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We investigate in detail the 21 May 2004 flare using simultaneousobservations of the Nobeyama Radioheliograph, Nobeyama Radiopolarimeters,Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and Solar andHeliospheric Observatory (SOHO). The flare images in different spectral rangesreveal the presence of a well-defined single flaring loop in this event. Wehave simulated the gyrosynchrotron microwave emission using the recentlydeveloped interactive IDL tool GX Simulator. By comparing the simulationresults with the observations, we have deduced the spatial and spectralproperties of the non-thermal electron distribution. The microwave emission hasbeen found to be produced by the high-energy electrons ($>100$ keV) with arelatively hard spectrum ($\delta\simeq 2$); the electrons were stronglyconcentrated near the loop top. At the same time, the number of high-energyelectrons near the footpoints was too low to be detected in the RHESSI imagesand spatially unresolved data. The SOHO Extreme-ultraviolet Imaging Telescopeimages and the low-frequency microwave spectra suggest the presence of anextended "envelope" of the loop with lower magnetic field. Most likely, theenergetic electron distribution in the considered flare reflects the localized(near the loop top) particle acceleration (injection) process accompanied bytrapping and scattering.

Spatially-resolved Energetic Electron Properties for the 21 May 2004 Flare from Radio Observations and 3D Simulations