Rapid Variability in the SOL2011-08-04 Flare: Implications for Electron Acceleration

TL;DR

This study analyzes rapid, small-scale nonthermal emissions during a solar flare, revealing that electron acceleration occurs in a compact region via super-Dreicer electric fields, challenging stochastic acceleration models.

Contribution

It provides detailed constraints on electron acceleration mechanisms during a solar flare's initial phase, emphasizing the role of super-Dreicer electric fields over stochastic processes.

Findings

Rapid subpeaks are caused by short injections of nonthermal electrons.

Electrons are accelerated to hundreds of keV within 50 ms.

Acceleration is likely driven by super-Dreicer electric fields.

Abstract

Particle acceleration in solar flares remains an outstanding problem in solar physics. It is yet unclear which of the acceleration mechanisms dominates and how exactly is the excessive magnetic energy transferred to the nonthermal and other forms of energy. We emphasize, that the ultimate acceleration mechanism must be capable of efficiently working in the most extreme conditions, such as the shortest detected time scales and the highest acceleration efficiency. Here we focus on detailed multiwavelength analysis of a very initial phase of the SOL2011-08-04 flare, which demonstrated prominent short subpeaks of nonthermal emission during filament eruption associated with the flare. We demonstrate that the three-dimensional configuration of the flare, combined with timing and spectral behavior of the rapidly varying component, put very stringent constraints on the acceleration regime. Specifically, the rapid subpeaks are generated by short injections of nonthermal electrons with a reasonably hard, single power-law spectrum and a relatively narrow spread of pitch-angles along the mean magnetic field. The acceleration site is a compact volume located near the top of extended coronal loop(s). The electrons are accelerated up to several hundreds of keV promptly, with the characteristic acceleration time shorter than 50 ms. We show, that these properties are difficult to reconcile with widely adopted stochastic acceleration models, while the data inescapably require acceleration by a super-Dreicer electric field, whether regular or random.