Local re-acceleration and a modified thick target model of solar flare electrons

TL;DR

This paper proposes a local re-acceleration model for solar flare electrons that enhances X-ray emission efficiency, reduces electron beam density and anisotropy issues, and aligns with observed timing and variability in solar flare data.

Contribution

It introduces the local re-acceleration thick target model (LRTTM) involving current sheet cascades, supported by MHD and test particle simulations, offering a novel explanation for solar flare electron dynamics.

Findings

Re-acceleration increases electron lifetime and X-ray yield.

LRTTM reduces electron beam density and anisotropy requirements.

Model aligns with observed timing and variability in solar flares.

Abstract

The collisional thick target model (CTTM) of solar hard X-ray (HXR) bursts has become an almost 'Standard Model' of flare impulsive phase energy transport and radiation. However, it faces various problems in the light of recent data, particularly the high electron beam density and anisotropy it involves.} {We consider how photon yield per electron can be increased, and hence fast electron beam intensity requirements reduced, by local re-acceleration of fast electrons throughout the HXR source itself, after injection.} {We show parametrically that, if net re-acceleration rates due to e.g. waves or local current sheet electric (${\cal E}$) fields are a significant fraction of collisional loss rates, electron lifetimes, and hence the net radiative HXR output per electron can be substantially increased over the CTTM values. In this local re-acceleration thick target model (LRTTM) fast electron number requirements and anisotropy are thus reduced. One specific possible scenario involving such re-acceleration is discussed, viz, a current sheet cascade (CSC) in a randomly stressed magnetic loop.} {Combined MHD and test particle simulations show that local ${\cal E}$ fields in CSCs can efficiently accelerate electrons in the corona and and re-accelerate them after injection into the chromosphere. In this HXR source scenario, rapid synchronisation and variability of impulsive footpoint emissions can still occur since primary electron acceleration is in the high Alfv\'{e}n speed corona with fast re-acceleration in chromospheric CSCs. It is also consistent with the energy-dependent time-of-flight delays in HXR features.