The Acceleration and Confinement of Energetic Electrons by a Termination Shock in a Magnetic Trap: An Explanation for Nonthermal Loop-top Sources during Solar Flares

Abstract

Nonthermal loop-top sources in solar flares are the most prominent observational signature that suggests energy release and particle acceleration in the solar corona. Although several scenarios for particle acceleration have been proposed, the origin of the loop-top sources remains unclear. Here we present a model that combines a large-scale magnetohydrodynamic simulation of a two-ribbon flare with a particle acceleration and transport model for investigating electron acceleration by a fast-mode termination shock at the looptop. Our model provides spatially resolved electron distribution that evolves in response to the dynamic flare geometry. We find a concave-downward magnetic structure located below the flare termination shock, induced by the fast reconnection downflows. It acts as a magnetic trap to confine the electrons at the looptop for an extended period of time. The electrons are energized significantly as they cross the shock front, and eventually build up a power-law energy arXiv:1911.08064v1 [astro-ph.SR] 19 Nov 2019 2 spectrum extending to hundreds of keV. We suggest that this particle acceleration and transport scenario driven by a flare termination shock is a viable interpretation for the observed nonthermal loop-top sources.

Multiple Sources of Solar High-energy Protons

“…Particle acceleration in the flare's impulsive phase proceeds in the environment where the newly reconnected magnetic field lines emanating from the CME-trailing current sheet interact with the underlying flare arcades. Particles can be accelerated there in collapsing magnetic traps (Somov & Kosugi 1997) or by Fermi-type acceleration in the "termination" shocks formed by reconnection outflows of the CMEtrailing current sheet (Masuda et al 1994;Chen et al 2015;Kong et al 2019). The flare environment is rich in turbulence, waves, and a fractal electric field, which have also been suggested as possible particle acceleration mechanisms (see the review by Zharkova et al 2011, and references therein).…”

A model of double coronal hard X-ray sources in solar flares

“…We model the acceleration and transport of electrons by coupling the Parker transport equation (Parker 1965) with a macroscopic MHD simulation of the solar flare. This general framework has 5 been discussed previously (Kong et al 2019(Kong et al , 2020Li et al 2018) and here we only provide a salient description.…”

“…Due to the dramatic difference between the kinetic scales (∼1-10 cm for electrons and ∼1-10 m for protons) and the macroscopic flare scale (∼ 10 8 m), numerical modeling of particle acceleration in solar flares has been very challenging. Recently, we have developed a macroscopic kinetic model for energetic particles that combines a particle acceleration and transport model with the MHD simulation of solar flares (Kong et al 2019(Kong et al , 2020. Kong et al (2019) showed that electrons can be efficiently accelerated by the TS and a magnetic trap plays a critical role in both confining and accelerating electrons in the looptop region.…”

“…Recently, we have developed a macroscopic kinetic model for energetic particles that combines a particle acceleration and transport model with the MHD simulation of solar flares (Kong et al 2019(Kong et al , 2020. Kong et al (2019) showed that electrons can be efficiently accelerated by the TS and a magnetic trap plays a critical role in both confining and accelerating electrons in the looptop region. The model for the first time numerically reproduces the necessary electron acceleration and spatial distribution for nonthermal looptop sources.…”

Shock Propagation and Associated Particle Acceleration in the Presence of Ambient Solar-Wind Turbulence

“…Observations of interplanetary shocks provide a unique opportunity to systematically understand the system where shocks interact with large-scale turbulence and accelerate energetic particles. Understanding how these processes work is also important to many other space, solar and astrophysical systems (Giacalone, 2017a;Kong et al, 2019b).…”