Turbulence in the Sub-Alfvénic Solar Wind

The Parker Solar Probe (PSP) entered a region of sub-Alfvénic 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 Alfvénic fluctuations, a consequence of the highly field-aligned flow that renders quasi-2D fluctuations unobservable to PSP. We extend Taylor's hypothesis to sub- and super-Alfvénic flows. Spectra for the fluctuating forward and backward Elsässer variables (z±, respectively) are presented, showing that z+ modes dominate z− by an order of magnitude or more, and the z+ spectrum is a power law in frequency (parallel wavenumber) f−3/2 (${k}_{\parallel }^{-3/2}$) compared to the convex z− spectrum with f−3/2 (${k}_{\parallel }^{-3/2}$) at low frequencies, flattening around a transition frequency (at which the nonlinear and Alfvén timescales are balanced) to f−1.25 at higher frequencies. The observed spectra are well fitted using a spectral theory for nearly incompressible magnetohydrodynamics assuming a wavenumber anisotropy ${k}_{\perp }\sim {k}_{\parallel }^{3/4}$, that the z+ fluctuations experience primarily nonlinear interactions, and that the minority z− fluctuations experience both nonlinear and Alfvénic interactions with z+ fluctuations. The density spectrum is a power law that resembles neither the z± 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-Alfvénic intervals are similar.