Increasing Lifetime of Recurrent Sunspot Groups within the Greenwich Photoheliographic Results

TL;DR

This study uses machine learning to analyze long-term changes in recurrent sunspot group lifetimes from historical data, revealing a century-long increase followed by a decline, with implications for solar activity understanding.

Contribution

The paper introduces a machine learning approach to identify recurrent sunspot groups in historical data, enabling analysis of lifetime changes over a century.

Findings

Sunspot group lifetime increased by 1.4 days between 1915 and 1940.

Recurrent sunspot groups exhibit a larger Gnevyshev-Waldmeier Relationship.

Evidence suggests a decline in sunspot group lifetime after 1950.

Abstract

Long-lived (>20 days) sunspot groups extracted from the Greenwich Photo- heliographic Results (GPR) are examined for evidence of decadal change. The problem of identifying sunspot groups which are observed on consecutive solar rotations (recurrent sunspot groups) is tackled by first constructing manually an example dataset of recurrent sunspot groups and then using machine learning to generalise this subset to the whole GPR. The resulting dataset of recurrent sunspot groups is verified against previous work by A. Maunder and other Royal Greenwich Observatory (RGO) compilers. Recurrent groups are found to exhibit a slightly larger value for the Gnevyshev-Waldmeier Relationship than the value found by Petrovay and van Driel-Gesztelyi (Solar Phys. 51, 25, 1997), who used recurrence data from the Debrecen Photoheliographic Results. Evidence for sunspot group lifetime change over the previous century is observed within recurrent groups. A lifetime increase of 1.4 between 1915 and 1940 is found, which closely agrees with results from Blanter et al. (Solar Phys. 237, 329, 2006). Furthermore, this increase is found to exist over a longer period (1915 to 1950) than previously thought and provisional evidence is found for a decline between 1950 and 1965. Possible applications of machine-learning procedures to the analysis of historical sunspot observations, the determination of the magnetic topology of the solar corona and the incidence of severe space-weather events are outlined briefly.