Cyclic Changes in Solar Rotation Inferred From Temporal Changes in the Mean Magnetic Field

Time–frequency variability of the solar mean magnetic field (SMMF) was studied, based on a continuous wavelet analysis. The rotational modulation of the SMMF dominates the wavelet spectrum at 27–30 and 13.5-day time scales. The rotational variation, in turn, is amplitude-modulated by the quasi-biennial periodicity in the SMMF. This is caused by magnetic field eruptions. Rigidly rotating modes appear in the time–longitude distribution of the large-scale magnetic field that is plotted from a deconvolution of the SMMF time series with a Carrington period. These rotational modes coexist and transform into one another over an 11-yr cycle. The modes with periods of 27.8–28.0 days dominate the phase of activity rise, whereas the 27-day rotational mode dominates the declining phase of the 11-yr cycle. The rotational modes with periods of 29–30 days occurred episodically. Most of the features in the time–longitude distribution of the SMMF are identifiable with those in similar diagrams of the solar background magnetic fields. They represent a combined effect of the background magnetic fields from both hemispheres. Eruptions of magnetic fields lead to dramatic changes in the picture of solar rotation and correlate well with the polarity asymmetry in the SMMF signal. The polarity asymmetry in the SMMF time series exhibits both long-term changes and a 22-yr cyclic behaviour, depending on the reversals of the global magnetic field in cycles 20–23.