THE BEHAVIOR OF THE 17 GHz SOLAR RADIUS AND LIMB BRIGHTENING IN THE SPOTLESS MINIMUM XXIII/XXIV

The current solar minimum has surprised the entire solar community because the spotless period is presently almost 2-3 years longer than the usual minima. To better understand this, we studied the variation of the solar radius and the polar limb brightening at 17 GHz, comparing the results from the minimum at the end of cycle XXIII with those of the previous one. Daily maps obtained by the Nobeyama Radioheliograph (NoRH) from 1992 through 2010 were analyzed. Whereas the variation of the solar radius at radio frequencies indicates the heating of the solar atmosphere due to solar activity, the limb brightening intensity depends on the organization of the polar magnetic field of the Sun, including the global dipole and the features formed around it. These features are more prominent during minima periods. As a common result, researchers have observed a decrease in both radius and limb brightness intensity at 17 GHz during the present minimum when compared with the previous one. The mean solar radius is 0.''9 ± 0.''6 smaller and the limb brightening reduced its intensity by around 20%. Both decrements are interpreted in terms of the weaker solar chromospheric activity of the present cycle. Measurement of the radius and limb brightening at 17 GHz can be used as an alternative solar activity index and should be included in the set of parameters used to predict future cycles.