Acceleration, magnetic fluctuations and cross-field transport of energetic electrons in a solar flare loop

TL;DR

This study uses RHESSI observations to analyze energetic electron behavior in a solar flare loop, revealing that magnetic turbulence influences electron acceleration and transport, with turbulence levels peaking during maximum electron acceleration.

Contribution

It provides the first observational evidence linking magnetic turbulence to the evolution of energetic electrons in a dense solar flare loop.

Findings

Electrons are continuously accelerated near the loop apex.

Cross-field electron transport is consistent with magnetic turbulence.

Magnetic fluctuation energy density exceeds that of non-thermal electrons.

Abstract

Plasma turbulence is thought to be associated with various physical processes involved in solar flares, including magnetic reconnection, particle acceleration and transport. Using Ramaty High Energy Solar Spectroscopic Imager ({\it RHESSI}) observations and the X-ray visibility analysis, we determine the spatial and spectral distributions of energetic electrons for a flare (GOES M3.7 class, April 14, 2002 23$:$55 UT), which was previously found to be consistent with a reconnection scenario. It is demonstrated that because of the high density plasma in the loop, electrons have to be continuously accelerated about the loop apex of length $\sim 2\times 10^9$cm and width $\sim 7\times 10^8$cm. Energy dependent transport of tens of keV electrons is observed to occur both along and across the guiding magnetic field of the loop. We show that the cross-field transport is consistent with the presence of magnetic turbulence in the loop, where electrons are accelerated, and estimate the magnitude of the field line diffusion coefficient for different phases of the flare. The energy density of magnetic fluctuations is calculated for given magnetic field correlation lengths and is larger than the energy density of the non-thermal electrons. The level of magnetic fluctuations peaks when the largest number of electrons is accelerated and is below detectability or absent at the decay phase. These hard X-ray observations provide the first observational evidence that magnetic turbulence governs the evolution of energetic electrons in a dense flaring loop and is suggestive of their turbulent acceleration.