Flare Energy Release at the Magnetic Field Polarity Inversion Line During the M1.2 Solar Flare of 2015 March 15. I. Onset of Plasma Heating and Electron Acceleration

We present the study of the SOL2015-03-15 M1.2 flare, revealing acceleration of electrons and plasma heating in the sheared twisted magnetic structure at the polarity inversion line (PIL). The purpose is to make an analysis of nonthermal electron dynamics and plasma heating in the highly stressed magnetic loops interacting at the PIL by using X-ray (RHESSI), microwave (NoRH and NoRP), ultraviolet (SDO/AIA), and optical (Hinode/SOT) observations. It is found that the most probable scenario for the energy release at the PIL is the tether-cutting magnetic reconnection between the low-lying (3 Mm above the photosphere) magnetic loops within a twisted magnetic flux rope. Energetic electrons with the hardest spectrum appeared at the onset of plasma heating up to the superhot temperature of 40 MK. These electrons are localized in a thin magnetic channel with a width of around 0.5 Mm and a high average magnetic field of about 1200 G. The plasma beta in the superhot region is less than 0.01. The estimated density of accelerated electrons is about 10^9 cm-3, which is much less than the superhot plasma density. The energy density flux of nonthermal electrons is estimated up to 3×10^12 erg cm-2 s-1, which is much higher than in the currently available radiative hydrodynamic models. These results revealed that one needs to develop new self-consistent flare models reproducing 3D magnetic reconnection at the PIL with strong magnetic field, spatial filamentation of energy release, formation of high-energy density populations of nonthermal electrons, and the appearance of the superhot plasma.