The Magnetic Classification of Solar Active Regions 1992 - 2015

TL;DR

This study analyzes the variation of Mount Wilson magnetic classifications of solar active regions from 1992 to 2015, revealing significant changes correlated with the solar cycle and providing insights into the formation of complex active regions.

Contribution

It provides the first comprehensive statistical analysis of magnetic classifications over multiple solar cycles using modern observations.

Findings

Alpha and beta classes constitute about 20% and 80% of active regions.

The fraction of alpha regions increases near solar minima.

Complex classifications gamma and delta are more common during solar maxima.

Abstract

The purpose of this letter is to address a blind-spot in our knowledge of solar active region statistics. To the best of our knowledge there are no published results showing the variation of the Mount Wilson magnetic classifications as a function of solar cycle based on modern observations. We show statistics for all active regions reported in the daily Solar Region Summary from 1992 January 1 to 2015 December 31. We find that the $\alpha$ and $\beta$ class active regions (including all sub-groups e.g. $\beta\gamma$, $\beta\delta$) make up fractions of approximately 20% and 80% of the sample respectively. This fraction is relatively constant during high levels of activity, however, an increase in the $\alpha$ fraction to about 35% and and a decrease in the $\beta$ fraction to about 65% can be seen near each solar minimum and is statistically significant at the 2-$\sigma$ level. Over 30% of all active regions observed during the years of solar maxima were appended with the classifications $\gamma$ and/or $\delta$, while these classifications account for only a fraction of a percent during the years near the solar minima. This variation in the active region types indicates that the formation of complex active regions may be due to the pileup of frequent emergence of magnetic flux during solar maximum, rather than the emergence of complex, monolithic flux structures.