Study of magnetic field topology of active region 12192 using an extrapolated non-force-free magnetic field
A. Prasad, R. Bhattacharyya, Q. Hu, S. S. Nayak, Sanjay Kumar
LLong Term Datasets for the Understanding of Solar and StellarMagnetic CyclesProceedings IAU Symposium No. 340, 2018D. Banerjee, J. Jiang, K. Kusano & S. Solanki c (cid:13) Study of magnetic field topology of activeregion 12192 using an extrapolatednon-force-free magnetic field
A. Prasad , R. Bhattacharyya , Q. Hu , S. S. Nayak & SanjayKumar Udaipur Solar Observatory, Physical Research Laboratory Center for Space Plasma and Aeronomic Research, The University of Alabama in Huntsville Post Graduate Department of Physics, Patna University
Abstract.
The solar active region (AR) 12192 was one of the most flare productive region ofsolar cycle 24, which produced many X-class flares; the most energetic being an X3.1 flare onOctober 24, 2014 at 21:10 UT. Customarily, such events are believed to be triggered by mag-netic reconnection in coronal magnetic fields. Here we use the vector magnetograms from solarphotosphere, obtained from Heliospheric Magnetic Imager (HMI) to investigate the magneticfield topology prior to the X3.1 event, and ascertain the conditions that might have causedthe flare. To infer the coronal magnetic field, a novel non-force-free field (NFFF) extrapolationtechnique of the photospheric field is used, which suitably mimics the Lorentz forces presentin the photospheric plasma. We also highlight the presence of magnetic null points and quasi-separatrix layers (QSLs) in the magnetic field topology, which are preferred sites for magneticreconnections and discuss the probable reconnection scenarios.
Keywords. magnetohydrodynamics (MHD) – Sun: activity – Sun: corona – Sun: flares – Sun:magnetic fields – Sun: photosphere
1. Introduction
The solar corona represents a magnetized plasma with high electrical conductivitywhose evolution is determined by magnetohydrodynamics (MHD). The large magneticReynolds number R M (= vL/η , in usual notations) of the corona ensures that the mag-netic field lines (MFLs) to remain tied to fluid parcels during their evolution. In contrast,instances of various eruptive events (flares and coronal mass ejections) occurring in thecorona are believed to be signatures of magnetic reconnection: the topological rearrange-ment of magnetic field lines along with the conversion of magnetic energy into heat andthe kinetic energy of mass motion. Currently, the coronal fields are numerically extrap-olated from the photospheric magnetic fields, as direct coronal measurements are notavailable. Here we present a model for coronal magnetic fields derived from the varia-tional principle of the minimum energy dissipation rate (Hu et al. 2008; Prasad et al.2017) formulated by superposing three linear-force-free fields and its application for theX3.1 flare event of AR 12192 on October 24th 2014 at 21:10 UT.
2. Results
For the NFFF extrapolation, we select the vector magnetogram at 20:46 UT obtainedfrom HMI on board the Solar Dynamics Observatory (SDO), roughly 20 minutes priorto the flare. In Fig. 1a, we show the large-scale sheared MFLs (shown in blue) nearthe polarity-inversion line and also the MFLs constituting the spine-fan structure of a1 a r X i v : . [ a s t r o - ph . S R ] A p r Prasad et al.three-dimensional (3D) null point (shown in red). Importantly, these MFLs demarcatethe QSLs (regions of high squashing-factor Q ; c.f. Fig. 1b). It is well known that 3D nullsand QSLs are preferred sites for magnetic reconnections (Pontin 2012). (a) (b) Figure 1 The extrapolated MFLs for AR 12192 overlaid with (a) vertical component ofmagnetic field Bz and (b) log of the squashing-factor Q . The values for B z are scaled bya factor of 200. (a) (b) Figure 2 The extrapolated MFLs for AR 12192 overlaid with images of (a) AIA 131 ˚A at21:02 UT and (b) AIA 1600 ˚A at 21:28 UT.In Fig. 2, we overlay the above MFLs with the extreme-ultra-violet (EUV) 131 ˚A chan-nel and the ultra-violet (UV) 1600 ˚A channel images from the Atmospheric ImagingAssembly (AIA) at 21:02 UT and 21:28 UT respectively, when the brightening in thesechannels are maximum. The near accurate correspondence between the brightening ob-served in the AIA channels with the MFL footpoints validate the NFFF extrapolationspresented here. The dynamics of the MFLs leading to the flare require full MHD simu-lations with these NFFF as initial fields. This would be presented elsewhere in future.