What determines the radio polar brightening?

¹ CRAAM, Instituto Nacional de Pesquisas Espaciais, São José dos Campos, SP 12201-970, Brasil e-mail: caius@craam.mackenzie.br
² CRAAM, Universidade Presbiteriana Mackenzie, São Paulo, SP 01302-907, Brasil

Received: 17 March 2005
Accepted: 9 May 2005

Abstract

In order to explain the bright patches of emission near the poles of 17 GHz solar maps, we have applied a previously developed atmospheric model based on radio observations. This 2-D model, which includes spicules, yields results in good agreement with brightness temperature values at the disk center, radius, and limb brightening 17 GHz observations at equatorial and intermediate regions. Nevertheless, the intensity of discrete bright patches observed near the poles in 17 GHz maps (as bright as 40% over the quiet Sun), can only be explained by holes in the spicule forest. These regions without spicules probably reflect the presence of polar faculae, which inhibit the appearance of spicules. Moreover, the absence of spicules over faculae explains the anti-correlation between the mean polar limb brightening and the solar cycle, because the polar faculae are known be anti-correlated with the solar cycle. Results from the model with spicule-less regions showed that: 1) for bare regions of the same size (5°), the limb brightening increases with the pole proximity; 2) larger regions yield more intense limb brightening; 3) the average height of the spicules greatly influences the solar radius; 4) the maximum excess brightness temperature of 40% is in very good agreement with the polar limb observations above bright patches.