Properties of the solar velocity field indicated by motions of coronal bright points

¹ Hvar Observatory, Faculty of Geodesy, University of Zagreb, Kačićeva 26, 10000 Zagreb, Croatia e-mail: [romanb; vruzdjak]@geodet.geof.hr
² Kiepenheuer-Institut für Sonnenphysik (KIS), Schöneckstr. 6, 79104 Freiburg, Germany e-mail: rbrajsa@kis.uni-freiburg.de; hw@kis.uni-freiburg.de
³ Observatoire Royal de Belgique (ORB), Av. Circulaire 3, 1180 Bruxelles, Belgium e-mail: Frederic.Clette@oma.be; hochedez@oma.be

Corresponding author: B. Vršnak, bvrsnak@geodet.geof.hr

Received: 22 January 2003
Accepted: 27 March 2003

Abstract

Full-disc solar images obtained with the Extreme Ultraviolet Imaging Telescope (EIT) on board the Solar and Heliospheric Observatory (SOHO) are used to analyse properties of the solar velocity field by tracing coronal bright points from June 4, 1998 to May 22, 1999. Rotation velocity residuals, meridional motions and their relationship are investigated. Zones of slow and fast rotation found in motions of coronal bright points are consistent with the pattern of torsional oscillations, indicating that the statistical velocity pattern of bright point motions reflects the large-scale plasma flows. A complex pattern of meridional motion is deduced: The equatorward flows are found to dominate at low () and high () latitudes, whereas at mid-latitudes () a poleward flow is inferred. The complete data set shows no significant correlation between rotation residuals and meridional motions. However, when a subsample of coronal bright points including only the “point-like structures” (predominantly young bright points) is considered, a statistically significant correlation is found. On average, faster tracers show equatorward motion and the slower ones show poleward motion. Such a segregation is reflected in a statistically significant covariance of the rotation residuals and meridional velocities in the order of -1000 m² s^-2, revealing an equatorward transport of angular momentum. The negative value of the covariance is provided by the high velocity tail in the velocity distribution of point-like structures, representing less than 15% of the population. The latitude dependence of the covariance can be expressed as  m² s^-2 covering the range –60°.