  We present the study of SOL2015-03-15 M1.2 flare, revealing acceleration of
electrons and plasma heating in the sheared twisted magnetic structure in the
polarity inversion line (PIL). The scope is to make the analysis of nonthermal
electrons dynamics and plasma heating in the highly stressed magnetic loops
interacting in the PIL by using X-ray, microwave, ultraviolet, and optical
observations. It is found that the most probable scenario for the energy
release in 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 were appeared at the
onset of plasma heating up to the super-hot temperature of 40 MK. These
electrons are localized in a thin magnetic channel with width of around 0.5 Mm
with high average magnetic field of about 1200 G. The plasma beta in the
super-hot region is less than 0.01. The estimated density of accelerated
electrons is about 10^9 cm^-3 that is much less than the super-hot plasma
density. The energy density flux of non-thermal electrons is estimated up to
3x10^12 ergs cm^-2s^-1 that is much higher than in the currently available
radiative hydrodynamic models. These results revealed that one need to develop
new self-consisting flare models reproducing 3D magnetic reconnection in the
PIL with strong magnetic field, spatial filamentation of energy release,
formation of high energy density populations of nonthermal electrons and
appearance of the super-hot plasma.
