Wave Composition, Propagation, and Polarization of MHD Turbulence within 0.3AU as Observed by PSP

TL;DR

This study analyzes the wave composition, propagation, and polarization of MHD turbulence within 0.3AU of the Sun using PSP data, revealing how wavevector directions and mode energies vary with scale and distance.

Contribution

It provides the first probability density function of wavevector directions in the inner heliosphere and details the scale-dependent wave mode composition and anisotropy of solar wind turbulence.

Findings

Wavevectors cluster quasi-parallel for small scales and shift to quasi-perpendicular at larger scales.

Alfvén mode dominates across all scales and distances.

Inward fast mode energy decreases with distance, inward/outward slow modes increase with distance.

Abstract

Turbulence, a ubiquitous phenomenon in interplanetary space, is crucial for the energy conversion of space plasma at multiple scales. This work focuses on the propagation, polarization and wave composition properties of the solar wind turbulence within 0.3AU, and its variation with heliocentric distances at MHD scales (from 10s to 1000s in the spacecraft frame). We present the probability density function of propagation wavevectors (${\rm{PDF}}(k_\parallel,k_\perp)$) for solar wind turbulence winthin 0.3 AU for the first time: (1) wavevectors cluster quasi-(anti-)parallel to the local background magnetic field for $kd_{\rm i}<0.02$, where $d_{\rm i}$ is the ion inertial length; (2) wavevectors shift to quasi-perpendicular directions for $kd_{\rm i}>0.02$. Based on our wave composition diagnosis, we find that: the outward/anti-sunward Alfv\'en mode dominates over the whole range of scales and distances, the spectral energy density fraction of the inward/sunward fast mode decreases with distance, and the fractional energy densities of the inward and outward slow mode increase with distance. The outward fast mode and inward Alfv\'en mode represent minority populations throughout the explored range of distances and scales. On average, the degree of anisotropy of the magnetic fluctuations defined with respect to the minimum variation direction decreases with increasing scale, with no trend in distance at all scales. Our results provide comprehensive insight into the scenario of transport and transfer of the solar wind fluctuations/turbulence in the inner heliosphere.