Relationship between Hard and Soft X-ray Emission Components of a Solar Flare

TL;DR

This study investigates the relationship between thermal and nonthermal X-ray emission components in a solar flare, using time derivative methods to identify a consistent break energy and analyze the evolution of pivot energies.

Contribution

It introduces a regularized method to determine break energies and examines the evolution of pivot energies, providing insights into the thermal and nonthermal emission relationship in solar flares.

Findings

Break energy around 11 keV consistent with spectral analysis.

Errors in break energy determination are mainly due to data limitations.

Pivot energies show variation between rise and decay phases.

Abstract

X-ray observations of solar flares routinely reveal an impulsive high-energy and a gradual low-energy emission component, whose relationship is one of the key issues of solar flare study. The gradual and impulsive emission components are believed to be associated with, respectively, the thermal and nonthermal components identified in spectral fitting. In this paper, a prominent about 50 second hard X-ray (HXR) pulse of a simple GOES class C7.5 flare on 20 February 2002 is used to study the association between high energy, non-thermal and impulsive evolution, and low energy, thermal and gradual evolution. We use regularized methods to obtain time derivatives of photon fluxes to quantify the time evolution as a function of photon energy, obtaining a break energy between impulsive and gradual behavior. These break energies are consistent with a constant value of about 11 keV in agreement with those found spectroscopically between thermal and non-thermal components, but the relative errors of the former are greater than 15% and much greater than the a few percent errors found from the spectral fitting. These errors only weakly depend on assuming an underlying spectral model for the photons, pointing to the current data being inadequate to reduce the uncertainties rather than there being a problem associated with an assumed model. The time derivative method is used to test for the presence of a 'pivot energy' in this flare. Although these pivot energies are marginally consistent with a constant value of about 9 keV, its values in the HXR rise phase appear to be lower than those in the decay phase.