MULTIBAND VLA OBSERVATIONS OF SOLAR ACTIVE REGIONS - IMPLICATIONS FORTHE DISTRIBUTION OF CORONAL PLASMA

We present high-quality radio observations of a solar active region (N OAA/USAF region 5131) made by the Very Large Array in the 0.33, 1.5, 4 .7, and 8.4 GHz frequency bands. The observations were made during the LAU-sanctioned International Solar Month on 1988 September 1 and 4. I n addition to the radio maps, the database includes images from the So lar Maximum Mission Soft X-ray Polychromator in the Fe XVII line, phot ospheric magnetograms, and H alpha filtergrams. Because coverage in on ly a single wavelength is available, the soft X-ray data are used for qualitative comparisons. We identify those areas of the active region from which the radio emission is predominantly due to thermal bremsstr ahlung emission. To account for the observed four-point radio spectra in these areas, we examine three different models, which characterize the corona in terms of a single plasma component, two components, and multiple components. The latter differs somewhat from past models. In particular, we take explicit account of the highly inhomogeneous struc ture of active regions through a multilayer slab model in which the th ermal distribution is constrained by the generic form of the different ial emission measure. While multicomponent models provide the best agr eement with the observations, we find that a complete model must inclu de both free-free and gyroresonance opacity. We compare our radio data and model results with those resulting from previous studies.