MHD Wave Propagation & Composition & Polarization in inner heliosphere observed by PSP

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 10 s to 1000 s in the spacecraft frame). For the propagation properties, we find at 0.166AU<R<0.243AU that: (1) At scales smaller than 0.02 (kd_i < 0.02, where d_i is the ion inertial length), the propagation angles of the wavevectors cluster around 150° relative to the local background magnetic field. (2) At 0.02 < kd_i < 0.1, the propagation angles shift to concentrate around 90°. Based on our wave composition diagnosis, we find that: the outward/anti-sunward Alfvén 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 slow mode increases with distance. The inward Alfvén mode, the outward fast mode and the outward slow 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 nature of the MHD fluctuations of the solar wind within 0.3AU, which helps to understand the transport of MHD turbulence in the inner heliosphere.