Free access article

A&A 374, 309-315 (2001)
DOI: 10.1051/0004-6361:20010694

Solar differential rotation determined by tracing coronal bright points in SOHO-EIT images

I. Interactive and automatic methods of data reduction

R. Brajsa^{1, 2}, H. Wöhl¹, B. Vrsnak², V. Ruzdjak², F. Clette³ and J.-F. Hochedez<sup>3

1</sup>  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, Kaciceva 26, 10000 Zagreb, Croatia
    e-mail: romanb@geodet.geof.hr; bvrsnak@geodet.geof.hr; vruzdjak@geodet.geof.hr
³  Observatoire Royal de Belgique (ORB), Ave. Circulaire 3, 1180 Bruxelles, Belgium
    e-mail: Frederic.Clette@oma.be; hochedez@oma.be

(Received 19 February 2001 / Accepted 4 May 2001 )

Abstract
Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) were used to analyse solar differential rotation determined by tracing coronal bright points. Two different procedures were developed and compared: an interactive and an automatic method. The interactive method is based on the visual tracing of coronal bright points in consecutive images using computer programs written in the Interactive Data Language (IDL). The automatic method relies on the IDL procedure "Regions Of Interest (ROI) segmentation" which is used to detect and follow bright points in triplets of consecutive images. The test-results obtained applying both methods by different persons who performed tracing are presented and compared. The advantages and disadvantages of the two methods are discussed.

Key words: Sun: corona -- Sun: UV radiation -- Sun: rotation

Offprint request: R. Brajsa, rbrajsa@kis.uni-freiburg.de

© ESO 2001

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