Long-term Variations of Solar Differential Rotation and Sunspot Activity: Revisited

TL;DR

This study re-analyzes long-term data on solar differential rotation and sunspot activity, revealing secular trends such as decreasing equatorial rotation rate and increasing sunspot area, with implications for understanding solar dynamics.

Contribution

It provides a comprehensive re-analysis of historical data, identifying secular trends in solar rotation parameters and sunspot activity with statistical significance.

Findings

Equatorial rotation rate shows a secular decrease since cycle 12.

Sunspot area exhibits a secular increase since cycle 12.

Negative correlation between sunspot activity level and equatorial rotation.

Abstract

Long-term variations of solar differential rotation and sunspot activity are investigated through re-analyzing the data on parameters of the differential rotation law obtained by Makarov, Tlatov, and Callebaut (1997), Javaraiah, Bertello, and Ulrich (2005a, b), and Javaraiah et al. (2009). Our results indicate that the solar surface rotation rate at the Equator (indicated by the A parameter of the standard solar rotation law) shows a secular decrease since cycle 12 onwards, given by about $1\,-\,1.5\times10^{-3}$($deg\ day^{-1} year^{-1}$). The B parameter of the standard differential rotation law seems to also show a secular decrease since cycle 12 onwards, but of weak statistical significance. The rotation rate averaged on latitudes ($0^{o}\,--\,40^{o}$) does not show a secular trend of statistical significance. Moreover, the average sunspot area shows a secular increase of statistical significance since cycle 12 onwards, while a negative correlation is found between the level of sunspot activity (indicated by the average sunspot area) and the solar equatorial rotation in the long run.