Low frequency radio observations of the `quiet' corona during the descending phase of sunspot cycle 24

TL;DR

This study analyzes the 'quiet' solar corona during the declining phase of sunspot cycle 24 using low-frequency radio observations, revealing a steady decrease in coronal size and electron density.

Contribution

It provides the first statistical analysis of the quiet corona's size and electron density decline during this solar cycle phase using simultaneous 53 MHz and 80 MHz radio data.

Findings

Coronal diameters shrank steadily at both frequencies.

Electron density in the corona decreased over time.

Radio observations agree with white-light coronagraph estimates.

Abstract

We carried out a statistical study of the `quiet' solar corona during the descending phase of the sunspot cycle 24 (i.e. 2015 January - 2019 May) using data obtained with the Gauribidanur RAdioheliograPH (GRAPH) at 53 MHz and 80 MHz simultaneously. Our results show that the equatorial (east-west) diameters of the solar corona at the above two frequencies shrunk steadily. The decrease was found to be due to a gradual reduction in the coronal electron density ($N_{e}$). Independent estimates of $N_{e}$ in the equatorial region of the `background' corona using white-light coronagraph observations indicate a decline consistent with our findings.