Turbulence in the Sub-Alfv\'enic Solar Wind

TL;DR

This paper analyzes low-frequency turbulence in the sub-Alfvénic solar wind observed by Parker Solar Probe, revealing primarily Alfvénic fluctuations and extending turbulence theories to sub- and super-Alfvénic flows.

Contribution

It provides the first detailed turbulence analysis in the sub-Alfvénic solar wind and extends the Taylor hypothesis to these flow regimes.

Findings

Observed predominantly Alfvénic fluctuations due to field alignment.

Spectra fit a nearly incompressible MHD theory with wave number anisotropy.

Density spectrum suggests advected entropic modes, not magnetosonic.

Abstract

Parker Solar Probe (PSP) entered a region of the sub-Alfvenic solar wind during encounter 8 and we present the first detailed analysis of low-frequency turbulence properties in this novel region. The magnetic field and flow velocity vectors were highly aligned during this interval. By constructing spectrograms of the normalized magnetic helicity, cross helicity, and residual energy, we find that PSP observed primarily Alfvenic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor hypothesis to sub and super Alfvenic flows. Spectra for the fluctuating forward and backward Elsasser variables are presented. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wave number anisotropy. The density spectrum is a power law that resembles neither the Elsasser spectra nor the compressible magnetic field spectrum, suggesting that these are advected entropic rather than magnetosonic modes and not due to the parametric decay instability. Spectra in the neighboring modestly super Alfvenic intervals are similar.