Energy Release and Particle Acceleration in Flares: Summary and Future Prospects

TL;DR

This paper summarizes RHESSI observations of energy release and particle acceleration in solar flares, highlighting new insights into electron and ion acceleration mechanisms, magnetic reconnection, and their relation to solar energetic particle events.

Contribution

It introduces new observational evidence and theoretical understanding of magnetic reconnection and particle acceleration processes in solar flares, based on RHESSI data.

Findings

Low energy cut-off of electron spectrum below tens of keV

Detection of high coronal hard X-ray source with energetic electrons

Correlation between flare acceleration and solar energetic particle events

Abstract

RHESSI measurements relevant to the fundamental processes of energy release and particle acceleration in flares are summarized. RHESSI's precise measurements of hard X-ray continuum spectra enable model-independent deconvolution to obtain the parent electron spectrum. Taking into account the effects of albedo, these show that the low energy cut-off to the electron power-law spectrum is typically below tens of keV, confirming that the accelerated electrons contain a large fraction of the energy released in flares. RHESSI has detected a high coronal hard X-ray source that is filled with accelerated electrons whose energy density is comparable to the magnetic-field energy density. This suggests an efficient conversion of energy, previously stored in the magnetic field, into the bulk acceleration of electrons. A new, collisionless (Hall) magnetic reconnection process has been identified through theory and simulations, and directly observed in space and in the laboratory; it should occur in the solar corona as well, with a reconnection rate fast enough for the energy release in flares. The reconnection process could result in the formation of multiple elongated magnetic islands, that then collapse to bulk-accelerate the electrons, rapidly enough to produce the observed hard X-ray emissions. RHESSI's pioneering {\gamma}-ray line imaging of energetic ions, revealing footpoints straddling a flare loop arcade, has provided strong evidence that ion acceleration is also related to magnetic reconnection. Flare particle acceleration is shown to have a close relationship to impulsive Solar Energetic Particle (SEP) events observed in the interplanetary medium, and also to both fast coronal mass ejections and gradual SEP events.