First Synoptic Maps of Photospheric Vector Magnetic Field from SOLIS/VSM: Non-Radial Magnetic Fields and Hemispheric Pattern of Helicity

TL;DR

This study presents the first synoptic maps of the photospheric vector magnetic field from SOLIS/VSM, analyzing hemispheric helicity patterns and their relation to magnetic field strength during solar cycle 24.

Contribution

It provides the first Carrington maps of the vector magnetic field and investigates hemispheric helicity patterns, including differences between weak and strong magnetic fields.

Findings

Hemispheric helicity rule is evident during cycle 24's rising phase.

Strong magnetic fields follow the hemispheric helicity rule, weak fields show opposite trend.

No evidence of reversal of the hemispheric helicity rule at cycle start.

Abstract

We use daily full-disk vector magnetograms from Vector Spectromagnetograph (VSM) on Synoptic Optical Long-term Investigations of the Sun (SOLIS) system to synthesize the first Carrington maps of the photospheric vector magnetic field. We describe these maps and make a comparison of observed radial field with the radial field estimate from LOS magnetograms. Further, we employ these maps to study the hemispheric pattern of current helicity density, Hc, during the rising phase of the solar cycle 24. Longitudinal average over the 23 consecutive solar rotations shows a clear signature of the hemispheric helicity rule, i.e. Hc is predominantly negative in the North and positive in South. Although our data include the early phase of cycle 24, there appears no evidence for a possible (systematic) reversal of the hemispheric helicity rule at the beginning of cycle as predicted by some dynamo models. Further, we compute the hemispheric pattern in active region latitudes (-30 deg \le \theta \le 30 deg) separately for weak (100< |B_r| <500 G)and strong (|B_r|>1000 G) radial magnetic fields. We find that while the current helicity of strong fields follows the well-known hemispheric rule (i.e., \theta . Hc < 0), H_c of weak fields exhibits an inverse hemispheric behavior (i.e., \theta . Hc > 0) albeit with large statistical scatter. We discuss two plausible scenarios to explain the opposite hemispheric trend of helicity in weak and strong field region.