Polar and limb brightening in ALMA images

Written by Ivica Skokic on . Posted in News

thumb Sudar limb brighteningIn a series of papers our team analyzed the effects of limb brightening and polar brightening in ALMA solar images. Limb brightening is an effect analogous to the familiar limb darkening visible in images of the solar photosphere which is explained as a consequence of seeing deeper (and hotter) layers of the photosphere near the center of the Sun. Near the limb, due to geometry, a shallower (and colder) photospheric layers are observed and hence they appear darker. However, as explained in the paper by Sudar et al. "Centre to Limb Brightness Variations from ALMA Full Disk Solar Images",  in chromospheric images of the Sun made with ALMA, due to inversion of the temperature profile when the temperature starts to rise again towards corona, an opposite effect is observed - limb brightening. In the paper, authors develop a method to remove the center-to-limb function from the data. In two papers by Sudar et al. "Polar Brightening in ALMA Full-disk Solar Images" and by Selhorst et al. "Solar Polar Brightening and Radius at 100 and 230 GHz Observed by ALMA", polar brightenings of the Sun at ALMA radio frequencies are studied and compared with several theoretical models. This can help in constraining different models and parameters of the solar atmosphere.