Evolution of Magnetic Helicity and Energy Spectra of Solar Active Regions

TL;DR

This study investigates the evolution of magnetic energy and helicity spectra in solar active regions, revealing how spectral indices and helicity signs change during development and solar cycle phases.

Contribution

It introduces an isotropic Fourier-based method to analyze magnetic spectra and their spectral indices in active regions, highlighting deviations from classical turbulence models.

Findings

Spectral indices of magnetic energy and helicity are lower than 5/3.

Magnetic helicity tends to increase with larger energy-carrying scales.

NOAA 11515's helicity violates hemispheric sign rules, linked to large-scale fields.

Abstract

We adopt an isotropic representation of the Fourier-transformed two-point correlation tensor of the magnetic field to estimate the magnetic energy and helicity spectra as well as current helicity spectra of two individual active regions (NOAA 11158 and NOAA 11515) and the change of the spectral indices during their development as well as during the solar cycle. The departure of the spectral indices of magnetic energy and current helicity from 5/3 are analyzed, and it is found that it is lower than the spectral index of the magnetic energy spectrum. Furthermore, the fractional magnetic helicity tends to increase when the scale of the energy-carrying magnetic structures increases. The magnetic helicity of NOAA 11515 violates the expected hemispheric sign rule, which is interpreted as an effect of enhanced field strengths at scales larger than 30-60Mm with opposite signs of helicity. This is consistent with the general cycle dependence, which shows that around the solar maximum the magnetic energy and helicity spectra are steeper, emphasizing the large-scale field.