Semi-Analytical Hybrid Model of Sequential Particle Acceleration in Flares

Abstract:

Understanding how particles are accelerated in flares has been a long-sought goal in Heliophysics. It is currently impossible to self-consistently unify flare models of particle energization over magnetohydrodynamics (MHD) and kinetic regimes because they operate at scales that differ by 10 orders of magnitude. Here, we describe our efforts to bridge this theoretical gap by combining global flare simulations and analytical kinetic theory. Simulation data provide ambient conditions for the particle distributions to be evolved in energy and pitch angle. Large-scale islands created by sporadic flare reconnection contract, energizing the ambient particles mostly through the betatron mechanism and supplemented by the Fermi mechanism. Hypothesized sequential boosts to the particle energy yield power-law–like spectra. We have developed a fully analytical model that characterizes the power-law properties (e.g., spectral indexes and power law breaks) as functions of very few physical parameters. This model explains key characteristics of observed flare hard X-ray spectra, as well as the underlying accelerated-electron properties.