Solar differential rotation in the period 1964–2016 determined by the Kanzelhöhe data set

¹ Physics DepartmentUniversity of Rijeka, Radmile Matejčić 2, 51000 Rijeka, Croatia
e-mail: ipoljancic@phy.uniri.hr
² Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia
³ Zagreb Astronomical Observatory, Opatička 22, 10000 Zagreb, Croatia
⁴ Kanzelhöhe Observatory for Solar and Environmental Research, University of Graz, Kanzelhöhe 19, 9521 Treffen am Ossiacher See, Austria
⁵ Institute for Geophysics, Astrophysics and Meteorology, Institute of Physics, University of Graz, Universitätsplatz 5, 8010 Graz, Austria
⁶ Astronomical Institute of the Czech Academy of Sciences, Fričova 298, 25165 Ondřejov, Czech Republic
⁷ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany

Received: 26 April 2017
Accepted: 5 July 2017

Abstract

Context. Kanzelhöhe Observatory for Solar and Environmental Research (KSO) provides daily multispectral synoptic observations of the Sun using several telescopes. In this work we made use of sunspot drawings and full disk white light CCD images.

Aims. The main aim of this work is to determine the solar differential rotation by tracing sunspot groups during the period 1964–2016, using the KSO sunspot drawings and white light images. We also compare the differential rotation parameters derived in this paper from the KSO with those collected fromf other data sets and present an investigation of the north – south rotational asymmetry.

Methods. Two procedures for the determination of the heliographic positions were applied: an interactive procedure on the KSO sunspot drawings (1964–2008, solar cycles Nos. 20–23) and an automatic procedure on the KSO white light images (2009–2016, solar cycle No. 24). For the determination of the synodic angular rotation velocities two different methods have been used: a daily shift (DS) method and a robust linear least-squares fit (rLSQ) method. Afterwards, the rotation velocities had to be converted from synodic to sidereal, which were then used in the least-squares fitting for the solar differential rotation law. A comparison of the interactive and automatic procedures was performed for the year 2014.

Results. The interactive procedure of position determination is fairly accurate but time consuming. In the case of the much faster automatic procedure for position determination, we found the rLSQ method for calculating rotational velocities to be more reliable than the DS method. For the test data from 2014, the rLSQ method gives a relative standard error for the differential rotation parameter B that is three times smaller than the corresponding relative standard error derived for the DS method. The best fit solar differential rotation profile for the whole time period is ω(b) = (14.47 ± 0.01)−(2.66 ± 0.10)sin²b (deg/day) for the DS method and ω(b) = (14.50 ± 0.01)−(2.87 ± 0.12)sin²b (deg/day) for the rLSQ method. A barely noticeable north – south asymmetry is observed for the whole time period 1964–2016 in the present paper. Rotation profiles, using different data sets, presented by other authors for the same time periods and the same tracer types, are in good agreement with our results.

Conclusions. The KSO data set used in this paper is in good agreement with the Debrecen Photoheliographic Data and Greenwich Photoheliographic Results and is suitable for the investigation of the long-term variabilities in the solar rotation profile. Also, the quality of the KSO sunspot drawings has gradually increased during the last 50 yr.