What determines the radio polar brightening?

Abstract

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.

Association of Radio Polar Cap Brightening with Bright Patches and Coronal Holes

“…The simulations were based on the temperature and density distributions proposed in the SSC atmospheric model (Selhorst et al 2005a), with modifications to include a coronal hole atmospheric model and magnetic loops as the sources of the radio bright patches.…”

“…The atmospheric model Selhorst et al (2005a) proposed an atmospheric model (hereafter referred to as the SSC model) with the distributions of temperature and density (electron and proton) as a…”

“…• of latitude are not able to reproduce the intense bright patches observed in the NoRH maps (Selhorst et al 2005b) and the presence of coronal holes reduce the expected limb brightening at 17 GHz (Figure 4), other solar features should be acting inside the coronal holes to increase their brightness temperature at 17 GHz (Gopalswamy et al 1999;Oliveira e Silva et al 2016). To simulate the observed 17 GHz bright patches, we introduced small magnetic loops inside coronal hole regions (Figure 2).…”

The Chromospheric Solar Limb Brightening at Radio, Millimeter, Sub-Millimeter, and Infrared Wavelengths

“…Further investigations conducted by Selhorst et al (2005) showed that the spicules could modulate the morphology of the limb brightening profile. However, the empirical chromospheric model used by Selhorst et al (2005) is not necessarily a good approximation for the physical conditions in the lower chromosphere (Carlsson & Stein 2002;De la Luz et al 2011) when considering a fully ionized chromosphere. Observations of the solar limb at Hα (Skogsrud et al 2014) clearly show spicules in the upper chromosphere.…”

“…The role of the fine structure involved in the quiet Sun emission (spicules) and its relation to the limb brightening at millimeter wavelengths was discussed in the first semi-empirical models (Fuerst et al 1979;Ahmad & Kundu 1981). Further investigations conducted by Selhorst et al (2005) showed that the spicules could modulate the morphology of the limb brightening profile. However, the empirical chromospheric model used by Selhorst et al (2005) is not necessarily a good approximation for the physical conditions in the lower chromosphere (Carlsson & Stein 2002;De la Luz et al 2011) when considering a fully ionized chromosphere.…”