X-ray, EUV, and centimetric observations of solar active regions - an empirical model for bright radio sources

Coordinated observations of solar active regions in X-rays, EUV, and 2.8 cm radio waves, and current-free extrapolations of the photospheric magnetic field to coronal levels are used to determine the emission mechanism of bright radio sources. High spatial resolution X-ray and EUV observations of the active regions show that the temperature and density structure of the atmosphere above the sunspots differs greatly from that above the plages. Extended low brightness temperature radio sources, associated with plage areas, are consistent with thermal free-free emission from the transition region and the corona above the active centers. High brightness temperature radio sources, associated with sunspots, cannot be explained by thermal free-free emissions either in hot, dense loops or in the cooler, lower pressure loops, observed emanating from sunspot umbrae. There is evidence that thermal gyroresonance absorption at the second and third harmonics of the gyrofrequency can produce sunspots associated with bright radio sources. The empirical models of the coronal loops, based on energy balance, and the effects of electric currents flowing in the corona above the sunspots are discussed.