Spectral Hardening of Large Solar Flares

TL;DR

This study analyzes RHESSI data from five large solar flares, revealing that spectral hardening occurs gradually and is linked to a single, evolving acceleration process rather than separate mechanisms.

Contribution

It provides a detailed spectral and spatial analysis showing the smooth transition between impulsive and gradual phases in large solar flares, suggesting a common acceleration process.

Findings

Spectral hardening occurs during the decay phase of flares.

Impulsive and gradual phases are connected by a smooth transition.

Both phases are explained by a single evolving acceleration mechanism.

Abstract

RHESSI observations are used to quantitatively study the hard X-ray evolution in 5 large solar flares selected for spectral hardening in the course of the event. The X-ray bremsstrahlung emission from non-thermal electrons is characterized by two spectroscopically distinct phases: impulsive and gradual. The impulsive phase usually consists of several emission spikes following a soft-hard-soft spectral pattern, whereas the gradual stage manifests itself as spectral hardening while the flux slowly decreases. Both the soft-hard-soft (impulsive) phase and the hardening (gradual) phase are well described by piecewise linear dependence of the photon spectral index on the logarithm of the hard X-ray flux. The different linear parts of this relation correspond to different rise and decay phases of emission spikes. The temporal evolution of the spectra is compared with the configuration and motion of the hard X-ray sources in RHESSI images. These observations reveal that the two stages of electron acceleration causing these two different behaviors are closely related in space and time. The transition between the impulsive and gradual phase is found to be smooth and progressive rather than abrupt. This suggests that they arise because of a slow change in a common accelerator rather than being caused by two independent and distinct acceleration processes. We propose that the hardening during the decay phase is caused by continuing particle acceleration with longer trapping in the accelerator before escape.