A Quantity Characterising Variation of Observed Magnetic Twist of Solar Active Regions

TL;DR

This study introduces the parameter $R_{J_z}$ to quantify magnetic twist variation in solar active regions and explores its relationship with the hemispheric sign rule of helicity, revealing phases linked to flare activity.

Contribution

The paper proposes a new parameter $R_{J_z}$ for characterizing magnetic twist variation and analyzes its correlation with the hemispheric sign rule and flare activity in active regions.

Findings

$R_{J_z}$ correlates with adherence to the hemispheric sign rule.

Active regions with $R_{J_z}<0.5$ tend to obey the sign rule.

Flare-productive phases show dynamic changes in $R_{J_z}$.

Abstract

An alternative parameter $R_{J_z}$ is introduced as the ratio of one of two kinds of opposite-sign current to the total current and investigate the relationship between the quantity and the hemispheric sign rule of helicity (HSR) that is established by a series of previous statistical studies. The classification of current in each hemisphere is according to the following rule: If the product of the current and the corresponding longitudinal field component contributes a consistent sign with reference to the HSR, it is called "HSR-compliant" current, or else it is called "HSR-noncompliant" current. Firstly, the consistence between the butterfly diagram of the $R_{J_z}$ and the current helicity was obtained in a statistical study. Active regions with $R_{J_z}$ smaller than 0.5 tend to obey the HSR whereas those with $R_{J_z}$ greater than 0.5 tend to disobey the HSR. The "HSR-compliant" current systems have 60\% probability of realization compared to 40\% of "HSR-noncompliant" current systems. Overall, the HSR is violated for active regions in which the "HSR-noncompliant" current is greater than the "HSR-compliant" current. Secondly, the $R_{J_z}$ parameter was subsequently used to study the evolution of current systems in the case analyses of flare-productive active regions NOAA AR 11158 and 11283. It is found that there were "$R_{J_z}$-quasi-stationary" phase that is relatively flare quiescent and "$R_{J_z}$-dynamic" phase that is covered by the occurrence of large flares.