Multifrequency observations of a solar microwave burst with two-dimensional spatial resolution

We report the first observations of a major solar flare with the OVRO frequency-agile interferometer using three baselines. The spatial information from the three baselines, coupled with the technique of frequency synthesis, has allowed us to obtain two-dimensional positions of multiple microwave sources at several frequency ranges in the flare. The spatial structure of the radio burst is dominated by a primary source at the location of the Hα emission, and a secondary source without associated Hα emission, located more than 3′ away near the leader sunspots of the active region. We obtain source size and brightness temperature spectra in the primary source, at the time of the peak of the burst. The source size is found to decrease rapidly with increasing frequency, but the brightness temperature spectrum can be well fitted by gyrosynchrotron emission from a nonthermal distribution of electrons with power-law index δ = 4.8. The secondary source is found to be consistent with optically thin gyrosynchrotron emission from a small number of electrons in the high field of the leader sunspots. In addition to the gyrosynchrotron emission, a source of coherent plasma emission at low frequencies is found near the leader spots of the active region, with o-mode polarization 85% or greater, and brightness temperature greater than 10⁹ K. The spectral and spatial complexity of the burst is surprising in view of the apparent simplicity of the Hα emission and shows the importance of both spatial and spectral coverage in interpreting microwave bursts.