Velocity-space Turbulent Cascade in the Near-Sun Solar Wind: First Insights from the Parker Solar Probe Mission

Abstract In space plasmas, the rarity of collisions leads to complex structures in the velocity space where a turbulent cascade of the velocity distribution function (VDF) fluctuations is thought to occur. Previous studies have explored this phenomenon using the Hermite decomposition of the ion VDF in both magnetosheath data and numerical simulations. In this work, we investigate the Hermite spectrum of the ion VDFs measured by the Parker Solar Probe in the inner heliosphere. We analyze a super-Alfvénic stream at a radial distances of R ≈ 28 R ⊙ and a sub-Alfvénic stream at R ≈ 11 R ⊙ , the former characterized by a prevalence of VDFs with suprathermal beams (also known as hammerhead distributions). The Hermite analysis is also compared with various proxies of energization and dissipation, in order to establish a connection between turbulent cascades in real space and those in the velocity space. A qualitative agreement between the energization proxies and the Hermite analysis is observed. The results are suggestive of the presence of a dual cascade in real and velocity space.