Statistical properties of coronal hole rotation rates: Are they linked to the solar interior?

¹ Center for Mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, 200 B, 3001 Leuven, Belgium
² Abastumani Astrophysical Observatory at Ilia State University, University St. 2, Tbilisi, Georgia
³ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: bidzina.shergelashvili@oeaw.ac.at
⁴ Combinatorial Optimization and Decision Support, KU Leuven campus Kortrijk, E. Sabbelaan 53, 8500 Kortrijk, Belgium
⁵ New Jersey Institute of Technology, Newark, NJ 07103, USA
⁶ Institute of Physics, IGAM, University of Graz, Universitätsplatz 5, 8010 Graz, Austria

Received: 30 December 2016
Accepted: 25 May 2017

Abstract

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 CHs 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 SDO/AIA193 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.