Helicity observations of active regions during the exchange period of Solar Cycle 24 and 25

Abstract

ABSTRACT Using vector magnetic field data from the Helioseismic and Magnetic Imager instrument onboard the Solar Dynamics Observatory, we study the signs of helicity (magnetic twist αav, z-component of current helicity Hc) and tilt angle of 85 sample active regions (ARs) that appeared on the central solar disc (within ±45° from disc centre) between December 2018 and November 2020. This time range spans the exchange period of Solar Cycle 24 and 25. The main findings are as follows: (1) As a whole, 62 per cent of sample ARs follow the helicity hemispherical sign rule, and our observational results do not show any hemispherical rule sign reversal at the end or beginning of a Solar Cycle. (2) Interestingly, there is no significant statistical relationship between helicity and tilt angle, as in contrast with the general idea on the conservation of magnetic helicity in the solar convection zone inferred by the relationship between the photospheric helicity and tilt angle. It is also found that the hemispherical tendency of helicity and tilt angle is more obvious for ARs at high latitudes, and the hemispherical preference is more obvious for ARs with magnetic twist and writhe of the opposite signs than for ARs with the same signs.