A multisource limb flare observed at multiple radio wavelengths

A flare with several radio sources occurred on the solar limb at 2155 UT on 1989 June 20. It was observed by the Very Large Array (VLA) and the Owens Valley Radio Observatory (OVRO). The VLA data consisted of images at 1.4 GHz, while OVRO provided spectral and spatial information over the range 1-15 GHz. We develop a new gyrosynchrotron model to analyze the sources observed at flare peak. This model differs from many previous ones in that it contains spatial variations of both the magnetic field and accelerated particle density. It uses a new gyrosynchrotron approximation which is valid at very low harmonics of the gyrofrequency. For the first time we find that the cause of the change of microwave source size with frequency in the event studied was due primarily to the spatial variation of the accelerated electron density. This is contrary to the common assumption that the variation in size is due to a nonuniform magnetic field. We also investigate a polarized source which brightened later in the flare, finding that it could have been due to plasma radiation or gyrosynchrotron emission.