Wave-particle interactions in non-uniform plasma and the interpretation of Hard X-ray spectra in solar flares

TL;DR

This study demonstrates that wave-particle interactions in non-uniform plasma significantly alter the evolution of energetic electrons in solar flares, affecting the interpretation of hard X-ray spectra and potentially leading to overestimations of electron populations.

Contribution

It introduces a numerical simulation of electron relaxation considering wave interactions in non-uniform plasma, revealing deviations from traditional collisional models.

Findings

Wave-particle interactions accelerate electrons beyond collisional predictions.

Langmuir wave spectrum evolution enhances non-thermal electron populations.

HXR spectral interpretation may overestimate electron energies and numbers.

Abstract

Context. High energy electrons accelerated during solar flare are abundant in the solar corona and in the interplanetary space. Commonly, the number and the energy of non-thermal electrons at the Sun is estimated using hard X-ray (HXR) spectral observations (e.g. RHESSI) and a single-particle collisional approximation. Aims. To investigate the role of the spectrally evolving Langmuir turbulence on the population of energetic electrons in the solar corona. Methods. We numerically simulate the relaxation of a power-law non-thermal electron population in a collisional inhomogeneous plasma including wave-particle, and wave-wave interactions. Results. The numerical simulations show that the long-time evolution of electron population above 20 keV deviates substantially from the collisional approximation when wave-particle interactions in non-uniform plasma are taken into account. The evolution of Langmuir wave spectrum towards smaller wavenumbers, due to large-scale density fluctuations and wave-wave interactions, leads to an effective acceleration of electrons. Furthermore, the time-integrated spectrum of non-thermal electrons, which is normally observed with HXR above 20 keV, is noticeably increased due to acceleration of non-thermal electrons by Langmuir waves. Conclusions. The results show that the observed HXR spectrum, when interpreted in terms of collisional relaxation, can lead to an overestimated number and energy of energetic electrons accelerated in the corona.