A Comparison of Solar Cycle Variations in the Equatorial Rotation Rates of the Sun's Subsurface, Surface, Corona, and Sunspot Groups

Abstract

Using the Solar Optical Observing Network (SOON) sunspot-group data for the period 1985 – 2010, the variations in the annual mean equatorial-rotation rates of the sunspot groups are determined and compared with the known variations in the solar equatorial-rotation rates determined from the following data: i) the plasma rotation rates at 0.94R⊙,0.95R⊙,…,1.0R⊙ measured by the Global Oscillation Network Group (GONG) during the period 1995 – 2010, ii) the data on the soft-X-ray corona determined from Yohkoh/SXT full-disk images for the years 1992 – 2001, iii) the data on small bright coronal structures (SBCS) that were traced in Solar and Heliospheric Observatory (SOHO)/EIT images during the period 1998 – 2006, and iv) the Mount Wilson Doppler-velocity measurements during the period 1986 – 2007. A large portion (up to ≈ 30∘ latitude) of the mean differential-rotation profile of the sunspot groups lies between those of the internal differential-rotation rates at 0.94R⊙ and 0.98R⊙. The variation in the yearly mean equatorial-rotation rate of the sunspot groups seems to be lagging behind that of the equatorial-rotation rate determined from the GONG measurements by one to two years. The amplitude of the GONG measurements is very small. The solar-cycle variation in the equatorial-rotation rate of the solar corona closely matches that determined from the sunspot-group data. The variation in the equatorial-rotation rate determined from the Mount Wilson Doppler-velocity data closely resembles the corresponding variation in the equatorial-rotation rate determined from the sunspot-group data that included the values of the abnormal angular motions (> |3∘| day−1) of the sunspot groups. Implications of these results are pointed out.