Time Evolution of the Energy Spectra of Accelerated Electrons and Hard X-Rays from Local Sources of Solar Flares

Abstract

Modern observations of solar flares in hard X-rays are carried out with a high spatial resolution of ~2–4 arcsec with the RHESSI satellite. In this way one can identify, at least for powerful events, the local spatial structure of hard X-ray sources. The structure of flare X2.8 SOL 2013-5-13T15:50, as detected from RHESSI data and from observations made using Nobeyama and SDO, includes three bright local hard X-ray sources: one at the loop top and two at the footpoints of the flaring loop. The goal of this work is to obtain the energy spectra of hard X-rays, determine the spectrum of delays in hard X-ray radiation, reconstruct the spectra of accelerated electrons for each local source, and determine the parameters of the electron beam and flare plasma. The electron spectra are reconstructed by the forward-fitting method and the Tikhonov regularized inversion. The results of the analysis suggest that explaining the bright hard X-ray source high in the corona requires, simultaneously, a high magnetic field gradient in the footpoints with B_max/B₀ > 7 (in a symmetrical configuration), a background plasma density of n₀ > 10¹⁰ cm^–3, and the presence of turbulence.