Multifrequency observations of a solar active region during a partial eclipse

Spatially-resolved microwave observations of a complex active region at 16 frequencies in the range 1.4-8 GHz are presented. One-dimensional profiles of the active region at each frequency were obtained with the Owens Valley frequency-agile interferometer, with 2.6-arcsec spatial resolution provided by the lunar limb during a partial eclipse. Simultaneous two-dimensional maps were obtained with the VLA at 1.45 and 4.9 GHz. The combination of spatial and spectral resolution allows the direct observation of the change in emission mechanism from predominantly free-free emission from dense active region loops at low frequencies to gyroresonance emission from regions of intense magnetic fields at high frequencies. The change occurs, for this active region, at about 3 GHz. The implications of the brightness temperature spectrum for the coronal structure of the active region are discussed.