The influence of spicules in the solar radius at multiple radio wavelengths

In this work, we analyze observations of the solar radius at 22 and 43 GHz obtained with the 13.7 m antenna of the Itapetinga Radio Observatory (Atibaia, Brazil) and at submillimeter-wave frequencies, 212 and 405 GHz, obtained by the Solar Submillimeter-wave Telescope (SST) (El Leoncito, San Juan, Argentina). The radius is defined as the limb position where the intensity is equal to half of the quiet Sun value. These measured radii are then compared with those predicted by a model of the solar atmosphere proposed by Selhorst, Silva, and Costa (2005). The results show that at 22 and 43 GHz, the emission comes from regions high in the chromosphere. Furthermore, the Itapetinga observations yield radii of 985" ± 5" and 981" ± 6", at 22 and 43 GHz respectively, consistent with the theoretical positions in the atmosphere. On other hand, the submillimeter observations resulted in a mean radius of 972" ± 3" and 975" ± 5" at 212 and 405 GHz, respectively, considered equal within the uncertainties. The latter results can be explained by the origin of the emission being very close to the region of minimum temperature, between the photosphere and chromosphere. This is a dynamic region largely affected by many solar features, like spicules and plages.