Shape parameters of the solar corona from 1991 to 2016

Abstract

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

Coronal Flattening Index of Total Solar Eclipse on 20 April 2023 and the Prediction of Solar Cycle 25 Maximum

“…In general, the abundance of streamer and plume features scattered all around the solar disk indicates that the Sun was in the ascending phase of the cycle. This is consistent with the previous solar observations which showed symmetrical structures during the ascending phase toward the solar cycle maximum (Priyatikanto, 2016).…”

“…The shape is nearly elliptical as it extends from both sides. Meanwhile, during the solar maximum, the corona appears more symmetrical and rounded as streamers are found in nearly all parts of the Sun (Priyatikanto, 2016). The appearance of the corona reflects the complexity of the Sun's magnetic field during the solar maximum (Sadovenko and Pishkalo, 2007).…”

“…Several studies have been conducted regarding the analysis of the solar cycle based on the structure and shape of the corona observed during TSEs (Pishkalo, 2011;Pishkalo, 2022, and references therein). Based on the past TSE data, the calculated flattening index values have ranged from 0 to 0.4, and they are inversely correlated with the solar cycle (Pishkalo, 2011;Priyatikanto, 2016).…”

Application of historic datasets to understanding open solar flux and the 20th-century grand solar maximum. 2. Solar observations

“…There have been other, more quantitive, analyses of the form of the corona in eclipse images, for example using isophotes to define the "coronal flattening" index, ϵ. This has been generated for a number of eclipses by a variety of authors (Pishkalo, 2011;Marzouk et al, 2016;Priyatikanto, 2016;Rušin, 2017). The results from different studies of the same eclipse show some degree of agreement as well as some differences.…”