Low-frequency radio observations of the “quiet” corona during the descending phase of sunspot cycle 24

Abstract

We carried out a statistical study of the “quiet” solar corona during the descending phase of the Sunspot Cycle 24 (i.e., January 2015 to May 2019) using data obtained with the Gauribidanur RAdioheliograPH (GRAPH) at 53 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 (Ne). Independent estimates of Ne in the equatorial region of the “background” corona using white-light coronagraph observations indicate a decline consistent with our findings.