The Coronal Flattening Index at the 20 April 2023 Total Solar Eclipse and the Prediction of Solar Cycle 25

Abstract

The Ludendorff coronal flattening index is a quantitative parameter to analyze the global structure and shape of the corona. This index plays a crucial role in identifying solar magnetic activity and estimating the phase of the solar cycle. We observed a total solar eclipse on 20 April 2023 in Timor-Leste and obtained a Ludendorff coronal flattening index of \(0.109\pm 0.025\) by analyzing isophotes in white-light coronal images. Based on the composite image of the corona, streamers and plumes were observed extending in various directions across the solar disk, indicating that the Sun was in the ascending phase of its cycle. To establish the relationship between the coronal flattening index and the solar cycle phase, historical total solar eclipse data (1893 – 2013) were analyzed, focusing on smoothed sunspot numbers and flattening indices during the ascending phase. Two datasets, designated as “full” and “conservative”, were constructed considering temporal constraints relative to solar maxima and minima. The coronal morphology observed during the 20 April 2023 total solar eclipse corresponded to a premaximum phase, with values of \(0.673\pm 0.172\) and \(0.613\pm 0.171\) for the full and conservative datasets, respectively. We also developed a multilinear correlation and polynomial regression of second order models to predict the peak amplitude of the current solar cycle using both datasets. The full dataset predicted a peak on 3 December 2024 with amplitudes of \(173\pm 23\) and \(163\pm 21\) for the respective models. Conversely, the conservative dataset predicted a peak on 30 May 2025 with amplitudes of \(180\pm 24\) and \(180\pm 25\) for the respective models. These findings suggest that Solar Cycle 25 will likely be stronger than Solar Cycle 24.