Context. The present paper discusses results of a statistical study of the characteristics of coronal hole (CH) rotation in order to find connections to the internal rotation of the Sun. Aims. The goal is to measure CH rotation rates and study their distribution over latitude and their area sizes. In addition, the CH rotation rates are compared with the solar photospheric and inner layer rotational profiles. Methods. We study coronal holes observed within ±60 • latitude and longitude from the solar disc centre during the time span from the 1 January 2013 to 20 April 2015, which includes the extended peak of solar cycle 24. We used data created by the Spatial Possibilistic Clustering Algorithm (SPoCA), which provides the exact location and characterisation of solar coronal holes using S DO/AIA 193 Å channel images. The CH rotation rates are measured with four-hour cadence data to track variable positions of the CH geometric centre.Results. North-south asymmetry was found in the distribution of coronal holes: about 60 percent were observed in the northern hemisphere and 40 percent were observed in the southern hemisphere. The smallest and largest CHs were present only at high latitudes. The average sidereal rotation rate for 540 examined CHs is 13.86 (±0.05) • /d. Conclusions. The latitudinal characteristics of CH rotation do not match any known photospheric rotation profile. The CH angular velocities exceed the photospheric angular velocities at latitudes higher than 35-40 degrees. According to our results, the CH rotation profile perfectly coincides with tachocline and the lower layers of convection zone at around 0.71 R ⊙ ; this indicates that CHs may be linked to the solar global magnetic field, which originates in the tachocline region.
The properties of the differential rotation of the Sun are investigated by using Hα filaments as tracers. Annual average angular velocities of 716 quiescent filaments are determined from Hα photoheliograms of the Abastumani Astrophysical Observatory film collection for the years 1957-1993. The existence of north-south (N-S) asymmetry in Hα filaments rotation is confirmed statistically. The connection of asymmetry with the solar activity cycles is established. It is found that the northern hemisphere rotates faster during the even cycles (20 and 22) while the rotation of southern hemisphere dominates in odd ones (cycles 19 and 21). The mechanism of the solar activity should be responsible for the N-S asymmetry of the solar differential rotation. A theoretical explanation for the N-S asymmetry in the Sun's rotation is offered. It is suggested that the asymmetry in the rotation of the two hemispheres of the Sun is balanced by the dynamo mechanism, which acts in parallel to the mechanism offered here. It is concluded that the N-S asymmetry of the solar rotation should cause a difference in activity level between the northern and southern hemispheres.
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