Microwave Polar Brightening and Its Connection to Polar Coronal Holes

TL;DR

This study analyzes 17 GHz microwave polar brightening over 26 years, revealing strong correlations with polar coronal hole areas and magnetic fields, and links to small-scale loop structures and bright points.

Contribution

It provides a comprehensive long-term analysis connecting microwave polar brightening with polar magnetic fields and coronal hole evolution using extensive observational data.

Findings

Microwave PB peak temperature correlates with PCH area.

Enhanced microwave emission regions are linked to small-scale loops and CBPs.

Microwave PB shows a strong relationship with polar magnetic field strength.

Abstract

Polar brightening (PB) observed at microwave frequencies serves as an important probe to study the thermal and magnetic properties in the Sun's polar regions. Building on earlier studies that linked microwave PB to polar faculae, small-scale loops, and the polar coronal holes (PCHs), we present a comprehensive analysis of the long-term behaviour of 17 GHz microwave PB and its relation to polar magnetic field and coronal hole evolution. Using daily Nobeyama Radioheliograph observations spanning 1992 to 2018, we quantify microwave PB peak temperature variations and compare them with the temporal evolution of PCH area extracted from SDO/AIA-based SPoCA coronal hole catalogues during the period 2010-2018. We also examine the correspondence between microwave PB and the polar magnetic field to assess the nature of their association. Our results show a strong correlation between microwave PB peak temperature and PCH area, as well as with the polar magnetic-field strength. In addition, we found that regions of enhanced microwave emission are frequently associated with small-scale loop structures, consistent with Coronal Bright Points (CBPs), which are often associated with the eruption of jets. Overall, this study aims to investigate the impact of coronal holes, polar magnetic fields, and small-scale polar activity on polar brightening observed at 17 GHz and its long-term evolution.