Shape Parameters of 1991 to 2016 Solar Corona

TL;DR

This study analyzes solar corona images from 1991 to 2016 to understand how the coronal flattening profile varies with solar activity, revealing anti-correlations with sunspot numbers and insights into coronal brightness changes.

Contribution

It introduces a method to model coronal flattening profiles using polynomial fits and analyzes their variation over multiple solar cycles, linking structural changes to solar activity.

Findings

Coronal flattening index and $R_{max}$ are anti-correlated with sunspot numbers.

Most coronal profiles can be modeled with a second-order polynomial.

Variation in $R_{max}$ reflects changes in coronal brightness profile over the solar equator.

Abstract

The global structure of solar corona observed in optical window is governed by the global magnetic field with different characteristics over solar activity cycle. Ludendorff flattening index becomes a popular measure of the global structure of solar corona as observed during eclipse. In this study, 15 digital images of solar corona from 1991 to 2016 were analyzed in order to construct the coronal flattening profiles as a function of radius. In most of the cases, the profile can be modeled with 2nd order polynomial function so that the radius with maximum flattening index ($R_{\text{max}}$) can be determined. Along with this value, Ludendorff index ($a+b$) was also calculated. Both Ludendorff index and $R_{\text{max}}$ show anti-correlation with monthly sunspot number, though the $R_{\text{max}}$ values are more scattered. The variation of $R_{\text{max}}$ can be regarded as the impact of changing coronal brightness profile over equator.