Long-Term Variations in the Growth and Decay Rates of Sunspot Groups

TL;DR

This study analyzes long-term data on sunspot groups to reveal significant variations in their growth and decay rates over a 135-year period, highlighting a notable decrease in growth rates during recent solar cycles.

Contribution

It provides a comprehensive analysis of long-term variations in sunspot group growth and decay rates using combined historical and recent data, revealing cyclical patterns and recent declines.

Findings

Growth rates vary by about 35% over 60 years.

Decay rates vary by about 13% over 60 years.

Growth rate decreased significantly from the start of Cycle 23 to 2008.

Abstract

Using the combined Greenwich (1874-1976) and Solar Optical Observatories Network (1977-2009) data on sunspot groups, we study the long-term variations in the mean daily rates of growth and decay of sunspot groups. We find that the minimum and the maximum values of the annually averaged daily mean growth rates are ~52% per day and ~183% per day, respectively, whereas the corresponding values of the annually averaged daily mean decay rates are ~21% per day and ~44% per day, respectively. The average value (over the period 1874-2009) of the growth rate is about 70% more than that of the decay rate. The growth and the decay rates vary by about 35% and 13%, respectively, on a 60-year time-scale. From the beginning of Cycle 23 the growth rate is substantially decreased and near the end (2007-2008) the growth rate is lowest in the past about 100 years.