[en] PNST (Programme National Soleil-Terre/Sun-Earth National Program) is dedicated to analysis of the Sun-Earth system, from generation of the solar magnetic field, flares and coronal mass ejections, until impact on the terrestrial magnetosphere, ionosphere and thermosphere. Research activities carried out in the frame of Programme National Soleil-Terre (PNST) rely on both ground-based and space-borne instruments. One of the main objectives of PNST is to stimulate coordinated studies and to optimize scientific return of these instruments. The 2009 PNST prospective colloquium comprised 9 sessions: 1 - Eruptive activity in plasmas; 2 - Particles heating and acceleration; 3 - Energy transfers at different scales and turbulence; 4 - Coupling between the different envelopes; 5 - Sun-Earth relations and space meteorology; 6 - Sun and star prototypes; 7 - Databases, services; 8 - Instrumentation; 9 - Prospective. This document is the book of abstracts of the colloquium

[en] 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.

[en] 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.

[en] 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 A (Fe IX and Fe X) channel and brightening in the 193 and 211 A (Fe XII-XIV) channels along the entire front, while there is weak 131 A (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.

[en] 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.

[en] 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.

[en] 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

[en] 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.

[en] 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.

[en] 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.