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

Abstract

We have detected several periodicities in the solar equatorial rotation rate of sunspot groups in the catalog 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. We have compared the results from the fast Fourier transform (FFT), the maximum entropy method (MEM), and the Morlet wavelet power-spectra of the equatorial rotation rates determined from SOON and DPD sunspot-group data during the period 1986 – 2007 with those of the Mount Wilson Doppler-velocity data during the same period determined by Javaraiah et al. (Solar Phys.257, 61, 2009). We have also compared the power-spectra computed from the DPD and the combined GPR and SOON sunspot-group data during the period 1974 – 2014 to those from the GPR sunspot-group data during the period 1931 – 1973. 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 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, 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.