Flare-productive active regions
Year: 2019
Type: article
Abstract: Strong solar flares and coronal mass ejections, here defined not only as the bursts of electromagnetic radiation but as the entire process in which magnetic energy is released through magnetic reconnection and plasma instability, emanate from active regions (ARs) in which high magnetic non-potentiality resides in a wide variety of forms. This review focuses on the formation and evolution of flare-productive ARs from both observational and theoretical points of view. Starting from a general introduction of the genesis of ARs and solar flares, we give an overview of the key observational features during the long-term evolution in the pre-flare state, the rapid changes in the magnetic field associated with the flare occurrence, and the physical mechanisms behind these phenomena. Our picture of flare-productive ARs is summarized as follows: subject to the turbulent convection, the rising magnetic flux in the interior deforms into a complex structure and gains high non-potentiality; as the flux appears on the surface, an AR with large free magnetic energy and helicity is built, which is represented by $$\delta $$ -sunspots, sheared polarity inversion lines, magnetic flux ropes, etc; the flare occurs when sufficient magnetic energy has accumulated, and the drastic coronal evolution affects magnetic fields even in the photosphere. We show that the improvement of observational instruments and modeling capabilities has significantly advanced our understanding in the last decades. Finally, we discuss the outstanding issues and future perspective and further broaden our scope to the possible applications of our knowledge to space-weather forecasting, extreme events in history, and corresponding stellar activities.
Source: Living Reviews in Solar Physics
Authors: Shin Toriumi, Haimin Wang
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: diamond
Awards: G1210760574, SCIENCE GOALS AND OBJECTIVES: OBSERVATIONAL AND THEORETICAL STUDIES IN RECENT YEARS HAVE DEMONSTRATED THAT THE INITIATION AND PROPAGATION OF SOLAR ERUPTIONS LEADING TO CORONAL MASS EJECTIONS (CMES) ARE CLOSELY ASSOCIATED WITH THE STRUCTURE OF BOTH THE CORE FIELDS AND THE LARGER SCALE FIELDS ABOVE THEM. IT HAS BEEN FOUND THAT WHILE MOST OF ERUPTIONS EVENTUALLY DEVELOP INTO CMES MANY END UP WITH CONFINED (OR FAILED) ERUPTIONS. THE PHYSICAL MECHANISMS UNDERLYING THE FILAMENT ACTIVATION AND THE ENSUING ERUPTION ARE CLOSELY RELATED TO THE KINK AND TORUS FLUX ROPE INSTABILITIES WHICH ARE DEPENDENT ON THE PROPERTIES OF THE CORE FLUX ROPE FIELDS AND THE OVERLYING ARCADE FIELDS. ALTERNATIVELY AN ERUPTION MIGHT BE DUE TO MAGNETIC RECONNECTION THAT MAY FORM NEW UNSTABLE FLUX ROPES. FINALLY IT IS RECENTLY DEMONSTRATED THAT MANY EVENTS GENERATE QUASI-CIRCULAR FLARES. THEY DO NOT FIT INTO CLASSICAL TWO-RIBBON FLARE MODEL BUT CAN BE EXPLAINED BY SPINE-FAN 3-D STRUCTURE. IN THIS PROPOSAL WE EMPLOY A UNIQUE APPROACH BASED ON A COMBINATION OF NUMERICAL SIMULATIONS OBSERVATIONS AND LABORATORY EXPERIMENTS TO ATTACK THE PROBLEMS OF THE TRIGGERING AND EVOLUTION OF SOLAR ERUPTIONS. WE PLAN TO ANSWER THE FOLLOWING SPECIFIC BUT RELATED QUESTIONS: (1) WHAT ARE IMPORTANT FACTORS IN STORING MAGNETIC ENERGY AND TRIGGERING SOLAR ERUPTIONS? (2) WHY SOME OF THE ERUPTIONS LEAD TO MAJOR CMES AND SOME OF THEM ARE CONFINED? (3) HOW DO THE MAGNETIC FIELDS EVOLVE DURING AND AFTER ERUPTIVE OR CONFINED EVENTS? METHODOLOGY: (1) PRE-ERUPTION CONDITIONS: FOR SELECTED EVENTS INCLUDING BOTH ERUPTIVE AND CONFINED ERUPTIONS THE PROPERTIES OF THE CORE FIELDS IN FLUX ROPES AND THE ARCADE FIELDS ABOVE THE FLUX ROPES WILL BE DERIVED USING THREE DIFFERENT NON-LINEAR FORCE-FREE FIELD (NLFFF) EXTRAPOLATIONS BASED ON SDO/HMI VECTOR MAGNETOGRAMS OF THE SOURCE ACTIVE REGIONS. 3D MAGNETIC SKELETON AND FULL BLOWN TOPOLOGY ANALYSIS OF THE TIME EVOLUTION OF PRE-ERUPTIVE CONFIGURATIONS WILL BE ANALYZED. FLOWS AND MAGNETIC STRUCTURE IN FLARE CORE REGIONS WILL BE DERIVED USING NEW SOLAR TELESCOPE AT BBSO. IN ADDITION WE WILL CORRELATE RECONNECTION PARAMETERS DERIVED FROM RIBBONS WITH Q FACTORS AT 3D QSLS. (2) MHD MODELING. TWO INDEPENDENT CODES WILL BE RUN FOR SAME EVENTS. (2A) WE WILL CARRY OUT PARAMETER STUDY OF CRITICAL TORUS N-INDEX OF OVERLAYING ARCADE AND GEOMETRIES OF FLUX ROPES IN KLIEM S MHD CODE INCORPORATING BOTH IDEALIZED CONFIGURATIONS AND DATA-CONSTRAINED FLUX-ROPE EMBEDDED MODELS FROM THE CFA THUS PRODUCING DATA-CONSTRAINED MHD SIMULATIONS. (2B) THE DATA-DRIVEN UAH MODEL IS USED TO NUMERICALLY SOLVE THE FULL SET OF TIME-DEPENDENT 3D MHD EQUATIONS WITH THE BOTTOM BOUNDARY CONDITIONS DRIVEN BY OBSERVED PHOTOSPHERIC MAGNETOGRAMS. OUTPUTS IN TWO MODELS ARE COMPARED AND USED IN A COMPLEMENTARY FASHION. (3) THE OBSERVATIONAL AND MODELING RESULTS WILL BE COMPARED WITH THE PPPL EXPERIMENTAL RESULTS. THE PROPERTIES OF THE CORE AND STRAPPING FIELDS IN THE LAB EXPERIMENTS FOR STABLE AND UNSTABLE REGIMES WILL BE COMPARED WITH SPECIFIC CONFIGURATIONS FOR ERUPTIVE AND CONFINED EVENTS. THE RESEARCH IS CLOSELY RELEVANT TO ONE OF THE HELIOPHYSICS DECADAL SURVEY GOALS: DETERMINE THE ORIGINS OF THE SUN'S ACTIVITY AND PREDICT THE VARIATIONS IN THE SPACE ENVIRONMENT. OUR PROPOSED RESEARCH WILL CLEARLY ANSWER SOME OF THE MOST CRITICAL QUESTIONS ABOUT THE INITIATION AND DEVELOPMENT OF SOLAR ERUPTIONS WHICH WILL EXTEND THE FRONTIER OF THE EXISTING KNOWLEDGE ON THE ENERGY RELEASE PHYSICS OF FLARES/CMES AND ASSOCIATED CORONAL FIELD RECONFIGURATION., 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, G2446448267, G2653762360, G4338246854, G4956240462, G5554240381, High Resolution Studies of Solar Activity Using the 1.6-meter Telescope in Big Bear, STUDYING THE MAGNETIC FIELD STRUCTURE AND TOPOLOGY OF CIRCULAR RIBBON FLARES
OpenAlex ID: W2941659446