Properties of the Acceleration Regions in Several Loop-structured Solar Flares

TL;DR

This study uses RHESSI X-ray data to analyze electron acceleration regions in solar flares, estimating plasma densities, filling factors, and acceleration rates to better understand flare energetics and particle acceleration mechanisms.

Contribution

It introduces a method to estimate the properties of acceleration regions in solar flares, including filling factors and specific acceleration rates, from imaging spectroscopy data.

Findings

Filling factors mostly between 0.1 and 1.0, with a mean around 0.20.

Average specific acceleration rate at 20 keV is approximately 6.0 x 10^{-3} electrons s^{-1} per ambient electron.

Results have implications for understanding the electrodynamics and acceleration processes in solar flares.

Abstract

Using {\em RHESSI} hard X-ray imaging spectroscopy observations, we analyze electron flux maps for a number of extended coronal loop flares. For each event, we fit a collisional model with an extended acceleration region to the observed variation of loop length with electron energy $E$, resulting in estimates of the plasma density in, and longitudinal extent of, the acceleration region. These quantities in turn allow inference of the number of particles within the acceleration region and hence the filling factor $f$ -- the ratio of the emitting volume to the volume that encompasses the emitting region(s). We obtain values of $f$ that lie mostly between 0.1 and 1.0; the (geometric) mean value is $f = 0.20 \times \div 3.9$, somewhat less than, but nevertheless consistent with, unity. Further, coupling information on the number of particles in the acceleration region with information on the total rate of acceleration of particles above a certain reference energy (obtained from spatially-integrated hard X-ray data) also allows inference of the specific acceleration rate (electron s$^{-1}$ per ambient electron above the chosen reference energy). We obtain a (geometric) mean value of the specific acceleration rate $\eta(20$ keV) $ = (6.0 \times / \div 3.4) \times 10^{-3}$ electrons s$^{-1}$ per ambient electron; this value has implications both for the global electrodynamics associated with replenishment of the acceleration region and for the nature of the particle acceleration process.