Global analysis of active longitudes of sunspots

Abstract

Context. Active longitudes have been found in various manifestations of solar activity. The longitudinal distribution of, e.g., sunspots and solar X-ray flares shows two persistent preferred longitudes separated by roughly 180 degrees. We previously studied solar X-ray flares using an improved version of a dynamic, differentially rotating coordinate system and found enhanced rotational asymmetry and rotation parameter values that are consistent for the three classes of X-ray flares. Aims. We aim to find the optimal values of rotation parameters of active longitudes of sunspots for several different time intervals and separately for the two solar hemispheres. Methods. We perform a global study of the longitudinal location of sunspots (all sunspots and first appearance sunspots) using a refined version of a dynamic, differentially rotating coordinate system. Results. We find that the rotation parameters for sunspots are in good agreement with those obtained for X-ray flares using the same method. The improved method typically finds somewhat faster equatorial rotation with better accuracy. The improved treatment also leads to a larger non-axisymmetry. Rotation parameters for all sunspots and first appearances closely agree with each other, but nonaxisymmetry is systematically larger for all sunspots than for first appearances, suggesting that strong fields follow more closely the pattern of active longitudes. The refined method emphasizes hemispheric differences in rotation. Over the whole interval, the mean rotation in the southern hemisphere is slower than in the north. We also find significant temporal variability in the two rotation parameters over the 136-year interval. Interestingly, the long-term variations (trends and residual oscillations) in solar rotation are roughly the opposite in the northern and southern hemispheres. Conclusions. Rotation parameters vary differently with time in the northern and southern hemispheres. Both sunspots and flares strongly suggest that the northern hemisphere rotated considerably faster but the southern hemisphere slightly slower than the Carrington rotation rate during the last three solar cycles.

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

“…The horizontal line gives the average (14.130 ± 0.030 deg/day) of the annual Ω 17 values in the two hemispheres over the whole time period. This value agrees well with our previous result (Zhang et al 2011b). Figure 1 shows that the rotation rates vary around the mean by up to ±(3−4)%.…”

“…We also studied how the weighting of sunspots by their areas can affects the best-fit rotation parameters and the evolution of solar hemispheric rotation. In some previous studies (Usoskin et al 2005;Zhang et al 2011b), sunspots were weighted by their area in the merit function (see below; this method is called here the weight method). Weighting sunspots by their areas emphasizes the effect of large sunspots when finding the best-fit values of the rotation parameters.…”

Active Longitude and Solar Flare Occurrences

“…By filtering the data with this limit, 53% of the CRs shows exactly one significant longitudinal belt; meanwhile, the remaining fraction of the data did not show strong in-homogeneous properties. Many studies have adressed the existence of the co-dominant longitude that has a phase shift of 180°from the dominant peak (Usoskin et al 2005;Zhang et al 2007Zhang et al , 2011a. However, we have found that the secondary AL is not, or is hardly detectable, after applying a high-pass filter to sort out the insignificant peaks.…”

“…The presumptions of the applied methods are different and they may have a considerable impact on the results. Possible presumptions are the following: a rigidly rotating frame carrying an active longitudinal domain as reported by Balthasar & Schüssler (1983), Kitchatinov & Olemskoy (2005), and Ivanov (2007); persistent ALs under the influence of differential rotation presented, e.g., by Usoskin et al (2005) and Zhang et al (2011a); two ALs at a distance of 180°as in Usoskin et al (2005). There were also skeptic views: Pelt et al (2005) showed that some results may be artefacts of the applied methods, Henney & Durney (2005) even questioned the existence of ALs.…”

The magnetic field in the solar atmosphere

“…Associated active features, including flares (Bai 2003;Zhang et al 2011) and coronal streamers (Li 2011) as well as superARs (i.e., ARs associated to repeated flaring and mass ejections, cf., Tian et al 2002) were found to be distributed inhomogeneously in solar longitude. They seemingly relate to "active nests" or "active longitudes", which had originally been postulated based on similar trends seen in surface magnetic field observations.…”