Magnetic Helicity Flux across Solar Active Region Photospheres: I. Hemispheric Sign Preference in Solar Cycle 24

TL;DR

This study analyzes magnetic helicity flux in solar active regions during cycle 24, revealing hemispheric preferences and their dependence on solar cycle phase, latitude, and magnetic flux, supporting dynamo theories involving Coriolis force and differential rotation.

Contribution

First comprehensive analysis of magnetic helicity flux across thousands of active regions during solar cycle 24, linking hemispheric sign preference to physical processes like the Coriolis force and differential rotation.

Findings

63-65% of active regions follow hemispheric sign preference

HSP increases to 70-80% during early cycle phases and at higher latitudes

HSP correlates with larger magnetic flux and plasma flow speeds

Abstract

A hemispheric preference in the dominant sign of magnetic helicity has been observed in numerous features in the solar atmosphere: i.e., left-handed/right-handed helicity in the northern/southern hemisphere. The relative importance of different physical processes which may contribute to the observed hemispheric sign preference (HSP) of magnetic helicity is still under debate. Here, we estimate magnetic helicity flux ($dH/dt$) across the photospheric surface for 4,802 samples of 1,105 unique active regions (ARs) that appeared over an 8-year period from 2010 to 2017 during solar cycle 24, using photospheric vector magnetic field observations by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The estimates of $dH/dt$ show that 63% and 65% of the investigated AR samples in the northern and southern hemispheres, respectively, follow the HSP. We also find a trend that the HSP of $dH/dt$ increases from ~50-60% up to ~70-80% as ARs (1) appear at the earlier inclining phase of the solar cycle or higher latitudes; (2) have larger values of $|dH/dt|$, the total unsigned magnetic flux, and the average plasma flow speed. These observational findings support the enhancement of the HSP mainly by the Coriolis force acting on a buoyantly rising and expanding flux tube through the turbulent convection zone. In addition, the differential rotation on the solar surface as well as the tachocline $\alpha$-effect of flux-transport dynamo may reinforce the HSP for ARs at higher latitudes.