Microwave emission from a sunspot - II. The center-to-limb variation

A series of microwave observations of a sunspot in the active region NOAA 4741 was made with the Owens Valley Solar Array for the purpose of investigating the center-to-limb variation of both the spectral and spatial brightness distribution. In this investigation, several properties of the sunspot microwave radiation are found. First, sunspot microwave emission appears in two typical profiles depending on the heliocentric position of the spot: either the ring structure near disk center or single-peak structure near the limb. Second, the brightness temperature at high, optically thin frequencies (>6 GHz) increases slightly as the spot approaches the limb, which we interpret as being due to the increase of the gyroresonance opacity of the field lines near the spot center as they gain greater viewing angles. Third, the center-to-limb variation of the gyroresonance spectrum seems to be mostly characterized by a change of effective harmonic, which accompanies a discontinuous change of the degree of polarization. Fourth, a change of spectrum from gyroresonance to free-free emission is found in the passage of the spot over the solar limb, which gives a determination of the height of the gyroresonance layer to confirm its location low in the corona of the active region.