A precise measurement of the solar differential rotation by tracing small bright coronal structures in SOHO-EIT images^*

Results and comparisons for the period 1998–2006

¹ Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, 79104 Freiburg, Germany e-mail: hw@kis.uni-freiburg.de
² Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia e-mail: romanb@geof.hr
³ Institut für Physik, IGAM, Universität Graz, Universitätsplatz 5, 8010 Graz, Austria e-mail: arnold.hanslmeier@uni-graz.at
⁴ SIDC, Royal Observatory of Belgium, Ave. Circulaire 3, 1180 Brussels, Belgium e-mail: sgissot@oma.be

Received: 6 August 2009
Accepted: 1 February 2010

Abstract

Aims. We precisely determine the solar rotation velocity during most of the 23rd solar cycle, in the years 1998-2006. We measure the solar differential rotation by tracing small bright coronal structures (SBCS) in SOHO-EIT images.

Methods. The 28.4 nm EIT channel was used and positions of more than 55 000 structures were measured applying an interactive and improved automatic method of data reduction.

Results. We achieve the closest representation of the observational data when all three solar differential rotation parameters are used and obtain the formula ω (b) = 14.499 (±0.006) – 2.54 (±0.06) sin² b – 0.77 (±0.09) sin⁴b. This result represents the sidereal rotation velocity in deg day^-1 and is produced by the automatic method applied in 1998-2006. A north-south rotational asymmetry and a rigid component of the solar rotation at high latitudes were found.

Conclusions. A more differential rotation profile of SBCS than of sunspots and sunspot groups was found. The rotation velocity of SBCS is very similar to those obtained by small photospheric magnetic features. The north-south rotational asymmetry of SBCS was interpreted with a model of the relationship between solar rotation and activity. The rigid component of the solar rotation at high latitudes, identifiable only from the results of the automatic method, was related to larger structures mostly identified by that method, in contrast to the interactive method, which detected smaller structures.