Short-Term Variations in the Equatorial Rotation Rate of Sunspot Groups

TL;DR

This study identifies multiple short-term and mid-term periodicities in the equatorial rotation rate of sunspot groups over several decades, revealing their association with solar cycle phases and providing insights into solar rotational dynamics.

Contribution

The paper uncovers specific periodicities in the solar equatorial rotation rate using long-term sunspot and velocity data, highlighting their temporal variations and relation to solar cycle phases.

Findings

Detected ~250-day periodicity mainly around 1991 and solar minima.

Identified ~1.4-year periodicity in velocity data during 1990-1995.

Found ~1.6-year and ~1.8-year periodicities around 1933-1955 and 1943.

Abstract

We have detected several periodicities in the solar equatorial rotation rate of sunspot groups in the Greenwich Photoheliographic Results (GPR) during the period 1931-1976, the Solar Optical Observing Network (SOON) during the period 1977-2014, and the Debrecen Photoheliographic Data (DPD) during the period 1974-2014. Our results suggest a ~250-day period in the equatorial rotation rate determined from both the Mt. Wilson Doppler-velocity data and the sunspot-group data during 1986-2007. However, a wavelet analysis reveals that this periodicity appears mostly around 1991 in the velocity data, while it is present in most of the solar cycles covered by the sunspot-group data, mainly near the minimum epochs of the solar cycles. We also found the signature of a period of ~1.4 years period in the velocity data during 1990-1995, and in the equatorial rotation rate of sunspot groups mostly around the year 1956. The equatorial rotation rate of sunspot groups reveals a strong ~1.6-year periodicity around 1933 and 1955 and a weaker one around 1976, and a strong ~1.8-year periodicity around 1943. Our analysis also suggests periodicities of ~5 years, ~7 years, and ~17 years as well as some other short-term periodicities. However, short-term periodicities are mostly present at the time of solar minima. Hence, short-term periodicities cannot be confirmed because of the larger uncertainty in the data.