Sub-Ion Scale Alfvenic Turbulence in the Near-Sun Solar Wind: New Insights from Parker Solar Probe

Parker Solar Probe (PSP) has revealed the near-Sun solar wind to be a highly dynamical environment supporting a rich variety of kinetic phenomena, including waves, instabilities and turbulence. These fundamental physical processes ultimately regulate the flow and dissipation of energy within the solar corona, with important ramifications for solar wind heating and acceleration, a problem which remains poorly understood. In this study, we use high-resolution magnetic field and ion measurements from PSP to probe the nature of Alfvénic turbulence and other kinetic processes ongoing at sub-ion scales in the near-Sun solar wind. We first investigate the anisotropy of magnetic field fluctuations in a highly Alfvénic wind stream close to perihelion at 35 solar radii, computing power, magnetic helicity and magnetic compressibility spectra as a function of the sampling angle between the local mean field and the plasma flow velocity. By comparing our results with a model spectrum of linear Alfven waves, we find that the spacecraft observations are consistent with a transition from an Alfvenic to kinetic Alfvenic turbulent cascade approaching the proton gyro-radius, as previously reported in simulations and elsewhere in the heliosphere. In addition, we find new evidence for a possible second transition in the turbulence at even smaller scales, as suggested by a subsequent decrease in the amplitude of the coherent magnetic helicity signature toward zero, following a peak close to the proton gyro-radius. We also investigate the dependence of these spectral properties on radial distance from the Sun while accounting for sampling effects arising from the unique nature of PSPs orbit. We find that our results are largely independent of distance in wind streams with similar plasma properties and solar source region. We also find that an apparent increasing dominance of Alfven ion-cyclotron waves toward smaller radial distances is likely due to spacecraft sampling effects rather than a significant increase in the amount of wave activity. Finally, we discuss the possible physical interpretations of the transition to a state of zero magnetic helicity and the potential implications for sub-ion scale turbulence in the solar wind.