Consistent long-term variation in the hemispheric asymmetry of solar rotation

¹ Department of PhysicsUniversity of Oulu, 90914 Oulu, Finland
e-mail: liyun.zhang@oulu.fi
² Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
³ Sodankylä Geophysical Observatory, University of Oulu, 99600 Tähtelä, Finland

Received: 5 November 2012
Accepted: 26 February 2013

Abstract

Context. Solar active longitudes and their rotation have been studied for a long time using various forms of solar activity. However, the results on the long-term evolution of rotation rates and the hemispheric asymmetry obtained by earlier authors differ significantly from each other.

Aims. We aim to find a consistent result on the long-term migration of active longitudes of sunspots in 1877−2008 separately for the two hemispheres.

Methods. We used a dynamic, differentially rotating reference system to determine the best-fit values of the differential rotation parameters of active longitudes for each year in 1877−2008. With these parameters we determined the momentary rotation rates at the reference latitude of 17° and calculated the non-axisymmetries of active longitudes. We repeated this with five different fit intervals and two weighting methods and compared the results.

Results. The evolution of solar surface rotation in each hemisphere suggests a quasi-periodicity of about 80−90 years. The long-term variations of solar rotation in the northern and southern hemisphere have a close anti-correlation, leading to a significant 80−90-year quasi-periodicity in the north-south asymmetry of solar rotation. The north-south asymmetry of solar rotation is found to have an inverse relationship with the area of large sunspots. The latitudinal contrast of differential rotation is also found to be anti-correlated with the sunspot area. Different fit and weight methods yield similar results.

Conclusions. Our results give strong evidence for the anti-correlation of the rotation of the two solar hemispheres. The long-term oscillation of solar rotation suggests that a systematic interchange of angular momentum takes place between the two hemispheres at a period of about 80−90 years.