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

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.

Spatiotemporal Energy Partitioning in a Nonthermally Dominated Two-loop Solar Flare

“…This likely points at the existence of two connecting loops in the horizontal direction. Loop-loop interactions have been proposed by Tajima et al (1987); Hanaoka (1999) to drive energy release in solar flares with multi-loop morphology; this scenario is further supported by simulations of loop-loop interaction (Kašparová et al 2005) and numerous observations of two-loop flare configurations (e.g., Fleishman et al 2016;Altyntsev et al 2019).…”

Incoherent Solar Radio Emission

“…Other publications that report pitch-angle anisotropies of the electrons that emit gyrosynchrotron emission include , , Fleishman et al (2003); Fleishman (2006), Altyntsev et al (2008Altyntsev et al ( , 2019, Tzatzakis et al (2008), Reznikova et al (2009), and Charikov et al (2017). Generally speaking, when pitch-angle anisotropy is present it is not correct to derive the energy spectrum of the energetic electrons from the slope of the optically thin part of the gyrosynchrotron spectrum (see Fleishman and Melnikov, 2003a,b, and the discussion in section 6.1).…”

Spectral Observations of After-Flare Loops in the Active Area Noaa 11515

“…Из анализа космических наблюдений Солнца в жестком рентгеновском излучении и из наземных радионаблюдений к настоящему времени стало известно, что значительная часть энергии, выделяющейся в течение вспышек, обусловлена нетепловыми процессами, связанными с ускоренными электронами. Механизм ускорения электронов до сих пор неизвестен [1]. В видимом диапазоне спектра таких наблюдений мало.…”