To Rain or Not to Rain: Correlating GOES Flare Class and Coronal Rain Statistics

TL;DR

This study statistically analyzes the occurrence of coronal rain after solar flares across different GOES classes, revealing correlations with flare strength, solar cycle activity, and loop characteristics, and suggesting new insights into solar flare dynamics.

Contribution

It provides the first comprehensive statistical overview of coronal rain occurrence in relation to GOES flare class and solar cycle, highlighting key thresholds and correlations.

Findings

Coronal rain occurs in nearly all X and most M-class flares.

Occurrence of rain drops sharply below C5 flare class.

Positive correlation between GOES class and rain duration.

Abstract

Post-flare arcades are well-known components of solar flare evolution, which have been observed for several decades. Coronal rain, cascades of catastrophically-cooled plasma, outline the loops and provide eye-catching evidence of the recent flare. These events are acknowledged to be common, but the scientific literature does not include any statistical overview documenting just how common the phenomenon actually is. This study reviews Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO AIA) observations of 241 flares collected from the Space Weather Prediction Center (SWPC) database between 2011 and 2018. The flares cover the entire strength range of the C, M, and X GOES classes, and are distributed evenly across the SDO-observed majority of Solar Cycle 24. We find that post-flare arcade rain occurs for nearly all X and most M-class flares, but that it tapers off rapidly within C-class flares. There appears to be a cut-off point around C5, below which the occurrence of post-flare arcade rain drops significantly. There is also a general positive correlation between GOES class and the average duration of post-flare rain events. Post-flare arcade rain events in C-class flares appear to track with the sunspot number, providing a potential new tool for estimating, if not predicting, solar cycle strength. Furthermore, condensations appear to be suppressed in the shortest-length arcade loops of any class observed, suggesting that active region heating is height-constrained. These results open up further avenues for future research, including new methods to estimate energy deposition and to gain greater insight into steady active region heating.