Solar surface rotation: N-S asymmetry and recent speed-up

¹ ReSoLVE Centre of Excellence, Department of PhysicsUniversity of Oulu, 90014 Oulu, Finland
e-mail: liyun.zhang@oulu.fi
² Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, 100875 Beijing, PR China
³ Sodankylä Geophysical Observatory, University of Oulu, 90014 Oulu, Finland

Received: 16 October 2014
Accepted: 20 January 2015

Abstract

Context. The relation between solar surface rotation and sunspot activity is still not fully resolved. The sunspot activity has been significantly reduced in solar cycle 24, and several solar activity indices and flux measurements experienced unprecedentedly low levels during the last solar minimum.

Aims. We aim to reveal the momentary variation of solar surface rotation, especially during the recent years of reduced solar activity.

Methods. We used a dynamic, differentially rotating reference system to determine the best-fit annual values of the differential rotation parameters of active longitudes of solar X-ray flares and sunspots in 1977–2012.

Results. The evolution of the rotation of solar active longitudes obtained from observing X-ray flares and sunspots is very similar. Both hemispheres have increased their rotation rate since the late 1990s, with the southern hemisphere rotating slightly faster than the north. In the 1980s, rotation in the northern hemisphere was considerably faster, but it experienced a major decrease in the early 1990s. On the other hand, little change was found in the rotation of the southern hemisphere during these decades. This led to a positive asymmetry in the north-south rotation rate in the early part of the time interval studied.

Conclusions. The rotation of both hemispheres has been speeding up at roughly the same rate since the late 1990s, with the southern hemisphere rotating slightly faster than the northern hemisphere. This period coincides with the start of a significant weakening of the solar activity, as observed in sunspots and several other solar, interplanetary, and geomagnetic parameters.