The Specific Acceleration Rate in Loop-structured Solar Flares -- Implications for Electron Acceleration Models

TL;DR

This study analyzes electron acceleration in solar flare loops using RHESSI data, inferring parameters like plasma density and acceleration region size, and compares observed acceleration rates with various theoretical models.

Contribution

It provides the first detailed analysis of the specific acceleration rate in extended coronal loop flares and compares observationally inferred rates with multiple acceleration models.

Findings

Inferred mean specific acceleration rate is ~10^{-2} s^{-1} above 20 keV.

Filling factors are consistent with a value of unity.

Observed acceleration rates are compared with models including sub-Dreicer, super-Dreicer, and stochastic acceleration.

Abstract

We analyze electron flux maps based on RHESSI hard X-ray imaging spectroscopy data for a number of extended coronal loop flare events. For each event, we determine the variation of the characteristic loop length $L$ with electron energy $E$, and we fit this observed behavior with models that incorporate an extended acceleration region and an exterior "propagation" region, and which may include collisional modification of the accelerated electron spectrum inside the acceleration region. The models are characterized by two parameters: the plasma density $n$ in, and the longitudinal extent $L_0$ of, the acceleration region. Determination of the best-fit values of these parameters permits inference of the volume that encompasses the acceleration region and of the total number of particles within it. It is then straightforward to compute values for the emission filling factor and for the {\it specific acceleration rate} (electrons s$^{-1}$ per ambient electron above a chosen reference energy). For the 24 events studied, the range of inferred filling factors is consistent with a value of unity. The inferred mean value of the specific acceleration rate above $E_0=20$ keV is $\sim10^{-2}$ s$^{-1}$, with a 1$\sigma$ spread of about a half-order-of-magnitude above and below this value. We compare these values with the predictions of several models, including acceleration by large-scale, weak (sub-Dreicer) fields, by strong (super-Dreicer) electric fields in a reconnecting current sheet, and by stochastic acceleration processes.