THE STRUCTURE OF THE SOLAR CORONA ABOVE SUNSPOTS AS INFERRED FROM RADIO, X-RAY, AND MAGNETIC-FIELD OBSERVATIONS

We present observations of a solar active region, NOAA/USAF no. 7123, during 1992 April 3-10. The database includes high-angular-resolution radio, soft X-ray, magnetograph, and Ha observations. The radio observ ations include VLA maps in the Stokes I and V parameters at 4.7 and 8. 4 GHz. The soft X-ray observations were obtained by the Soft X-Ray Tel escope on board the Yohkoh satellite, the magnetograms were obtained a t Kitt Peak, Mt. Wilson, and Big Bear, and the Ha data were obtained a t Big Bear. The lead sunspot in the active region is studied here. In particular, the polarization properties and brightness temperature spe ctrum are used to constrain the thermal structure of the corona over t he sunspot. It is found that the 4.7 GHz emission of the sunspot is po larized in the sense of the ordinary mode, in contradiction with simpl e gyroresonance models that predict that the spot should be polarized in the sense of the extraordinary mode. We model the spectral and temp oral evolution of the polarization structure in two frequencies, 4.7 a nd 8.4 GHz, using gyroresonance models to fit one-dimensional brightne ss temperature profiles across the spot in each polarization and frequ ency. The constraints provided by the X-ray and magnetic field observa tions help us to derive a qualitatively self-consistent picture for th e daily evolution of the spot. We attribute the excess of the o-mode e mission to the magnetic field configuration and to the temperature inh omogeneities across the spot. Namely, we find that (1) the umbral and penumbral environments are distinct, with the X-rays and the o-mode ra dio emission coming from the hotter penumbral loops, while the observe d x-mode emission originates from the cooler umbral loops; (2) there e xist temperature inhomogeneities in both the radial and vertical direc tion over the spot; and (3) the umbral magnetic field remains more con fined in the corona than that predicted by a dipole model. Instead, a field configuration based on the magnetohydrostatic equilibrium model of Low gives a better agreement with the observations.