The spectral evolution of impulsive solar X-ray flares. II.Comparison of observations with models

TL;DR

This study compares observed spectral evolution of solar X-ray flares with simple acceleration models, finding that a stochastic acceleration model fits most events but requires unphysically high energies for some, highlighting limitations in current models.

Contribution

It provides a comprehensive comparison of spectral evolution data with simple electron acceleration models, identifying their successes and limitations in explaining solar flare observations.

Findings

A stochastic acceleration model fits about 77% of events.

Unphysically high electron acceleration rates are needed for some events.

Simple models struggle to explain all observed spectral behaviors.

Abstract

We study the evolution of the spectral index and the normalization (flux) of the non-thermal component of the electron spectra observed by RHESSI during 24 solar hard X-ray flares. The quantitative evolution is confronted with the predictions of simple electron acceleration models featuring the soft-hard-soft behaviour. The comparison is general in scope and can be applied to different acceleration models, provided that they make predictions for the behavior of the spectral index as a function of the normalization. A simple stochastic acceleration model yields plausible best-fit model parameters for about 77% of the 141 events consisting of rise and decay phases of individual hard X-ray peaks. However, it implies unphysically high electron acceleration rates and total energies for the others. Other simple acceleration models such as constant rate of accelerated electrons or constant input power have a similar failure rate. The peaks inconsistent with the simple acceleration models have smaller variations in the spectral index. The cases compatible with a simple stochastic model require typically a few times 10^36 electrons accelerated per second at a threshold energy of 18 keV in the rise phases and 24 keV in the decay phases of the flare peaks.