Hemispheric Helicity Trend for Solar Cycle 24

TL;DR

This study analyzes the hemispheric helicity trend in solar active regions during solar cycles 23 and 24 using Hinode satellite data, revealing patterns, variations, and potential underlying mechanisms of magnetic helicity.

Contribution

It provides the first detailed comparison of helicity trends between two solar cycles with high-precision Hinode data, highlighting the weak hemispheric rule and differences between strong and weak fields.

Findings

Active regions in cycle 24 follow the hemispheric helicity rule.

The overall sample follows the hemispheric sign rule.

Helicity signs differ between strong and weak magnetic fields.

Abstract

Using vector magnetograms obtained with the Spectro-polarimeter (SP) on aboard Hinode satellite, we studied two helicity parameters (local twist and current helicity) of 64 active regions occurred in the descending phase of solar cycle 23 and the ascending phase of solar cycle 24. Our analysis gives the following results. (1) The 34 active regions of the solar cycle 24 follow the so-called hemispheric helicity rule, whereas the 30 active regions of the solar cycle 23 do not. (2) When combining all 64 active regions as one sample, they follow the hemispheric helicity sign rule as in most other observations. (3) Despite with the so-far most accurate measurement of vector magnetic field given by SP/Hinode, the rule is still weak with large scatters. (4) The data show evidence of different helicity signs between strong and weak fields, confirming previous result from a large sample of ground-based observations. (5) With two example sunspots we show that the helicity parameters change sign from the inner umbra to the outer penumbra, where the sign of penumbra agrees with the sign of the active region as a whole. From these results, we speculate that both the Sigma-effect (turbulent convection) and the dynamo have contributed in the generation of helicity, whereas in both cases turbulence in the convection zone has played a significant role.