Comparative analysis of nonthermal emissions and electron transport in a solar flare

We study nonthermal emission in a solar flare occurring on 2003 May 29 using RHESSI hard X-ray (HXR) and Nobeyama microwave observations. This flare shows several typical behaviors of HXR and microwave emission: time delay of microwave peaks relative to HXR peaks, loop-top microwave and footpoint HXR sources, and a harder electron energy distribution from the microwave spectrum than from the HXR spectrum. In addition, we found that the time profile of the spectral index of the higher energy (greater than or similar to 100 keV) HXRs is similar to that of the microwaves, and is delayed relative to that of the lower energy (greater than or similar to 100 keV) HXRs. We interpret these observations in terms of an electron transport model called trap-plus-precipitation. We numerically solved the spatially homogeneous Fokker-Planck equation to determine electron evolution in energy and pitch-angle space. By comparing observations with the behavior of HXR and microwave emission predicted by the model, we examine the pitch-angle distribution of the electrons injected into the flare site. We find that the observed spectral variations can be qualitatively explained if the injected electrons have a pitch-angle distribution concentrated perpendicular to the magnetic field lines rather than an isotropic distribution.