Solar coronal rotation determined by X-ray bright points in Hinode/XRT and Yohkoh/SXT full-disc images

Abstract

Aims. Our aim is to identify and trace the X-ray bright points (XBPs) over the disc and use them as tracers to determine the coronal rotation. This investigation will help to clarify and understand several issues: (i) whether the corona rotates differentially; (ii) whether the rotation depends on the sizes of the XBPs; and (iii) whether there is a dependence on phases of the solar magnetic cycle. Methods: We analysed the daily full-disc soft X-ray images observed with (i) X-Ray Telescope (XRT) onboard the Hinode mission during January, March, and April 2007; and (ii) Soft X-ray Telescope (SXT) onboard the Yohkoh from 1992 to 2001 using SSW in IDL. We used the tracer method to trace the passage of XBPs over the solar disc with the help of overlaying grids and derived the sidereal angular rotation velocity and the coordinates (latitude and longitude) of the XBPs. Results: We have determined the position of a large number of XBPs, both in Hinode/XRT and Yohkoh/SXT images, and followed them over the solar disc as a function of time. We derived the coronal sidereal angular rotation velocity and compared it with heliocentric latitude and as a function of solar activity cycle. In addition, we measured the sizes of all the XBPs and related them to the coronal rotation. The important results derived from these investigations are: (i) the solar corona rotates differentially like the photosphere and chromosphere; (ii) the sidereal angular rotation velocity is independent of the sizes of the XBPs; (iii) the sidereal angular rotation velocity does not depend on phases of the solar magnetic cycle; and (iv) the differential rotation of the corona is present throughout the solar magnetic cycle.