Hale cycle in solar hemispheric radio flux and sunspots: Evidence for a northward-shifted relic field

Space Climate Group, Space Physics and Astronomy Research Unit, University of Oulu, PO Box 8000 90014 Oulu, Finland
e-mail: kalevi.mursula@oulu.fi

Received: 25 January 2023
Accepted: 30 April 2023

Abstract

Context. Solar and heliospheric parameters occasionally depict notable differences between the northern and southern solar hemisphere. Although the hemispheric asymmetries of some heliospheric parameters vary systematically with the Hale cycle, this has not been found to be commonly valid for solar parameters. Also, no verified physical mechanism exists that can explain possible systematic hemispheric asymmetries.

Aims. We use a novel method of high heliolatitudinal vantage points to increase the fraction of one hemisphere in solar 10.7 cm radio fluxes and sunspot numbers. We aim to explore the possibility that solar radio fluxes and sunspot numbers, the two most fundamental solar parameters, depict systematic, possibly mutually similar patterns in their hemispheric activities during the last 75 yr.

Methods. We used three different sets of time intervals with increasing mean heliographic latitude and calculated corresponding hemispheric high-latitude radio fluxes and sunspot numbers. We also normalized these fluxes by yearly means in order to study the variation of fluxes in the two hemispheres over the whole 75 yr time interval.

Results. We find that cycle-maximum radio fluxes and sunspot numbers in each odd solar cycle (19, 21, 23) are larger at northern high latitudes than at southern high latitudes, while maximum fluxes and numbers in all even cycles (18, 20, 22 24) are larger at southern high latitudes than at northern high latitudes. This alternation indicates a new form of systematic, Hale-cycle-related variation in solar activity. Hemispheric differences at cycle maxima are 15% for radio flux and 23% for sunspot numbers, on average. The difference is largest during cycle 19 and smallest in cycle 24. Normalized radio fluxes depict a dominant Hale-cycle variation in both hemispheres, with an opposite phase and overall amplitude of about 5% in the north and 4% in the south. Thus, there is systematic Hale-cycle alternation in magnetic flux emergence in both hemispheres.

Conclusions. The hemispheric Hale cycle in flux emergence can be explained if there is a northward-directed relic magnetic field, which is slightly shifted northward. In that case, in odd cycles, the northern hemisphere is enhanced more than the southern hemisphere, and in even cycles, the northern hemisphere is reduced more than the southern hemisphere, establishing the observed hemispheric alternation. The temporal change of asymmetry during the seven cycles can be explained if the relic shift oscillates at the 210 yr Suess/deVries period, which also provides a physical cause to this periodicity. Gleissberg cycles are explained as off-equator excursions of the relic, each Gleissberg cycle forming one half of the full relic shift oscillation cycle. Having a relic field in the Sun also offers interesting possibilities for century-scale forecasting of solar activity.