Abstract:
We have used the daily values of the equatorial rotation rate determined from the Mt. Wilson daily Doppler-velocity measurements during the period 3 December 1985 – 5 March 2007 to search for periodicities in the solar equatorial rotation rate on time scales shorter than 11 years. After the daily values have been binned into 61-day intervals, a cosine fit with a period of one year was applied to the sequence to remove any seasonal trend. The spectral properties of this sequence were then investigated by using standard Fourier analysis, maximum-entropy methods, and a Morlet-wavelet analysis. From the analysis of the Fourier power spectrum we detected peaks with periodicities around 7.6, 2.8, and 1.47 years and 245, 182, and 158 days, but none of them were at a statistically significant level. In the Morlet-wavelet analysis the ≈1.47-year periodicity is detected only for 1990 (i.e., near the maximum of cycle 22) and near the end of cycle 22 in 1995. From the same wavelet analysis we found some evidence for the existence of a 2.8-year periodicity and a 245-day periodicity in the equatorial rotation rate around the years 1990 and 1992, respectively. In the data taken during the period 1996 – 2007, when the Mt. Wilson spectrograph instrumentation was more stable, we were unable to detect any signal from the wavelet analysis. Thus, the detected periodicities during the period before 1996 could be artifacts of frequent changes in the Mt. Wilson spectrograph instrumentation. However, the temporal behavior of most of the activity phenomena during cycles 22 (1986 – 1996) and 23 (after 1997) is considerably different. Therefore, the presence of the aforementioned short-term periodicities during the last cycle and absence of them in the current cycle may, in principle, be real temporal behavior of the solar rotation during these cycles.
