Characteristics of the Accelerated Electrons Moving along the Loop Derived from Cyclical Microwave Brightenings at the Footpoints

TL;DR

This study estimates the pitch-angle distribution and reflection behavior of accelerated electrons during a solar flare by analyzing cyclical microwave brightenings, revealing their bouncing motion along magnetic field lines.

Contribution

It introduces a method to estimate electron pitch angles from microwave source propagation velocities during solar flares, providing new insights into electron acceleration and reflection processes.

Findings

Estimated electron velocities of 77,000 and 90,000 km/s.

Electron pitch angles estimated at 69-80 degrees.

Most electrons reflected at the footpoint, indicating bouncing motion.

Abstract

Many particles are accelerated during solar flares. To understand the acceleration and propagation processes of electrons, we require the pitch-angle distributions of the particles. The pitch angle of accelerated electrons has been estimated from the propagation velocity of a nonthermal microwave source archived in Nobeyama Radioheliograph data. We analyzed a flare event (an M-class flare on 2014 October 22) showing cyclical microwave brightenings at the two footpoint regions. Assuming that the brightenings were caused by the accelerated electrons, we approximated the velocity parallel to the magnetic field of the accelerated electrons as 77,000 and 90,000 km/s. The estimated pitch angle of the accelerated electrons is 69-80 degrees and the size of the loss cone at the footpoint (estimated from the magnetic field strength in the nonlinear force-free field model) is approximately 43 degrees. Most of the accelerated electrons could be reflected at the footpoint region. This feature can be interpreted as brightenings produced by bouncing motion of the accelerated electrons.