STUDYING THE MAGNETIC FIELD STRUCTURE AND TOPOLOGY OF CIRCULAR RIBBON FLARES

CHROMOSPHERIC FLARE RIBBONS ARE ONE OF THE MOST DIRECT OBSERVATIONAL LINKS TO THE MAGNETIC RECONNECTION REGION AND CAN PROVIDE UNIQUE INFORMATION ON THE 3D MAGNETIC GEOMETRY OF THE ENERGY RELEASE SITE AND CHANNELS. ONLY RECENTLY FLARES HAVING A CIRCULAR RIBBON HAVE STARTED TO BE REPORTED AND GAINED INCREASING ATTENTION THANKS TO HIGH-RESOLUTION OBSERVATIONS FROM SDO. IN INTERPRETING THIS KIND OF EVENTS THE FAN-SPINE MAGNETIC TOPOLOGY IS USUALLY INVOKED WHERE THE DOME-SHAPED FAN PORTRAYS THE CLOSED SEPARATRIX SURFACE AND THE INNER AND OUTER SPINE FIELD LINES LAY IN DIFFERENT CONNECTIVITY DOMAINS AND PASS THROUGH A CORONAL NULL POINT. INTRIGUINGLY IN SOME CIRCULAR-RIBBON FLARES AN ACTIVE FILAMENT/FLUX ROPE CAN BE EMBEDDED UNDER THE FAN DOME. SINCE MAGNETIC NULL POINTS ARE COMMON STRUCTURES IN THE CORONA RELATED TO BOTH ERUPTIVE AND CONFINED FLARES AS WELL AS CORONAL JETS STUDYING CIRCULAR-RIBBON FLARES AND THE RELATED MAGNETIC STRUCTURE/TOPOLOGY WILL ADVANCE OUR KNOWLEDGE OF SOLAR ERUPTIONS IN GENERAL. THE STUDIES SO FAR ARE NEVERTHELESS STILL VERY LIMITED IN THE SENSE THAT THE MAGNETIC STRUCTURAL EVOLUTION OF CIRCULAR-RIBBON FLARES ESPECIALLY RELATED TO THE EVENT TRIGGERING AND PROGRESSION IS NOT FULLY UNDERSTOOD. MOREOVER THE DETAILED INTERACTION PROCESS BETWEEN THE EMBEDDED FILAMENT AND THE FAN-SPINE FIELD HAS NOT YET BEEN ADDRESSED. FROM A PRELIMINARY EVENT SURVEY WE HAVE COMPILED A LIST OF OVER 20 CIRCULAR-RIBBON FLARES (C-CLASS AND UP) AND MORE SUCH EVENTS ARE EXPECTED AS SDO PROVIDES NEARLY CONTINUOUS COVERAGE. THIS MAKES IT AN OPPORTUNE MOMENT TO QUANTITATIVELY ADDRESS THE FOLLOWING QUESTIONS BY DETAILED CASE BY CASE AS WELL AS COMPARATIVE STUDIES. WE PLAN THE SCIENTIFIC OBJECTIVES OF THIS PROPOSAL AS FOLLOWS. 1. USING HIGH-RESOLUTION OBSERVATIONS (EUV/UV FROM SDO/AIA AND IRIS AND OPTICAL FROM BBSO/GST) AIDED BY THE 3D CORONAL FIELD MODELING (NONLINEAR FORCE-FREE FIELD EXTRAPOLATION AND FLUX ROPE INSERTION BASED ON SDO/HMI VECTOR FIELD DATA AND DYNAMICAL MAGNETOFRICTIONAL AND MHD SIMULATIONS) WE WILL STUDY THE TEMPORAL EVOLUTION OF THE MAGNETIC FIELD STRUCTURE AND PHOTOSPHERIC FLOW FIELD LEADING TO THE INITIATION OF CIRCULAR-RIBBON FLARES AND ALSO THE DYNAMICS OF CIRCULAR FLARE RIBBONS RELATED TO THE NULL-POINT RECONNECTION. 2. SPECIAL EMPHASIS WILL BE PLACED ON EVENTS IN WHICH AN ACTIVE FILAMENT IS EMBEDDED IN THE FAN-SPINE TOPOLOGY. DIFFERENT FROM REGULAR CIRCULAR-RIBBON FLARES THAT ARE USUALLY CONFINED THESE EVENTS CAN BE ERUPTIVE. USING BOTH MULTIWAVELENGTH OBSERVATIONS AND CORONAL MAGNETIC FIELD MODELING OF THE FAN-SPINE AND FILAMENT FIELDS WE WILL ELUCIDATE THE INTERACTION BETWEEN THE ACTIVE FILAMENT AND FANSPINE MAGNETIC FIELDS AND DISENTANGLE THE POSSIBLE CAUSAL RELATIONSHIP BETWEEN THE OBSERVED FILAMENT ERUPTION (STANDARD PICTURE) AND CIRCULAR-RIBBON FLARE (OCCURRING IN THE PROCESS OF NULL-POINT RECONNECTION). THE PROPOSED INVESTIGATION WILL EXTENSIVELY USE DATA FROM CURRENT NASA SPACECRAFT. COMPLEMENTARY HIGH-RESOLUTION OBSERVATIONS FROM BIG BEAR SOLAR OBSERVATORY WILL ENHANCE THE STUDY. THIS TIMELY RESEARCH INSPIRED BY OUR NEW RESULTS ON CIRCULAR-RIBBON FLARES CAN HELP TO ADDRESS THE FUNDAMENTAL PHYSICS CONCERNING THE TRIGGERING AND PROGRESSION OF ALL SOLAR FLARES AND ERUPTIONS. THIS IS CLOSELY RELEVANT TO ONE OF THE HELIOPHYSICS DECADAL SURVEY GOALS: DETERMINE THE ORIGINS OF THE SUN'S ACTIVITY AND CHARACTERIZE A FUNDAMENTAL PROCESS IN ASTROPHYSICS I.E. RECONNECTION