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

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.

Properties of sunspot cycles and hemispheric wings since the 19th century

“…The difference is positive . Relative (grey; right y-axis) and absolute (black; left y-axis) difference between the wing-averaged monthly GSN in the northern wing and the southern wing for cycles 7-10 and 12-24. during most of the previous century and depicts negative values at the end of the 19th and 20th century, indicating a systematic long-term variation in the hemispheric asymmetry, which is in a close agreement with earlier results (Zhang et al 2013). There is also some indication of a shorter variation of the difference at the period of about 5 cycles.…”

On the Long-Term Modulation of Solar Differential Rotation

“…Javaraiah et al (2005) and Suzuki (2012) showed that the cycleaveraged B varies quasi-periodically over a long-term span of eight or nine cycles, similar to the Gleissberg cycle found for sunspot number. Zhang et al (2013) found a similar quasi-periodic variation of 80 -90 years in the rotation rate of a latitude zone where sunspots are found to be prolific. At present, the value of the period seems to depend on the averaging time span and is open for further study.…”

“…They further suggested the possibility of an antisymmetric torsional oscillation of the Sun. Recently, Zhang et al (2013), with the analysis of the Greenwich data and USAF/NOAA (1877 -2008), found evidence of anti-correlation of the rotation in the two hemispheres and oscillatory behavior of the asymmetry at a period of 80 -90 years. However, previous results do not seem to be consistent, probably due to the difficulty of determining the rotation parameters in sunspot minimum phases, where numbers of sunspots may be very low.…”

Observations of the Solar Corona from Space

“…Furthermore, a systematic long-term north-south asymmetry of the sunspot rotation has been found, which oscillates with about a 80-90 year period. The hemispheric anticorrelation peaks around 1920 (fastest rotation in the southern hemisphere) and 1980 (fastest rotation in the northern hemisphere) and goes through a minimum in cycles 18-20 (Zhang et al 2013). These results point to the existence of a weak coupling of the magnetic field rotation in the opposite hemispheres, that is, a weak interdependence of the field systems originating in the two hemispheres.…”