Radius variations over a solar cycle

We report the analysis of solar radius determination for more than 3800 maps at 17 GHz from the Nobeyama Radioheliograph (NoRH) over a solar cycle (1992–2003). The aim of this work is to determine the radius dependence on solar activity at 17 GHz. This study was divided into two parts: (i) the mean solar radius calculated using the coordinates around the solar limb, and (ii) the mean polar radius using only the coordinates within ±30° of the poles. While a good correlation between the mean solar radius and the sunspot number was found, the polar radius variations were found to be anti-correlated with sunspot number during this period. Nevertheless, the results showed the polar radius to have the same temporal evolution as the polar limb brightening at 17 GHz throughout the solar cycle, indicating a close relationship between them. A variation of 3'' was found for the mean radius from minimum to solar maximum, while the polar radius presented a variation of 1'' during the same period. Moreover, the distribution of center-to-limb distances showed a large increase in the equatorial regions with a double peaked profile during periods of maximum activity. From this work, we have concluded that the measurements of the solar radius at radio frequencies are influenced by various solar features, which make the definition of the solar radius a difficult task. Despite this, the study of solar radius is able to provide a good indication of the changes in the solar atmosphere.