The Turbulence Magnetic Helicity Signature in the Interplanetary Medium: A Blackman–Tukey and Morlet Wavelet Analysis

Abstract

Abstract Interplanetary turbulence shows a spectral magnetic helicity signature whose properties could depend on the nature and dissipation of proton-scale fluctuations. A database of Wind spacecraft intervals of turbulence and helicity signatures is evaluated. Spectra are computed by both the Blackman–Tukey method and the Morlet wavelet method. A global mean magnetic field is used in each case, and the wavelet spectrum is averaged over time to facilitate comparison with the Blackman–Tukey spectrum. The maximum magnitude of the signature normalized by the trace of the magnetic spectral power has a frequency that correlates well between the two methods. The sign of the signature is also the same with both approaches, but the magnitudes differ. Statistically, the mean magnitudes of each method do agree, and the difference of individual magnitudes is assigned to uncertainties within each method. The Morlet wavelet method obtains fewer noisy signatures with a tighter overall correlation between magnetic helicity and cross-helicity; however, no trend is confirmed between helicity and the ratio of plasma to magnetic pressure. Subdivision of the analyzed intervals establishes that the helicity signature is persistent but variable. A portion of the variability comes from cross-helicity and possibly from the cascade rate. The observed magnetic helicity is compared to model and simulation results. Two-dimensional hybrid simulations yield results for the magnetic helicity magnitude that are larger than the mean values observed under similar conditions.