Solar Microwave Bursts and Injection Pitch‐Angle Distribution of Flare Electrons

Abstract

We calculate the time variation of the energy and pitch angle ofhigh-energy electrons injected into a magnetic loop and subsequentlytrapped there because of magnetic mirroring. We use the evolvingdistribution in the calculation of gyrosynchrotron emission, as an aidto interpretation of a particular microwave burst observed using theOwens Valley Solar Array (OVSA) during a GOES class C2.8 flare on 1993June 3. The electrons are assumed to have a Gaussian pitch-angledistribution, whose width and mean pitch angle are calculated as theyevolve in time, taking into account the electron energy loss and aspecific magnetic loop structure set as a model for the target activeregion. Various temporal behaviors of the microwave spectrum are foundas a function of injection and trap conditions, which can be used toinfer some of the injection properties directly from the observedmicrowave spectra. As a main result we found that initial pitch-angledistribution plays an important role in the microwave spectralevolution. This is largely due to the fact that pitch-angle diffusion ofelectrons under Coulomb collisions markedly differs at those electronenergies to which the microwave spectrum is sensitive. This effectcannot be reproduced by adjusting the trap properties and thereforecould be used to determine whether the initial pitch-angle distributionis isotropic or narrowly beamed. The microwave burst spectra observedduring the 1993 June 3 flare are found to be most consistent with thehypothesis of an initially narrow beamed injection ( =5minutes) in terms of a transport effect rather than accelerationcharacteristics. The physical connection of the proposed microwave modelto hard X-ray models in thin/thick targets is briefly discussed.