Association of radio polar cap brightening with bright patches and coronal holes

TL;DR

This study models radio bright patches near solar poles as small magnetic loops within coronal holes, explaining their observed characteristics and their association with coronal holes and limb brightening.

Contribution

The paper introduces a model of small magnetic loops as the source of polar bright patches at 17 GHz, aligning simulations with observational data.

Findings

Bright patches are caused by unresolved small magnetic loops with dense, hot plasma.

Loops with increased plasma density and temperature in the lower atmosphere produce bright patches.

Small loops near the limb can merge with limb brightening, affecting observed brightness profiles.

Abstract

Radio-bright regions near the solar poles are frequently observed in Nobeyama Radioheliograph (NoRH) maps at 17 GHz, and often in association with coronal holes. However, the origin of these polar brightening has not been established yet. We propose that small magnetic loops are the source of these bright patches, and present modeling results that reproduce the main observational characteristics of the polar brightening within coronal holes at 17 GHz. The simulations were carried out by calculating the radio emission of the small loops, with several temperature and density profiles, within a 2D coronal hole atmospheric model. If located at high latitudes, the size of the simulated bright patches are much smaller than the beam size and they present the instrument beam size when observed. The larger bright patches can be generated by a great number of small magnetic loops unresolved by the NoRH beam. Loop models that reproduce bright patches contain denser and hotter plasma near the upper chromosphere and lower corona. On the other hand, loops with increased plasma density and temperature only in the corona do not contribute to the emission at 17 GHz. This could explain the absence of a one-to-one association between the 17 GHz bright patches and those observed in extreme ultraviolet. Moreover, the emission arising from small magnetic loops located close to the limb may merge with the usual limb brightening profile, increasing its brightness temperature and width.