The centenary variations in the solar corona shape in accordance with the observations during the minimal activity epoch

Abstract

Aims. We investigate the long-term change in coronal large-scale structure at periods of minimum solar activity from 1878 to 2008. Methods. A parameter γ that characterizes the angle between high-latitude boundaries of the large coronal streamers at a distance of 2 R⊙ is used to quantify the large-scale coronal structure at minimum solar activity. The comparative analysis of the solar corona during the minimum epochs in activity cycles 12 to 24 shows that the index has been slow and steadily changing during the past 130 years. Results. The maximum value of the index occurred during activity cycles 17 to 19, i.e. around 1950, which was the period when the global magnetic field of the Sun was closest to a dipole configuration. During activity minima close to the years 1900 and 2000, the large-scale coronal structure corresponded to a quadrupole configuration. The reasons for the variations in the solar coronal structure and its relation with long-term variations in the geomagnetic indices and Gleissberg cycle are discussed. © 2010 ESO.