Scale-dependence of magnetic helicity in the solar wind

TL;DR

This study analyzes magnetic helicity in the solar wind using Ulysses data, revealing scale-dependent sign changes, hemispheric differences, and variations with solar activity, advancing understanding of solar wind magnetic properties.

Contribution

It introduces a Fourier-based method to measure magnetic helicity at different scales in the solar wind, utilizing high-latitude Ulysses data under the assumption of homogeneity.

Findings

Magnetic helicity changes sign at specific wavenumbers depending on distance from the Sun.

Small-scale magnetic helicity is positive in the northern hemisphere and negative in the southern.

Helicity declines towards solar minimum, indicating solar cycle dependence.

Abstract

We determine the magnetic helicity, along with the magnetic energy, at high latitudes using data from the Ulysses mission. The data set spans the time period from 1993 to 1996. The basic assumption of the analysis is that the solar wind is homogeneous. Because the solar wind speed is high, we follow the approach first pioneered by Matthaeus et al. (1982, Phys. Rev. Lett. 48, 1256) by which, under the assumption of spatial homogeneity, one can use Fourier transforms of the magnetic field time series to construct one-dimensional spectra of the magnetic energy and magnetic helicity under the assumption that the Taylor frozen-in-flow hypothesis is valid. That is a well-satisfied assumption for the data used in this study. The magnetic helicity derives from the skew-symmetric terms of the three-dimensional magnetic correlation tensor, while the symmetric terms of the tensor are used to determine the magnetic energy spectrum. Our results show a sign change of magnetic helicity at wavenumber k~2 AU^{-1} (or frequency nu~2 uHz) at distances below 2.8 AU and at k~30 AU^{-1} (or nu~25 uHz) at larger distances. At small scales the magnetic helicity is positive at northern heliographic latitudes and negative at southern latitudes. The positive magnetic helicity at small scales is argued to be the result of turbulent diffusion reversing the sign relative to what is seen at small scales at the solar surface. Furthermore, the magnetic helicity declines toward solar minimum in 1996. The magnetic helicity flux integrated separately over one hemisphere amounts to about 10^{45} Mx^2/cycle at large scales and to a 3 times lower value at smaller scales.