hook then appeared at the end of the elongated ribbon with hot and twisted loops rooted there. After the eruption of these hot loops, the ribbon hook expanded and later contracted. We interpret these observations in the framework of the recent three-dimensional (3D) extensions to the standard solar flare model predicting the drift of the flux rope footpoints. The hot sigmoidal loop is interpreted as the flux rope, whose footpoints drift during the eruption. While the deformation and drift of the new hook can be described by the model, the displacement of the flux rope footpoint from the filament to that of the erupting flux rope indicate that the hook evolution can be more complex than those captured by the model. Title: Velocities of Flare Kernels and the Mapping Norm of Field Line Connectivity Authors: Lörinčík, Juraj; Aulanier, Guillaume; Dudík, Jaroslav; Zemanová, Alena; Dzifčáková, Elena Bibcode: 2019ApJ...881...68L Altcode: 2019arXiv190601880L We report on observations of flare ribbon kernels during the 2012 August 31 filament eruption. In the 1600 and 304 Å channels of the Atmospheric Imaging Assembly, flare kernels were observed to move along flare ribbons at velocities v of up to 450 km s-1. Kernel velocities were found to be roughly anticorrelated with strength of the magnetic field. An apparent slipping motion of the flare loops was observed in the 131 Å only for the slowest kernels moving through the strong-B region. In order to interpret the observed relation between B LOS and v , we examined the distribution of the norm N, a quantity closely related to the slippage velocity. We calculated the norm N of the quasi-separatrix layers (QSLs) in MHD model of a solar eruption adapted to the magnetic environment that qualitatively agrees to that of the observed event. We found that both the modeled N and velocities of kernels reach their highest values in the same weak-field regions, one located in the curved part of the ribbon hook and the other in the straight part of the conjugate ribbon located close to a parasitic polarity. Contrariwise, lower values of the kernel velocities are seen at the tip of the ribbon hook, where the modeled N is low. Because the modeled distribution of N matches the observed dynamics of kernels, this supports the notion that the kernel motions can be interpreted as a signature of QSL reconnection during the eruption. Title: Flare Reconnection-driven Magnetic Field and Lorentz Force Variations at the Sun’s Surface Authors: Barczynski, Krzysztof; Aulanier, Guillaume; Masson, Sophie; Wheatland, Michael S. Bibcode: 2019ApJ...877...67B Altcode: 2019arXiv190405447B During eruptive flares, vector magnetograms show an increasing horizontal magnetic field and downward Lorentz force in the Sun’s photosphere around the polarity-inversion line (PIL). This behavior has often been associated with the implosion conjecture and has been interpreted as the result of either momentum conservation while the eruption moves upward or of the contraction of flare loops. We characterize the physical origin of these observed behaviors by analyzing a generic 3D magnetohydrodynamics simulation of an eruptive flare. Even though the simulation was not designed to recover the magnetic field and Lorentz force properties, it is fully consistent with them, and it provides key additional information for understanding them. The area where the magnetic field increases gradually develops between current ribbons, which spread away from each other and are connected to the coronal region. This area is merely the footprint of the coronal post-flare loops, whose contraction increases their shear field component and the magnetic energy density, in line with the ideal induction equation. For simulated data, we computed the Lorentz force density map by applying the method used in observations. We obtained an increase in the downward component of the Lorentz force density around the PIL, consistent with observations. However, this significantly differs from the Lorentz force density maps that are obtained directly from the 3D magnetic field and current. These results altogether question previous interpretations that were based on the implosion conjecture and momentum conservation with the coronal mass ejection, and rather imply that the observed increases in photospheric horizontal magnetic fields result from the reconnection-driven contraction of sheared flare loops. Title: Sheared Magnetic Arcades and the Pre-eruptive Magnetic Configuration of Coronal Mass Ejections: Diagnostics, Challenges and Future Observables Authors: Patsourakos, Spiros; Vourlidas, A.; Anthiochos, S. K.; Archontis, V.; Aulanier, G.; Cheng, X.; Chintzoglou, G.; Georgoulis, M. K.; Green, L. M.; Kliem, B.; Leake, J.; Moore, R. L.; Nindos, A.; Syntelis, P.; Torok, T.; Yardley, S. L.; Yurchyshyn, V.; Zhang, J. Bibcode: 2019shin.confE.194P Altcode: Our thinking about the pre-eruptive magnetic configuration of Coronal Mass Ejections has been effectively dichotomized into two opposing and often fiercely contested views: namely, sheared magnetic arcades and magnetic flux ropes. Finding a solution to this issue will have important implications for our understanding of CME initiation. We first discuss the very value of embarking into the arcade vs. flux rope dilemma and illustrate the corresponding challenges and difficulties to address it. Next, we are compiling several observational diagnostics of pre-eruptive sheared magnetic arcades stemming from theory/modeling, discuss their merits, and highlight potential ambiguities that could arise in their interpretation. We finally conclude with a discussion of possible new observables, in the frame of upcoming or proposed instrumentation, that could help to circumvent the issues we are currently facing. Title: Modeling the Effect of Mass-draining on Prominence Eruptions Authors: Jenkins, Jack M.; Hopwood, Matthew; Démoulin, Pascal; Valori, Gherardo; Aulanier, Guillaume; Long, David M.; van Driel-Gesztelyi, Lidia Bibcode: 2019ApJ...873...49J Altcode: 2019arXiv190110970J Quiescent solar prominences are observed within the solar atmosphere for up to several solar rotations. Their eruption is commonly preceded by a slow increase in height that can last from hours to days. This increase in the prominence height is believed to be due to their host magnetic flux rope transitioning through a series of neighboring quasi-equilibria before the main loss of equilibrium that drives the eruption. Recent work suggests that the removal of prominence mass from a stable, quiescent flux rope is one possible cause for this change in height. However, these conclusions are drawn from observations and are subject to interpretation. Here, we present a simple model to quantify the effect of “mass-draining” during the pre-eruptive height evolution of a solar flux rope. The flux rope is modeled as a line current suspended within a background potential magnetic field. We first show that the inclusion of mass, up to 1012 kg, can modify the height at which the line current experiences loss of equilibrium by up to 14%. Next, we show that the rapid removal of mass prior to the loss of equilibrium can allow the height of the flux rope to increase sharply and without an upper bound as it approaches its loss-of-equilibrium point. This indicates that the critical height for the loss of equilibrium can occur at a range of heights depending explicitly on the amount and evolution of mass within the flux rope. Finally, we demonstrate that for the same amount of drained mass, the effect on the height of the flux rope is up to two orders of magnitude larger for quiescent prominences than for active region prominences. Title: Generalization of the Magnetic Field Configuration of Typical and Atypical Confined Flares Authors: Joshi, Navin Chandra; Zhu, Xiaoshuai; Schmieder, Brigitte; Aulanier, Guillaume; Janvier, Miho; Joshi, Bhuwan; Magara, Tetsuya; Chandra, Ramesh; Inoue, Satoshi Bibcode: 2019ApJ...871..165J Altcode: 2018arXiv181101228J Atypical flares cannot be naturally explained with standard models. To predict such flares, we need to define their physical characteristics, in particular, their magnetic environment, and identify pairs of reconnected loops. Here, we present in detail a case study of a confined flare preceded by flux cancellation that leads to the formation of a filament. The slow rise of the noneruptive filament favors the growth and reconnection of overlying loops. The flare is only of C5.0 class but it is a long duration event. The reason is that it is comprised of three successive stages of reconnection. A nonlinear force-free field extrapolation and a magnetic topology analysis allow us to identify the loops involved in the reconnection process and build a reliable scenario for this atypical confined flare. The main result is that a curved magnetic polarity inversion line in active regions is a key ingredient for producing such atypical flares. A comparison with previous extrapolations for typical and atypical confined flares leads us to propose a cartoon for generalizing the concept. Title: Drifting of the line-tied footpoints of CME flux-ropes Authors: Aulanier, Guillaume; Dudík, Jaroslav Bibcode: 2019A&A...621A..72A Altcode: 2018arXiv181104253A Context. Bridging the gap between heliospheric and solar observations of eruptions requires the mapping of interplanetary coronal mass ejection (CME) footpoints down to the Sun's surface. But this not straightforward. Improving the understanding of the spatio-temporal evolutions of eruptive flares requires a comprehensive standard model. But the current model is only two-dimensional and cannot address the question of interplanetary CME footpoints.
Aims: Existing 3D extensions to the standard model show that flux-rope footpoints are surrounded by curved-shaped quasi-separatrix layer (QSL) footprints that can be related with hook-shaped flare-ribbons. We build upon this finding and further address the joint questions of their time-evolution, and of the formation of flare loops at the ends of the flaring polarity inversion line (PIL) of the erupting bipole, which are both relevant for flare understanding in general and for interplanetary CME studies in particular.
Methods: We calculated QSLs and relevant field lines in an MHD simulation of a torus-unstable flux-rope. The evolving QSL footprints are used to define the outer edge of the flux rope at different times, and to identify and characterize new 3D reconnection geometries and sequences that occur above the ends of the flaring PIL. We also analyzed flare-ribbons as observed in the extreme ultraviolet by SDO/AIA and IRIS during two X-class flares.
Results: The flux-rope footpoints are drifting during the eruption, which is unexpected due to line-tying. This drifting is due to a series of coronal reconnections that erode the flux rope on one side and enlarge it on the other side. Other changes in the flux-rope footpoint-area are due to multiple reconnections of individual field lines whose topology can evolve sequentially from arcade to flux rope and finally to flare loop. These are associated with deformations and displacements of QSL footprints, which resemble those of the studied flare ribbons.
Conclusions: Our model predicts continuous deformations and a drifting of interplanetary CME flux-rope footpoints whose areas are surrounded by equally evolving hooked-shaped flare-ribbons, as well as the formation of flare loops at the ends of flaring PILs which originate from the flux-rope itself, both of which being due to purely three-dimensional reconnection geometries. The observed evolution of flare-ribbons in two events supports the model, but more observations are required to test all its predictions. Title: Fast velocities of flare ribbon kernels and ribbon elongation in a quescent filament eruption of 2012 August 31 observed by SDO/AIA Authors: Lörinčík, Juraj; Dudík, Jaroslav; Kašparová, Jana; Aulanier, Guillaume; Zemanová, Alena; Dzifčáková, Elena Bibcode: 2018csc..confE..63L Altcode: We report on SDO observations of an eruption of a quiescent filament from 2012 August 31. In the 1600 Å filter channel of AIA, flare ribbons were observed to elongate at velocities up to 480 km s^{-1} and flare kernels move along a ribbon at velocity of ≈ 260 km s^{-1}. In order to investigate the emission observed in the 1600 Å channel, we used synthetic spectra modeled using CHIANTI and RADYN models of flare atmospheres with beam parameters constrained using fits of RHESSI spectra. We found out that depending on parameters of heating of a flare model, thickness of a region where the emission of the 1600 Å filter channel originates ranges between 10^{-2} and 10^{2} km. Information on dimensions of the formation region were then utilized to estimate densities in flare ribbons using inversions of the emission measure. These were found to range between 10^{10} - 4.10^{12} cm^{-3} for flare atmospheres heated by beams of different parameters. Together with B_{LOS} data from SDO/HMI, diagnosed densities were used to calculate Alfvén velocities in observed ribbons. These can be as small as 17 km s^{-1} for flare ribbons observed in region of weak magnetic field at latter stages of heating. This finding suggests that elongation of ribbons and motion of kernels might not be related to waves. Motions along the PIL are well-described in the 3D model of solar eruptions of Aulanier et al. 2013 (A&A, 543, 110). However, EUV observations of flare loops revealed that velocity of their apparent slipping motion is much lower than velocity of elongation of a ribbon, which is observed in a close vicinity. Therefore, observed phenomena can not be directly related to super-Alvénic regime of magnetic slipping reconnection introduced in the 3D model. Title: The nature of imploding loops during solar eruptions as revealed by MHD simulations and AIA observations Authors: Aulanier, Guillaume; Dudik, Jaroslav; Zucarello, F. P.; Demoulin, Pascal; Schmieder, Brigitte Bibcode: 2018csc..confE..19A Altcode: Over the last years AIA revealed the frequent occurence of contracting loops at the flanks of erupting active regions. Those have often been interpreted as an evidence of the implosion conjecture that relates magnetic energy decreases with volume contractions in the Sun's corona. So as to unveil the physical nature of these features we carried out observational analyses of two solar eruptions observed with AIA with different projection angles, which we coupled with new analyses of a generic zero-beta MHD simulation of an asymmetric eruption driven by the torus instability, that was not designed for this particular study. The simulation does display contracting loops in general. And the synthetic time-slices of the simulation, when rotated to the right projections, do match the observed ones. But in the simulation these inward motions are not due to any volume contraction. Instead they are associated with two large-scale quasi-incompressible coronal-vortices. Those develop at the flanks of the erupting flux ropes, as most of the compressive component of the flow is evacuated away by an Alfven wave in the early stages of the eruption. We argue that this behavior is merely a magnetic version of the usual pressure-driven formation of vortex rings in hydrodyanmics. This result implies that during a solar eruption, the free magnetic-energy from the pre-erupting active-region is converted not only in the flare and the CME, but is also "lost" in the generation of these two large-scale coronal vortices. Title: Importance of the Hα Visibility and Projection Effects for the Interpretation of Prominence Fine-structure Observations Authors: Gunár, Stanislav; Dudík, Jaroslav; Aulanier, Guillaume; Schmieder, Brigitte; Heinzel, Petr Bibcode: 2018ApJ...867..115G Altcode: We construct a new 3D Whole-prominence Fine-structure (WPFS) model based on a prominence magnetic field configuration designed to qualitatively approximate the morphology of a quiescent prominence observed on 2010 June 22. The model represents an entire prominence with its numerous fine structures formed by a prominence plasma located in dips in the prominence magnetic field. We use the constructed 3D model and employ a radiative-transfer-based Hα visualization method to analyze the Hα visibility of prominence fine structures and its effect on the perceived morphology of observed and modeled prominences. We qualitatively compare three techniques used for visualization of modeled prominences—visualizations drawing magnetic dips up to a height of 1 pressure scale height, drawing the full extent of magnetic dips, and the synthetic Hα visualization—and discuss their suitability for direct comparison between models and observations of prominences and filaments. We also discuss the role of visibility of the prominence fine structures in the estimation of the total height of prominences, which may indicate the height of pre-erupting flux ropes. This parameter is critical for the observational determination of the flux-rope stability. In addition, we employ the WPFS model to assess the effects caused by a projection of the naturally three-dimensional and heterogeneous prominences onto a two-dimensional plane of the sky. We discuss here how the morphological structures of prominences differ when observed in projections from different viewing angles. We also discuss the shapes of the dipped magnetic field lines and the perceived projection of motions of prominence fine structures along such field lines. Title: Flare reconnection driven magnetic field and Lorentz force variations at the Sun's surface Authors: Barczynski, Krzysztof; Aulanier, Guillaume; Masson, Sophie; Wheatland, Michael S. Bibcode: 2018csc..confE..27B Altcode: We show that the simulation is fully consistent with the observed increase of the photospheric horizontal magnetic field and electric currents around flaring PILs. The simulation also finds that the surface integral coming from the volume integral of the Maxwell stress tensor, as usually used in observational data analysis as the proxy of the Lorentz force, shows an increased downard component in the photosphere, as observed. But we also find that this proxy is significantly different from the true Lorentz force, which does not reveal this downward component. This result questions every previous interpretation based on the implosion conjecture and momentum conservation. However based on the analysis of the induction equation in the simulation, we unveil that the increase of the horizontal magnetic filed around active region PILs during eruptions is solely and exclusively result of the flare reconnection-driven contraction of flare loops. Title: Signature of flux ropes before and after eruptions: electric currents in active regions Authors: Schmieder, Brigitte; Aulanier, Guillaume; Dalmasse, Kévin; Janvier, Miho; Gilchrist, Stuart; Zhao, Jie; Dudik, Jaroslav Bibcode: 2018cosp...42E3026S Altcode: Solar observations, nonlinear force-free field extrapolations relying on these observations, and three-dimensional magnetohydrodynamic (MHD) models indicate the presence of electric currents in the pre-eruption state and in the course of eruptions of solar magnetic structures which are interpreted as flux ropes (sigmoids, filaments, cavities).The MHD models are able to explain the net currents in active regions by the existence of strong magnetic shear along the polarity inversion lines, thus confirming previous observations. The models have also captured the essence of the behavior of electric currents in active regions during solar eruptions, predicting current-density increases and decreases inside flare ribbons and in the interior of expanding flux ropes, respectively.The observed photospheric current-density maps, inferred from vector magnetic field observations, exhibit whirling ribbon patterns similar to the MHD model results, which are interpreted as the signatures of flux ropes and of quasi-separatrix layers (QSLs) between the magnetic systems in active regions. We will show how observations can confirm enhancement of the total current in these QSLs during the eruptions, and how these observations can be used to investigate whether current density decrease can be seen at the footpoints of erupting flux ropes Title: Can 3D whole-prominence fine structure models be used for assessment of the prominence plasma mass and distribution prior to the onset of CMEs? Authors: Gunár, Stanislav; Schmieder, Brigitte; Aulanier, Guillaume; Anzer, Ulrich; Heinzel, Petr; Mackay, Duncan; Dudik, Jaroslav Bibcode: 2018cosp...42E1316G Altcode: Two complex 3D models of entire prominences including their numerous fine structures were recently developed. The first 3D Whole-Prominence Fine Structure (WPFS) model was developed by Gunár and Mackay. The second 3D WPFS model was put forward by Gunár, Aulanier, Dudík, Heinzel, and Schmieder. These 3D prominence models combine simulations of the 3D magnetic field configuration of an entire prominence with a detailed description of the prominence plasma. The plasma is located in magnetic dips in hydrostatic equilibrium and is distributed along hundreds of fine structures. The assumed prominence plasma has realistic density and temperature distributions including the prominence-corona transition region.These 3D WPFS models allow us to study the distribution and the mass of the prominence plasma contained in prominence magnetic field configurations. These can be crucial during the onset and early evolution of CMEs. Moreover, prominence plasma represents a bulk of the material ejected by CMEs into the interplanetary space. Here, we investigate the potential of using the 3D WPFS models for assessment of the role the prominence plasma plays in the initiation and evolution of CMEs. Title: Interactions of Twisted Ω-loops in a Model Solar Convection Zone Authors: Jouve, L.; Brun, A. S.; Aulanier, G. Bibcode: 2018ApJ...857...83J Altcode: 2018arXiv180304709J This study aims at investigating the ability of strong interactions between magnetic field concentrations during their rise through the convection zone to produce complex active regions at the solar surface. To do so, we perform numerical simulations of buoyant magnetic structures evolving and interacting in a model solar convection zone. We first produce a 3D model of rotating convection and then introduce idealized magnetic structures close to the bottom of the computational domain. These structures possess a certain degree of field line twist and they are made buoyant on a particular extension in longitude. The resulting twisted Ω-loops will thus evolve inside a spherical convective shell possessing large-scale mean flows. We present results on the interaction between two such loops with various initial parameters (mainly buoyancy and twist) and on the complexity of the emerging magnetic field. In agreement with analytical predictions, we find that if the loops are introduced with opposite handedness and same axial field direction or the same handedness but opposite axial field, they bounce against each other. The emerging region is then constituted of two separated bipolar structures. On the contrary, if the loops are introduced with the same direction of axial and peripheral magnetic fields and are sufficiently close, they merge while rising. This more interesting case produces complex magnetic structures with a high degree of non-neutralized currents, especially when the convective motions act significantly on the magnetic field. This indicates that those interactions could be good candidates to produce eruptive events like flares or CMEs. Title: On the Occurrence of Thermal Nonequilibrium in Coronal Loops Authors: Froment, C.; Auchère, F.; Mikić, Z.; Aulanier, G.; Bocchialini, K.; Buchlin, E.; Solomon, J.; Soubrié, E. Bibcode: 2018ApJ...855...52F Altcode: 2018arXiv180204010F Long-period EUV pulsations, recently discovered to be common in active regions, are understood to be the coronal manifestation of thermal nonequilibrium (TNE). The active regions previously studied with EIT/Solar and Heliospheric Observatory and AIA/SDO indicated that long-period intensity pulsations are localized in only one or two loop bundles. The basic idea of this study is to understand why. For this purpose, we tested the response of different loop systems, using different magnetic configurations, to different stratifications and strengths of the heating. We present an extensive parameter-space study using 1D hydrodynamic simulations (1020 in total) and conclude that the occurrence of TNE requires specific combinations of parameters. Our study shows that the TNE cycles are confined to specific ranges in parameter space. This naturally explains why only some loops undergo constant periodic pulsations over several days: since the loop geometry and the heating properties generally vary from one loop to another in an active region, only the ones in which these parameters are compatible exhibit TNE cycles. Furthermore, these parameters (heating and geometry) are likely to vary significantly over the duration of a cycle, which potentially limits the possibilities of periodic behavior. This study also confirms that long-period intensity pulsations and coronal rain are two aspects of the same phenomenon: both phenomena can occur for similar heating conditions and can appear simultaneously in the simulations. Title: Solar Active Region Electric Currents Before and During Eruptive Flares Authors: Schmieder, Brigitte; Aulanier, Guillaume Bibcode: 2018GMS...235..391S Altcode: 2019arXiv190304050S This chapter discusses electric currents in the preeruption state and in the course of eruptions of solar magnetic structures, using information from solar observations, nonlinear force-free field extrapolations relying on these observations, and three-dimensional magnetohydrodynamic (MHD) models. The discussion addresses the issue of neutralized versus nonneutralized currents in active regions and concludes that MHD models are able to explain nonneutralized currents in active regions by the existence of strong magnetic shear along the polarity inversion lines, thus confirming previous observations that already contained this result. The models have also captured the essence of the behavior of electric currents in active regions during solar eruptions, predicting current-density increases and decreases inside flare ribbons and in the interior of expanding flux ropes, respectively. The observed photospheric current-density maps, inferred from vector magnetic field observations, exhibit whirling ribbon patterns similar to the MHD model results, which are interpreted as the signatures of flux ropes and of quasi-separatrix layers (QSLs) between the magnetic systems in active regions. Enhancement of the total current in these QSLs during the eruptions and decreasing current densities at the footpoint of erupting flux ropes, has been confirmed in the observations. Title: Geoeffective events through solar cycles Authors: Schmieder, Brigitte; Aulanier, Guillaume Bibcode: 2018IAUS..340..255S Altcode: Extreme solar storms are well known in the historical databases. Since the modern era, it has been possible to associate clearly geomagnetic disturbances with solar events (flares, SEP, CMEs). In the recent solar cycles the geoeffective events (number and strength) are decreasing. As an example, in the 2002 maximum activity year, we present how many flares, and CMEs were geoeffective. Based on observations and simulations, we discuss on the size of sunspots and the field strength to get more energetic flares (> 1032 ergs) in the near future. Title: Slippage of Jets Explained by the Magnetic Topology of NOAA Active Region 12035 Authors: Joshi, R.; Schmieder, B.; Chandra, R.; Aulanier, G.; Zuccarello, F. P.; Uddin, W. Bibcode: 2017SoPh..292..152J Altcode: 2017arXiv170902791J We present the investigation of 11 recurring solar jets that originated from two different sites (site 1 and site 2) close to each other (≈11 Mm) in NOAA active region (AR) 12035 during 15 - 16 April 2014. The jets were observed by the Atmospheric Imaging Assembly (AIA) telescope on board the Solar Dynamics Observatory (SDO) satellite. Two jets were observed by the telescope of the Aryabhatta Research Institute of Observational Sciences (ARIES), Nainital, India, in Hα . On 15 April, flux emergence is strong in site 1, while on 16 April, flux emergence and cancellation mechanisms are involved in both sites. The jets of both sites have parallel trajectories and move to the south with a speed between 100 and 360 km s−1. The jets of site 2 occurred during the second day have a tendency to move toward the jets of site 1 and merge with them. We conjecture that the slippage of the jets could be explained by the complex topology of the region, which included a few low-altitude null points and many quasi-separatrix layers (QSLs), which could intersect with one another. Title: Expanding and Contracting Coronal Loops as Evidence of Vortex Flows Induced by Solar Eruptions Authors: Dudík, J.; Zuccarello, F. P.; Aulanier, G.; Schmieder, B.; Démoulin, P. Bibcode: 2017ApJ...844...54D Altcode: 2017arXiv170604783D Eruptive solar flares were predicted to generate large-scale vortex flows at both sides of the erupting magnetic flux rope. This process is analogous to a well-known hydrodynamic process creating vortex rings. The vortices lead to advection of closed coronal loops located at the peripheries of the flaring active region. Outward flows are expected in the upper part and returning flows in the lower part of the vortex. Here, we examine two eruptive solar flares, the X1.1-class flare SOL2012-03-05T03:20 and the C3.5-class SOL2013-06-19T07:29. In both flares, we find that the coronal loops observed by the Atmospheric Imaging Assembly in its 171 Å, 193 Å, or 211 Å passbands show coexistence of expanding and contracting motions, in accordance with the model prediction. In the X-class flare, multiple expanding and contracting loops coexist for more than 35 minutes, while in the C-class flare, an expanding loop in 193 Å appears to be close by and cotemporal with an apparently imploding loop arcade seen in 171 Å. Later, the 193 Å loop also switches to contraction. These observations are naturally explained by vortex flows present in a model of eruptive solar flares. Title: Transition from eruptive to confined flares in the same active region Authors: Zuccarello, F. P.; Chandra, R.; Schmieder, B.; Aulanier, G.; Joshi, R. Bibcode: 2017A&A...601A..26Z Altcode: 2017arXiv170202477Z Context. Solar flares are sudden and violent releases of magnetic energy in the solar atmosphere that can be divided into two classes: eruptive flares, where plasma is ejected from the solar atmosphere resulting in a coronal mass ejection (CME), and confined flares, where no CME is associated with the flare.
Aims: We present a case study showing the evolution of key topological structures, such as spines and fans, which may determine the eruptive versus non-eruptive behavior of the series of eruptive flares followed by confined flares, which all originate from the same site.
Methods: To study the connectivity of the different flux domains and their evolution, we compute a potential magnetic field model of the active region. Quasi-separatrix layers are retrieved from the magnetic field extrapolation.
Results: The change in behavior of the flares from one day to the next - from eruptive to confined - can be attributed to the change in orientation of the magnetic field below the fan with respect to the orientation of the overlaying spine rather than an overall change in the stability of the large-scale field.
Conclusions: Flares tend to be more confined when the field that supports the filament and the overlying field gradually becomes less anti-parallel as a direct result of changes in the photospheric flux distribution, being themselves driven by continuous shearing motions of the different magnetic flux concentrations.

Movies associated to Figs. 2, 3, and 5 are available at http://www.aanda.org Title: Vortex and Sink Flows in Eruptive Flares as a Model for Coronal Implosions Authors: Zuccarello, F. P.; Aulanier, G.; Dudík, J.; Démoulin, P.; Schmieder, B.; Gilchrist, S. A. Bibcode: 2017ApJ...837..115Z Altcode: 2017arXiv170200199Z Eruptive flares are sudden releases of magnetic energy that involve many phenomena, several of which can be explained by the standard 2D flare model and its realizations in 3D. We analyze a 3D magnetohydrodynamics simulation, in the framework of this model, that naturally explains the contraction of coronal loops in the proximity of the flare sites, as well as the inflow toward the region above the cusp-shaped loops. We find that two vorticity arcs located along the flanks of the erupting magnetic flux rope are generated as soon as the eruption begins. The magnetic arcades above the flux rope legs are then subjected to expansion, rotation, or contraction depending on which part of the vortex flow advects them. In addition to the vortices, an inward-directed magnetic pressure gradient exists in the current sheet below the magnetic flux rope. It results in the formation of a sink that is maintained by reconnection. We conclude that coronal loop apparent implosions observed during eruptive flares are the result of hydromagnetic effects related to the generation of vortex and sink flows when a flux rope moves in a magnetized environment. Title: Long-period Intensity Pulsations in Coronal Loops Explained by Thermal Non-equilibrium Cycles Authors: Froment, C.; Auchère, F.; Aulanier, G.; Mikić, Z.; Bocchialini, K.; Buchlin, E.; Solomon, J. Bibcode: 2017ApJ...835..272F Altcode: 2017arXiv170101309F In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon that can occur when the heating is both highly stratified and quasi-constant. Unambiguous observational identification of TNE would thus permit us to strongly constrain heating scenarios. While TNE is currently the standard interpretation of coronal rain, the long-term periodic evolution predicted by simulations has never been observed. However, the detection of long-period intensity pulsations (periods of several hours) has been recently reported with the Solar and Heliospheric Observatory/EIT, and this phenomenon appears to be very common in loops. Moreover, the three intensity-pulsation events that we recently studied with the Solar Dynamics Observatory/Atmospheric Imaging Assembly (AIA) show strong evidence for TNE in warm loops. In this paper, a realistic loop geometry from linear force-free field (LFFF) extrapolations is used as input to 1D hydrodynamic simulations. Our simulations show that, for the present loop geometry, the heating has to be asymmetrical to produce TNE. We analyze in detail one particular simulation that reproduces the average thermal behavior of one of the pulsating loop bundle observed with AIA. We compare the properties of this simulation with those deduced from the observations. The magnetic topology of the LFFF extrapolations points to the presence of sites of preferred reconnection at one footpoint, supporting the presence of asymmetric heating. In addition, we can reproduce the temporal large-scale intensity properties of the pulsating loops. This simulation further strengthens the interpretation of the observed pulsations as signatures of TNE. This consequently provides important information on the heating localization and timescale for these loops. Title: Successive Magnetic Reconnections Observed during Sympathetic Eruptions Authors: Chandra Joshi, Navin; Schmieder, Brigitte; Magara, Tetsuya; Guo, Yang; Aulanier, Guillaume Bibcode: 2017psio.confE.120C Altcode: No abstract at ADS Title: Solar physics: When the tail wags the dog Authors: Aulanier, Guillaume Bibcode: 2016NatPh..12..998A Altcode: Solar eruptions are triggered by magnetic stress building up in the corona due to the motion of the Sun's dense surface. New observations reveal that these eruptions can, in turn, induce the rotational motion of sunspots. Title: Long-period Intensity Pulsations as the Manifestation of the Heating Stratification and Timescale in Coronal Loops Authors: Froment, Clara; Auchère, Frédéric; Aulanier, Guillaume; Mikić, Zoran; Bocchialini, Karine; Buchlin, Eric; Solomon, Jacques Bibcode: 2016usc..confE..47F Altcode: In solar coronal loops, thermal non-equilibrium (TNE) is a phenomenon that can occur when the heating is both highly-stratified and quasi-constant. Unambiguous observational identification of TNE would thus permit to strongly constrain heating scenarios. Up to now, while TNE is the standard interpretation of coronal rain, it was not believed to happen commonly in warm coronal loops. Recently, the detection of long-period intensity pulsations (periods of several hours) has been reported with SoHO/EIT. This phenomenon appears to be very common in loops (Auchère et al. 2014). In Froment et al. 2015, three intensity-pulsation events studied with SDO/AIA, show strong evidence for TNE in warm loops. We use realistic loop geometries from LFFF extrapolations for one of these events are used as input to a 1D hydrodynamic simulation of TNE. A highly-stratified heating function is chosen to reproduce the observed period of pulsation and temperature of the loops. With these conditions, the heating function has to be asymmetric. The magnetic topology of the LFFF extrapolations points to the presence of sites of preferred reconnection at one footpoint, supporting the presence of asymmetric heating. We compared the properties of the simulated loop with the properties deduced from observations. We found that the 1D hydrodynamic simulation can reproduce the large temporal scale intensity properties of the pulsating loops (Froment et al. 2016, submitted). This simulation further strengthen the interpretation of the observed pulsations as signatures of TNE. This implies that the heating for these loops is highly-stratified and that the frequency of the heating events must be high compared to the typical cooling time. Title: Evidence of flux rope and sigmoid in Active Regions prior eruptions Authors: Schmieder, Brigitte; Aulanier, Guillaume; Janvier, Miho; Bommier, Veronique; Dudik, Jaroslav; Gilchrist, Stuart; Zhao, Jie Bibcode: 2016cosp...41E1750S Altcode: In the solar corona, the magnetic field is dominant, and the current density vector is nearly aligned with the magnetic field lines for strong and stressed field regions. Stressed and highly twisted flux ropes are at the origin of eruptive events such as flares and coronal mass ejections, which inject material into the interplanetary medium. The standard three dimensional (3D) flare model predicts the complex evolution of flare loops and the flux rope before the eruption. Flux ropes are not directly observed in the corona, however it has started to be possible to detect their footprints in the photosphere. Recent high spatial and temporal resolution spectro-polarimeters have allowed us to compute the photospheric electric currents and follow their evolution. Characteristics pattern like J-shaped ribbons indicate the presence of a flux rope before the flare. The results confirm the predictions of the 3D MHD standard model of eruptive flares. It is interesting to compare the magnetic helicity of the ejected flux rope with the in situ measurements of the corresponding ICME at L1. We will show some examples (February 15 2011, July 12 2012, Sept 10 2014). Title: Erratum: "Hooked Flare Ribbons and Flux-rope Related QSL Footprints"(2016, ApJ, 823, 62) Authors: Zhao, Jie; Gilchrist, Stuart A.; Aulanier, Guillaume; Schmieder, Brigitte; Pariat, Etienne; Li, Hui Bibcode: 2016ApJ...825...80Z Altcode: No abstract at ADS Title: 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: Slipping Magnetic Reconnection, Chromospheric Evaporation, Implosion, and Precursors in the 2014 September 10 X1.6-Class Solar Flare Authors: Dudík, Jaroslav; Polito, Vanessa; Janvier, Miho; Mulay, Sargam M.; Karlický, Marian; Aulanier, Guillaume; Del Zanna, Giulio; Dzifčáková, Elena; Mason, Helen E.; Schmieder, Brigitte Bibcode: 2016ApJ...823...41D Altcode: 2016arXiv160306092D We investigate the occurrence of slipping magnetic reconnection, chromospheric evaporation, and coronal loop dynamics in the 2014 September 10 X-class flare. Slipping reconnection is found to be present throughout the flare from its early phase. Flare loops are seen to slip in opposite directions toward both ends of the ribbons. Velocities of 20-40 km s-1 are found within time windows where the slipping is well resolved. The warm coronal loops exhibit expanding and contracting motions that are interpreted as displacements due to the growing flux rope that subsequently erupts. This flux rope existed and erupted before the onset of apparent coronal implosion. This indicates that the energy release proceeds by slipping reconnection and not via coronal implosion. The slipping reconnection leads to changes in the geometry of the observed structures at the Interface Region Imaging Spectrograph slit position, from flare loop top to the footpoints in the ribbons. This results in variations of the observed velocities of chromospheric evaporation in the early flare phase. Finally, it is found that the precursor signatures, including localized EUV brightenings as well as nonthermal X-ray emission, are signatures of the flare itself, progressing from the early phase toward the impulsive phase, with the tether-cutting being provided by the slipping reconnection. The dynamics of both the flare and outlying coronal loops is found to be consistent with the predictions of the standard solar flare model in three dimensions. Title: Chain Reconnections Observed in Sympathetic Eruptions Authors: Joshi, Navin Chandra; Schmieder, Brigitte; Magara, Tetsuya; Guo, Yang; Aulanier, Guillaume Bibcode: 2016ApJ...820..126J Altcode: 2016arXiv160207792J The nature of various plausible causal links between sympathetic events is still a controversial issue. In this work, we present multiwavelength observations of sympathetic eruptions, associated flares, and coronal mass ejections (CMEs) occurring on 2013 November 17 in two close active regions. Two filaments, I.e., F1 and F2, are observed in between the active regions. Successive magnetic reconnections, caused for different reasons (flux cancellation, shear, and expansion) have been identified during the whole event. The first reconnection occurred during the first eruption via flux cancellation between the sheared arcades overlying filament F2, creating a flux rope and leading to the first double-ribbon solar flare. During this phase, we observed the eruption of overlying arcades and coronal loops, which leads to the first CME. The second reconnection is believed to occur between the expanding flux rope of F2 and the overlying arcades of filament F1. We suggest that this reconnection destabilized the equilibrium of filament F1, which further facilitated its eruption. The third stage of reconnection occurred in the wake of the erupting filament F1 between the legs of the overlying arcades. This may create a flux rope and the second double-ribbon flare and a second CME. The fourth reconnection was between the expanding arcades of the erupting filament F1 and the nearby ambient field, which produced the bi-directional plasma flows both upward and downward. Observations and a nonlinear force-free field extrapolation confirm the possibility of reconnection and the causal link between the magnetic systems. Title: The Apparent Critical Decay Index at the Onset of Solar Prominence Eruptions Authors: Zuccarello, F. P.; Aulanier, G.; Gilchrist, S. A. Bibcode: 2016ApJ...821L..23Z Altcode: A magnetic flux rope (MFR) embedded in a line-tied external magnetic field that decreases with height as {z}-n is unstable to perturbations if the decay index of the field n is larger than a critical value. The onset of this instability, called torus instability, is one of the main mechanisms that can initiate coronal mass ejections. Since flux ropes often possess magnetic dips that can support prominence plasma, this is also a valuable mechanism to trigger prominence eruptions. Magnetohydrodynamic (MHD) simulations of the formation and/or emergence of MFRs suggest a critical value for the onset of the instability in the range [1.4-2]. However, detailed observations of prominences suggest a value in the range [0.9-1.1]. In this Letter, by using a set of MHD simulations, we show why the large discrepancy between models and observations is only apparent. Our simulations indeed show that the critical decay index at the onset of the eruption is n=1.4+/- 0.1 when computed at the apex of the flux rope axis, while it is n=1.1+/- 0.1 when it is computed at the altitude of the topmost part of the distribution of magnetic dips. The discrepancy only arises because weakly twisted curved flux ropes do not have dips up to the altitude of their axis. Title: Role of "X point'' in Flares and Filament Interactions Authors: Schmieder, B.; Zuccarello, F. P.; Aulanier, G.; Chandra, R.; Joshi, N. C.; Filippov, B. Bibcode: 2016CEAB...40...35S Altcode: Explaining the trigger and energy release processes of flares is a fundamental problem of solar physics. It is commonly held that magnetic reconnection plays a key role in converting magnetic energy into other forms of energy. In 2D magnetic field configurations, when oppositely directed magnetic fields are brought together they may reconnect thereby releasing stored magnetic energy eventually resulting in a flare. In 3D configurations, the magnetic topology should be considered and the reconnection is favored at the intersection of magnetic quasi-separatrix layers, which is an extension of what is called "X point" in 2D. The evolution of key topological structures, such as null point, spines and fans may determine the eruptive behavior of a flare. The presence of a null point can be very important. We present a few examples, i.e., a flare with a circular flare ribbon and the interaction of two parallel filaments.

In the case of flux rope destabilization and triggering eruption due to the torus instability, the important parameter is the decay index and not the topology itself. However the complexity of active regions leads to interpretations where different mechanisms may be intermixed. The breakout mechanism, which requires a quadrupolar configuration with QSLs and separator where the reconnection can occur, is present in many models. That is not always a sufficient condition to explain the eruptive flares. For one case study, the different behaviour of a series of eruptive flares followed by confined flares, all originating in the same site has been attributed to the change of orientation of the magnetic field below the fan with respect to the orientation of the spine. Flares tend to be more confined when the two fields become less antiparallel. Title: From Coronal Observations to MHD Simulations, the Building Blocks for 3D Models of Solar Flares (Invited Review) Authors: Janvier, M.; Aulanier, G.; Démoulin, P. Bibcode: 2015SoPh..290.3425J Altcode: 2015SoPh..tmp...63J; 2015arXiv150505299J Solar flares are energetic events taking place in the Sun's atmosphere, and their effects can greatly impact the environment of the surrounding planets. In particular, eruptive flares, as opposed to confined flares, launch coronal mass ejections into the interplanetary medium, and as such, are one of the main drivers of space weather. After briefly reviewing the main characteristics of solar flares, we summarise the processes that can account for the build-up and release of energy during their evolution. In particular, we focus on the development of recent 3D numerical simulations that explain many of the observed flare features. These simulations can also provide predictions of the dynamical evolution of coronal and photospheric magnetic field. Here we present a few observational examples that, together with numerical modelling, point to the underlying physical mechanisms of the eruptions. Title: Critical Decay Index at the Onset of Solar Eruptions Authors: Zuccarello, F. P.; Aulanier, G.; Gilchrist, S. A. Bibcode: 2015ApJ...814..126Z Altcode: 2015arXiv151003713Z Magnetic flux ropes are topological structures consisting of twisted magnetic field lines that globally wrap around an axis. The torus instability model predicts that a magnetic flux rope of major radius R undergoes an eruption when its axis reaches a location where the decay index -d({ln}{B}{ex})/d({ln}R) of the ambient magnetic field Bex is larger than a critical value. In the current-wire model, the critical value depends on the thickness and time evolution of the current channel. We use magnetohydrodynamic simulations to investigate whether the critical value of the decay index at the onset of the eruption is affected by the magnetic flux rope’s internal current profile and/or by the particular pre-eruptive photospheric dynamics. The evolution of an asymmetric, bipolar active region is driven by applying different classes of photospheric motions. We find that the critical value of the decay index at the onset of the eruption is not significantly affected by either the pre-erupitve photospheric evolution of the active region or the resulting different magnetic flux ropes. As in the case of the current-wire model, we find that there is a “critical range” [1.3-1.5], rather than a “critical value” for the onset of the torus instability. This range is in good agreement with the predictions of the current-wire model, despite the inclusion of line-tying effects and the occurrence of tether-cutting magnetic reconnection. Title: Flare-CME Models: An Observational Perspective (Invited Review) Authors: Schmieder, B.; Aulanier, G.; Vršnak, B. Bibcode: 2015SoPh..290.3457S Altcode: 2015SoPh..tmp...64S Eruptions, flares, and coronal mass ejection (CMEs) are due to physical phenomena mainly driven by an initially force-free current-carrying magnetic field. We review some key observations relevant to the current theoretical trigger mechanisms of the eruption and to the energy release via reconnection. Sigmoids observed in X-rays and UV, as well as the pattern (double J-shaped) of electric currents in the photosphere show clear evidence of the existence of currents parallel to the magnetic field and can be the signature of a flux rope that is detectable in CMEs. The magnetic helicity of filaments and active regions is an interesting indirectly measurable parameter because it can quantify the twist of the flux rope. On the other hand, the magnetic helicity of the solar structures allows us to associate solar eruptions and magnetic clouds in the heliosphere. The magnetic topology analysis based on the 3D magnetic field extrapolated from vector magnetograms is a good tool for identifying the reconnection locations (null points and/or the 3D large volumes - hyperbolic flux tube, HFT). Flares are associated more with quasi-separatrix layers (QSLs) and HFTs than with a single null point, which is a relatively rare case. We review various mechanisms that have been proposed to trigger CMEs and their observable signatures: by "breaking" the field lines overlying the flux rope or by reconnection below the flux rope to reduce the magnetic tension, or by letting the flux rope to expand until it reaches a minimum threshold height (loss of equilibrium or torus instability). Additional mechanisms are commonly operating in the solar atmosphere. Examples of observations are presented throughout the article and are discussed in this framework. 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: Slipping reconnection and chromospheric evaporation in the 10 September 2014 flare Authors: Dudík, Jaroslav; Janvier, Miho; Polito, Vanessa; Mulay, Sargam; Del Zanna, Giulio; Mason, Helen; Aulanier, Guillaume Bibcode: 2015IAUGA..2252237D Altcode: We study the occurrence of slipping reconnection in the long-duration X-class flare of 2014 September 10. From the start, the flare shows apparent slippage of hot Fe XXI flare loops observed in the 131A channel of SDO/AIA. Using the time-distance plots, we show that the slipping motion of the flare loops proceeds in counter directions in both flare ribbons. Simultaneous IRIS Fe XXI observations show the occurrence of chromospheric evaporation at brightening kernels that are involved in the slipping reconnection of AIA loops. This happens also during a flux-rope breakout accompanied by a faint 'magnetic implosion' of a coronal loop. Based on the 3D MHD flare model, we argue that the 'implosion' is caused by the erupting flux rope pushing the neighbouring loops aside, with the low-lying loops being squeezed. Title: Formation of a rotating jet during the filament eruption on 2013 April 10-11 Authors: Filippov, B.; Srivastava, A. K.; Dwivedi, B. N.; Masson, S.; Aulanier, G.; Joshi, N. C.; Uddin, W. Bibcode: 2015MNRAS.451.1117F Altcode: 2015MNRAS.451.5636F; 2015arXiv150501615F We analyse multiwavelength and multiviewpoint observations of a helically twisted plasma jet formed during a confined filament eruption on 2013 April 10-11. Given a rather large-scale event with its high spatial and temporal resolution observations, it allows us to clearly understand some new physical details about the formation and triggering mechanism of twisting jet. We identify a pre-existing flux rope associated with a sinistral filament, which was observed several days before the event. The confined eruption of the filament within a null-point topology, also known as an Eiffel tower (or inverted-Y) magnetic field configuration results in the formation of a twisted jet after the magnetic reconnection near a null point. The sign of helicity in the jet is found to be the same as that of the sign of helicity in the filament. Untwisting motion of the reconnected magnetic field lines gives rise to the accelerating plasma along the jet axis. The event clearly shows the twist injection from the pre-eruptive magnetic field to the jet. 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: Can we explain atypical solar flares? Authors: Dalmasse, K.; Chandra, R.; Schmieder, B.; Aulanier, G. Bibcode: 2015A&A...574A..37D Altcode: 2014arXiv1410.8194D Context. We used multiwavelength high-resolution data from ARIES, THEMIS, and SDO instruments to analyze a non-standard, C3.3 class flare produced within the active region NOAA 11589 on 2012 October 16. Magnetic flux emergence and cancellation were continuously detected within the active region, the latter leading to the formation of two filaments.
Aims: Our aim is to identify the origins of the flare taking the complex dynamics of its close surroundings into account.
Methods: We analyzed the magnetic topology of the active region using a linear force-free field extrapolation to derive its 3D magnetic configuration and the location of quasi-separatrix layers (QSLs), which are preferred sites for flaring activity. Because the active region's magnetic field was nonlinear force-free, we completed a parametric study using different linear force-free field extrapolations to demonstrate the robustness of the derived QSLs.
Results: The topological analysis shows that the active region presented a complex magnetic configuration comprising several QSLs. The considered data set suggests that an emerging flux episode played a key role in triggering the flare. The emerging flux probably activated the complex system of QSLs, leading to multiple coronal magnetic reconnections within the QSLs. This scenario accounts for the observed signatures: the two extended flare ribbons developed at locations matched by the photospheric footprints of the QSLs and were accompanied with flare loops that formed above the two filaments, which played no important role in the flare dynamics.
Conclusions: This is a typical example of a complex flare that can a priori show standard flare signatures that are nevertheless impossible to interpret with any standard model of eruptive or confined flare. We find that a topological analysis, however, permitted us to unveil the development of such complex sets of flare signatures.

Movies associated to Figs. 1, 3, and 9 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/574/A37 Title: VizieR Online Data Catalog: Movies of 2012-10-16 solar flare (Dalmasse+, 2015) Authors: Dalmasse, K.; Chandra, R.; Schmieder, B.; Aulanier, G. Bibcode: 2015yCat..35740037D Altcode: 2015yCat..35749037D Part of the observations of NOAA 11589 presented here were obtained with the Atmospheric Imaging Assembly imager (AIA) and the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) satellite. The AIA instrument observes the Sun over a wide range of temperatures from the photosphere to the corona. The pixel size of the AIA images is 0.6". In this study, we considered the 1600, 304, 193, and 171Å data. The magnetic field in the AR was studied by using the line-of-sight magnetograms of the HMI instrument, which observes the full disk with a pixel size of 0.5".

We also used ground-based observations of the AR obtained with the Indian telescope from the Aryabhatta Research Institute of observational Sciences (ARIES) and with the French Telescope Heliographique pour l'Etude du Magnetisme et des Instabilites Solaires (THEMIS). The 15-cm f/15 Coude telescope of the ARIES, operating in Nainital (India), observes in the Hα line with a spatial resolution of 0.58". The THEMIS telescope, operating in Tenerife (Canary Islands), allows a simultaneous mapping of the Hα emission and the full Stokes parameters in the Fe 6302.5Å of a field of view of about 240"x100" in one hour.

(2 data files). 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: Distribution of electric currents in source regions of solar eruptions Authors: Torok, Tibor; Leake, James E.; Titov, Viacheslav; Archontis, Vasilis; Mikic, Zoran; Linton, Mark; Dalmasse, Kevin; Aulanier, Guillaume; Kliem, Bernhard Bibcode: 2014AAS...22431202T Altcode: There has been a long-lasting debate on the question of whether or not electric currents in the source regions of solar eruptions are neutralized. That is, whether or not the direct coronal currents connecting the photospheric polarities in such regions are surrounded by return currents of equal amount and opposite direction. In order to address this question, we consider several mechanisms of source region formation (flux emergence, photospheric shearing/twisting flows, and flux cancellation) and quantify the evolution of the electric currents, using 3D MHD simulations. For the experiments conducted so far, we find a clear dominance of the direct currents over the return currents in all cases in which the models produce significant magnetic shear along the source region's polarity inversion line. This suggests that pre-eruptive magnetic configurations in strongly sheared active regions and filament channels carry substantial net currents. We discuss the implications of this result for the modeling of solar eruptions. Title: Detection of Coherent Structures in Photospheric Turbulent Flows Authors: Chian, Abraham C. -L.; Rempel, Erico L.; Aulanier, Guillaume; Schmieder, Brigitte; Shadden, Shawn C.; Welsch, Brian T.; Yeates, Anthony R. Bibcode: 2014ApJ...786...51C Altcode: 2013arXiv1312.2405C We study coherent structures in solar photospheric flows in a plage in the vicinity of the active region AR 10930 using the horizontal velocity data derived from Hinode/Solar Optical Telescope magnetograms. Eulerian and Lagrangian coherent structures (LCSs) are detected by computing the Q-criterion and the finite-time Lyapunov exponents of the velocity field, respectively. Our analysis indicates that, on average, the deformation Eulerian coherent structures dominate over the vortical Eulerian coherent structures in the plage region. We demonstrate the correspondence of the network of high magnetic flux concentration to the attracting Lagrangian coherent structures (aLCSs) in the photospheric velocity based on both observations and numerical simulations. In addition, the computation of aLCS provides a measure of the local rate of contraction/expansion of the flow. Title: Slipping Magnetic Reconnection during an X-class Solar Flare Observed by SDO/AIA Authors: Dudík, J.; Janvier, M.; Aulanier, G.; Del Zanna, G.; Karlický, M.; Mason, H. E.; Schmieder, B. Bibcode: 2014ApJ...784..144D Altcode: 2014arXiv1401.7529D We present SDO/AIA observations of an eruptive X-class flare of 2012 July 12, and compare its evolution with the predictions of a three-dimensional (3D) numerical simulation. We focus on the dynamics of flare loops that are seen to undergo slipping reconnection during the flare. In the Atmospheric Imaging Assembly (AIA) 131 Å observations, lower parts of 10 MK flare loops exhibit an apparent motion with velocities of several tens of km s-1 along the developing flare ribbons. In the early stages of the flare, flare ribbons consist of compact, localized bright transition-region emission from the footpoints of the flare loops. A differential emission measure analysis shows that the flare loops have temperatures up to the formation of Fe XXIV. A series of very long, S-shaped loops erupt, leading to a coronal mass ejection observed by STEREO. The observed dynamics are compared with the evolution of magnetic structures in the "standard solar flare model in 3D." This model matches the observations well, reproducing the apparently slipping flare loops, S-shaped erupting loops, and the evolution of flare ribbons. All of these processes are explained via 3D reconnection mechanisms resulting from the expansion of a torus-unstable flux rope. The AIA observations and the numerical model are complemented by radio observations showing a noise storm in the metric range. Dm-drifting pulsation structures occurring during the eruption indicate plasmoid ejection and enhancement of the reconnection rate. The bursty nature of radio emission shows that the slipping reconnection is still intermittent, although it is observed to persist for more than an hour. Title: Distribution of Electric Currents in Solar Active Regions Authors: Török, T.; Leake, J. E.; Titov, V. S.; Archontis, V.; Mikić, Z.; Linton, M. G.; Dalmasse, K.; Aulanier, G.; Kliem, B. Bibcode: 2014ApJ...782L..10T Altcode: 2014arXiv1401.2931T There has been a long-standing debate on the question of whether or not electric currents in solar active regions are neutralized. That is, whether or not the main (or direct) coronal currents connecting the active region polarities are surrounded by shielding (or return) currents of equal total value and opposite direction. Both theory and observations are not yet fully conclusive regarding this question, and numerical simulations have, surprisingly, barely been used to address it. Here we quantify the evolution of electric currents during the formation of a bipolar active region by considering a three-dimensional magnetohydrodynamic simulation of the emergence of a sub-photospheric, current-neutralized magnetic flux rope into the solar atmosphere. We find that a strong deviation from current neutralization develops simultaneously with the onset of significant flux emergence into the corona, accompanied by the development of substantial magnetic shear along the active region's polarity inversion line. After the region has formed and flux emergence has ceased, the strong magnetic fields in the region's center are connected solely by direct currents, and the total direct current is several times larger than the total return current. These results suggest that active regions, the main sources of coronal mass ejections and flares, are born with substantial net currents, in agreement with recent observations. Furthermore, they support eruption models that employ pre-eruption magnetic fields containing such currents. 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 current variations and 3D magnetic configuration of coronal jets Authors: Schmieder, Brigitte; Harra, Louise K.; Aulanier, Guillaume; Guo, Yang; Demoulin, Pascal; Moreno-Insertis, Fernando, , Prof Bibcode: 2014cosp...40E2928S Altcode: Coronal jets (EUV) were observed by SDO/AIA on September 17, 2010. HMI and THEMIS measured the vector magnetic field from which we derived the magnetic flux, the phostospheric velocity and the vertical electric current. The magnetic configuration was computed with a non linear force-free approach. The phostospheric current pattern of the recurrent jets were associated with the quasi-separatrix layers deduced from the magnetic extrapolation. The large twisted near-by Eiffel-tower-shape jet was also caused by reconnection in current layers containing a null point. This jet cannot be classified precisely within either the quiescent or the blowout jet types. We will show the importance of the existence of bald patches in the low atmosphere Title: A confined flare above filaments Authors: Dalmasse, K.; Chandra, R.; Schmieder, B.; Aulanier, G. Bibcode: 2014IAUS..300..227D Altcode: 2013arXiv1310.0667D We present the dynamics of two filaments and a C-class flare observed in NOAA 11589 on 2012 October 16. We used the multi-wavelength high-resolution data from SDO, as well as THEMIS and ARIES ground-based observations. The observations show that the filaments are progressively converging towards each other without merging. We find that the filaments have opposite chirality which may have prevented them from merging. On October 16, a C3.3 class flare occurred without the eruption of the filaments. According to the standard solar flare model, after the reconnection, post-flare loops form below the erupting filaments whether the eruption fails or not. However, the observations show the formation of post-flare loops above the filaments, which is not consistent with the standard flare model. We analyze the topology of the active region's magnetic field by computing the quasi-separatrix layers (QSLs) using a linear force-free field extrapolation. We find a good agreement between the photospheric footprints of the QSLs and the flare ribbons. We discuss how slipping or slip-running reconnection at the QSLs may explain the observed dynamics. 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: The physical mechanisms that initiate and drive solar eruptions Authors: Aulanier, Guillaume Bibcode: 2014IAUS..300..184A Altcode: 2013arXiv1309.7329A Solar eruptions are due to a sudden destabilization of force-free coronal magnetic fields. But the detailed mechanisms which can bring the corona towards an eruptive stage, then trigger and drive the eruption, and finally make it explosive, are not fully understood. A large variety of storage-and-release models have been developed and opposed to each other since 40 years. For example, photospheric flux emergence vs. flux cancellation, localized coronal reconnection vs. large-scale ideal instabilities and loss of equilibria, tether-cutting vs. breakout reconnection, and so on. The competition between all these approaches has led to a tremendous drive in developing and testing all these concepts, by coupling state-of-the-art models and observations. Thanks to these developments, it now becomes possible to compare all these models with one another, and to revisit their interpretation in light of their common and their different behaviors. This approach leads me to argue that no more than two distinct physical mechanisms can actually initiate and drive prominence eruptions: the magnetic breakout and the torus instability. In this view, all other processes (including flux emergence, flux cancellation, flare reconnection and long-range couplings) should be considered as various ways that lead to, or that strengthen, one of the aforementioned driving mechanisms. 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: Hot Spine Loops and the Nature of a Late-phase Solar Flare Authors: Sun, Xudong; Hoeksema, J. Todd; Liu, Yang; Aulanier, Guillaume; Su, Yingna; Hannah, Iain G.; Hock, Rachel A. Bibcode: 2013ApJ...778..139S Altcode: 2013arXiv1310.1438S The fan-spine magnetic topology is believed to be responsible for many curious features in solar explosive events. A spine field line links distinct flux domains, but direct observation of such a feature has been rare. Here we report a unique event observed by the Solar Dynamic Observatory where a set of hot coronal loops (over 10 MK) connected to a quasi-circular chromospheric ribbon at one end and a remote brightening at the other. Magnetic field extrapolation suggests that these loops are partly tracers of the evolving spine field line. Continuous slipping- and null-point-type reconnections were likely at work, energizing the loop plasma and transferring magnetic flux within and across the fan quasi-separatrix layer. We argue that the initial reconnection is of the "breakout" type, which then transitioned to a more violent flare reconnection with an eruption from the fan dome. Significant magnetic field changes are expected and indeed ensued. This event also features an extreme-ultraviolet (EUV) late phase, i.e., a delayed secondary emission peak in warm EUV lines (about 2-7 MK). We show that this peak comes from the cooling of large post-reconnection loops beside and above the compact fan, a direct product of eruption in such topological settings. The long cooling time of the large arcades contributes to the long delay; additional heating may also be required. Our result demonstrates the critical nature of cross-scale magnetic coupling—topological change in a sub-system may lead to explosions on a much larger scale. Title: Twisting solar coronal jet launched at the boundary of an active region Authors: Schmieder, B.; Guo, Y.; Moreno-Insertis, F.; Aulanier, G.; Yelles Chaouche, L.; Nishizuka, N.; Harra, L. K.; Thalmann, J. K.; Vargas Dominguez, S.; Liu, Y. Bibcode: 2013A&A...559A...1S Altcode: 2013arXiv1309.6514S
Aims: A broad jet was observed in a weak magnetic field area at the edge of active region NOAA 11106 that also produced other nearby recurring and narrow jets. The peculiar shape and magnetic environment of the broad jet raised the question of whether it was created by the same physical processes of previously studied jets with reconnection occurring high in the corona.
Methods: We carried out a multi-wavelength analysis using the EUV images from the Atmospheric Imaging Assembly (AIA) and magnetic fields from the Helioseismic and Magnetic Imager (HMI) both on-board the Solar Dynamics Observatory, which we coupled to a high-resolution, nonlinear force-free field extrapolation. Local correlation tracking was used to identify the photospheric motions that triggered the jet, and time-slices were extracted along and across the jet to unveil its complex nature. A topological analysis of the extrapolated field was performed and was related to the observed features.
Results: The jet consisted of many different threads that expanded in around 10 minutes to about 100 Mm in length, with the bright features in later threads moving faster than in the early ones, reaching a maximum speed of about 200 km s-1. Time-slice analysis revealed a striped pattern of dark and bright strands propagating along the jet, along with apparent damped oscillations across the jet. This is suggestive of a (un)twisting motion in the jet, possibly an Alfvén wave. Bald patches in field lines, low-altitude flux ropes, diverging flow patterns, and a null point were identified at the basis of the jet.
Conclusions: Unlike classical λ or Eiffel-tower-shaped jets that appear to be caused by reconnection in current sheets containing null points, reconnection in regions containing bald patches seems to be crucial in triggering the present jet. There is no observational evidence that the flux ropes detected in the topological analysis were actually being ejected themselves, as occurs in the violent phase of blowout jets; instead, the jet itself may have gained the twist of the flux rope(s) through reconnection. This event may represent a class of jets different from the classical quiescent or blowout jets, but to reach that conclusion, more observational and theoretical work is necessary. Title: The standard flare model in three dimensions. III. Slip-running reconnection properties Authors: Janvier, M.; Aulanier, G.; Pariat, E.; Démoulin, P. Bibcode: 2013A&A...555A..77J Altcode: 2013arXiv1305.4053J Context. A standard model for eruptive flares aims at describing observational 3D features of the reconnecting coronal magnetic field. Extensions to the 2D model require the physical understanding of 3D reconnection processes at the origin of the magnetic configuration evolution. However, the properties of 3D reconnection without null point and separatrices still need to be analyzed.
Aims: We focus on magnetic reconnection associated with the growth and evolution of a flux rope and associated flare loops during an eruptive flare. We aim at understanding the intrinsic characteristics of 3D reconnection in the presence of quasi-separatrix layers (QSLs), how QSL properties are related to the slip-running reconnection mode in general, and how this applies to eruptive flares in particular.
Methods: We studied the slip-running reconnection of field lines in a magnetohydrodynamic simulation of an eruptive flare associated with a torus-unstable flux rope. The squashing degree and the mapping norm are two parameters related to the QSLs. We computed them to investigate their relation with the slip-running reconnection speed of selected field lines.
Results: Field lines associated with the flux rope and the flare loops undergo a continuous series of magnetic reconnection, which results in their super-Alfvénic slipping motion. The time profile of their slippage speed and the space distribution of the mapping norm are shown to be strongly correlated. We find that the motion speed is proportional to the mapping norm. Moreover, this slip-running motion becomes faster as the flux rope expands, since the 3D current layer evolves toward a current sheet, and QSLs to separatrices.
Conclusions: The present analysis extends our understanding of the 3D slip-running reconnection regime. We identified a controlling parameter of the apparent velocity of field lines while they slip-reconnect, enabling the interpretation of the evolution of post flare loops. This work completes the standard model for flares and eruptions by giving its 3D properties. Title: Hot Spine Loops and the Nature of a Late-Phase Solar Flare Authors: Sun, Xudong; Hoeksema, J. T.; Liu, Y.; Aulanier, G.; Su, Y.; Hannah, I.; Hock, R. Bibcode: 2013SPD....4420103S Altcode: The fan-spine magnetic topology is believed to be responsible for many curious emission signatures in solar explosive events. A spine field line links topologically distinct flux domains and possibly their evolutionary trends, but direct observation of such structure has been rare. Here we report a unique event observed by the Solar Dynamic Observatory (SDO) where a set of hot coronal loops (over 10 MK) that developed during the rising phase of the flare connected to a quasi-circular chromospheric ribbon at one end and a remote brightening at the other. Magnetic field extrapolation suggests these loops are partly tracers of the evolving spine field line. The sequential brightening of the ribbon, the apparent shuffling loop motion, and the increasing volume occupied by the hot loops suggest that continuous slipping- and null-point-type reconnections were at work, energizing the loop plasma and transferring magnetic flux within and across the dome-shaped, fan quasi-separatrix layer (QSL). We argue that the initial reconnection is of the "break-out" type, which transitioned to more violent flare reconnection nearing the flare peak with an eruption from the fan dome. Significant magnetic field changes are expected and indeed ensued, which include a shift of the QSL footprint, an increase of the horizontal photospheric field, and de-shearing of the coronal loops. This event also features an extreme-ultraviolet (EUV) late phase -- a second emission peak observed in the warm EUV lines (about 2--7 MK) up to 1--2 hours after the soft X-ray peak. We show that this peak comes from the large post-flare arcades beside and above the compact fan dome, a direct product of eruption in such topological settings. Cooling of these large arcades naturally explains the sequential delay of the late-phase peaks in increasingly cooler EUV lines, and the estimated theoretical cooling time is compatible with observation. Our result demonstrates the critical nature of cross-scale magnetic coupling -- minor topological change in a sub-system may lead to explosions on a much larger scale. Title: Solar filament eruptions and their physical role in triggering coronal mass ejections Authors: Schmieder, B.; Démoulin, P.; Aulanier, G. Bibcode: 2013AdSpR..51.1967S Altcode: 2012arXiv1212.4014S Solar filament eruptions play a crucial role in triggering coronal mass ejections (CMEs). More than 80% of eruptions lead to a CME. This correlation has been studied extensively during the past solar cycles and the last long solar minimum. The statistics made on events occurring during the rising phase of the new solar cycle 24 is in agreement with this finding. Both filaments and CMEs have been related to twisted magnetic fields. Therefore, nearly all the MHD CME models include a twisted flux tube, called a flux rope. Either the flux rope is present long before the eruption, or it is built up by reconnection of a sheared arcade from the beginning of the eruption. Title: SDO/AIA Prominence physical conditions Authors: Schmieder, B.; Parenti, S.; Dudik, J.; Aulanier, G.; Heinzel, P.; Zapior, M.; Golub, L. Bibcode: 2013enss.confE..27S Altcode: SDO/AIA has carried out continuous observations of prominences in multiple wavelengths, with high spatial and temporal resolution. These data provide us an opportunity to understand the physical conditions and dynamics of prominences. The surprising brightness of prominences in some coronal lines has been well explained by the presence of transition region lines in the bandpass of the filters (171 A, 131 A), a result that leads us to revise our model of the transition region of prominences and to consider a relatively dense transition region in some prominence evolutionary phases or in some viewing orientation. An additional aspect of prominence dynamics will be presented with a new quasi-static MHD model proposed for bubbles and plumes. We propose an alternative to the interpretation that thermal instabilities are responsible for the formation of bubbles. The bubbles are found to correspond to magnetic separatrices formed by emerging magnetic field close to prominence footpoints. Title: Behavior of the vertical current during the X2 flare of 2011 February 15 observed by SDO/HMI, compared to a line-tied zero-beta resistive MHD simulation Authors: Bommier, Veronique; Aulanier, Guillaume Bibcode: 2013enss.confE..79B Altcode: The level-1 data of SDO/HMI have been inverted with the UNNOFIT inversion code (Bommier et al., 2007, A&A, 464, 323), which differs from VFISV about the magnetic filling factor modeling. More realistic field inclinations are obtained outside the active region. The spatial resolution seems to be also better. UNNOFIT is enabled for the taking into account of gradients of radial velocity, responsible for asymmetry of the Stokes profiles (Molodij et al., 2011, A&A, 531, A139). The ambiguity has been solved with the ME0 code of Metcalf, Leka, Barnes & Crouch. We present the movie of 4 hours of observation, the flare occurring at middle. Two current ribbons of opposite polarity are visible along the magnetic neutral line, in the vertical density current map. The negative one strengthens and radially expands from the flare center at the eruption moment. A similar current ribbon pair is visible in a 3D line-tied zero-beta resistive MHD flare simulation with the OHM code (Aulanier et al. , 2012, A&A, 543, A110). The two ribbons part from the flare center during the flare, similarly to what observed on 2011 February 15 at 02:00 with SDO/HMI. Title: Global dynamics of subsurface solar active regions Authors: Jouve, L.; Brun, A. S.; Aulanier, G. Bibcode: 2013ApJ...762....4J Altcode: 2012arXiv1211.7251J We present three-dimensional numerical simulations of a magnetic loop evolving in either a convectively stable or unstable rotating shell. The magnetic loop is introduced into the shell in such a way that it is buoyant only in a certain portion in longitude, thus creating an Ω-loop. Due to the action of magnetic buoyancy, the loop rises and develops asymmetries between its leading and following legs, creating emerging bipolar regions whose characteristics are similar to those of observed spots at the solar surface. In particular, we self-consistently reproduce the creation of tongues around the spot polarities, which can be strongly affected by convection. We further emphasize the presence of ring-shaped magnetic structures around our simulated emerging regions, which we call "magnetic necklace" and which were seen in a number of observations without being reported as of today. We show that those necklaces are markers of vorticity generation at the periphery and below the rising magnetic loop. We also find that the asymmetry between the two legs of the loop is crucially dependent on the initial magnetic field strength. The tilt angle of the emerging regions is also studied in the stable and unstable cases and seems to be affected both by the convective motions and the presence of a differential rotation in the convective cases. Title: The standard flare model in three dimensions. II. Upper limit on solar flare energy Authors: Aulanier, G.; Démoulin, P.; Schrijver, C. J.; Janvier, M.; Pariat, E.; Schmieder, B. Bibcode: 2013A&A...549A..66A Altcode: 2012arXiv1212.2086A Context. Solar flares strongly affect the Sun's atmosphere as well as the Earth's environment. Quantifying the maximum possible energy of solar flares of the present-day Sun, if any, is thus a key question in heliophysics.
Aims: The largest solar flares observed over the past few decades have reached energies of a few times 1032 erg, possibly up to 1033 erg. Flares in active Sun-like stars reach up to about 1036 erg. In the absence of direct observations of solar flares within this range, complementary methods of investigation are needed to assess the probability of solar flares beyond those in the observational record.
Methods: Using historical reports for sunspot and solar active region properties in the photosphere, we scaled to observed solar values a realistic dimensionless 3D MHD simulation for eruptive flares, which originate from a highly sheared bipole. This enabled us to calculate the magnetic fluxes and flare energies in the model in a wide paramater space.
Results: Firstly, commonly observed solar conditions lead to modeled magnetic fluxes and flare energies that are comparable to those estimated from observations. Secondly, we evaluate from observations that 30% of the area of sunspot groups are typically involved in flares. This is related to the strong fragmentation of these groups, which naturally results from sub-photospheric convection. When the model is scaled to 30% of the area of the largest sunspot group ever reported, with its peak magnetic field being set to the strongest value ever measured in a sunspot, it produces a flare with a maximum energy of ~6 × 1033 erg.
Conclusions: The results of the model suggest that the Sun is able to produce flares up to about six times as energetic in total solar irradiance fluence as the strongest directly observed flare of Nov. 4, 2003. Sunspot groups larger than historically reported would yield superflares for spot pairs that would exceed tens of degrees in extent. We thus conjecture that superflare-productive Sun-like stars should have a much stronger dynamo than in the Sun. Title: Magnetic Topology of Bubbles in Quiescent Prominences Authors: Dudík, J.; Aulanier, G.; Schmieder, B.; Zapiór, M.; Heinzel, P. Bibcode: 2012ApJ...761....9D Altcode: We study a polar-crown prominence with a bubble and its plume observed in several coronal filters by the SDO/AIA and in Hα by the MSDP spectrograph in Białków (Poland) to address the following questions: what is the brightness of prominence bubbles in EUV with respect to the corona outside of the prominence and the prominence coronal cavity? What is the geometry and topology of the magnetic field in the bubble? What is the nature of the vertical threads seen within prominences? We find that the brightness of the bubble and plume is lower than the brightness of the corona outside of the prominence, and is similar to that of the coronal cavity. We constructed linear force-free models of prominences with bubbles, where the flux rope is perturbed by inclusion of parasitic bipoles. The arcade field lines of the bipole create the bubble, which is thus devoid of magnetic dips. Shearing the bipole or adding a second one can lead to cusp-shaped prominences with bubbles similar to the observed ones. The bubbles have complex magnetic topology, with a pair of coronal magnetic null points linked by a separator outlining the boundary between the bubble and the prominence body. We conjecture that plume formation involves magnetic reconnection at the separator. Depending on the viewing angle, the prominence can appear either anvil-shaped with predominantly horizontal structures, or cusp-shaped with predominantly vertical structuring. The latter is an artifact of the alignment of magnetic dips with respect to the prominence axis and the line of sight. Title: X-ray and ultraviolet investigation into the magnetic connectivity of a solar flare Authors: Reid, H. A. S.; Vilmer, N.; Aulanier, G.; Pariat, E. Bibcode: 2012A&A...547A..52R Altcode: 2012arXiv1210.2916R We investigate the X-ray and UV emission detected by RHESSI and TRACE in the context of a solar flare on the 16th November 2002 with the goal of better understanding the evolution of the flare. We analysed the characteristics of the X-ray emission in the 12-25 and 25-50 keV energy range while we looked at the UV emission at 1600 Å . The flare appears to have two distinct phases of emission separated by a 25-s time delay, with the first phase being energetically more important. We found good temporal and spatial agreement between the 25-50 keV X-rays and the most intense areas of the 1600 Å UV emission. We also observed an extended 100-arcsec < 25 keV source that appears coronal in nature and connects two separated UV ribbons later in the flare. Using the observational properties in X-ray and UV wavelengths, we propose two explanations for the flare evolution in relation to the spine/fan magnetic field topology and the accelerated electrons. We find that a combination of quasi separatrix layer reconnection and null-point reconnection is required to account for the observed properties of the X-ray and UV emission. Title: Reconstruction of 3D Coronal Magnetic Structures from THEMIS/MTR and Hinode/SOT Vector Maps Authors: Schmieder, B.; Guo, Y.; Aulanier, G.; Démoulin, P.; Török, T.; Bommier, V.; Wiegelmann, T.; Gosain, S. Bibcode: 2012ASPC..454..363S Altcode: Coordinated campaigns using THEMIS, Hinode, and other instruments have allowed us to study the magnetic fields of faculae, filaments, and active regions. In a first case, we modelled the 3D magnetic field in a flaring active region with a nonlinear force-free field extrapolation, using magnetic vectors observed by THEMIS/MTR as boundary condition. In order to construct a consistent bottom boundary for the model, we first removed the 180 degree ambiguity of the transverse fields and minimized the force and torque in the observed vector fields. We found a twisted magnetic flux rope, well aligned with the polarity inversion line and a part of an Hα filament, and located where a large flare is initiated about two hours later. In a second case, Hinode/SOT allowed us to detect fine flux concentrations in faculae, while MTR provided us with magnetic information at different levels in the atmosphere. The polarimetry analysis of the MTR and SOT data gave consistent results, using both UNNOFIT and MELANIE inversion codes. 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: Can bubbles in quiescent prominences be purely magnetic phenomena? Authors: Dudik, Jaroslav; Schmieder, Brigitte; Aulanier, Guillaume; Zapior, Maciej; Heinzel, Petr Bibcode: 2012cosp...39..486D Altcode: 2012cosp.meet..486D We present a model of the magnetic field constituting quiescent prominences. The model assumes a linear force-free field with a weakly twisted flux-tube in an OX/OF topology perturbed by presence of parasitic polarities within the filament channel. The parasitic polarities locally create the cusp-shaped prominences with bubbles exactly as those observed by the SDO/AIA and Bialkow Observatory. We find that the observations are best reproduced if the parasitic bipoles are sheared with respect to the main inversion line. We show that the bubbles are in fact constituted by the arcade-like field lines, as opposed to that of the prominence, which is created by magnetic dips. A pair of null points is always associated with the parasitic bipole. These null points are connected by a separator passing through the prominence bubble. We show how the presence of an additional parasitic bipole moves the separator to the boundary between the bubble and the rest of the prominence, producing a topology favorable for reconnection and possibly for the formation of plumes. Title: The standard flare model in three dimensions. I. Strong-to-weak shear transition in post-flare loops Authors: Aulanier, G.; Janvier, M.; Schmieder, B. Bibcode: 2012A&A...543A.110A Altcode: Context. The standard CSHKP model for eruptive flares is two-dimensional. Yet observational interpretations of photospheric currents in pre-eruptive sigmoids, shear in post-flare loops, and relative positioning and shapes of flare ribbons, all together require three-dimensional extensions to the model.
Aims: We focus on the strong-to-weak shear transition in post-flare loops, and on the time-evolution of the geometry of photospheric electric currents, which occur during the development of eruptive flares. The objective is to understand the three-dimensional physical processes, which cause them, and to know how much the post-flare and the pre-eruptive distributions of shear depend on each other.
Methods: The strong-to-weak shear transition in post-flare loops is identified and quantified in a flare observed by STEREO, as well as in a magnetohydrodynamic simulation of CME initiation performed with the OHM code. In both approaches, the magnetic shear is evaluated with field line footpoints. In the simulation, the shear is also estimated from ratios between magnetic field components.
Results: The modeled strong-to-weak shear transition in post-flare loops comes from two effects. Firstly, a reconnection-driven transfer of the differential magnetic shear, from the pre- to the post-eruptive configuration. Secondly, a vertical straightening of the inner legs of the CME, which induces an outer shear weakening. The model also predicts the occurrence of narrow electric current layers inside J-shaped flare ribbons, which are dominated by direct currents. Finally, the simulation naturally accounts for energetics and time-scales for weak and strong flares, when typical scalings for young and decaying solar active regions are applied.
Conclusions: The results provide three-dimensional extensions to the standard flare model. These extensions involve MHD processes that should be tested with observations. Title: How to build-up realistic 3D magnetic field environments for eruptive flares? Authors: Aulanier, Guillaume Bibcode: 2012cosp...39...71A Altcode: 2012cosp.meet...71A The standard model in 2D for solar eruptive flares is based on magnetic reconnection at an X-point between open field lines, which produces closed post-flare loops at the footpoints of which ribbons emissions take place. So as to understand the observed morphology and dynamics of postflare loops and ribbons, it is required to go beyond the standard model. This implies to know the 3D geometry of the reconnecting magnetic fields. This passes through the modeling of the dynamic expansion of current-carrying loops, which eventually turn into a coronal mass ejection (CME). On the one hand the first model proposed for CME triggering -the loss of equilibrium model- used the physical approach of electric wires and Laplace. But on the other hand, the wire approach must be replaced by the physics of MHD in the solar corona. Therefore, this paradigm has led to some uncertainty for several decades, regarding the nature of the most physically viable eruptions mechanisms for CMEs. A large variety of semi-analytical and numerical storage-and-release MHD models have been developped in the past 20 years or so. While all these models rely on the slow increase of currents and/or the slow decrease of the restraining magnetic tension preceding the eruption, they all put the emphazis on different physical mechanisms, both to achieve the pre-eruptive evolution, and to suddenly trigger and later drive the eruption. Nevertheless, all the models share many common features. And all the later describe many individual observed aspects of solar eruptions. It is therefore not always clear which of all the suggested mechanisms, if any, do really account for the triggering of observed events in general. Also, these mechanisms should arguably not be as numerous as the models themselves, owing to the common occurence of eruptions. In order to shed some light on this challenging -but still unripe- topic, I will first compare the driving and stabilizing forces in both the wire and the MHD views. I will then discuss the applicability of the MHD models to the Sun, and most of all I will re-order the most sensitive ones ina common frame, so as to find common denominators. I will end on a brief report of the shape of post-flare loops and ribbons, as they develop in one of these CME models. 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: Moving towards a comprehensive model for active region outflows Authors: Aulanier, Guillaume; Del Zanna, Giulio; Bradshaw, Stephen Bibcode: 2012cosp...39...72A Altcode: 2012cosp.meet...72A Coronal outflows located at the edges of strong magnetic flux concentrations seem to be a common and persistent property of solar active regions. They have been reported and discussed using both direct imaging in EUV and SXR, as well as using Doppler measurements from EUV spectroscopy. Due to their potential role in feeding the solar wind with extra mass and momentum, which is one of the primary goal of the upcoming Solar Orbiter mission, coronal outflows have received a broad attention since more than a decade. But for the genuine reader, the nature of the observed motions, as well as their physical origins, still look to be very unclear. I will review some of the most debated interpretations for these features, such as : Do the `warm' outflows, reported in imagery, which are in fact associated with redshifts seen in spectroscopy, correspond to upward-traveling waves ? Do the redshifts indicate a global mass circulation along the chromosphere-coronal axis ? Do the non-steady, quasi-periodic, and relatively faster disturbances, that are superposed on quasi-steady and relatively-slower upflows, correspond to magnetoacoustic waves, to spicule transient jets, or to some other bulk flows ? More generally, do the assymetric `hot' line profiles reported in several observations really highlight different plasmas along the LOS, and thus require the development of specific models ? Do the increasing Dopplershifts with line temperature formation indicate accelerating upflows, or do they simply show different flows along different flux tubes ? Do all the observed outflows feed the solar wind, or do some of them end up as downflows ? In the latter case, does the mass fall back at remote coronal locations, or right towards the underlying chromosphere ? Is the continuing growth of active regions responsible for the flows, and if yes are the flows induced by high-altitude reconnection or by low altitude squashing of the plasma ? Owing to the intrinsic limitations of the EUV spectroscopic observations, new studies have investigated this intriguing phenomenon, using others methods of investigation : magnetic field extrapolations, topological analyses, radiative hydrodynamic simulations, calculation of synthetic line profiles, and last but not least, radio observations as well as in-situ solar wind plasma measurements. I will argue that these studies, when reviewed alltogether, appear to provide an important step towards an integrated model for coronal outflows, even though they do not solve all the aforementioned issues. 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: What are the physical mechanisms of eruptions and CMEs? Authors: Schmieder, Brigitte; Aulanier, Guillaume Bibcode: 2012AdSpR..49.1598S Altcode: CMEs are due to physical phenomena that drive both, eruptions and flares in active regions. Eruptions/CMEs must be driven from initially force-free current-carrying magnetic field. Twisted flux ropes, sigmoids, current lanes and pattern in photospheric current maps show a clear evidence of currents parallel to the magnetic field. Eruptions occur starting from equilibria which have reached some instability threshold. Revisiting several data sets of CME observations we identified different mechanisms leading to this unstable state from a force free field. Boundary motions related to magnetic flux emergence and shearing favor the increase of coronal currents leading to the large flares of November 2003. On the other hand, we demonstrated by numerical simulations that magnetic flux emergence is not a sufficient condition for eruptions. Filament eruptions are interpreted either by a torus instability for an event occurring during the minimum of solar activity either by the diffusion of the magnetic flux reducing the tension of the restraining arcade. We concluded that CME models (tether cutting, break out, loss of equilibrium models) are based on these basic mechanisms for the onset of CMEs. Title: Electric current density and related sigmoid in an active region Authors: Joulin, V.; Schmieder, B.; Aulanier, G.; Bommier, V. Bibcode: 2012EAS....55..143J Altcode: Using THEMIS vector magnetograms we measured vertical electric current density in the leading sunspot of NOAA 11127 active region during its disk passage. The current structures evolve versus time. MHD modelling allows us to explain the spiral pattern by torsion. We found observational visible counterparts in the SDO/AIA 335 A images. The field lines are visible as loops in the AIA images. When the torsion is increasing, a sigmoid is observed. In the present event observed on November 24th 2010, we find that reconnection is also necessary to explain their sigmoidal shape. Title: New perspectives on solar prominences Authors: Schmieder, B.; Aulanier, G. Bibcode: 2012EAS....55..149S Altcode: Recent observations of prominences obtained with high spatial and temporal resolution instruments, on board satellites (Hinode, SDO) as well as on the ground (SST) have provided very intriguing movies and open a new area for understanding the nature of prominences. The main topics are still debate: formation, dynamics, and characteristics of the plasma in the core and in the transition zone between the prominence and corona. We will review briefly the recent advances made in these topics, observationally as well as theoretically. 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: On the Nature of Prominence Bubbles and Plumes Authors: Schmieder, B.; Zapior, M.; Heinzel, P.; Aulanier, G. Bibcode: 2012ASPC..456...77S Altcode: An example of bubble and related plume, using time series in selected SDO/AIA channels and co-temporal MSDP observations in the Hα line obtained at Wroclaw-Bialkow observatory on April 20, 2011 is presented. The formation of bubbles may be due to emerging arcades in the filament channel, below the prominence. After discussing on MHD modeling, we conclude that bubbles and plumes are parts of the corona observed through gaps/windows in the prominence. Title: 3D MHD Simulation of Current Intensification along Serpentine Emerging Magnetic Fields Authors: Pariat, E.; Masson, S.; Aulanier, G. Bibcode: 2012ASPC..455..177P Altcode: The high resolution observations of the Hinode instruments have revealed many important features of the magnetic flux evolution and its interaction with the solar plasma in emerging flux regions. The high intermittency of the magnetic field distribution in interspot regions confirms the serpentine topology adopted by the magnetic field as it cross the solar photosphere. Precise information about the evolution of localized brightenings, usually called Ellerman bombs (EBs), typical events of emerging flux regions, have been gathered by Hinode: the link between EBs and the magnetic topology, the EBs detailed spectral time evolution and their relation with other dynamic events such as small scale jets, etc. Ellerman bombs are believed to be the observational signatures of the multiple magnetic reconnections which enable the magnetic field to emerge further up and magnetically structure the corona above active regions. This work is part of a world-wide effort to model the emergence of magnetic field forming solar active regions. Using a data-driven, three-dimensional (3D) magnetohydrodynamic (MHD) numerical simulation of a flux emergence region, we study the development of 3D electric current sheets. We show that these currents buildup along the 3D serpentine magnetic-field structure as a result of photospheric diverging horizontal line-tied motions that emulate the observed photospheric evolution. We study which types of motion and magnetic topology lead to the highest current intensification and therefore to the highest reconnection probability. We discuss how these currents can explain the formation of Ellerman bombs, facilitate the flux emergence, and account for some observed pattern of emerging flux regions. Title: Topological tools for the analysis of active region filament stability Authors: DeLuca, Edward E.; Savcheva, A.; van Ballegooijen, A.; Pariat, E.; Aulanier, G.; Su, Y. Bibcode: 2012decs.confE..64D Altcode: The combination of accurate NLFFF models and high resolution MHD simulations allows us to study the changes in stability of an active region filament before a CME. Our analysis strongly supports the following sequence of events leading up to the CME: first there is a build up of magnetic flux in the filament through flux cancellation beneath a developing flux rope; as the flux rope develops a hyperbolic flux tube (HFT) forms beneath the flux rope; reconnection across the HFT raises the flux rope while adding addition flux to it; the eruption is triggered when the flux rope becomes torus-unstable. The work applies topological analysis tools that have been developed over the past decade and points the way for future work on the critical problem of CME initiation in solar active regions. We will discuss the uses of this approach, current limitations and future prospects. Title: Interchange Slip-Running Reconnection and Sweeping SEP Beams Authors: Masson, S.; Aulanier, G.; Pariat, E.; Klein, K. -L. Bibcode: 2012SoPh..276..199M Altcode: 2011arXiv1109.5678M We present a new model to explain how particles (solar energetic particles; SEPs), accelerated at a reconnection site that is not magnetically connected to the Earth, could eventually propagate along the well-connected open flux tube. Our model is based on the results of a low-β resistive magnetohydrodynamics simulation of a three-dimensional line-tied and initially current-free bipole, which is embedded in a non-uniform open potential field. The topology of this configuration is that of an asymmetric coronal null point, with a closed fan surface and an open outer spine. When driven by slow photospheric shearing motions, field lines, initially fully anchored below the fan dome, reconnect at the null point, and jump to the open magnetic domain. This is the standard interchange mode as sketched and calculated in 2D. The key result in 3D is that reconnected open field lines located in the vicinity of the outer spine keep reconnecting continuously, across an open quasi-separatrix layer, as previously identified for non-open-null-point reconnection. The apparent slipping motion of these field lines leads to formation of an extended narrow magnetic flux tube at high altitude. Because of the slip-running reconnection, we conjecture that if energetic particles would be traveling through, or be accelerated inside, the diffusion region, they would be successively injected along continuously reconnecting field lines that are connected farther and farther from the spine. At the scale of the full Sun, owing to the super-radial expansion of field lines below 3 R, such energetic particles could easily be injected in field lines slipping over significant distances, and could eventually reach the distant flux tube that is well-connected to the Earth. Title: Large Field-of-View Spectropolarimetric Observations with a Large Aperture Telescope Authors: Molodij, G.; Aulanier, G. Bibcode: 2012SoPh..276..451M Altcode: In the context of the increasing interest in large-aperture telescopes dedicated to the Sun, we present a study to evaluate the adaptive-optics system limitations on the requirements expected for magnetic-field extrapolations and data-driven MHD simulations of active regions. The questions we address include: What is the size of the field of view at high spatial resolution for a four-meter class telescope with a spectrograph? What is the impact of the selected spectral domain on the performance in relation to the aforementioned scientific goals? We show that the visible-wavelength domain remains difficult to explore with ground-based telescopes using classical adaptive optics systems. The field of view obtained will be only a few arcseconds at the diffraction limit for most of the time. We review alternative configurations of adaptive-optics systems for different telescope apertures and wavelength domains which could be considered for practical implementation in derivations of the magnetic field from polarimetric observations. Title: Adaptive optics system performances and large field of view spectropolarimetric observations Authors: Aulanier, G.; Molodij, G. Bibcode: 2011sf2a.conf..395A Altcode: In the context of the increasing interest for large aperture telescope dedicated to the Sun such as EST or ATST projects, we (the Guillaumes) present a study to evaluate the adaptive optics system limitations in regard of the scientific requirements expected for magnetic field extrapolations and data-driven MHD simulations of active regions. The questions we address are: what is the size of the field of view at high spatial resolution for a 4 meter class telescope with a spectrograph, what is the impact of the selected spectral domain on the performances in relation to the scientific goals aforementioned ? We show that the visible wavelength domain still remains difficult to explore with ground-based telescope using a classical adaptive optics system. The field of view obtained will be only few arcsecs at diffraction limit for the most part of the observation time. Title: A Reconnection-driven Rarefaction Wave Model for Coronal Outflows Authors: Bradshaw, S. J.; Aulanier, G.; Del Zanna, G. Bibcode: 2011ApJ...743...66B Altcode: We conduct numerical experiments to determine whether interchange reconnection at high altitude coronal null points can explain the outflows observed as blueshifts in coronal emission lines at the boundaries between open and closed magnetic field regions. In this scenario, a strong, post-reconnection pressure gradient forms in the field-aligned direction when dense and hot, active region core loops reconnect with neighboring tenuous and cool, open field lines. We find that the pressure gradient drives a supersonic outflow and a rarefaction wave develops in both the open and closed post-reconnection magnetic field regions. We forward-model the spectral line profiles for a selection of coronal emission lines to predict the spectral signatures of the rarefaction wave. We find that the properties of the rarefaction wave are consistent with the observed velocity versus temperature structure of the corona in the outflow regions, where the velocity increases with the formation temperature of the emission lines. In particular, we find excellent agreement between the predicted and observed Fe XII 195.119 Å spectral line profiles in terms of the blueshift (10 km s-1), full width at half-maximum (83 mÅ) and symmetry. Finally, we find that Ti < Te in the open field region, which indicates that the interchange reconnection scenario may provide a viable mechanism and source region for the slow solar wind. 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: A filament supported by different magnetic field configurations Authors: Guo, Y.; Schmieder, B.; Démoulin, P.; Wiegelmann, T.; Aulanier, G.; Török, T.; Bommier, V. Bibcode: 2011IAUS..273..328G Altcode: A nonlinear force-free magnetic field extrapolation of vector magnetogram data obtained by THEMIS/MTR on 2005 May 27 suggests the simultaneous existence of different magnetic configurations within one active region filament: one part of the filament is supported by field line dips within a flux rope, while the other part is located in dips within an arcade structure. Although the axial field chirality (dextral) and the magnetic helicity (negative) are the same along the whole filament, the chiralities of the filament barbs at different sections are opposite, i.e., right-bearing in the flux rope part and left-bearing in the arcade part. This argues against past suggestions that different barb chiralities imply different signs of helicity of the underlying magnetic field. This new finding about the chirality of filaments will be useful to associate eruptive filaments and magnetic cloud using the helicity parameter in the Space Weather Science. Title: Coronal heating and flaring in QSLs Authors: Aulanier, Guillaume Bibcode: 2011IAUS..273..233A Altcode: Quasi-Separatrix Layers (QSLs) are 3D geometrical objects that define narrow volumes across which magnetic field lines have strong, but finite, gradients of connectivity from one footpoint to another. QSLs extend the concept of separatrices, that are topological objects across which the connectivity is discontinuous. Based on analytical arguments, and on magnetic field extrapolations of the Sun's coronal force-free field above observed active regions, it has long since been conjectured that QSLs are favorable locations for current sheet (CS) formation, as well as for magnetic reconnection, and therefore are good predictors for the locations of magnetic energy release in flares and coronal heating. It is only up to recently that numerical MHD simulations and solar observations, as well as a laboratory experiment, have started to address the validity of these conjectures. When put all together, they suggest that QSL reconnection is involved in the displacement of EUV and SXR brightenings along chromospheric flare ribbons, that it is related with the heating of EUV coronal loops, and that the dissipation of QSL related CS may be the cause of coronal heating in initially homogeneous, braided and turbulent flux tubes, as well as in coronal arcades rooted in the slowly moving and numerous small-scale photospheric flux concentrations, both in active region faculae and in the quiet Sun. The apparent ubiquity of QSL-related CS in the Sun's corona, which will need to be quantified with new generation solar instruments, also suggests that QSLs play an important role in stellar's atmospheres, when their surface radial magnetic fields display complex patterns. 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: 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: A single picture for solar coronal outflows and radio noise storms Authors: Del Zanna, G.; Aulanier, G.; Klein, K. -L.; Török, T. Bibcode: 2011A&A...526A.137D Altcode: We propose a unified interpretation for persistent coronal outflows and metric radio noise storms, two phenomena typically observed in association with quiescent solar active regions. Our interpretation is based on multi-wavelength observations of two such regions as they crossed the meridian in May and July 2007. For both regions, we observe a persistent pattern of blue-shifted coronal emission in high-temperature lines with Hinode/EIS, and a radio noise storm with the Nançay Radioheliograph. The observations are supplemented by potential and linear force-free extrapolations of the photospheric magnetic field over large computational boxes, and by a detailed analysis of the coronal magnetic field topology. We find true separatrices in the coronal field and null points high in the corona, which are preferential locations for magnetic reconnection and electron acceleration. We suggest that the continuous growth of active regions maintains a steady reconnection across the separatrices at the null point. This interchange reconnection occurs between closed, high-density loops in the core of the active region and neighbouring open, low-density flux tubes. Thus, the reconnection creates strong pressure imbalances which are the main drivers of plasma upflows. Furthermore, the acceleration of low-energy electrons in the interchange reconnection region sustains the radio noise storm in the closed loop areas, as well as weak type III emission along the open field lines. For both active regions studied, we find a remarkable agreement between the observed places of persistent coronal outflows and radio noise storms with their locations as predicted by our interpretation. 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: 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: Multiwavelength Observations of Small-scale Reconnection Events Triggered by Magnetic Flux Emergence in the Solar Atmosphere Authors: Guglielmino, S. L.; Bellot Rubio, L. R.; Zuccarello, F.; Aulanier, G.; Vargas Domínguez, S.; Kamio, S. Bibcode: 2010ApJ...724.1083G Altcode: 2010arXiv1007.4657G The interaction between emerging magnetic flux and the pre-existing ambient field has become a "hot" topic for both numerical simulations and high-resolution observations of the solar atmosphere. The appearance of brightenings and surges during episodes of flux emergence is believed to be a signature of magnetic reconnection processes. We present an analysis of a small-scale flux emergence event in NOAA 10971, observed simultaneously with the Swedish 1 m Solar Telescope on La Palma and the Hinode satellite during a joint campaign in 2007 September. Extremely high-resolution G-band, Hα, and Ca II H filtergrams, Fe I and Na I magnetograms, EUV raster scans, and X-ray images show that the emerging region was associated with chromospheric, transition region and coronal brightenings, as well as with chromospheric surges. We suggest that these features were caused by magnetic reconnection at low altitude in the atmosphere. To support this idea, we perform potential and linear force-free field extrapolations using the FROMAGE service. The extrapolations show that the emergence site is cospatial with a three-dimensional null point, from which a spine originates. This magnetic configuration and the overall orientation of the field lines above the emerging flux region are compatible with the structures observed in the different atmospheric layers and remain stable against variations of the force-free field parameter. Our analysis supports the predictions of recent three-dimensional numerical simulations that energetic phenomena may result from the interaction between emerging flux and the pre-existing chromospheric and coronal field. Title: Criteria for Flux Rope Eruption: Non-equilibrium Versus Torus Instability Authors: Démoulin, P.; Aulanier, G. Bibcode: 2010ApJ...718.1388D Altcode: 2010arXiv1006.1785D The coronal magnetic configuration of an active region typically evolves quietly for a few days before becoming suddenly eruptive and launching a coronal mass ejection (CME). The precise origin of the eruption is still under debate. The loss of equilibrium, or an ideal magnetohydrodynamic (MHD) instability such as torus instability are among the several mechanisms that have proposed to be responsible for the sudden eruptions. Distinct approaches have also been formulated for limited cases having circular or translation symmetry. We revisit the previous theoretical approaches setting them in the same analytical framework. The coronal field results from the contribution of a non-neutralized current channel added to a background magnetic field, which in our model is the potential field generated by two photospheric flux concentrations. The evolution on short Alfvénic timescale is governed by ideal MHD. We first show analytically that the loss of equilibrium and the stability analysis are two different views of the same physical mechanism. Second, we identify that the same physics is involved in the instabilities of circular and straight current channels. Indeed, they are just two particular limiting cases of more general current paths. A global instability of the magnetic configuration is present when the current channel is located at a coronal height, h, large enough so that the decay index of the potential field, ∂ln |B p|/∂ln h, is larger than a critical value. At the limit of very thin current channels, previous analysis has found critical decay indices of 1.5 and 1 for circular and straight current channels, respectively. However, with current channels being deformable and as thick as that expected in the corona, we show that this critical index has similar values for circular and straight current channels, and is typically in the range [1.1,1.3]. Title: Coexisting Flux Rope and Dipped Arcade Sections Along One Solar Filament Authors: Guo, Y.; Schmieder, B.; Démoulin, P.; Wiegelmann, T.; Aulanier, G.; Török, T.; Bommier, V. Bibcode: 2010ApJ...714..343G Altcode: We compute the three-dimensional magnetic field of an active region in order to study the magnetic configuration of active region filaments. The nonlinear force-free field model is adopted to compute the magnetic field above the photosphere, where the vector magnetic field was observed by THEMIS/MTR on 2005 May 27. We propose a new method to remove the 180° ambiguity of the transverse field. Next, we analyze the implications of the preprocessing of the data by minimizing the total force and torque in the observed vector fields. This step provides a consistent bottom boundary condition for the nonlinear force-free field model. Then, using the optimization method to compute the coronal field, we find a magnetic flux rope along the polarity inversion line. The magnetic flux rope aligns well with part of an Hα filament, while the total distribution of the magnetic dips coincides with the whole Hα filament. This implies that the magnetic field structure in one section of the filament is a flux rope, while the other is a sheared arcade. The arcade induced a left-bearing filament in the magnetic field of negative helicity, which is opposite to the chirality of barbs that a flux rope would induce in a magnetic field of the same helicity sign. The field strength in the center of the flux rope is about 700 G, and the twist of the field lines is ~1.4 turns. Title: Physics of Solar Prominences: II—Magnetic Structure and Dynamics Authors: Mackay, D. H.; Karpen, J. T.; Ballester, J. L.; Schmieder, B.; Aulanier, G. Bibcode: 2010SSRv..151..333M Altcode: 2010SSRv..tmp...32M; 2010arXiv1001.1635M Observations and models of solar prominences are reviewed. We focus on non-eruptive prominences, and describe recent progress in four areas of prominence research: (1) magnetic structure deduced from observations and models, (2) the dynamics of prominence plasmas (formation and flows), (3) Magneto-hydrodynamic (MHD) waves in prominences and (4) the formation and large-scale patterns of the filament channels in which prominences are located. Finally, several outstanding issues in prominence research are discussed, along with observations and models required to resolve them. Title: Formation of Torus-Unstable Flux Ropes and Electric Currents in Erupting Sigmoids Authors: Aulanier, G.; Török, T.; Démoulin, P.; DeLuca, E. E. Bibcode: 2010ApJ...708..314A Altcode: We analyze the physical mechanisms that form a three-dimensional coronal flux rope and later cause its eruption. This is achieved by a zero-β magnetohydrodynamic (MHD) simulation of an initially potential, asymmetric bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and sub-Alfvénic, line-tied shearing motions in the photosphere. As in similar models, flux-cancellation-driven photospheric reconnection in a bald-patch (BP) separatrix transforms the sheared arcades into a slowly rising and stable flux rope. A bifurcation from a BP to a quasi-separatrix layer (QSL) topology occurs later on in the evolution, while the flux rope keeps growing and slowly rising, now due to shear-driven coronal slip-running reconnection, which is of tether-cutting type and takes place in the QSL. As the flux rope reaches the altitude at which the decay index -∂ln B/∂ln z of the potential field exceeds ~3/2, it rapidly accelerates upward, while the overlying arcade eventually develops an inverse tear-drop shape, as observed in coronal mass ejections (CMEs). This transition to eruption is in accordance with the onset criterion of the torus instability. Thus, we find that photospheric flux-cancellation and tether-cutting coronal reconnection do not trigger CMEs in bipolar magnetic fields, but are key pre-eruptive mechanisms for flux ropes to build up and to rise to the critical height above the photosphere at which the torus instability causes the eruption. In order to interpret recent Hinode X-Ray Telescope observations of an erupting sigmoid, we produce simplified synthetic soft X-ray images from the distribution of the electric currents in the simulation. We find that a bright sigmoidal envelope is formed by pairs of J-shaped field lines in the pre-eruptive stage. These field lines form through the BP reconnection and merge later on into S-shaped loops through the tether-cutting reconnection. During the eruption, the central part of the sigmoid brightens due to the formation of a vertical current layer in the wake of the erupting flux rope. Slip-running reconnection in this layer yields the formation of flare loops. A rapid decrease of currents due to field line expansion, together with the increase of narrow currents in the reconnecting QSL, yields the sigmoid hooks to thin in the early stages of the eruption. Finally, a slightly rotating erupting loop-like feature (ELLF) detaches from the center of the sigmoid. Most of this ELLF is not associated with the erupting flux rope, but with a current shell that develops within expanding field lines above the rope. Only the short, curved end of the ELLF corresponds to a part of the flux rope. We argue that the features found in the simulation are generic for the formation and eruption of soft X-ray sigmoids. Title: A Flaring Twisted Emerging Flux Region Authors: Chandra, R.; Schmieder, B.; Aulanier, G.; Malherbe, J. M. Bibcode: 2010ASSP...19..523C Altcode: 2010mcia.conf..523C We present signatures of the emergence of a twisted flux tube in decaying active region NOAA AR10365 on 27 May 2003. When the magnetic flux tube is twisted, an asymmetry appears in the magnetogram because of the contribution of the azimuthal component to the observed vertical component of the field. In this case, the vertical component produces two "tongues" (Fig. 1, left). The twist of the flux tube is revealed by the photospheric longitudinalmagnetic field pattern: diverging flows of opposite polarities, elongated polarities with a "tongue" shape. The asymmetry of the opposite polarities is interpreted as right-hand twist of the emerging flux tube. Title: Magnetic Flux Rope Eruption: Non Equilibrium versus Torus Instability Authors: Demoulin, Pascal; Aulanier, Guillaume; Toeroek, Tibor; Deluca, Edward Bibcode: 2010cosp...38.1855D Altcode: 2010cosp.meet.1855D The coronal magnetic configuration of an active region typically evolves quietly during few days before becoming suddenly eruptive and launching a CME. The precise origin of the eruption is still debated. Among other mechanisms, it has been long proposed that a loss of equilibrium, or an ideal MHD instability such as the torus instability, could be responsible for the sudden eruptivity. We first revisit both approaches with simple analytical models as well as with a 3D MHD simulation of an initially potential bipolar field, which evolves by means of simultaneous slow magnetic field diffusion and shearing motions in the photosphere. Reconnection of sheared arcade leads to the formation of a twisted flux rope, which corresponds to an electric current channel. We find that the electric current distribution and the field-line organization present in the MHD simulation provide an explanation for the recent X-rays Hinode observations of erupting sigmoidal regions. Next, we show analytically that the loss of equilibrium and the torus instability are two different views of the same physical mechanism. We compare the instability thresholds in the limit of straight and circular current channels, finding that they are closely comparable for thick current channels (as present in the MHD simulation and as expected in the corona) while these thresholds are well distinct at the limit of very thin current channels (as typically found in previous studies). Finally, including photospheric line tying of the current channel in the analytical models permits to have a closer comparison between instability thresholds found analytically and by the MHD simulation. Title: What triggers coronal mass ejections ? Authors: Aulanier, Guillaume Bibcode: 2010cosp...38.1854A Altcode: 2010cosp.meet.1854A Coronal mass ejections (CMEs) are large clouds of highly magnetized plasma. They are ac-celerated from the solar atmosphere into interplanetary space by the Lorentz force, which is associated to their strong current-carrying magnetic fields. Both theory and observations lead to the inevitable conclusion that the launch of a CME must result from the sudden release of free magnetic energy, which has slowly been accumulated in the corona for a long time before the eruption. Since the incomplete, but seminal, loss-of-equilibrium model was proposed by van Tend and Kuperus (1978), a large variety of analytical and numerical storage-and-release MHD models has been put forward in the past 20 years or so. All these models rely on the slow increase of currents and/or the slow decrease of the restraining magnetic tension preceding the eruption. But they all put the emphazis on different physical mechanisms to achieve this preeruptive evolution, and to suddenly trigger and later drive a CME. Nevertheless, all these models actually share many common features, which all describe many individual observed aspects of solar eruptions. It is therefore not always clear which of all the suggested mecha-nisms do really account for the triggering of observed CMEs in general. Also, these mechanisms should arguably not be as numerous as the models themselves, owing to the common occurence of CMEs. In order to shed some light on this challenging, but unripe, topic, I will attempt to rediscuss the applicability of the models to the Sun, and to rethink the most sensitive ones in a common frame, so as to find their common denominator. I will elaborate on the idea that many of the proposed triggering mechanisms may actually only be considered as different ways to apply a "last push", which puts the system beyond its eruptive threshold. I will argue that, in most cases, the eruptive threshold is determined by the vertical gradient of the magnetic field in the low-β corona, just like the usual convection threshold is determined by the temperature gradient in a stratified medium. Title: How skeletons turn into quasi-separatrix layers in source models Authors: Restante, A. L.; Aulanier, G.; Parnell, C. E. Bibcode: 2009A&A...508..433R Altcode: Context: In situations where there are no magnetic null points located above a reference photospheric plane, and when the photospheric magnetic field is modeled by discrete flux concentrations, the magnetic connectivity is defined by the magnetic skeleton of the configuration. For a continuous distribution of non-zero photospheric flux, the connectivity is defined by quasi-separatrix layers (QSLs). Both the magnetic skeleton and QSLs can account for current sheet formation and dissipation. Observationally, though, only some portions of the skeleton are found to be related to flare ribbons, which are generally associated with QSL footpoints.
Aims: In potential magnetic source models, a transition from the skeleton to QSLs has been shown to occur when the sources are displaced below the photospheric plane. The objective of this paper is to understand the topological and geometrical nature of this transition, and to derive rules to predict which parts of a given skeleton will give rise to QSLs.
Methods: We consider magnetic configurations, derived from potential magnetic sources, which possess no coronal null points. We have calculated their skeletons, composed of null points, spine field lines and separatrix (fan) surfaces. Choosing a reference photospheric plane above the sources, we have calculated their QSL footprints.
Results: As already known, the latter mostly match with subphotospheric spine field lines since, above these lines, field lines tend to diverge as a result of approaching a null and lying either side of the separatrix surface extending out of from this null. However, many non-spine related QSL footprints are also found, which we call branches. They correspond to the intersection with the photosphere of portions of fan field lines which “branch” away from the sources and result in QSLs due to the inclination of the coronal field lines.
Conclusions: Our findings allow a better geometrical understanding of the relations between QSLs and skeletons. We show that in the absence of coronal null points, spines, as well as specific portions of fans as calculated in standard potential source models, are good predictors for the location of QSL footprints and of flare ribbons. Title: Formation, Interaction and Merger of an Active Region and a Quiescent Filament Prior to Their Eruption on 19 May 2007 Authors: Bone, L. A.; van Driel-Gesztelyi, L.; Culhane, J. L.; Aulanier, G.; Liewer, P. Bibcode: 2009SoPh..259...31B Altcode: We report observations of the formation of two filaments - one active and one quiescent, and their subsequent interactions prior to eruption. The active region filament appeared on 17 May 2007, followed by the quiescent filament about 24 hours later. In the 26 hour interval preceding the eruption, which occurred at around 12:50 UT on 19 May 2007, we see the two filaments attempting to merge and filament material is repeatedly heated suggesting magnetic reconnection. The filament structure is observed to become increasingly dynamic preceding the eruption with two small hard X-ray sources seen close to the active part of the filament at around 01:38 UT on 19 May 2007 during one of the activity episodes. The final eruption on 19 May at about 12:51 UT involves a complex CME structure, a flare and a coronal wave. A magnetic cloud is observed near Earth by the STEREO-B and WIND spacecraft about 2.7 days later. Here we describe the behaviour of the two filaments in the period prior to the eruption and assess the nature of their dynamic interactions. Title: Fan-Spine Topology Formation Through Two-Step Reconnection Driven by Twisted Flux Emergence Authors: Török, T.; Aulanier, G.; Schmieder, B.; Reeves, K. K.; Golub, L. Bibcode: 2009ApJ...704..485T Altcode: 2009arXiv0909.2235T We address the formation of three-dimensional nullpoint topologies in the solar corona by combining Hinode/X-ray Telescope (XRT) observations of a small dynamic limb event, which occurred beside a non-erupting prominence cavity, with a three-dimensional (3D) zero-β magnetohydrodynamics (MHD) simulation. To this end, we model the boundary-driven "kinematic" emergence of a compact, intense, and uniformly twisted flux tube into a potential field arcade that overlies a weakly twisted coronal flux rope. The expansion of the emerging flux in the corona gives rise to the formation of a nullpoint at the interface of the emerging and the pre-existing fields. We unveil a two-step reconnection process at the nullpoint that eventually yields the formation of a broad 3D fan-spine configuration above the emerging bipole. The first reconnection involves emerging fields and a set of large-scale arcade field lines. It results in the launch of a torsional MHD wave that propagates along the arcades, and in the formation of a sheared loop system on one side of the emerging flux. The second reconnection occurs between these newly formed loops and remote arcade fields, and yields the formation of a second loop system on the opposite side of the emerging flux. The two loop systems collectively display an anenome pattern that is located below the fan surface. The flux that surrounds the inner spine field line of the nullpoint retains a fraction of the emerged twist, while the remaining twist is evacuated along the reconnected arcades. The nature and timing of the features which occur in the simulation do qualititatively reproduce those observed by XRT in the particular event studied in this paper. Moreover, the two-step reconnection process suggests a new consistent and generic model for the formation of anemone regions in the solar corona. Title: Surface magnetic fields on two accreting TTauri stars: CVCha and CRCha Authors: Hussain, G. A. J.; Collier Cameron, A.; Jardine, M. M.; Dunstone, N.; Ramirez Velez, J.; Stempels, H. C.; Donati, J. -F.; Semel, M.; Aulanier, G.; Harries, T.; Bouvier, J.; Dougados, C.; Ferreira, J.; Carter, B. D.; Lawson, W. A. Bibcode: 2009MNRAS.398..189H Altcode: 2009arXiv0905.0914H; 2009MNRAS.tmp..997H We have produced brightness and magnetic field maps of the surfaces of CVCha and CRCha: two actively accreting G- and K-type TTauri stars in the ChamaeleonI star-forming cloud with ages of 3-5Myr. Our magnetic field maps show evidence for strong, complex multipolar fields similar to those obtained for young rapidly rotating main-sequence stars. Brightness maps indicate the presence of dark polar caps and low-latitude spots - these brightness maps are very similar to those obtained for other pre-main-sequence and rapidly rotating main-sequence stars.

Only two other classical TTauri stars have been studied using similar techniques so far: V2129Oph and BPTau. CVCha and CRCha show magnetic field patterns that are significantly more complex than those recovered for BPTau, a fully convective TTauri star.

We discuss possible reasons for this difference and suggest that the complexity of the stellar magnetic field is related to the convection zone; with more complex fields being found in TTauri stars with radiative cores (V2129Oph, CVCha and CRCha). However, it is clearly necessary to conduct magnetic field studies of TTauri star systems, exploring a wide range of stellar parameters in order to establish how they affect magnetic field generation, and thus how these magnetic fields are likely to affect the evolution of TTauri star systems as they approach the main sequence. Title: Evidence of Magnetic Helicity in Emerging Flux and Associated Flare Authors: Chandra, R.; Schmieder, B.; Aulanier, G.; Malherbe, J. M. Bibcode: 2009SoPh..258...53C Altcode: 2009arXiv0906.1210C The aim of this paper is to look at the magnetic helicity structure of an emerging active region and show that both emergence and flaring signatures are consistent with a same sign for magnetic helicity. We present a multiwavelength analysis of an M1.6 flare occurring in the NOAA active region 10365 on 27 May 2003, in which a large new bipole emerges in a decaying active region. The diverging flow pattern and the "tongue" shape of the magnetic field in the photosphere with elongated polarities are highly suggestive of the emergence of a twisted flux tube. The orientation of these tongues indicates the emergence of a flux tube with a right-hand twist (i.e., positive magnetic helicity). The flare signatures in the chromosphere are ribbons observed in Hα by the MSDP spectrograph in the Meudon solar tower and in 1600 Å by TRACE. These ribbons have a J shape and are shifted along the inversion line. The pattern of these ribbons suggests that the flare was triggered by magnetic reconnection at coronal heights below a twisted flux tube of positive helicity, corresponding to that of the observed emergence. It is the first time that such a consistency between the signatures of the emerging flux through the photosphere and flare ribbons has been clearly identified in observations. Another type of ribbons observed during the flare at the periphery of the active region by the MSDP and SOHO/EIT is related to the existence of a null point, which is found high in the corona in a potential field extrapolation. We discuss the interpretation of these secondary brightenings in terms of the "breakout" model and in terms of plasma compression/heating within large-scale separatrices. Title: Current Buildup in Emerging Serpentine Flux Tubes Authors: Pariat, E.; Masson, S.; Aulanier, G. Bibcode: 2009ApJ...701.1911P Altcode: The increase of magnetic flux in the solar atmosphere during active-region formation involves the transport of the magnetic field from the solar convection zone through the lowest layers of the solar atmosphere, through which the plasma β changes from >1 to <1 with altitude. The crossing of this magnetic transition zone requires the magnetic field to adopt a serpentine shape also known as the sea-serpent topology. In the frame of the resistive flux-emergence model, the rising of the magnetic flux is believed to be dynamically driven by a succession of magnetic reconnections which are commonly observed in emerging flux regions as Ellerman bombs. Using a data-driven, three-dimensional (3D) magnetohydrodynamic numerical simulation of flux emergence occurring in active region 10191 on 2002 November 16-17, we study the development of 3D electric current sheets. We show that these currents buildup along the 3D serpentine magnetic-field structure as a result of photospheric diverging horizontal line-tied motions that emulate the observed photospheric evolution. We observe that reconnection can not only develop following a pinching evolution of the serpentine field line, as usually assumed in two-dimensional geometry, but can also result from 3D shearing deformation of the magnetic structure. In addition, we report for the first time on the observation in the UV domain with the Transition Region and Coronal Explorer (TRACE) of extremely transient loop-like features, appearing within the emerging flux domain, which link several Ellermam bombs with one another. We argue that these loop transients can be explained as a consequence of the currents that build up along the serpentine magnetic field. Title: The Nature of Flare Ribbons in Coronal Null-Point Topology Authors: Masson, S.; Pariat, E.; Aulanier, G.; Schrijver, C. J. Bibcode: 2009ApJ...700..559M Altcode: Flare ribbons are commonly attributed to the low-altitude impact, along the footprints of separatrices or quasi-separatrix layers (QSLs), of particle beams accelerated through magnetic reconnection. If reconnection occurs at a three-dimensional coronal magnetic null point, the footprint of the dome-shaped fan surface would map a closed circular ribbon. This paper addresses the following issues: does the entire circular ribbon brighten simultaneously, as expected because all fan field lines pass through the null point? And since the spine separatrices are singular field lines, do spine-related ribbons look like compact kernels? What can we learn from these observations about current sheet formation and magnetic reconnection in a null-point topology? The present study addresses these questions by analyzing Transition Region and Coronal Explorer and Solar and Heliospheric Observatory/Michelson Doppler Imager observations of a confined flare presenting a circular ribbon. Using a potential field extrapolation, we linked the circular shape of the ribbon with the photospheric mapping of the fan field lines originating from a coronal null point. Observations show that the flare ribbon outlining the fan lines brightens sequentially along the counterclockwise direction and that the spine-related ribbons are elongated. Using the potential field extrapolation as initial condition, we conduct a low-β resistive magnetohydrodynamics simulation of this observed event. We drive the coronal evolution by line-tied diverging boundary motions, so as to emulate the observed photospheric flow pattern associated with some magnetic flux emergence. The numerical analysis allows us to explain several observed features of the confined flare. The vorticity induced in the fan by the prescribed motions causes the spines to tear apart along the fan. This leads to formation of a thin current sheet and induces null-point reconnection. We also find that the null point and its associated topological structure is embedded within QSLs, already present in the asymmetric potential field configuration. We find that the QSL footprints correspond to the observed elongated spine ribbons. Finally, we observe that before and after reconnecting at the null point, all field lines undergo slipping and slip-running reconnection within the QSLs. Field lines, and therefore particle impacts, slip or slip-run according to their distance from the spine, in directions and over distances that are compatible with the observed dynamics of the ribbons. Title: Coronal loops, flare ribbons and aurora during slip-running Authors: Schmieder, Brigitte; Aulanier, Guillaume; Démoulin, Pascal; Pariat, Etienne Bibcode: 2009EP&S...61..565S Altcode: 2009EP&S...61L.565S Solar two ribbon flares are commonly explained by magnetic field reconnections in the low corona. During the reconnection energetic particles (electrons and protons) are accelerated from the reconnection site. These particles are following the magnetic field lines down to the chromosphere. As the plasma density is higher in these lower layers, there are collisions and emission of radiation. Thus bright ribbons are observed at both ends of flare loops. These ribbons are typically observed in Hα and in EUV with SoHO and TRACE. As the time is going, these ribbons are expanding away of each other. In most studied models, the reconnection site is a separator line, where two magnetic separatrices intersect. They define four distinct connectivity domains, across which the magnetic connectivity changes discontinuously. In this paper, we present a generalization of this model to 3D complex magnetic topologies where there are no null points, but quasi-separatrices layers instead. In that case, while the ribbons spread away during reconnection, we show that magnetic field lines can quickly slip along them. We propose that this new phenomenon could explain fast extension of Hα and TRACE 1600 Å ribbons, fast moving HXR footpoints along the ribbons as observed by RHESSI, and that it is observed in soft X rays with Hinode/XRT. Title: Evidence for a Pre-Eruptive Twisted Flux Rope Using the Themis Vector Magnetograph Authors: Canou, A.; Amari, T.; Bommier, V.; Schmieder, B.; Aulanier, G.; Li, H. Bibcode: 2009ApJ...693L..27C Altcode: Although there is evidence that twisted structures form during large-scale eruptive events, it is not yet clear whether these exist in the pre-eruptive phase as twisted flux ropes (TFRs) in equilibrium. This question has become a major issue since several theoretical mechanisms can lead to the formation of TFRs. These models consider either the evolution of a coronal configuration driven by photospheric changes or the emergence of TFR from the convection zone. We consider as a target for addressing this issue the active region NOAA AR 10808 known at the origin of several large-scale eruptive phenomena, and associated with the emergence of a δ-spot. Using the THEMIS vector magnetogram as photospheric boundary conditions for our nonlinear force-free reconstruction model of the low corona and without any other assumption, we show that the resulting pre-eruptive configuration exhibits a TFR above the neutral line of the emerging δ-spot. In addition, the free magnetic energy of this configuration could even be large enough to explain such resulting large-scale eruptive events. Title: Solar prominences Authors: Schmieder, Brigitte; Aulanier, Guillaume; Török, Tibor Bibcode: 2009IAUS..257..223S Altcode: Solar filaments (or prominences) are magnetic structures in the corona. They can be represented by twisted flux ropes in a bipolar magnetic environment. In such models, the dipped field lines of the flux rope carry the filament material and parasitic polarities in the filament channel are responsible for the existence of the lateral feet of prominences.

Very simple laws do exist for the chirality of filaments, the so-called “filament chirality rules”: commonly dextral/sinistral filaments corresponding to left- (resp. right) hand magnetic twists are in the North/South hemisphere. Combining these rules with 3D weakly twisted flux tube models, the sign of the magnetic helicity in several filaments were identified. These rules were also applied to the 180° disambiguation of the direction of the photospheric transverse magnetic field around filaments using THEMIS vector magnetograph data (López Ariste et al. 2006). Consequently, an unprecedented evidence of horizontal magnetic support in filament feet has been observed, as predicted by former magnetostatic and recent MHD models.

The second part of this review concerns the role of emerging flux in the vicinity of filament channels. It has been suggested that magnetic reconnection between the emerging flux and the pre-existing coronal field can trigger filament eruptions and CMEs. For a particular event, observed with Hinode/XRT, we observe signatures of such a reconnection, but no eruption of the filament. We present a 3D numerical simulation of emerging flux in the vicinity of a flux rope which was performed to reproduce this event and we briefly discuss, based on the simulation results, why the filament did not erupt. Title: Magnetic helicity and solar prominence formation Authors: Aulanier, G.; Schmieder, B. Bibcode: 2008sf2a.conf..543A Altcode: Simple laws have long-since been put forward from the chirality of observed features to derive the direction of the axial magnetic field inside solar filaments. These are the so-called ``filament chirality rules''. Here, we report on two uses of these rules applied to THEMIS and SVST observations and to MHD simulations. Being the first to apply these rules to the 180° disambiguation of the direction of the photospheric transverse magnetic field around filaments, we found the unprecedented evidence of magnetic support in filament feet, as predicted by former magnetostatic and recent MHD models. By combining these rules with 3D weakly twisted flux tube models, we identified the sign of the magnetic helicity in several filaments. Following their interactions with one another over a few days, we found that the observational condition for two filaments to merge is that their flux tubes must have the same helicity sign. We theoretically recovered these results, by conducting a parametric study of 3D numerical MHD simulations of sheared bipoles. This study also provided new conditions for filament merging, in yet-unobserved configurations in which sheared bipoles are oppositely oriented. 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: Photospheric flows around a quiescent filament at Large and small scale and their ffects on filament destabilization Authors: Roudier, Th.; Malherbe, J. M.; Švanda, M.; Molodij, G.; Keil, S.; Sütterlin, P.; Schmieder, B.; Bommier, V.; Aulanier, G.; Meunier, N.; Rieutord, M.; Rondi, S. Bibcode: 2008sf2a.conf..569R Altcode: We study the influence of large and small scales photospheric motions on the destabilization of an eruptive filament, observed on October 6, 7, and 8, 2004 as part of an international observing campaign (JOP 178). Large-scale horizontal flows are invetigated from a series of MDI/SOHO full-disc Dopplergrams and magnetograms from THEMIS. Small-scale horizontal flows were derived using local correlation tracking on TRACE satellite, Dutch Open Telescope (DOT) and The Dunn Solar telescope (DST) data. The topology of the flow field changed significantly during the filament eruptive phase, suggesting a possible coupling between the surface flow field and the coronal magnetic field. We measured an increase of the shear below the point where the eruption starts and a decrease in shear after the eruption. We conclude that there is probably a link between changes in surface flow and the disappearance of the eruptive filament. Title: The MHD coupling between coronal dynamics and photospheric motions Authors: Grappin, R.; Aulanier, G.; Pinto, R. Bibcode: 2008A&A...490..353G Altcode: Context: Whether it be the heating problem or the destabilization of coronal structures, use is often made of the so-called “line-tying” boundary conditions, which amounts to imposing the photospheric velocity at the photosphere as a boundary condition for coronal dynamics. Directly coupling the low beta coronal evolution to prescribed photospheric motions of the magnetic footpoints allows strong magnetic energy accumulation in the corona. But this amounts to ignoring possible feedback from the coronal loops on photospheric motions, a neglect that is commonly justified by the strong density contrast between the photosphere and the corona. On the other hand, the energy injected into the corona comes from the photosphere, so in principle the coronal loop might act as a conduit communicating photospheric dynamics from one region to another.
Aims: Our objective is to test the degree of validity of this line-tying approximation by considering the role of the dense photosphere explicitly.
Methods: We consider here a 1.5D MHD model of a magnetic loop including a strongly stratified solar-like atmosphere and consider free (instead of prescribed/line-tied) boundary conditions applied deep in the photosphere, so as to quantify the coupling between the photosphere and corona as determined by stratification. We give an initial kick to one of the footpoints in the form of an upwardly propagating Alfvénic perturbation rising from the lower boundary, and then allow waves to freely escape the numerical domain from the boundaries, seated deep in the photosphere.
Results: We find that the response of the loop differs in many aspects from what is predicted by the line-tied condition. a) The magnetic energy density available in the corona is limited to a value equal to the kinetic energy density in the photospheric motion. b) The initial velocity shear between the opposite loop footpoints vanishes after a time proportional to the loop length. The shear between the coronal boundaries on opposite sides of the loop is quasi-uniform and is relaxed slowly by Alfvén waves propagating downwards through the high-β photospheric layers. This process is insensitive to details of the thermal structure. c) Coronal loops are thus shown to exert a strong feedback on the photospheric dynamics, intermediate between friction and diffusion, instead of no reaction at all. Title: Magnetic field changes preceding filament eruptions and coronal mass ejections Authors: Schmieder, B.; Török, T.; Aulanier, G. Bibcode: 2008AIPC.1043..260S Altcode: Solar filaments (or prominences) can be represented by twisted flux ropes in a bipolar magnetic environment. In such models, the dipped field lines of the flux rope carry the filament material and parasitic polarities in the filament channel are responsible for the existence of the lateral feet of filaments. Most filaments eventually erupt, in many cases as part of a coronal mass ejection (CME). Such eruptions are often preceded by detectable changes in the photospheric magnetic field in the vicinity of the filament. We first review recent observations of such changes due to large-scale flows or variations of the background magnetic field, and we discuss their role in eruptions. We then focus on emerging flux in the vicinity of filament channels. It has been suggested that magnetic reconnection between the emerging flux and the pre-existing coronal field can trigger filament eruptions and CMEs. For a particular event, observed with Hinode/XRT, we observe signatures of such reconnection, but no eruption of the filament. We present a numerical simulation of this event and we briefly argue why no eruption took place in this case. Title: Multiple Ribbons of a M1.6 Flare Related to the Magnetic Configuration of the NOAA AR 365 Authors: Chandra, R.; Schmieder, B.; Aulanier, G.; Malherbe, M. J. Bibcode: 2008ESPM...12.2.65C Altcode: The aim of this paper is to understand the magnetic configuration of an active region NOAA 365 and its evolution before and after the occurrence of a M1.6 flare in order to understand the magnetic origin of the flare and its development at particularly the ribbons.

We analyze a M1.6 flare occurring in the active region NOAA 365, using space instruments (SOHO/MDI, EIT, TRACE, RHESSI) as well as ground based instruments i.e. the MSDP spectrograph of the Meudon solar tower. The analysis of the magnetic topology of the region is done by using a force-free field linear extrapolation code of the photospheric field proposed in the database FROMAGE.

Two series of ribbons are identified, signature of the reconnection, the main ribbons in the center of the active region and secondary ribbons at the periphery of the active region. The main ribbons have a `J' shape, typical shape for large emerging flux tube. The secondary ribbons are explained by the magnetic configuration. Title: Non-Linear Force-Free Field Modeling of a Solar Active Region Around the Time of a Major Flare and Coronal Mass Ejection Authors: De Rosa, M. L.; Schrijver, C. J.; Metcalf, T. R.; Barnes, G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.; Wheatland, M.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann, M.; Kusano, K.; Régnier, S.; Thalmann, J. Bibcode: 2008AGUSMSP31A..06D Altcode: Solar flares and coronal mass ejections are associated with rapid changes in coronal magnetic field connectivity and are powered by the partial dissipation of electrical currents that run through the solar corona. A critical unanswered question is whether the currents involved are induced by the advection along the photosphere of pre-existing atmospheric magnetic flux, or whether these currents are associated with newly emergent flux. We address this problem by applying nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA Active Region 10930 around the time of a powerful X3.4 flare in December 2006. We compute 14 NLFFF models using 4 different codes having a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We do find agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~1032~erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection. We discuss the relative merits of these models in a general critique of our present abilities to model the coronal magnetic field based on surface vector field measurements. Title: Topological Departures from Translational Invariance along a Filament Observed by THEMIS Authors: Dudík, J.; Aulanier, G.; Schmieder, B.; Bommier, V.; Roudier, T. Bibcode: 2008SoPh..248...29D Altcode: 2008SoPh..tmp...53D We study the topology of the 3D magnetic field in a filament channel to address the following questions: Is a filament always formed in a single flux tube? How does the photospheric magnetic field lead to filament interruptions and to feet formation? What is the relation between feet-related field lines and the parasitic polarities? What can topological analyses teach us about EUV filament channels? To do so, we consider a filament observed on 6 October 2004 with THEMIS/MTR, in Hα with the full line profile simultaneously and cospatially with its photospheric vector magnetic field. The coronal magnetic field was calculated from a "linear magnetohydrostatic" extrapolation of a composite THEMIS-MDI magnetogram. Its free parameters were adjusted to get the best match possible between the distribution of modeled plasma-supporting dips and the Hα filament morphology. The model results in moderate plasma β≤1 at low altitudes in the filament, in conjunction with non-negligible departures from force-freeness measured by various metrics. The filament here is formed by a split flux tube. One part of the flux tube is rooted in the photosphere aside an observed interruption in the filament. This splitted topology is due to strong network polarities on the edge of the filament channel, not to flux concentrations closer to the filament. We focus our study to the northwest portion of the filament. The related flux tube is highly fragmented at low altitudes. This fragmentation is due to small flux concentrations of two types. First, some locally distort the tube, leading to noticeable thickness variations along the filament body. Second, parasitic polarities, associated with filament feet, result in secondary dips above the related local inversion line. These dips belong to long field lines that pass below the flux tube. Many of these field lines are not rooted near the related foot. Finally, the present model shows that the coronal void interpretation cannot be ruled out to interpret the wideness of EUV filament channels. Title: Nonlinear Force-free Field Modeling of a Solar Active Region around the Time of a Major Flare and Coronal Mass Ejection Authors: Schrijver, C. J.; DeRosa, M. L.; Metcalf, T.; Barnes, G.; Lites, B.; Tarbell, T.; McTiernan, J.; Valori, G.; Wiegelmann, T.; Wheatland, M. S.; Amari, T.; Aulanier, G.; Démoulin, P.; Fuhrmann, M.; Kusano, K.; Régnier, S.; Thalmann, J. K. Bibcode: 2008ApJ...675.1637S Altcode: 2007arXiv0712.0023S Solar flares and coronal mass ejections are associated with rapid changes in field connectivity and are powered by the partial dissipation of electrical currents in the solar atmosphere. A critical unanswered question is whether the currents involved are induced by the motion of preexisting atmospheric magnetic flux subject to surface plasma flows or whether these currents are associated with the emergence of flux from within the solar convective zone. We address this problem by applying state-of-the-art nonlinear force-free field (NLFFF) modeling to the highest resolution and quality vector-magnetographic data observed by the recently launched Hinode satellite on NOAA AR 10930 around the time of a powerful X3.4 flare. We compute 14 NLFFF models with four different codes and a variety of boundary conditions. We find that the model fields differ markedly in geometry, energy content, and force-freeness. We discuss the relative merits of these models in a general critique of present abilities to model the coronal magnetic field based on surface vector field measurements. For our application in particular, we find a fair agreement of the best-fit model field with the observed coronal configuration, and argue (1) that strong electrical currents emerge together with magnetic flux preceding the flare, (2) that these currents are carried in an ensemble of thin strands, (3) that the global pattern of these currents and of field lines are compatible with a large-scale twisted flux rope topology, and (4) that the ~1032 erg change in energy associated with the coronal electrical currents suffices to power the flare and its associated coronal mass ejection. Title: A New Model for Propagating Parts of EIT Waves: A Current Shell in a CME Authors: Delannée, C.; Török, T.; Aulanier, G.; Hochedez, J. -F. Bibcode: 2008SoPh..247..123D Altcode: EIT waves are observed in EUV as bright fronts. Some of these bright fronts propagate across the solar disk. EIT waves are all associated with a flare and a CME and are commonly interpreted as fast-mode magnetosonic waves. Propagating EIT waves could also be the direct signature of the gradual opening of magnetic field lines during a CME. We quantitatively addressed this alternative interpretation. Using two independent 3D MHD codes, we performed nondimensional numerical simulations of a slowly rotating magnetic bipole, which progressively result in the formation of a twisted magnetic flux tube and its fast expansion, as during a CME. We analyse the origins, the development, and the observability in EUV of the narrow electric currents sheets that appear in the simulations. Both codes give similar results, which we confront with two well-known SOHO/EIT observations of propagating EIT waves (7 April and 12 May 1997), by scaling the vertical magnetic field components of the simulated bipole to the line of sight magnetic field observed by SOHO/MDI and the sign of helicity to the orientation of the soft X-ray sigmoids observed by Yohkoh/SXT. A large-scale and narrow current shell appears around the twisted flux tube in the dynamic phase of its expansion. This current shell is formed by the return currents of the system, which separate the twisted flux tube from the surrounding fields. It intensifies as the flux tube accelerates and it is co-spatial with weak plasma compression. The current density integrated over the altitude has the shape of an ellipse, which expands and rotates when viewed from above, reproducing the generic properties of propagating EIT waves. The timing, orientation, and location of bright and faint patches observed in the two EIT waves are remarkably well reproduced. We conjecture that propagating EIT waves are the observational signature of Joule heating in electric current shells, which separate expanding flux tubes from their surrounding fields during CMEs or plasma compression inside this current shell. We also conjecture that the bright edges of halo CMEs show the plasma compression in these current shells. Title: Slipping Magnetic Reconnection in Coronal Loops Authors: Aulanier, Guillaume; Golub, Leon; DeLuca, Edward E.; Cirtain, Jonathan W.; Kano, Ryouhei; Lundquist, Loraine L.; Narukage, Noriyuki; Sakao, Taro; Weber, Mark A. Bibcode: 2007Sci...318.1588A Altcode: Magnetic reconnection of solar coronal loops is the main process that causes solar flares and possibly coronal heating. In the standard model, magnetic field lines break and reconnect instantaneously at places where the field mapping is discontinuous. However, another mode may operate where the magnetic field mapping is continuous but shows steep gradients: The field lines may slip across each other. Soft x-ray observations of fast bidirectional motions of coronal loops, observed by the Hinode spacecraft, support the existence of this slipping magnetic reconnection regime in the Sun’s corona. This basic process should be considered when interpreting reconnection, both on the Sun and in laboratory-based plasma experiments. Title: What can We learn about Filaments from Vector Magnetograms? Authors: Schmieder, B.; Aulanier, G.; Lopez Ariste, A. Bibcode: 2007ASPC..369..137S Altcode: Theoretical MHD models of filaments have predicted the existence of dips in magnetic field lines supporting the cool filament plasma. Condensation would be one of the possible mechanism of filament formation. The controversy is still alive. Filament fine structures are of dynamical nature and injection of cold material (like surges) from the photosphere would be the proposed mechanism. With the presence of dips, the magnetic lines are predicted to be tangent to the photosphere, near the barbs, footpoints of the prominences. The injection model predicted nearly vertical structures of the barbs rooted in the minority polarities.

Measurements of vector magnetic field in filament channels can answer to these questions and will be possible with Solar-B.

Recent results have been obtained with THEMIS vector magnetograph in Tenerife, suggesting that dips are really present in filament channel leading to the conclusion that the observed magnetic topology in the photosphere fully supports models of prominences based on dips. Title: Eruptive and Compact Flares Authors: Schmieder, B.; Aulanier, G.; Delannée, C.; Berlicki, A. Bibcode: 2007AIPC..934...22S Altcode: Solar two ribbon flares are commonly explained by magnetic field reconnections in the high 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 after the flare bright ribbons are observed at both ends of 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 an X-point, 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 as observed by RHESSI, and that it is observed in soft X rays with Hinode/XRT. We also show how this concept can be applied to model the non-wave nature of EIT waves. Title: Unveiling the Magnetic Field Topology of Prominences Authors: López Ariste, A.; Aulanier, G. Bibcode: 2007ASPC..368..291L Altcode: A renewed effort is being dedicated to infer magnetic fields in prominences through spectropolarimetry of emission lines like the He D3 and 10830 Å lines. We will briefly review those present observational efforts pointing at how do they improve or disregard past attempts to measure magnetic fields in prominences and at how they try to answer some of the standing questions from theoretical models. A brief and gentle introduction to the Hanle effect and other subtle quantum phenomena is given, as they are key to present diagnostic techniques and, finally, an overview of future observational goals and wishes is presented. 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: Stationary parts of an EIT and Moreton wave: a topological model Authors: Delannée, C.; Hochedez, J. -F.; Aulanier, G. Bibcode: 2007A&A...465..603D Altcode: Context: EIT and Moreton waves came into focus in 1997, when a propagating disturbance on a large area of the solar disc was discovered. The process generating the EIT and Moreton waves has been frequently discussed.
Aims: On May 2, 1998, a halo CME was observed related to an EIT wave, a Moreton wave, a X1 flare, radio emission sources, and dimmings. We studied this event to find the relation between all these structures.
Methods: We use and co-align multi-wavelength observations and the online potential field source surface (pfss) package.
Results: The observed EIT and Moreton waves present some brightenings that remain at the same location. We relate the connectivity of the coronal potential magnetic field to the stationary brightenings. We find that the areas where the magnetic field lines have drastic jumps of connectivity are cospatial to the stationary brightenings of the waves.
Conclusions: .We conclude that the EIT and Moreton waves may be due to Joule heating resulting from the generation of electric currents in the neighboring area of the drastic jumps of magnetic connectivity, while the magnetic field lines are opening during a CME. Title: Magnetic flux tubes observed with THEMIS/MSDP Authors: Mein, P.; Mein, N.; Faurobert, M.; Aulanier, G.; Malherbe, J. -M. Bibcode: 2007A&A...463..727M Altcode: Aims:We use spectro-polarimetric THEMIS/MSDP data to investigate the 3D structure of solar magnetic-flux tubes across the upper photosphere.
Methods: Profiles of the sodium D1 line 589.6 nm are analysed by the bisector method at different wavelengths from the core to the wings, for several bright features. They are compared to synthetic profiles derived from 2D magnetic models of flux tubes and from the MULTI code for NLTE line profiles. Three different magnetic models of flux tubes are investigated. Model (I) consists of a single flux tube that compensates for the horizontal Lorentz forces exactly, while model (II) uses a compromise between horizontal and vertical components. Model (III), a conglomerate of thinner flux tubes, leads to the best agreement with observations.
Results: (1) The combination of seeing effects (small filling factor) with slopes of line profiles, which are different in the flux tubes and the neighbouring quiet sun, account for the decrease in observed magnetic field from line core to line wings in central parts of magnetic features, as well as the decrease in magnetic fluxes integrated over the whole magnetic features. (2) The expansion with height of single magnetic flux tubes (models I and II) accounts for the increase in the size of magnetic features from line wings to line core. (3) Pure thermodynamical criteria characterising Dopplershifts and line-intensity fluctuations of magnetic and non-magnetic features have been proven by observations.
Conclusions: . We could account for differential Zeeman effects along the D1 line profile by combining expansion of flux tubes with height, low gas pressure inside flux tubes, and small filling factor due to seeing effects. Better agreement with observations, in particular with respect to magnetic field amplitudes, will probably need 3D models that take velocity fields and horizontal gradients of temperature into account. Title: Magnetic flux tubes observed with THEMIS/MSDP . Authors: Mein, P.; Mein, N.; Faurobert, M.; Aulanier, G.; Malherbe, J. -M. Bibcode: 2007MmSAI..78...92M Altcode: We use 2D spectro-polarimetric data of the NaD1 line to investigate magnetic flux tubes at several levels of the solar photosphere:

- magnetic and non-magnetic bright features can be discriminated by simple criteria of intensities and dopplershifts.

- 2D magnetic models and NLTE line profiles are compared to observations : combination of seeing effects and departures between slopes of line profiles in flux tubes and neighbouring photosphere account for vertical gradients of line-of-sight (LOS) magnetic field measurements.

- Best qualitative agreements are obtained with clusters of magnetic flux tubes. Title: What is the role of magnetic null points in large flares? Authors: Schmieder, B.; Mandrini, C. H.; Démoulin, P.; Aulanier, G.; Li, H.; Berlicki, A. Bibcode: 2007AdSpR..39.1840S Altcode: We have performed the analysis of the magnetic topology of active region NOAA 10486 before two large flares occurring on October 26 and 28, 2003. The 3D extrapolation of the photospheric magnetic field shows the existence of magnetic null points when using two different methods. We use TRACE 1600 Å and 195 Å brightenings as tracers of the energy release due to magnetic reconnections. We conclude on the three following points: The small events observed before the flares are related to low lying null points. They are long lasting and associated with low energy release. They are not triggering the large flares.

On October 26, a high altitude null point is found. We look for bright patches that could correspond to the signatures of coronal reconnection at the null point in TRACE 1600 Å images. However, such bright patches are not observed before the main flare, they are only observed after it.

On October 28, four ribbons are observed in TRACE images before the X17 flare. We interpret them as due to a magnetic breakout reconnection in a quadrupolar configuration. There is no magnetic null point related to these four ribbons, and this reconnection rather occurs at quasi-separatrix layers (QSLs).

We conclude that the existence of a null point in the corona is neither a sufficient nor a necessary condition to give rise to large flares. Title: Evolving Photospheric Flux Concentrations and Filament Dynamic Changes Authors: Schmieder, B.; Aulanier, G.; Mein, P.; Ariste, A. López Bibcode: 2006SoPh..238..245S Altcode: 2006SoPh..tmp...64S We analyze the role of weak photospheric flux concentrations that evolve in a filament channel, in the triggering of dynamic changes in the shape of a filament. The high polarimetric sensitivity of THEMIS allowed us to detect weak flux concentrations (few Gauss) associated with the filament development. The synoptic instruments (MDI, SOLIS) even if their sensitivity is much less than THEMIS were useful to follow any subsequent strengthening of these flux concentrations after their identification in the THEMIS magnetograms. We found that (1) the northern part of the filament develops an Hα barb at the same time that weak minority polarity elements develop near a plage; (2) a section in the southern part of the Hα filament gradually disappears and later reforms at the same time that several mixed-polarity magnetic elements appear, then subsequently cancel or spread away from each other. These changes correspond to increases in EUV emission, as observed by TRACE, EIT, and CDS. This suggests that the plasma is temporarily heated along the filament spine. An idealized sequence of force-free models of this filament channel, based on plasma-supporting magnetic dips occurring in the windings of a very weakly twisted flux tube, naturally explains the evolution of its southern part as being due to changes in the topology of the coronal magnetic field as the photospheric flux concentrations evolve. 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: First observation of bald patches in a filament channel and at a barb endpoint Authors: López Ariste, A.; Aulanier, G.; Schmieder, B.; Sainz Dalda, A. Bibcode: 2006A&A...456..725L Altcode: The 3D magnetic field topology of solar filaments/prominences is strongly debated, because it is not directly measureable in the corona. Among various prominence models, several are consistent with many observations, but their related topologies are very different. We conduct observations to address this paradigm. We measure the photospheric vector magnetic field in several small flux concentrations surrounding a filament observed far from disc center. Our objective is to test for the presence/absence of magnetic dips around/below the filament body/barb, which is a strong constraint on prominence models, and that is still untested by observations. Our observations are performed with the THEMIS/MTR instrument. The four Stokes parameters are extracted, from which the vector magnetic fields are calculated using a PCA inversion. The resulting vector fields are then deprojected onto the photospheric plane. The 180° ambiguity is then solved by selecting the only solution that matches filament chirality rules. Considering the weakness of the resulting magnetic fields, a careful analysis of the inversion procedure and its error bars was performed, to avoid over-interpretation of noisy or ambiguous Stokes profiles. Thanks to the simultaneous multi-wavelength THEMIS observations, the vector field maps are coaligned with the Hα image of the filament. By definition, photospheric dips are identifiable where the horizontal component of the magnetic field points from a negative toward a positive polarity. Among six bipolar regions analyzed in the filament channel, four at least display photospheric magnetic dips, i.e. bald patches. For barbs, the topology of the endpoint is that of a bald patch located next to a parasitic polarity, not of an arcade pointing within the polarity. The observed magnetic field topology in the photosphere tends to support models of prominence based on magnetic dips located within weakly twisted flux tubes. Their underlying and lateral extensions form photospheric dips both within the channel and below barbs. Title: Is Pre-Eruptive Null Point Reconnection Required for Triggering Eruptions? Authors: Li, Hui; Schmieder, Brigitte; Aulanier, Guillaume; Berlicki, Arkadiusz Bibcode: 2006SoPh..237...85L Altcode: 2006SoPh..tmp...22L; 2006SoPh..tmp...18L We study the magnetic field evolution and topology of the active region NOAA 10486 before the 3B/X1.2 flare of October 26, 2003, using observational data from the French-Italian THEMIS telescope, the Michelson Doppler Imager (MDI) onboard Solar and Heliospheric Observatory (SOHO), the Solar Magnetic Field Telescope (SMFT) at Huairou Solar Observation Station (HSOS), and the Transition Region and Coronal Explorer (TRACE). Three dimensional (3D) extrapolation of photospheric magnetic field, assuming a potential field configuration, reveals the existence of two magnetic null points in the corona above the active region. We look at their role in the triggering of the main flare, by using the bright patches observed in TRACE 1600 Å images as tracers at the solar surface of energy release associated with magnetic reconnection at the null points. All the bright patches observed before the flare correspond to the low-altitude null point. They have no direct relationship with the X1.2 flare because the related separatrix is located far from the eruptive site. No bright patch corresponds to the high-altitude null point before the flare. We conclude that eruptions can be triggered without pre-eruptive coronal null point reconnection, and the presence of null points is not a sufficient condition for the occurrence of flares. We propose that this eruptive flare results from the loss of equilibrium due to persistent flux emergence, continuous photospheric motion and strong shear along the magnetic neutral line. The opening of the coronal field lines above the active region should be a byproduct of the large 3B/X1.2 flare rather than its trigger. Title: Solar Prominence Merging Authors: Aulanier, Guillaume; DeVore, C. Richard; Antiochos, Spiro K. Bibcode: 2006ApJ...646.1349A Altcode: In a recent paper, we described MHD simulations of the interaction between a pair of distinct prominences formed by the photospheric line-tied shearing of two separated dipoles. One case was typical of solar observations of prominence merging, in which the prominences have the same axial field direction and sign of magnetic helicity. For that configuration, we reported the formation of linkages between the prominences due to magnetic reconnection of their sheared fields. In this paper, we analyze the evolution of the plasma-supporting magnetic dips in this configuration. As the photospheric flux is being progressively sheared, dip-related chromospheric fibrils and high-altitude threads form and develop into the two prominences, which undergo internal oscillations. As the prominences are stretched farther along their axes, they come into contact and their sheared fluxes pass each other, and new dips form in the interaction region. The distribution of these dips increasingly fills the volume between the prominences, so that the two progenitors gradually merge into a single prominence. Our model reproduces typical observational properties reported from both high-cadence and daily observations at various wavelengths. We identify the multistep mechanism, consisting of a complex coupling between photospheric shear, coronal magnetic reconnection without null points, and formation of quasi bald patches, that is responsible for the prominence merging through dip creation. The resulting magnetic topology differs significantly from that of a twisted flux tube. Title: Electron Acceleration and Transport During the November 5, 1998 Solar Flare At ∼13:34 UT Authors: Trottet, G.; Correia, E.; Karlický, M.; Aulanier, G.; Yan, Y.; Kaufmann, P. Bibcode: 2006SoPh..236...75T Altcode: This paper deals with a detailed analysis of spectral and imaging observations of the November 5, 1998 (Hα 1B, GOES M1.5) flare obtained over a large spectral range, i.e., from hard X-rays to radiometric wavelengths. These observations allowed us to probe electron acceleration and transport over a large range of altitudes that is to say within small-scale (a few 103 km) and large-scale (a few 105 km) magnetic structures. The observations combined with potential and linear force-free magnetic field extrapolations allow us to show that: (i) Flare energy release and electron acceleration are basically driven by loop-loop interactions at two independent, low lying, null points of the active region magnetic field; (ii) <300 keV hard X-ray-producing electrons are accelerated by a different process (probably DC field acceleration) than relativistic electrons that radiate the microwave emission; and (iii) although there is evidence that hard X-ray and decimetric/metric radio-emitting electrons are produced by the same accelerator, the present observations and analysis did not allow us to find a clear and direct magnetic connection between the hard X-ray emitting region and the radio-emitting sources in the middle corona. 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: Magnetic Topology before a 3B/X1.2 Flare and the Associated CME Authors: Li, H.; Schmieder, B.; Aulanier, G.; Berlicki, A. Bibcode: 2006IAUS..233..399L Altcode: We extrapolated observed photospheric magnetic field before the 3B/X1.2 flare occurred in NOAA 10486 on 2003 October 26. Two magnetic null points exist above the active region, which have no crucial role in triggering the flare, even though enhanced TRACE EUV/UV emission was observed before the main flare due to magnetic reconnection near the lower-altitude null point. We conclude that this flare results from the loss of equilibrium due to persistent flux emergence and photospheric motion, and strong shear. The accompanying fast CME with velocity of ∼ 800 km s^{-1} shows quick mass pickup and energy increase in the low corona. Its kinetic energy is always larger than its potential energy. Title: Can we finally solve the problems of "Coronal Heating " and "Solar Wind Acceleration" in the Cosmic Vision era ? Authors: Maksimovic, M.; Appourchaux, T.; Aulanier, G.; Chust, T.; Dudok de Wit, T.; Klein, K. L.; Krasnoselskikh, V.; Louarn, P.; Roux, A.; Vial, J. C. Bibcode: 2006cosp...36.2999M Altcode: 2006cosp.meet.2999M Since the discovery of the corona s million-degree temperature in the 1940s and the supersonic solar wind in the early 1960s major efforts have been made to discover the physical mechanisms that could explain these two observations These efforts have led to a tremendous increase in our knowledge of our neighbour star the inner heliosphere and the Sun-Earth connections Unfortunately these efforts haven t allowed to provide a definitive answer to these questions Why such a situation On one hand the remote-sensing strategy has tried to probe the coronal properties by basically analysing the photons emitted or absorbed by the Sun s atmosphere With this regards limitations occur on both theoretical ground physics of the coupling between photons and plasma and experimental ground limited number of observables such as spectral lines or the hardly solvable problem of the line of sight integration On the other hand solar wind in-situ measurements have had access to the very detailed state of the local plasma properties full particles velocity distribution functions observations of the electromagnetic plasma fluctuations over a huge frequency range but at locations far from the corona and the solar wind acceleration regions Moreover it has been realized more recently that the magnetic field plays a fundamental role in shaping the low corona and channelling the energy inputs Unfortunately the measurement of the full magnetic vector in the corona is a very difficult enterprise and this lack of information hampers all on-going modelling efforts Given Title: Emergence of undulatory magnetic flux tubes by small scale reconnections Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2006AdSpR..38..902P Altcode: With Flare Genesis Experiment (FGE), a balloon borne observatory launched in Antarctica on January 2000, series of high spatial resolution vector magnetograms, Dopplergrams, and Hα filtergrams have been obtained in an emerging active region (AR 8844). Previous analyses of this data revealed the occurence of many short-lived and small-scale H α brightenings called 'Ellerman bombs' (EBs) within the AR. We performed an extrapolation of the field above the photosphere using the linear force-free field approximation. The analysis of the magnetic topology reveals a close connexion between the loci of EBs and the existence of "Bald patches" (BP) regions (BPs are regions where the vector magnetic field is tangential to the photosphere). Some of these EBs/BPs are magnetically connected by low-lying field lines, presenting a serpentine shape. This results leads us to conjecture that arch filament systems and active regions coronal loops do not result from the smooth emergence of large scale Ω-loops, but rather from the rise of flat undulatory flux tubes which get released from their photospheric anchorage by reconnection at BPs, which observational signature is Ellerman bombs. Title: The role of null points in large flares Authors: Schmieder, B.; Mandrini, C.; Démoulin, P.; Aulanier, G.; Li, H. Bibcode: 2006cosp...36..156S Altcode: 2006cosp.meet..156S We have performed the analysis of the magnetic topology of the active region NOAA 10486 before two large flares occurring on October 26 and 28 2003 The 3D extrapolation of photospheric magnetic field show the existence of magnetic null points We used TRACE 1600 A brightenings as tracers of the energy release during magnetic reconnections We conclude on the three following points 1 The observed small pre-events observed before the flares were related to low lying null points They were long lasting and associated with low energy release They were not triggering of the large flares 2 On October 26 a high altitude null point was detected We looked at the TRACE 1600 A images for bright patches corresponding to a possible coronal reconnection at the null point However no bright patch was observed before the main flare 3 On October 28 a breakout of the large scale overlaying magnetic field lines occurred but without the presence of a null point in the corona So the existence of a null point in the corona is not a sufficient and or necessary condition for getting large flares 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: Observational Evidence of Photoshperic Magnetic Dips in Filament Channels Authors: Schmieder, B.; López Ariste, A.; Aulanier, G. Bibcode: 2005ESASP.596E..19S Altcode: 2005ccmf.confE..19S No abstract at ADS Title: Solar Prominence Interactions Authors: DeVore, C. Richard; Antiochos, Spiro K.; Aulanier, Guillaume Bibcode: 2005ApJ...629.1122D Altcode: We report numerical simulations of the formation, interaction, and magnetic reconnection between pairs of solar prominences within the sheared-arcade model. Our experiments consider the four possible basic combinations of chiralities (identical or opposite) and axial magnetic fields (aligned or opposed) between the participating prominences. When the topology of the global flux system comprising the prominences and arcades is bipolar, so that a single polarity inversion line is shared by the two structures, then identical chiralities necessarily imply aligned axial fields, while opposite chiralities imply opposed axial fields. In the former case, external magnetic reconnections forming field lines linking the two prominences occur; in the latter, such reconnections are disfavored, and no linkage takes place. These results concur with empirical rules for prominence interactions. When the topology instead is quadrupolar, so that a second polarity inversion line crossing the first lies between the prominences, then the converse relation holds between chirality and axial-field alignment. External reconnections forming linking field lines now occur between prominences with opposite chiralities; they also occur, but result only in footpoint exchanges, between prominences with identical chiralities. These findings conflict with the accepted empirical rules but may not have been tested in observations to date. All of our model prominences, especially those that undergo linking reconnections, contain substantial magnetic shear and twist. Nevertheless, none exhibits any sign of onset of instability or loss of equilibrium that might culminate in an eruption. Title: Equilibrium and observational properties of line-tied twisted flux tubes Authors: Aulanier, G.; Démoulin, P.; Grappin, R. Bibcode: 2005A&A...430.1067A Altcode: We describe a new explicit three-dimensional magnetohydrodymanic code, which solves the standard zero-β MHD equations in Cartesian geometry, with line-tied conditions at the lower boundary and open conditions at the other ones. Using this code in the frame of solar active regions, we simulate the evolution of an initially potential and concentrated bipolar magnetic field, subject to various sub-Alfvénic photospheric twisting motions which preserve the initial photospheric vertical magnetic field. Both continuously driven and relaxation runs are performed. Within the numerical domain, a steep equilibrium curve is found for the altitude of the apex of the field line rooted in the vortex centers as a function of the twist. Its steepness strongly depends on the degree of twist in outer field lines rooted in weak field regions. This curve fits the analytical expression for the asymptotic behaviour of force-free fields of spherical axisymmetric dipoles subject to azimuthal shearing motions, as well as the curve derived for other line-tied twisted flux tubes reported in previous works. This suggests that it is a generic property of line-tied sheared/twisted arcades. However, contrary to other studies we never find a transition toward a non-equilibrium within the numerical domain, even for twists corresponding to steep regions of the equilibrium curve. The calculated configurations are analyzed in the frame of solar observations. We discuss which specific conditions are required for the steepness of the generic equilibrium curve to result in dynamics which are typical of both fast and slow CMEs observed below 3 R_⊙. We provide natural interpretations for the existence of asymmetric and multiple concentrations of electric currents in homogeneoulsy twisted sunspots, due to the twisting of both short and long field lines. X-ray sigmoids are reproduced by integrating the Joule heating term along the line-of-sight. These sigmoids have inverse-S shapes associated with negative force-free parameters α, which is consistent with observed rules in the northern solar hemisphere. We show that our sigmoids are not formed in the main twisted flux tube, but rather in an ensemble of low-lying sheared and weakly twisted field lines, which individually never trace the whole sigmoid, and which barely show their distorded shapes when viewed in projection. We find that, for a given bipolar configuration and a given twist, neither the α nor the altitude of the lines whose envelope is a sigmoid depends on the vortex size. Title: Research on a complex CME event including Hα, LASCO, radio and MDI observations Authors: Wang, S. J.; Maia, D.; Pick, M.; Aulanier, G.; Malherbe, J. -M.; Delaboudinière, J. -P. Bibcode: 2005AdSpR..36.2273W Altcode: We present our research on a fast and decelerating partial halo coronal mass ejection (CME) event detected in multi-wavelengths in the chromosphere and the corona on 14 October, 1999. The event involved a whole complex active area which spanned more than 40° of heliolongitude. It included a strong solar flare (XI/1N) and a complex eruptive filament within an active region of the entire complex. Especially, several radio sources were detected in the decimetric range prior to the CME by the Nançay Radioheliograph (NRH). A linear force-free field extrapolation of the Michelson Doppler Imager (MDI) magnetogram was performed to calculate the magnetic topology of the complex prior to the triggering of the event. The presence of a coronal null point combined with the occurrence of two distant and nearly simultaneous radio sources put strong arguments in favor of the generalized breakout model for the triggering of the eruption. The analysis of the subsequent development of the event suggests that large interconnecting loops were ejected together with the CME. Title: Flux tube emergence, from photosphere to corona Authors: Pariat, E.; Schmieder, B.; Aulanier, G. Bibcode: 2004sf2a.conf..103P Altcode: 2004sf2a.confE.339P From a campaign of multi-wavelength observations of an emerging active region, we have studied the dynamics of the solar atmosphere due to this emergence and the magnetic field topology of the active region. In addition with the observations obtained with Yohkoh, SOHO and TRACE, a balloon borne 80 cm telescope (Flare Genesis Experiment) provided us a series of high spatial resolution vector magnetograms. For the first time we highlight that magnetic flux tubes do not directly emerge with a large Omega-loop shape, as suggest the TRACE observations of the corona, but rather within an undulatory shape. We demonstrated that the resistive Parker instability allows the flux tube to go through the low atmosphere.

This result has been obtained by performing an extrapolation of the field above the active region. Title: Resistive Emergence of Undulatory Flux Tubes Authors: Pariat, E.; Aulanier, G.; Schmieder, B.; Georgoulis, M. K.; Rust, D. M.; Bernasconi, P. N. Bibcode: 2004ApJ...614.1099P Altcode: During its 2000 January flight, the Flare Genesis Experiment observed the gradual emergence of a bipolar active region, by recording a series of high-resolution photospheric vector magnetograms and images in the blue wing of the Hα line. Previous analyses of these data revealed the occurrence of many small-scale, transient Hα brightenings identified as Ellerman bombs (EBs). They occur during the flux emergence, and many of them are located near moving magnetic dipoles in which the vector magnetic field is nearly tangential to the photosphere. A linear force-free field extrapolation of one of the magnetograms was performed to study the magnetic topology of small-scale EBs and their possible role in the flux emergence process. We found that 23 out of 47 EBs are cospatial with bald patches (BPs), while 15 are located at the footpoints of very flat separatrix field lines passing through distant BPs. We conclude that EBs can be due to magnetic reconnection, not only at BP locations, but also along their separatrices, occurring in the low chromosphere. The topological analysis reveals, for the first time, that many EBs and BPs are linked by a hierarchy of elongated flux tubes showing aperiodic spatial undulations, whose wavelengths are typically above the threshold of the Parker instability. These findings suggest that arch filament systems and coronal loops do not result from the smooth emergence of large-scale Ω-loops from below the photosphere, but rather from the rise of undulatory flux tubes whose upper parts emerge because of the Parker instability and whose dipped lower parts emerge because of magnetic reconnection. EBs are then the signature of this resistive emergence of undulatory flux tubes. Title: Evolution and magnetic topology of the M 1.0 flare of October 22, 2002 Authors: Berlicki, A.; Schmieder, B.; Vilmer, N.; Aulanier, G.; Del Zanna, G. Bibcode: 2004A&A...423.1119B Altcode: In this paper we analyse an M 1.0 confined flare observed mainly during its gradual phase. We use the data taken during a coordinated observational campaign between ground based instruments (THEMIS and VTT) and space observatories (SoHO/CDS and MDI, TRACE and RHESSI). We use these multi-wavelength observations to study the morphology and evolution of the flare, to analyse its gradual phase and to understand the role of various heating mechanisms. During the flare, RHESSI observed emission only within the 3-25 keV spectral range. The RHESSI spectra indicate that the emission of the flare was mainly of thermal origin with a small non-thermal component observed between 10 and 20 keV. Nevertheless, the energy contained in the non-thermal electrons is negligible compared to the thermal energy of the flaring plasma. The temperature of plasma obtained from the fitting of the RHESSI X-ray spectra was between 8.5 and 14 MK. The lower temperature limit is typical for a plasma contained in post flare loops observed in X-rays. Higher temperatures were observed during a secondary peak of emission corresponding to a small impulsive event. The SoHO/CDS observations performed in EUV Fe XIX line also confirm the presence of a hot plasma at temperatures similar to those obtained from RHESSI spectra. The EUV structures were located at the same place as RHESSI X-ray emission. The magnetic topology analysis of the AR coming from a linear force-free field extrapolation explains the observed features of the gradual phase of the flare i.e. the asymmetry of the ribbons and their fast propagation. The combination of the multi-wavelength observations with the magnetic model further suggests that the onset of the flare would be due to the reconnection of an emerging flux in a sheared magnetic configuration. Title: Multi-wavelength flare study and magnetic configuration Authors: Schmieder, Brigitte; Berlicki, A.; Vilmer, N.; Aulanier, G.; Démoulin, P.; Mein, P.; Mandrini, C.; Deluca, E. Bibcode: 2004IAUS..223..397S Altcode: 2005IAUS..223..397S Recent results of two observation campaigns (October 2002 and October 2003) are presented with the objective of understanding the onset of flares and CMEs. The magnetic field was observed with THEMIS and MDI, the chromosphere with the MSDP operating on the German telescope VTT and on THEMIS, the EUV images with SOHO/CDS and TRACE, the X-ray with RHESSI. We show how important is the magnetic configuration of the active region to produce CMEs using two examples: the October 28 2003 X 17 flare and the October 22 2002 M 1.1 flare. The X 17 flare gave a halo CME while the M 1.1 flare has no corresponding CME. The magnetic topology analysis of the active regions is processed with a linear-force-free field configuration. 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 evolution and magnetic configuration study Authors: Berlicki, A.; Schmieder, B.; Aulanier, G.; Vilmer, N.; Yan, Y. H. Bibcode: 2004cosp...35.2200B Altcode: 2004cosp.meet.2200B We will present the analysis of M1.0 confined flare emission and evolution in the context of the topology of the coronal magnetic field. This flare was observed in NOAA 0162 on 22 October 2002. The multiwavelength data were taken during a coordinated observational campaign between ground based instruments and space observatories. The photospheric line-of-sight magnetic field observations were obtained with THEMIS and SOHO/MDI. We used these data to perform linear force-free field extrapolation of magnetic field into the corona. Our extrapolation provides an explanation of the appearance of H-alpha flare ribbons. An elongated shape of X-ray emission observed by Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) also follow the predicted shape of extrapolated field lines. Moreover, the X-ray emission observed by RHESSI permit to see thermal emission of coronal loops heated probably by non-thermal electrons, accelerated during the reconnection processes. The presence of non-thermal particles can be deduced from RHESSI X-ray spectra reconstructed during the gradual phase of the flare. On Huairou vector magnetograms of the AR we see that there was strong shear between one of main negative spot and the north small positive spot. The extrapolation with non-constant alpha force-free field model did not obtain any loop to connect these two spots. Title: 3D MHD models for XUV sigmoids Authors: Aulanier, G.; Démoulin, P.; Grappin, R.; Marqué, C. Bibcode: 2004cosp...35.3011A Altcode: 2004cosp.meet.3011A Solar sigmoids observed in XUV wavelengths have recently been identified as good probes of current-carrying magnetic fields which can precede the launch of a CME. However, the amplitude and the distribution of these pre-eruptive currents are yet very poorly known, since several 3D magnetostatic models are able to produce S-shaped field lines projected onto the photospheric plane, most of them implying that more or less twisted field lines trace the whole sigmoid. We will show two new families of magnetic models for XUV sigmoids: a generic model based on MHD calculations of line-tied twisted flux tubes, and a model for one observed sigmoid based on linear force-free field extrapolations. Both models predict that XUV sigmoids do not trace the central twisted flux tube, also that no single field line can ever trace the whole sigmoid, and finally that sigmoids may only show an ensemble of low-lying sheared field lines of various lengths, with several interruptions of connectivities all along the sigmoid depending on the shear distribution and on the local complexity of the photospheric field. So, these new models predict that sigmoids may not have a magnetic topology as simple as what is predicted by classic models, and that the derivation of the degree of twist from the shape of the sigmoid may not be directly doable, so that it may require the use of models. The 3D characteristic of these new XUV sigmoid models, combined with their synthetic transverse photospheric fields, provide new predictions and tools which will be very well suited for the analyzis of the upcoming STEREO and SOLAR-B combined observations of these structures. Title: Interpretation of a complex CME event: Coupling of scales in multiple flux systems Authors: Maia, D.; Aulanier, G.; Wang, S. J.; Pick, M.; Malherbe, J. -M.; Delaboudinière, J. -P. Bibcode: 2003A&A...405..313M Altcode: Using multi-wavelength observations, in particular in imagery, recorded by SOHO/LASCO-MDI, Yohkoh/SXT, the Meudon spectroheliograph and the Nançay radioheliograph, and performing a linear force-free field extrapolation, we analyzed the triggering and the development of a complex eruptive event in the chromosphere and in the corona. This event included an X1 class flare and an eruptive filament within an active region, but it also involved a whole active complex spanning over 40 degrees of heliolongitude. It resulted in a fast and decelerating partial halo CME, associated with a Moreton wave and a complex series of metric, decimetric and microwave radio bursts. The presence of a coronal null point combined with the occurrence of two distant and nearly simultaneous radio sources give strong arguments in favor of the generalized breakout model for the triggering of the eruption. The observations are consistent with the occurrence of magnetic reconnection at the null point three minutes before the start of the eruption, which is consistent with other observed CME precursors. The analysis of the subsequent development of the event suggests that large interconnecting loops were ejected together with the CME, and that secondary reconnections at low altitude probably occurred remotely in the active complex. Our results show that the triggering and evolution of this complex CME involved multiple magnetic flux systems over a large coronal volume surrounding the flare site, and that it resulted from the coupling of scales from narrow reconnection current sheets to very large inter active region magnetic connections. Title: Amplitude and orientation of prominence magnetic fields from constant-alpha magnetohydrostatic models Authors: Aulanier, G.; Démoulin, P. Bibcode: 2003A&A...402..769A Altcode: We analyze outputs from three-dimensional models for three observed filaments, which belong to the quiescent, intermediate and plage class respectively. Each model was calculated from a constant-alpha magnetohydrostatic extrapolation, assuming that the prominence material is located in magnetic dips, so that the field is nearly horizontal throughout the prominence body and feet. We calculate the spatial distribution of the magnetic field amplitude B and orientation theta with respect to the filament axis, neither of which were imposed a priori in the models. In accordance with past magnetic field measurements within prominence bodies, we also obtain nearly homogeneous magnetic fields, respectively of about B ~ 3, 14 and 40 G for the quiescent, intermediate and plage prominence, with a systematic weak vertical field gradient of partial B / partial z ~ 0.1-1.5 x 10-4 G km-1. We also find that the inverse polarity configuration is dominant with theta ~ -20o to 0o, which is slightly smaller than in some observations. We also report some other properties, which have either rarely or never been observed. We find at prominence tops some localized normal polarity regions with theta < +10o. At prominence bottoms below 20 Mm in altitude, we find stronger field gradients partial B / partial z ~ 1-10 x 10-4 G km-1 and a wider range of field directions theta ~ -90o to 0o. These properties can be interpreted by the perturbation of the prominence flux tube by strong photospheric polarities located in the neighborhood of the prominence. We also report some full portions of prominences that have the normal polarity. The latter are simply due to the local curvature of the filaments with respect to their average axis, which was used to define theta . These results could either be used as predictions for further testing of this class of models with new observations, or as quantitative tools for the interpretation of observations which show complex patterns. Title: Na I D1 Stokes V Asymmetries and Velocity Structure Around Sunspots Authors: Eibe, M. T.; Sánchez Almeida, J.; Mein, P.; Aulanier, G.; Malherbe, J. M. Bibcode: 2003ASPC..307..374E Altcode: No abstract at ADS Title: Magnetic field measurements with THEMIS Authors: Aulanier, G. Bibcode: 2003sf2a.conf...83A Altcode: 2003sf2a.confE..40A Some recent observations have shown that dark and wide EUV filament extensions observed on the disc at lambda < 912 A are due to the absorption of coronal and transition region lines in cool plasma condensations, located aside of filaments, that are unobservable in Hα. We present the results of our 3D magnetohydrostatic model for filaments, here applied to THEMIS and SoHO observations of such a wide EUV filament channel. The model was calculated from a MDI magnetogram as lower boundary conditions. We discuss the results of the model in terms of predictions for (i) the mass loading of CMEs during filament eruptions and (ii) the detectability of real filaments within dark, but possibly filamentless, EUV channels. We believe that both these issues will be relevant in the definition of future space instruments dedicated to 'space weather oriented' solar observations in EUV. Title: Spectropolarimetry of Solar Prominences Authors: Paletou, F.; Aulanier, G. Bibcode: 2003ASPC..307..458P Altcode: No abstract at ADS Title: On the Need of High-Resolution Spectropolarimetric Observations of Prominences Authors: Paletou, F.; Aulanier, G. Bibcode: 2003ASPC..286...45P Altcode: 2003ctmf.conf...45P No abstract at ADS Title: What can we learn from lfff magnetic extrapolations Authors: Schmieder, B.; Aulanier, G. Bibcode: 2003AdSpR..32.1875S Altcode: Observations of the Sun is done, up to now, in 2D and magnetic field is measured mainly in the photosphere. Magnetic extrapolation techniques allow us nevertheless to have a 3D view of the magnetic field. Different methods are available today. We present in this paper interesting and exploitable results obtained with linear force-free-field methods (lfff). The α parameter is assumed to be constant in the whole volume of computations. The results concern the global potential field in the corona (e.g. example of transequatorial loops), the magnetic topology of flaring active regions (e.g. flares and bald patch regions), twisted magnetic fields in filaments, and magnetic reconnection in emerging active region (e.g. Ellerman Bombs). We discuss on the limits of the validity of the methods. Title: The "careers in solar physics" session of the SPM10 meeting Authors: Aulanier, G.; Parenti, S.; Krijger, J. M. Bibcode: 2002ESASP.506..981A Altcode: 2002ESPM...10..981A; 2002svco.conf..981A During the SPM10 meeting held in Prague (Czech Republic) on September 9-14, 2002, a half-day 'young session' was organized on the topic of careers in solar physics. Several young researchers and senior scientists were invited to give oral contributions on the current advantages and difficulties attached to the current system for post-doctoral contracts. A scientist from USA also presented the American system for contractors, and an ESA representative presented the official position of ESA regarding funding researchers. From the talks as well as from the long open discussion which followed, it was widely agreed that several typical rules for EU post-doc contracts (their short duration, their mandatory mobility, their age limit and their administrative and financial difficulties) not only lead to serious problems in the private life of postdocs, but essentially can have serious drawbacks on the follow-up of long-term scientific developments, and could quickly result in a dramatic loss of expertise, from the scale of individual institutes to the European scientific community at large. Many participants and most of the young researchers naturally agreed that new longer-term, renewable and stable contracts are necessary. In order to create such types of contracts, several fund raising initiative achieveable by the scientific community were discussed. The development of better public outreach initiatives on the European scale was a possibility which federated most of the participants. The resulting conclusion on this session were transmitted to the new board of the Solar Physics Section of the EAS/SPS. 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. Title: Simulations of Interactions and Magnetic Reconnection Between Solar Filaments Authors: DeVore, C. R.; Antiochos, S. K.; Aulanier, G. Bibcode: 2002AAS...200.3720D Altcode: 2002BAAS...34..698D It has long been known that pairs of filaments near each other on the Sun's disk sometimes come into contact and interact. Under favorable conditions, the two structures apparently link up to form a single, larger filament. When conditions are unfavorable, on the other hand, the filaments appear to avoid each other and retain their distinct identities. Recent ground-based observational studies have shown that a key requirement for linkage to occur is that the two filaments possess the same chirality, or handedness. We have performed detailed numerical experiments of pairs of interacting filaments within the sheared-arcade model. In this model, the filament plasma resides in the magnetic hammock formed in a strongly sheared field held down by an overlying arcade. We considered four cases: like or unlike chirality of the two filaments, and like or unlike polarity of the vertical magnetic fields at their approaching ends. Only the case of like chirality and unlike polarity produces any significant reconfiguration. The magnetic structure is substantially modified, with reconnected field lines extending over the entire combined length of the filaments. Low, closed arcade fields form in the reconnection zone, forcing the newly linked filament fields above them to rise and form a magnetic 'aneurysm.' Our simple, bipolar configuration relaxes to a new equilibrium, consistent with those cases in which the linked structure is observed to persist stably after the interaction has passed. In the much more complex magnetic environment of the solar corona, on the other hand, newly linked filaments with such aneurysms sometimes are observed to erupt promptly and violently. The removal of the restraining arcade fields, by reconnection with the external field of the corona, is likely necessary for eruption to occur. This research was supported by NASA and ONR. Title: The magnetic nature of wide EUV filament channels and their role in the mass loading of CMEs Authors: Aulanier, G.; Schmieder, B. Bibcode: 2002A&A...386.1106A Altcode: Previous works have shown that dark and wide EUV filament channels observed at lambda < 912 Å, are due to absorption of EUV lines in cool plasma condensations that are not observed in Hα . We extend this interpretation and we address the issue of the possible injection of their mass into CMEs, through the magneto-hydrostatic modeling in 3D of one filament observed both in Hα and in EUV. The model parameters are fixed so as to match the Hα observations only. Further comparison of the model with the EUV observations reveal the magnetic nature of the absorbing plasma condensations. They are formed in magnetic dips, as for the filament itself. Opacity ratios and the hydrostatic condition imply that the dips must be filled by cool material up to 1700 km, which increases the filament mass by 50% as compared to Hα estimations. Far from the filament, the absorbing condensations are located below 4 Mm in altitude above the photosphere, on the edge of weak photospheric parasitic polarities, within the lower parts of long field lines overlaying the filament. By physical analogy with Hα filament feet, we redefined these extended regions as EUV feet. The broadening of the EUV filament channel is dominated by EUV feet, while the larger filling of dips plays a non-negligible but minor role. Further implications from this work are discussed, on the visibility and the geometry of the condensations, on the existence of EUV filament channels in the absence of filaments, on the loading of cool material into filament feet through bald patch reconnection and on the complex geometry of the upper prominence-corona transition region. The magnetic topology implies that during the filament eruption, the mass of its wide EUV feet will not contribute to the CME, whereas the extra mass provided by the large filling of dips in the filament flux tube will be loaded into the CME. Title: The Magnetic Helicity Injected by Shearing Motions Authors: Démoulin, P.; Mandrini, C. H.; Van Driel-Gesztelyi, L.; Lopez Fuentes, M. C.; Aulanier, G. Bibcode: 2002SoPh..207...87D Altcode: Photospheric shearing motions are one of the possible ways to inject magnetic helicity into the corona. We explore their efficiency as a function of their particular properties and those of the magnetic field configuration. Based on the work of M. A. Berger, we separate the helicity injection into two terms: twist and writhe. For shearing motions concentrated between the centers of two magnetic polarities the helicity injected by twist and writhe add up, while for spatially more extended shearing motions, such as differential rotation, twist and writhe helicity have opposite signs and partially cancel. This implies that the amount of injected helicity can change in sign with time even if the shear velocity is time independent. We confirm the amount of helicity injected by differential rotation in a bipole in the two particular cases studied by DeVore (2000), and further explore the parameter space on which this injection depends. For a given latitude, tilt and magnetic flux, the generation of helicity is slightly more efficient in young active regions than in decayed ones (up to a factor 2). The helicity injection is mostly affected by the tilt of the AR with respect to the solar equator. The total helicity injected by shearing motions, with both spatial and temporal coherence, is at most equivalent to that of a twisted flux tube having the same magnetic flux and a number of turns of 0.3. In the solar case, where the motions have not such global coherence, the injection of helicity is expected to be much smaller, while for differential rotation this maximum value reduces to 0.2 turns. We conclude that shearing motions are a relatively inefficient way to bring magnetic helicity into the corona (compared to the helicity carried by a significantly twisted flux tube). Title: Prominence Magnetic Dips in Three-Dimensional Sheared Arcades Authors: Aulanier, G.; DeVore, C. R.; Antiochos, S. K. Bibcode: 2002ApJ...567L..97A Altcode: We calculate the distribution of field-line dips in the three-dimensional sheared arcade model for prominence/filament magnetic fields. We consider both moderately and highly sheared configurations computed by fully time-dependent three-dimensional MHD simulations in which the field was relaxed to a static equilibrium end state. In agreement with previous low spatial resolution measurements of the magnetic field inside prominences, we find that for all configurations, the field in the great majority of the calculated dips exhibits inverse polarity. But for each configuration we also find well-defined narrow regions with stable dips of normal polarity. These tend to be located on the edges of the filament ends and at the top of the central part of the prominence. This distinctive mixture of normal/inverse polarity dips that we find in sheared arcades is not likely to be present in twisted flux rope prominence models. Therefore, our results provide a rigorous and unique observational test that can distinguish between the two classes of models, as well as new predictions for future high spatial resolution spectropolarimetric observations of filaments and prominences. Title: What is the source of the magnetic helicity shed by CMEs? The long-term helicity budget of AR 7978 Authors: Démoulin, P.; Mandrini, C. H.; van Driel-Gesztelyi, L.; Thompson, B. J.; Plunkett, S.; Kovári, Zs.; Aulanier, G.; Young, A. Bibcode: 2002A&A...382..650D Altcode: An isolated active region (AR) was observed on the Sun during seven rotations, starting from its birth in July 1996 to its full dispersion in December 1996. We analyse the long-term budget of the AR relative magnetic helicity. Firstly, we calculate the helicity injected by differential rotation at the photospheric level using MDI/SoHO magnetograms. Secondly, we compute the coronal magnetic field and its helicity selecting the model which best fits the soft X-ray loops observed with SXT/Yohkoh. Finally, we identify all the coronal mass ejections (CMEs) that originated from the AR during its lifetime using LASCO and EIT/SoHO. Assuming a one to one correspondence between CMEs and magnetic clouds, we estimate the magnetic helicity which could be shed via CMEs. We find that differential rotation can neither provide the required magnetic helicity to the coronal field (at least a factor 2.5 to 4 larger), nor to the field ejected to the interplanetary space (a factor 4 to 20 larger), even in the case of this AR for which the total helicity injected by differential rotation is close to the maximum possible value. However, the total helicity ejected is equivalent to that of a twisted flux tube having the same magnetic flux as the studied AR and a number of turns in the interval [0.5,2.0]. We suggest that the main source of helicity is the inherent twist of the magnetic flux tube forming the active region. This magnetic helicity is transferred to the corona either by the continuous emergence of the flux tube for several solar rotations (i.e. on a time scale much longer than the classical emergence phase), or by torsional Alfvén waves. Title: Vertical structure of sunspots from THEMIS observations Authors: Eibe, M. T.; Aulanier, G.; Faurobert, M.; Mein, P.; Malherbe, J. M. Bibcode: 2002A&A...381..290E Altcode: We have analysed two-dimensional spectro-polarimetric data taken with the MSDP observing mode of THEMIS in the Na I D1 line to investigate the height variation of the magnetic field in sunspot umbrae. From the Zeeman-induced circular polarization measured at individual MSDP channels within the line profile, maps of the longitudinal magnetic field have been computed. A method based on Response Functions has been developed to estimate the depth in the atmosphere at which the Zeeman measurements are originated, thus providing the line-of-sight field at different altitudes in the photosphere. The magnetogram corresponding to the deepest level has served as a boundary condition to perform the potential field extrapolation into the corona. We have found that the spatial distribution of vertical field gradient contours predicted from extrapolation is in qualitatively good agreement with that inferred from observations. Quantitatively, however, the longitudinal field gradients obtained with both methods differ about one order of magnitude, being larger for observations. The origin of this discrepancy has been discussed with respect to possible observation biases, as well as to idealizations used for field extrapolation. This is a crucial problem to be addressed in future work, and may have important implications for the physics of how the magnetic field evolves through sunspots and how the flux is distributed in the corona. Title: Energetics of the 18 May 1994 brightening event Authors: Tang, Y. H.; Li, Y. N.; Fang, C.; Schmieder, B.; Aulanier, G.; Demoulin, P. Bibcode: 2002AdSpR..30..557T Altcode: By using YOHKOH soft X-ray images, vector magnetograms and Hα filtergrams, the energetics of the brightening event of May 18, 1994 has been studied. It occurred in a nearly potential magnetic configuration as shown by the comparison between the magnetic extrapolation(linear force free field) and the large scale soft X-ray loops. This event is related to the emergence of a new magnetic flux. The brightening points of Hα and soft X-ray are located at computed separatrices associated with field lines which are tangent to the photosphere. This brightening may be a signature of reconnection taking place between the pre-existing non-potential loops and the new emerging small loops. The magnetic energy provided by reconnection is likely to be stored in the non-potential loops and shearing emerging flux. A phenomenological model is offered. Title: Helicity Loading and Dissipation: The Helicity Budget of AR 7978 from the Cradle to the Grave Authors: van Driel-Gesztelyi, L.; Démoulin, P.; Mandrini, C. H.; Plunkett, S.; Thompson, B.; Kövári, Zs.; Aulanier, G.; Young, A.; López Fuentes, M.; Poedts, S. Bibcode: 2002mwoc.conf..143V Altcode: An isolated active region was observed on the Sun during seven rotations, starting in July 1996. I will present a study of its magnetic field, concentrating on its helicity budget. The photospheric field is extrapolated into the corona in a linear force-free approach, using SOHO/MDI magnetograms and Yohkoh/SXT images, allowing us to compute, in a crude way, the relative coronal magnetic helicity of the active region. Using the observed magnetic field distribution (SOHO/MDI magnetograms) we also calculate the helicity injected by the differential rotation during seven solar rotations. Finally, using SOHO/LASCO and EIT as well as Yohkoh/SXT observations, we identify all the 26 CMEs which originated from this active region during its lifetime and using average values of the field and radius of magnetic clouds, we estimate the helicity which should be shed via CMEs. We compare these three values to evaluate the importance of the differential rotation relative to twisted flux emergence as a source of magnetic helicity. We find that the differential rotation can neither provide enough helicity to account for the diagnosed coronal heicity values, nor for the helicity carried away by CMEs. We suggest that the main source of the magnetic helicity must be the inherent twist of the magnetic flux tube forming the active region. This magnetic helicity is transferred to the corona either by a slow continuous emergence of the flux tube or by torsional Alfven waves, during several solar rotations. Title: What can we learn from magnetic extrapolation above active region and filament? Authors: Schmieder, B.; Aulanier, G. Bibcode: 2002cosp...34E.458S Altcode: 2002cosp.meetE.458S Observations of the Sun is done up to now in 2D. Magnetic extrapolation technique allows us to have a 3D view of the magnetic field lines. Global 3D structures can be visualized by using such computations. Assuming linear force-free-field configuration we have analyzed different active regions and filaments. In the lfff approximation thevalue is constant for all the field lines and is chosen in order to give the best global or local fit to the structures projected on the solar surface observed in H, in UV lines (EIT, TRACE) or in X rays (Yohkoh/SXT). With some examples we will show some important results that we have obtained: 1. the existence of a vertical gradient of the magnetic shear above active region with filament (Schmieder et al 1996). 2. the existence of the same value offitting the sheared filament and the spiral structure around a sunspot, the existence of dips in the magnetic field strength at the location of the filament channel in an AR (Schmieder et al 1991). 3. the validity of chirality laws with dextral/sinistral filaments and negative /positiveparameter according to the hemisphere (Aulanier et al 1999, 2000, Aulanier and Schmieder 2002) Title: Multi-wavelength research for the solar flare and associated coronal mass ejection event on 1999 October 14 Authors: Wang, S.; Maia, D.; Pick, M.; Aulanier, G. Bibcode: 2002cosp...34E.102W Altcode: 2002cosp.meetE.102W We present the results of our investigation on multi-wavelength observations of the strong solar flare (X1/1N) that occured in complex active regions on October 14, 1999. In association with this event a partial halo coronal mass ejection (CME) is observed by the Solar and Helospheric Observatory Large-Angle Spectrometric coronagraph (LASCO/SOHO). The observation of the CME is preceded by radio bursts detected by the Nançay radioheliograph (NRH) and other radio telescapes. In the decimeter range, the radioheliograph shows two noise storm (NS) sources prior to the event. After the onset of the flare four emitting sources are detected contributing to the radio outburst. Two of them were close to the two NS sources, but the other two were in the west part of the complex active regions. Also a Moreton wave is indentified in Hα data which propagated closely associated in time with the position evolution of the radio emitting source in southeast direction. The manifestations of global radio spectrum from 7.6 GHz to 1 MHz are a series of fast drifring bursts, metric wave type II burst and some type III bursts. The time of the CME at one solar radius is inferred that indicate the fact the CME is launched in close temporal proximity with the flare and the radio bursts. Then a line force-free field extrapolation is preformed using the Michelson Doppler Imager (MDI) data as boundary conditions. The topology of the reconstruction fields coincide well with the radio emitting sources. We interpret this event including CME as the results of a certain magnetic instability. Title: Relationships between CME's and prominences Authors: Schmieder, B.; van Driel-Gesztelyi, L.; Aulanier, G.; Démoulin, P.; Thompson, B.; De Forest, C.; Wiik, J. E.; Saint Cyr, C.; Vial, J. C. Bibcode: 2002AdSpR..29.1451S Altcode: We have studied the erupting prominences which were associated with coronal mass ejections during a series of campaigns involving both spacecraft and ground-based observatories. The evolution of the physical conditions within the prominences was established from Hα and magnetic field observations. Particular attention ahs been paid to the presence of mixed amgnetic polarity in the filament channel, the evolution of the shear of the large-scale magnetic field, and the existence of multiple magnetic inversion lines. We conclude that reconnection of large-scale coronal magnetic fields is responsible for both the CME and filament eruption. Title: Is the CME magnetic energy stored in coronal flux ropes ? Authors: Aulanier, G. Bibcode: 2001AGUSM..SH41C06A Altcode: MHD theory, coronograph and in situ solar wind observations are consistent with a flux rope like topology for many CME's. However, this does not permit to conclude that the pre-eruptive coronal magnetic field does have this topology. This is because any erupting fields must lead to the formation of a vertical current sheet, in which reconnection can form the so-called coronal postflare loops, as well as a large flux rope which surrounds the erupting fields, which are ejected alltogether. The only physical property of the pre-eruptive fields which is required for triggering a CME is that they must be ``stressed'', i.e. they must contain strong electric currents. As prominences are the largest and best observed stressed magnetic configurations of the solar atmosphere, and as they often lead to CME's, they provide a unique opportunity for studying the pre-CME magnetic field topology. Based on a compilation of old and recent results, I will show that both magnetic and multi-wavelength observations strongly support a uniform flux rope like topology for prominences. Then, I will compare the predicted flux/twist/helicity with the values measured in interplanetary magnetic clouds, and I will discuss the role of the large scale fields for the development of CME's. I will end on possible theoretical and observational prospectives which should be achieved for the study of stressed fields in prominences and in active regions (e.g. delta-spots). Title: Magnetic Evolution of a Long-Lived Active Region: The Sources of Magnetic Helicity Authors: Mandrini, C. H.; Démoulin, P.; van Driel-Gesztelyi, L.; Aulanier, G.; Thompson, B.; Plunkett, S.; Kövári, Zs. Bibcode: 2001ASPC..248..139M Altcode: 2001mfah.conf..139M No abstract at ADS Title: Model Prediction for an Observed Filament Authors: Aulanier, G.; Srivastava, N.; Martin, S. F. Bibcode: 2000ApJ...543..447A Altcode: This paper presents the results of a ``blind test'' for modeling the structure of an observed filament using the three-dimensional magnetohydrostatic model recently developed by Aulanier et al. in 1999. The model uses a constant shear α, and it takes into account the effects of pressure and gravity. The test consisted of predicting the structure of a filament (observed in the southern hemisphere) with a minimum observational input: only a line-of-sight magnetogram, with a straight line drawn on it to show the location of the filament, was provided. The filament was chosen by the observers (N. S. and S. F. M.) because it had a definite overall left-handed structural pattern known as sinistral, but the direction of component of the magnetic field along the filament axis was uncertain from the combination of Hα data and magnetograms. The modeler (G. A.) evaluated and fixed the values of some of the free parameters of the model while some others were varied in reasonable ranges. The Hα image of the filament was revealed only after the modeling. For α>0, the three-dimensional distribution of magnetic dips computed by the model fairly well reproduces the structure of the filament and its barbs. Moreover, the models for which α<0 do not match well the observations. This study then shows the first successful theoretical prediction for the magnetic field of an observed filament. It shows that the method based on the Aulanier et al. model is a powerful tool, not only for purposes of modeling, but also for prediction of the chirality, helicity, and morphology of observed filaments. Title: Initiation of CMEs: the role of magnetic twist Authors: van Driel-Gesztelyi, L.; Manoharan, P. K.; Démoulin, P.; Aulanier, G.; Mandrini, C. H.; Lopez-Fuentes, M.; Schmieder, B.; Orlando, S.; Thompson, B.; Plunkett, S. Bibcode: 2000JASTP..62.1437V Altcode: 2000JATP...62.1437V Recent multiwavelength observations, modelling results and theoretical developments indicate the importance of twisted magnetic configurations in solar active regions (ARs) in the initiation of coronal mass ejections (CMEs). Through multiwavelength analysis of a few representative events we make an attempt to provide constraints for CME models. The two events presented here in detail start with the expansion of sigmoids (S- or inverse S-shaped loops) observed in soft X-rays. Both events (on 25 October /1994 and 14 October /1995) occurred before the launch of the SOHO spacecraft, but indirect evidences (i.e. signatures of an outward propagation traced up to /~20 solar radii and an associated magnetic cloud) suggest that both of them were related to CMEs. We show evidence that sigmoids are the coronal manifestations of twisted magnetic flux tubes, which start expanding presumably due to a loss of equilibrium. It is noteworthy that the analysed CMEs occurred in a complex (not simply bipolar) magnetic environment and in all cases we found evidences of the interaction (magnetic reconnection) with the surrounding fields. We propose a scenario for sigmoid expansion related CME events and suggest that twisted magnetic configurations are good candidates for being source regions of CMEs. Title: The Topology and Evolution of the Bastille Day Flare Authors: Aulanier, G.; DeLuca, E. E.; Antiochos, S. K.; McMullen, R. A.; Golub, L. Bibcode: 2000ApJ...540.1126A Altcode: On 1998 July 14, a class M3 flare occurred at 12:55 UT in AR 8270 near disk center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a δ spot. Mees vector magnetograms show a strong shear localized near a portion of the closed neutral line around the parasitic polarity of the δ spot. Observations of the flare in 171, 195, and 1600 Å have been obtained by TRACE, with ~=40 s temporal and 0.5" spatial resolutions. They reveal that small-scale preflare loops above the sheared region expanded and disappeared for more than 1 hr before flare maximum. During the flare, bright loops anchored in bright ribbons form and grow. This occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the active region. This suggests that the flare was eruptive and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, with its associated ``spine'' field line, and its ``fan'' surface surrounding the parasitic polarity. We show that while the whole event occurs, the intersections of the ``fan'' and the ``spine'' with the photosphere brighten and move continuously. The interpretation of the event shows that the magnetic evolution of the eruptive flare is strongly coupled with its surrounding complex topology. We discuss evidence supporting a ``magnetic breakout'' process for triggering this eruptive flare. We finally conclude that multipolar fields cannot be neglected in the study and modeling of the origin of CMEs in the corona. Title: Hα and Soft X-Ray Brightening Events Caused by Emerging Flux Authors: Tang, Y. H.; Li, Y. N.; Fang, C.; Aulanier, G.; Schmieder, B.; Demoulin, P.; Sakurai, T. Bibcode: 2000ApJ...534..482T Altcode: By using Yohkoh soft X-ray images, vector magnetograms, and Hα filtergrams, the brightening event that occurred on 1994 May 18 has been studied in detail. It occurred in a nearly potential large-scale magnetic configuration as shown by the comparison between the magnetic extrapolation (linear force-free field) and the large-scale soft X-ray loops. This event is related to the emergence of a new magnetic flux of about 3×1020 Mx. The impulsive enhancement of the emerging flux occurs about 20 minutes before the peaks of the Hα and soft X-ray brightening and lasts for about 10 minutes. This brightening may be a signature of reconnection taking place between the preexisting nonpotential loops and the new emerging small loops. The magnetic energy provided by reconnection is likely to be stored in the nonpotential loops and the emerging flux as implied by the vector magnetograms. By using the electron temperature and the electron density of the brightening event derived from the analysis of the Yohkoh data, an energy budget has been estimated. The result indicates that the energy needed can be reasonably provided by magnetic reconnection. Title: The Topology and Evolution of the Bastille Day Flare Observed by TRACE Authors: Aulanier, G.; Antiochos, S. K.; DeLuca, E. E.; McMullen, R. A.; Golub, L. Bibcode: 2000SPD....31.1402A Altcode: 2000BAAS...32..846A On July 14, 1998, a class M3 flare occurred at 12:55 UT in AR 8270 near disc center. Kitt Peak line-of-sight magnetograms show that the flare occurred in a δ -spot. Mees vector magnetograms show a strong shear localized near a portion of the closed neutral line around the parasitic polarity of the δ -spot. Observations of the flare in 171 Angstroms, 195 Angstroms and 1600 Angstroms have been obtained by TRACE, with ~= 40 s temporal and 0.5 arcsec spatial resolutions. They reveal that small-scale pre-flare loops above the sheared region expanded and disappeared for more than one hour before flare maximum. During the flare, bright loops anchored in bright ribbons form and grow. This occurs while large-scale dimmings, associated with large expanding loops, develop on both sides of the AR. This suggests that the flare was eruptive, and was accompanied by a coronal mass ejection (CME). Magnetic field extrapolations reveal the presence of a null point in the corona, with its associated ``spine'' field line, and its ``fan'' surface surrounding the parasitic polarity. We show that while the whole event occurs, the intersections of the ``fan'' and the ``spine'' with the photosphere brighten and move continuously. The interpretation of the event shows that the magnetic evolution of the eruptive flare is strongly coupled with its surrounding complex topology. We discuss evidence supporting a ``magnetic breakout'' process for triggering this eruptive flare. We finally conclude that multipolar fields cannot be neglected in the study and modeling of the origin of CMEs in the corona. This work is supported, at SAO by a NASA contract to Lockheed-Martin, and at NRL by NASA and ONR. Title: Brightening Event in Hα and Soft X-Ray on May 18, 1994 Authors: Tang, Y. H.; Li, Y. N.; Schmieder, B.; Aulanier, G.; Demoulin, P.; Fang, C.; Sakurai, T. Bibcode: 2000AdSpR..25.1829T Altcode: By using Yohkoh soft X-ray images, vector magnetograms and Hα filtergrams, a brightening event that occurred on May 18, 1994 has been studied in detail. It occurred in a nearly potential large-scale magnetic configuration as shown by the comparisons between the magnetic extrapolations (linear force-free field) and the large-scale soft X-ray loops. This brightening event observed in Hα line and soft X-ray seems to be related to new emerging magnetic flux with an amount of photospheric magnetic flux of about 3× 1020 Mx. The emerging flux increases obviously about 20 minutes before the Hα and soft X-ray brightening, and lasts for about 103 s. This brightening may be a signature of reconnection taking place between the pre-existing loop and the new emerging small loops Title: 3-D Magnetic Configurations for Filaments and Flares: The Role of ``Magnetic Dips'' and ``Bald Patches'' Authors: Aulanier, G.; Schmieder, B.; van Driel-Gesztelyi, L.; Kucera, T.; Démoulin, P.; Fang, C.; Mein, N.; Vial, J. -C.; Mein, P.; Tang, Y. H.; Deforest, C. Bibcode: 2000AdSpR..26..485A Altcode: The 3-D magnetic configuration of a filament and of a low energy flare is reconstructed, using linear mag- netohydrostatic (lmhs) extrapolations. In both cases, we find observational signatures of energy release at the locations of computed ``bald patches'' separatrices, characterised by field lines which are tangent to the photosphere.The filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si IV with SOHO/SUMER. It is modeled as a twisted flux-tube deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed dipped field lines are in good agreement with the shape of the filament and its feet observed in Hα. Some ``bald patches'' (BPs) are present where the distribution of dips reaches the photosphere. We show that some of the large scale field lines rooted in BPs can be related to bright fine structures in Si IV. We propose that the plasma there is heated by ohmic dissipation from the currents expected to be present along the BP separatrices.The flare was observed on May 18, 1994, in soft X-rays with Yohkoh/SXT, and in Hα at Mitaka (Japan). The magnetic field is directly extrapolated from a photospheric magnetogram from Kitt Peak Observatory. The intersections with the photosphere of the computed separatrices match well the bright Hα ribbons. The later are associated to three BPs, with overlaying dipped field lines. We show that enhanced densities are present in these dips, which can be correlated with dark Hα fibrils.Both cases show the importance of dipped field lines and BPs in the solar atmosphere. Energy release via ohmic dissipation as well as reconnection along BP separatrices is proposed to provide heating observed as UV brightenings in filament channels and even as small flares Title: Cme Associated with Transequatorial Loops and a Bald Patch Flare Authors: Delannée, C.; Aulanier, G. Bibcode: 1999SoPh..190..107D Altcode: We study a flare which occurred on 3 November 1997 at 10:31 UT in the vicinity of a parasitic polarity of AR 8100. Using SOHO/EIT 195 Å observations, we identify the brightening of thin transequatorial loops connecting AR 8100 and AR 8102, and dimmings located between the two active regions. Difference images highlight the presence of a loop-like structure rooted near the flare location usually called an EIT wave. The coronal magnetic field derived from potential extrapolations from a SOHO/MDI magnetogram shows that the topology is complex near the parasitic polarity. There, a `bald patch' (defined as the locations where the magnetic field is tangent to the photosphere) is present. We conclude that the flare was a `bald patch flare'. Moreover, the extrapolation confirms that there is a large coronal volume filled with transequatorial field lines interconnecting AR 8100 and AR 8102, and overlaying the bald patch. Title: The Role of "Magnetic Dips" and "Bald Patches" for a Filament Observed by SOHO and GBO Authors: Aulanier, G.; Schmieder, B.; Kucera, T.; van Driel-Gesztelyi, L.; Démoulin, P.; Mein, N.; Vial, J. -C.; Mein, P. Bibcode: 1999ASPC..184..291A Altcode: The studied filament was observed on Sept. 25, 1996, in Hα with the MSDP on the German VTT, Tenerife, as well as in Si IV with SOHO/SUMER. The 3-D magnetic configuration of the filament channel is reconstructed, using linear magnetohydrostatic (lmhs) extrapolations from a SOHO/MDI magnetogram, which is modified by a background magnetic component constraining a twisted flux-tube. This flux-tube is deformed by the magnetic polarities observed with SOHO/MDI. The shape and location of the computed "dipped field lines" are in good agreement with the shape of the filament and its feet observed in Hα. Some "bald patches" (BPs) are present where the distribution of dips reaches the photosphere. We find observational signatures in Si IV brightenings of energy release at the locations of computed "bald patch separatrices", defined by field lines which are tangent to the photosphere. We propose that the plasma is there heated by ohmic dissipation from the expected currents in the BP separatrices. The results show the importance of "dipped field lines" and "bald patches" in filament channels. Title: Long-Term Magnetic Evolution of an AR and its CME Activity Authors: van Driel-Gesztelyi, L.; Mandrini, C. H.; Thompson, B.; Plunkett, S.; Aulanier, G.; Démoulin, P.; Schmieder, B.; de Forest, C. Bibcode: 1999ASPC..184..302V Altcode: Using SOHO/MDI full-disc magnetic maps, we follow the magnetic evolution of a solar active region for several months in the period of July-November 1996. We extrapolate the photospheric magnetic fields in the linear force-free approximation and match the modelled field lines with the soft X-ray loops observed with the Yohkoh/SXT in order to diagnose the coronal magnetic shear. We find that while the turbulent motions diffuse the flux, the differential rotation, and possibly twisted flux emergence, increase the magnetic shear. Flares are observed during the first three rotations, while CME events (observed by SOHO/EIT and LASCO) originate from this AR from its emergence throughout its decay. Several early CMEs, while none of the late CMEs, are related to flare events above the GOES B1 level. We find that the late CMEs occur when the magnetic shear, after accumulating for four rotations, reaches a high level and saturates. We propose that CME activity serves as a valve through which the AR could get rid of excess shear and helicity. Title: Some Advances in 3D-Magnetic Field Topology: an Observed Case of a "Bald Patch" Flare Authors: Démoulin, P.; Aulanier, G.; Schmieder, B. Bibcode: 1999ASPC..184...65D Altcode: The aim of this work is to bring observational evidences of the possible role of the "bald patch" topology in flaring events. A bald patch (or BP) is present along the photospheric inversion line where the field lines are curved-up, so when magnetic dips are present. The set of field lines associated to the BP define a separatrix where a current sheet may form. We find such configuration in AR 7722 where, on May 18, 1994, a sub-flare was observed in X-rays by Yohkoh/SXT and in Hα at NAOJ. Using the magnetohydrostatic equations derived by Low (1992), we model the magnetic field configuration by extrapolation of the Kitt Peak photospheric field, taking into account the effects of pressure and gravity. Hα flare kernels are shown to be located at the lower parts of the computed separatrices associated to bald patches (BPs). This is an evidence that BPs can be involved in flares, and that current sheets can be dissipated in low levels of the solar atmosphere. Title: A new emerging flux and brightening event. Authors: Li, Yining; Tang, Yuhua; Aulanier, G.; Schmieder, B.; Demoulin, P.; Fang, Cheng Bibcode: 1999PPMtO..18..131L Altcode: By using Yohkoh soft X-ray images, vector magnetograms and Hα filtergrams, a brightening event of Hα and soft X-ray of 18 May, 1994 has been studied in detail. It is shown that the event is obviously caused by new emerging flux. The brightening points of Hα and soft X-ray are located at computed separatrices associated to field lines which are tangent to the photosphere. This brightening event may be a signature of reconnection taking place between the pre-existing loops and the new emerging small loops in the lower solar atmosphere. Title: Filament channel structures in a SI IV line related to a 3d magnetic model Authors: Kucera, T. A.; Aulanier, G.; Schmieder, B.; Mein, N.; Vial, J. -C. Bibcode: 1999SoPh..186..259K Altcode: A recent 3D magnetic model of filament support (Aulanier and Démoulin, 1998) has shown that specific morphologies derived from the model, based on SOHO/MDI magnetograms, match quite well with the observations of a filament observed in Hα and Ca ii lines with the German telescope VTT in Tenerife on 25 September 1996 (Aulanier et al., 1998, 1999a). Some predictions of this model concern the filament channel. To continue the comparison of model and data, we have investigated the same filament region observed in ultraviolet by the SOHO spectrometers SUMER and CDS. The elongated EUV fine structures in the filament channel observed in the Si iv 1393.76 Å line by SUMER have similar orientations and locations to features predicted by the model of Aulanier et al. (1999a). These regions are near the bases of field lines which tangentially join to the photosphere in so called 'bald patches' and are parts of large arcades above the filament channel. In addition, we consider the Si iv Doppler shifts in these structures and compare them to what might be expected from the model field structure. Our study also suggests that the filament has a very low opacity in Si iv, lower than that of the O v line observed by CDS. Title: New 3-D magnetic model for prominences based on dipped field lines and compared with observations Authors: Aulanier, G.; Demoulin, P. Bibcode: 1999AAS...194.3103A Altcode: 1999BAAS...31R.868A Despite of the large number of 2D and 3D models for the magnetic fields in prominences, none has been succesfully directly compared with observational data untill recently, even if many of them still satisfied some well established observational results. This has led to a never ending controversy between theorists and observers, about the relevance of dipped field lines for mass support against gravity in these objects. Here we present a new 3D magnetohydrostatic model based on dipped field lines. The main body of the filament is formed by a twisted flux rope in a bipolar region, and we show how small parasitic polarities near the neutral line lead to a local perturbation of the magnetic fields in the corona, leading to the formation of lateral dips, which appear aside from the flux rope. We show that these are formed by local expansions of the fluxe rope, as well as by the bending of some surrounding sheared field lines, toward the photospheric parasitic polarities. We suggest that these lateral dips can form lateral feet of filaments. Moreover the global magnetic configuration shows in a natural fashion a very large number of typical observational aspects of prominences. Using a magnetogram obtained by SOHO/MDI on 09/25/96 as boundary conditions, we extrapolate the magnetic field under the assumptions of the model. Comparing the 3D distribution of dips with the morphology of the filament observed in Hα on the German VTT (Tenerife) at the same time, we show a good correlation between the dips and the filament body, as well as with its lateral feet. All these results suggest that the model is well adapted for the description of the magnetic field in filaments and their close environement. Furthermore its direct comparison with observations provide evidence in favor of dipped field lines in prominences. Title: 3-D magnetic configurations supporting prominences. III. Evolution of fine structures observed in a filament channel Authors: Aulanier, G.; Démoulin, P.; Mein, N.; van Driel-Gesztelyi, L.; Mein, P.; Schmieder, B. Bibcode: 1999A&A...342..867A Altcode: On September 25() th 1996, a quiescent filament located near the center disc (S2, E5) was observed on the German VTT (Tenerife) with the MSDP instrument, in the Hα line center and wings. SOHO/MDI line-of-sight magnetograms were co-aligned with the MSDP images, showing the position and evolution of the lateral feet of the filament in the vicinity of the parasitic magnetic polarities observed in the filament channel. Using the assumptions developed in the previous papers of this series related to the reconstruction of the 3-D magnetic configuration of filaments, we perform ``linear magnetohydrostatic'' extrapolations (taking into account the effects of plasma pressure and gravity) on the SOHO/MDI magnetograms. The main hypothesis is the presence of a twisted flux-tube located above the photospheric inversion line. Assuming that the parameters of the model do not need to be significantly modified during the evolution of the configuration for a duration of 1 day, we have shown that the 3-D distribution of dipped field lines is well correlated with Hα dark absorbing features in the filament channel: the filament itself, its lateral feet and some of the surrounding dark fibrils. In this way we confirm what was suggested in our earlier papers, i.e. that the feet are composed of the dipped portions of some field lines, which form a continuous pattern from the corona to the photosphere. We propose the same explanation for the magnetic configuration of some of the dark Hα fibrils in the channel. We show that the plasma effects are not responsible for the existence of most of the magnetic dips, however their inclusion helps to get a better correspondence between the model and the observations. We find that the average Hα Doppler velocities associated with the filament and with the chromospheric fibrils is of the order of a few hundred m s(-1) (though it can go locally up to 3 km s(-1) in the filament). These upward velocities are consistent with a quasi-static evolution of the magnetic configuration and with the support of dense plasma in magnetic dips. Title: The NOAA AR 6718 magnetic field extrapolation with localized current filaments. Authors: Karlický, M.; Démoulin, P.; Aulanier, G.; van Driel-Gesztelyi, L.; Hénoux, J. C.; Jirička, K. Bibcode: 1999joso.proc...97K Altcode: The 3-D extrapolation of magnetic field lines of the July 11, 1991 Kitt Peak magnetogram shows a differential magnetic field shear in the NOAA 6718 active region. A new combined extrapolation technique, which includes localized current filaments is suggested and applied in modelling of this shear. First, the potential field extrapolation is made and then force-free current paths for several current filaments in specific positions are computed. It is shown that with the electric current increase the localized magnetic field shear is increasing. The total electric current in 16 current filaments is considered up to 1.2×1012A. Title: Magnetohydrostatic Model of a Bald-Patch Flare Authors: Aulanier, G.; Démoulin, P.; Schmieder, B.; Fang, C.; Tang, Y. H. Bibcode: 1998SoPh..183..369A Altcode: On 18 May, 1994, a subflare was observed in AR 7722 in X-rays by Yohkoh/SXT and in Hα at National Astronomical Observatory of Japan. The associated brightenings are due to small-scale magnetic energy release, triggered by parasitic fluxes emerging and moving at the edge of leading sunspots. Using the magnetohydrostatic equations derived by Low (1992), we model the magnetic field configuration by extrapolation of the Kitt Peak photospheric field, taking into account the effects of pressure and gravity. Hα flare kernels are shown to be located at computed separatrices associated with field lines which are tangent to the photosphere, namely 'bald patches' (BPs). This is evidence that BPs can be involved in flares, and that current sheets can be dissipated in low levels of the solar atmosphere. The presence of dense plasma which is supported against gravity in the magnetic dips above BPs is correlated to dark elongated features observed in Hα. Mass flows in these flat fibrils are discussed in the context of energy release in the BP separatrices. The effect of the plasma on the computed magnetic configuration is shown to be of secondary importance with respect to the topology of the field. Title: 3-D magnetic configurations supporting prominences. II. The lateral feet as a perturbation of a twisted flux-tube Authors: Aulanier, G.; Demoulin, P.; van Driel-Gesztelyi, L.; Mein, P.; Deforest, C. Bibcode: 1998A&A...335..309A Altcode: In a previous paper we have shown that a twisted flux-tube is the most probable magnetic configuration supporting prominences. The model interprets many observations in a natural way (in particular the magnetic measurements in prominences and the chirality properties). Moreover, prominence feet appear as a direct consequence of the parasitic polarities present in the filament channel. Here we investigate further the link between feet and parasitic polarities by modelling explicitly these polarities. We show that the prominence lateral feet appear naturally, above secondary photospheric inversion lines and we describe the morphological change of feet as parasitic polarities evolve. This approach is applied to an observed filament in Hα with the MSDP on the German VTT (Tenerife) where SOHO/MDI magnetograms are available. We show that the shape of the prominence is defined by the distribution of the dips in the computed magnetic configuration. Then we analyse the topology of the magnetic field using the quasi-separatrix layers (QSLs) method. We describe the basic changes in the topology as the parasitic polarities evolve, in particular how the configuration pass from an OX to an OF topology. We find a correspondance between the computed QSLs and some of the chromospheric brightenings, observed around the feet of filaments in the y line (Ca II, 8542 Angstroms). It confirms the deduced magnetic configuration and shows that energy release is present at a low level in the complex topology of the filament configuration. Title: Modelisation tridimensionnelle de configurations magnetiques a l'equilibre appliquees aux observations d'eruptions et de protuberances solaires Title: Modelisation tridimensionnelle de configurations magnetiques a l'equilibre appliquees aux observations d'eruptions et de protuberances solaires Title: Three-dimensional modeling of equilibrium magnetic configurations applied to observations of solar eruptions and protuberances; Authors: Aulanier, Guillaume Bibcode: 1998PhDT.......229A Altcode: No abstract at ADS Title: Evolution of the Magnetic Field and Chromospheric Fine Structure in a Filament Channel Authors: van Driel-Gesztelyi, L.; Mein, P.; Mein, N.; Schmieder, B.; Malherbe, J. -M.; Aulanier, G.; Démoulin, P.; Deforest, C.; Staiger, J. Bibcode: 1998ASPC..155..321V Altcode: 1998sasp.conf..321V No abstract at ADS Title: Non potentiality of coronal loops above active regions Authors: Aulanier, G.; Schmieder, B.; Démoulin, P.; van Driel-Gesztelyi, L.; Deforest, C. Bibcode: 1998ASPC..155..105A Altcode: 1998sasp.conf..105A No abstract at ADS Title: Filament Disparition Brusque and CME - September 25-26, 1996 Event Authors: van Driel-Gesztelyi, L.; Schmieder, B.; Aulanier, G.; Demoulin, P.; Martens, P. C. H.; Zarro, D.; Deforest, C.; Thompson, B.; St. Cyr, C.; Kucera, T.; Burkepile, J. T.; White, O. R.; Hanaoka, Y.; Nitta, N. Bibcode: 1998ASPC..150..366V Altcode: 1998IAUCo.167..366V; 1998npsp.conf..366V No abstract at ADS Title: 3-D Modelling of a Filament Observed in Hα and with SOHO Authors: Aulanier, G.; Schmieder, B.; Démoulin, P.; Mein, N.; van Driel-Gesztelyi, L.; Mein, P.; Vial, J. C.; Deforest, C. Bibcode: 1998ESASP.417..217A Altcode: 1998cesh.conf..217A No abstract at ADS Title: 3-D Modelling of a Filament Observed in Hα and with SOHO/MDI Authors: Aulanier, G.; Démoulin, P.; van Driel-Gesztelyi, L.; Mein, P.; Deforest, C. Bibcode: 1998ASPC..155..326A Altcode: 1998sasp.conf..326A No abstract at ADS Title: 3-D Twisted Flux-Tube in a Linear Force-Free Equilibrium Authors: Aulanier, G.; Demoulin, P. Bibcode: 1998ASPC..150...86A Altcode: 1998npsp.conf...86A; 1998IAUCo.167...86A No abstract at ADS Title: 3-D magnetic configurations supporting prominences. I. The natural presence of lateral feet Authors: Aulanier, G.; Demoulin, P. Bibcode: 1998A&A...329.1125A Altcode: It is now commonly accepted that prominence plasma is supported in magnetic dips, in particular in twisted flux-tubes. But present two-dimensional models are unable to explain the observed presence and structure of prominences feet. This requires three-dimensional models. We modeled the field using linear force-free field equations. Combining a small number of harmonics, and using observational constraints, we have found the area in the parameter space where prominences are likely to be present. Then, adding 3-D harmonics, we show that feet appear periodically underneath the prominence body. For great helicity, the parameter space is mostly fulfilled by configurations which have feet alternating between both sides of the prominence axis, as observed. The theoretical photospheric field has a quasi-bipolar pattern and the prominence stands above a magnetic corridor containing only small parasitic polarities. The lateral feet are formed by dips in the vicinity of these small polarities. These configurations show in a natural fashion a number of well-established as well as more recent observational aspects of prominences, in particular the vector magnetic field measurements in prominences and the chirality patterns (the dextral/sinistral, right/left bearing, skew of the overlying coronal arcade and fibril organization in prominence channels). Title: Magnetic reconnection driven by emergence of sheared magnetic field. Authors: Schmieder, B.; Aulanier, G.; Demoulin, P.; van Driel-Gesztelyi, L.; Roudier, T.; Nitta, N.; Cauzzi, G. Bibcode: 1997A&A...325.1213S Altcode: Recurrent subflares (Class C) were observed in the NOAA 7608 active region on 27 October 1993. From multi-wavelength observations (white-light, magnetic field, H-alpha, X-ray), obtained during a coordinated campaign between Pic du Midi and Yohkoh, it appears that these flares were double ribbon flares caused by new flux emergence. As the flare begins, the X-ray emission observed with Yohkoh/SXT is loop-shaped with the axis almost parallel to the magnetic inversion line, while during the flare development, X-ray loops appear at the location of the emerging flux. The extrapolation of the photospheric magnetic field in a linear force-free field configuration allows identification of the magnetic configuration given by the flares. The Hα flare ribbons are located at the intersections of the computed quasi-separatrice layers (QSLs) with the chromosphere. We show that the initial loop-shaped X-ray emission region is in fact formed by several smaller loops directed in a nearly orthogonal direction with their feet anchored close to or in the Hα ribbons. During the flare development there are X-ray loops which represent only one foot of open or largescale magnetic loops. For the studied flares the puzzling soft X-rays observations could only be understood with the help of Hα and magnetic data combined with a modeling of the coronal magnetic field. Further, from the deduced magnetic field topology, the width of the QSLs and our present knowledge of 3-D magnetic reconnection, we conclude that the flare was due to magnetic reconnection driven by emergence of sheared magnetic field impacting in the pre-existing coronal field. Title: 3-D reconnection related to new emerging flux Authors: Schmeider, B.; Démoulin, P.; Aulanier, G.; Malherbe, J. M.; van Driel-Gesztelyi, L.; Mandrini, C. H.; Roudier, T.; Nitta, N.; Harra-Murnion, L. K. Bibcode: 1997AdSpR..19.1871S Altcode: We present evidences that emergence of new flux in the lower atmosphere leads to magnetic reconnection of field lines. In a first phase the phenomenon is observed in the chromosphere by the formation of dark filaments (arch filament system) which are overlaid by bright loops visible in soft X-rays. Different types of event appear according to the magnetic field configuration and the amount of energy involved. 3-D modelling of the photospheric magnetic field provides a new tool for understanding reconnection in real configurations. The observed chromospheric and coronal loops are good diagnostics for the modelling. We document our statement by examples obtained during coordinated campaigns with the Hα Multichannel Subtractive Double Pass spectrographs-MSDP (Pic du Midi and Tenerife) and the Yohkoh instruments. Title: Magnetic reconnection driven by an emerging flux. Authors: Aulanier, G.; Démoulin, P.; Schmieder, B.; Malherbe, J. M.; van Driel-Gesztelyi, L.; Roudier, T. Bibcode: 1997joso.proc...51A Altcode: No abstract at ADS Title: Differential Magnetic Field Shear in an Active Region Authors: Schmieder, B.; Demoulin, P.; Aulanier, G.; Golub, L. Bibcode: 1996ApJ...467..881S Altcode: The three-dimensional extrapolation of magnetic field lines from a magneto gram obtained at Kitt Peak allows us to understand the global structure of the NOAA active region 6718, as observed in X-rays with the Normal Incidence X-ray Telescope (NIXT) and in Hα with the Multichannel Subtractive Double Pass spectrograph (MSDP) in Meudon on 1991 July 11. This active region was in a quiet stage. Bright X-ray loops connect plages having field strengths of ∼300 G, while Hα fibriles connect penumbrae having strong spot fields to the surrounding network. Small, intense X-ray features in the moat region around a large spot, which could be called X-ray-bright points, are due mainly to the emergence of magnetic flux and merging of these fields with surrounding ones. A set of large-scale, sheared X-ray loops is observed in the central part of the active region. Based on the fit between the observed coronal structure and the field configurations (and assuming a linear force-free field), we propose a differential magnetic field shear model for this active region. The decreasing shear in outer portions of the active region may indicate a continual relaxation of the magnetic field to a lower energy state in the progressively older portions of the AR.