MICROWAVE STRUCTURE OF THE QUIET SUN AT 8.5 GHZ

We present multifrequency VLA observations of the quiet Sun near 8.5 GHz, obtained on 1988 September 7-8. The observations on the first day correspond to a decayed (spotless) active region, while those of the second day correspond to a region of primarily unipolar supergranular network. The full-day synthesis maps for both days show nearly perfect correspondence to Ha images, and to longitudinal magnetograms, as has been found previously at 4.9 GHz. A unique aspect of the observations of the decayed active region is the clear indication of coronal loops. The loops appear as regions of radio emission with no underlying longitudi-nal magnetic fields, are aligned with Hα fibrils in the photosphere, and connect regions of opposite magnetic polaritv. The emission can be successfully modeled as optically thin free-free emission from a coronal loop with a peak axial density of ∼2.4-2.8 × 109 cm-3, for an assumed coronal temperature of 1-2 × 106 K. The brightest sources other than active regions are close dipoles. Besides dipoles, the quiet chromosphere sources are found to fall into three classes: network elements, Tb = 1.1-2.0 × 104 K; cell interiors, 6000-9000 K, and filament channels, or horizontal field regions, 4000-6000 K. We discuss the significance of these measurements for existing chromospheric models, most of which give much higher brightness temperatures. Because the radio images correspond closely to the true structure of the chromosphere, they change as the magnetic fields do ; examples of this are given.