Bihelical spectrum of solar magnetic helicity and its evolution

TL;DR

This study analyzes the solar magnetic helicity spectrum over multiple years using advanced formalism, revealing bihelical spectra consistent with solar dynamo theory but also noting some deviations from expected patterns.

Contribution

It applies a new two-scale formalism to a large dataset, providing detailed insights into the evolution of solar magnetic helicity across solar cycle 24.

Findings

Bihelical spectra observed at different solar cycle phases.

Majority data supports large-scale dynamo with helical turbulence.

Over 20% of maps show deviations from expected helicity sign rule.

Abstract

Using a recently developed two-scale formalism to determine the magnetic helicity spectrum (Brandenburg et al. 2017), we analyze synoptic vector magnetograms built with data from the Vector Spectromagnetograph (VSM) instrument on the \emph{Synoptic Optical Long-term Investigations of the Sun} (SOLIS) telescope during January 2010-July 2016. In contrast to an earlier study using only three Carrington rotations, our analysis includes 74 synoptic Carrington rotation maps. We recover here bihelical spectra at different phases of solar cycle~24, where the net magnetic helicity in the majority of the data is consistent with a large-scale dynamo with helical turbulence operating in the Sun. More than $20\%$ of the analyzed maps, however, show violations of the expected sign rule.