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

Particle acceleration in solar flares remains an outstanding problem in solar physics. It is currently unclear which of the acceleration mechanisms dominates and how exactly the excessive magnetic energy is transferred to 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 timescales and the highest acceleration efficiency. Here we focus on a detailed multiwavelength analysis of the 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 the extended coronal loop(s). The electrons are promptly accelerated up to several hundreds of keV, 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.