Appearance of active regions at the end of solar cycle 24 and at the beginning of cycle 25

Abstract

The spatial-temporal picture of appearance of active regions and the relationship of their appearance with the structure and development of a large-scale magnetic field were studied during the transition from solar cycle 24 to 25. During this period, solar activity is low, and therefore the dynamics of a large-scale magnetic field in the appearance of new active regions is most noticeable. We have used SDO/HMI data on the longitudinal magnetic field to determine the time and heliographic coordinates of the origin of an active region, as well as daily WSO maps (Wilcox Solar Observatory) to compare with the structure of the large-scale magnetic field. We have obtained the following results. During the transition from one cycle to another, new active regions appeared in half of the cases in the polarity inversion line of the large-scale magnetic field, and almost exclusively at the Hale boundaries in the corresponding hemispheres and solar cycles. In other cases, the places of appearance were unipolar regions of the large-scale magnetic field without a clear advantage in the location of the field regions according to the Hale law. The formation of active regions is preceded or accompanied by changes in the structure of the large-scale magnetic field. At the same time, in the fine structure of the magnetic field in the photosphere we can observe an increase in the magnetic field network on a spatial scale of the size of supergranules and larger, as well as the appearance of small regions of a new magnetic field of both polarities. The appearing active regions were concentrated in two narrow longitudinal zones that covered both hemispheres of the Sun. The new cycle began in the same longitudinal zones, where the activity of the old cycle decayed.