摘要：Context. The standard model for eruptive flares has been extended to
three dimensions (3D) in the past few years. This model predicts typical
J-shaped photospheric footprints of the coronal current layer, forming
at similar locations as the quasi-separatrix layers (QSLs). Such a morphology is also
found for flare ribbons observed in the extreme ultraviolet (EUV) band, and in nonlinear
force-free field (NLFFF) magnetic field extrapolations and models.
Aims. We study the evolution of the photospheric traces of the current
density and flare ribbons, both obtained with the Solar Dynamics Observatory instruments.
We aim to compare their morphology and their time evolution, before and during the flare,
with the topological features found in a NLFFF model.
Methods. We investigated the photospheric current evolution during the
06 September 2011 X-class flare (SOL2011-09-06T22:20) occurring in NOAA AR 11283 from
observational data of the magnetic field obtained with the Helioseismic and Magnetic
Imager aboard the Solar Dynamics Observatory. We compared this evolution with that of the
flare ribbons observed in the EUV filters of the Atmospheric Imager Assembly. We also
compared the observed electric current density and the flare ribbon morphology with that
of the QSLs computed from the flux rope insertion method-NLFFF model.
Results. The NLFFF model shows the presence of a fan-spine configuration
of overlying field lines, due to the presence of a parasitic polarity, embedding an
elongated flux rope that appears in the observations as two parts of a filament. The QSL
signatures of the fan configuration appear as a circular flare ribbon that encircles the
J-shaped ribbons related to the filament ejection. The QSLs, evolved
via a magnetofrictional method, also show similar morphology and evolution as both the
current ribbons and the EUV flare ribbons obtained several times during the flare.
Conclusions. For the first time, we propose a combined analysis of the
photospheric traces of an eruptive flare, in a complex topology, with direct measurements
of electric currents and QSLs from observational data and a magnetic field model. The
results, obtained by two different and independent approaches 1) confirm previous results
of current increase during the impulsive phase of the flare and 2) show how NLFFF models
can capture the essential physical signatures of flares even in a complex magnetic field
topology.
关键词：Sun: flares;Sun: magnetic fields;magnetic reconnection;magnetohydrodynamics (MHD)
