A HEMISPHERIC HELICITY SIGN RULE INDICATED BY LARGE-SCALE PHOTOSPHERIC MAGNETIC FIELDS AT THREE PHASES OF SOLAR CYCLE 23

Abstract

The hemispheric sign rule of current helicity is examined for large-scale magnetic fields, extending to 60° high in solar latitudes, at three different phases of solar cycle 23 and using three different instruments. The current helicity is calculated from the vector magnetic fields derived respectively from the longitudinal full-disk magnetograms obtained by the Michelson Doppler Imager on board the Solar and Heliospheric Observatory as well as the Kitt Peak Vacuum Telescope and the Synoptic Long-term Investigations of the Sun at the National Solar Observatory at Kitt Peak. We have used the same technique introduced by Pevtsov & Latushko to retrieve the vector magnetic field but have applied a different method to calculate the current helicity. Our analysis strengthens evidence that the large-scale magnetic fields show a clear and consistent current helicity pattern that follows the established hemispheric rule, that is, a positive helicity sign in the southern hemisphere and a negative helicity sign in the northern hemisphere. This hemispheric sign pattern is present everywhere in the global magnetic field, extending to 60° high in solar latitudes, independent of the instruments and parameters used, and is evident at both solar maximum and minimum phases.