Simon White

A model of the atmosphere above a sunspot from radio observations

The results of VLA observations of an unusual source of microwave radiation, associated with the sunspot NOAA 7789 on October, 15, 1994, are presented. The fine structure of the source, which is a ring structure in intensity and polarization at frequencies 4.5 and 8.0 GHz, is discussed. It is shown that the features observed can be explained by a thermal cyclotron mechanism if the magnetic field is approximated by a vertical dipole buried under the photosphere, but the spatial distributions of kinetic temperature and electron density in the atmosphere above the sunspot differ considerably from the standard model. A two-dimensional source model (the dependences of the parameters on the height and distance from the center of the sunspot), which fits the observations at the above frequencies, is evolved. The principal physical result is that the data observed are explainable by the presence of an unexpectedly dense cool plasma in the atmosphere over the center of the umbra.