Inference of Flare Acceleration Region’s Physical Properties from High-Energy Observations

Understanding the conditions high in the corona where particles are accelerated to produce the observed flare high-energy emission is a long-standing goal in Heliophysics. This tenuous region cannot be easily probed with current observations to deduce the particle distributions, which makes testing models of flare particle acceleration not straightforward. Guidoni et al. (2022) proposed a semi-analytic model of sequential particle acceleration in large-scale magnetic islands (denoted accelerators) formed by flare reconnection. The model predicts the accelerated particles’ distribution spectral index and low-energy cutoff as functions of two physical average parameters of the acceleration region: (1) the accelerators’ energy gain, and (2) the fraction of particles that can hop between accelerators to receive sequential energy boosts. Our analysis of a few simulated flare islands estimated their average energy gain to be in the range of ~1-4.5, but the fraction of particles transferred among accelerators remains unconstrained. We will present estimates of both flare acceleration region’s physical parameters from electron spectral power-law indices and low-energy cutoffs inferred from high-energy photon spectra. We studied several NuSTAR microflares and a large number of RHESSI A- through X-class flares under the assumptions of the collisional thick target model and will discuss the uncertainties of our estimates based on these assumptions. These results should be further compared to those deduced from dedicated flare-acceleration simulation codes, such as kglobal or AMPS.