Comparison of the sidereal angular velocity of subphotospheric layers and small bright coronal structures during the declining phase of solar cycle 23

摘要

Context. We compare solar differential rotation of subphotospheric layersderived from local helioseismology analysis of GONG++ dopplergrams and the onederived from tracing small bright coronal structures (SBCS) using EIT/SOHOimages for the period August 2001 - December 2006, which correspond to thedeclining phase of solar cycle 23. Aims. The study aims to find a relationshipbetween the rotation of the SBCS and the subphotospheric angular velocity. Thenorthsouth asymmetries of both rotation velocity measurements are alsoinvestigated. Methods. Subphotospheric differential rotation was derived usingring-diagram analysis of GONG++ full-disk dopplergrams of 1 min cadence. Thecoronal rotation was derived by using an automatic method to identify and trackthe small bright coronal structures in EIT full-disk images of 6 hours cadence.Results. We find that the SBCS rotate faster than the considered uppersubphotospheric layer (3Mm) by about 0.5 deg/day at the equator. This resultjoins the results of several other magnetic features (sunspots, plages,faculae, etc.) with a higher rotation than the solar plasma. The rotation ratelatitudinal gradients of the SBCS and the subphotospheric layers are verysimilar. The SBCS motion shows an acceleration of about 0.005 deg/day/monthduring the declining phase of solar cycle 23, whereas the angular velocity ofsubsurface layers does not display any evident variation with time, except forthe well known torsional oscillation pattern. Finally, both subphotospheric andcoronal rotations of the southern hemisphere are predominantly larger thanthose of the northern hemisphere. At latitudes where the north-south asymmetryof the angular velocity increases (decreases) with activity for the SBCS, itdecreases (increases) for subphotospheric layers.