Transition of Solar Wind Turbulence from MHD to Kinetic Scales

Abstract

Turbulence is a ubiquitous process in space plasmas that could potentially explain the large temperatures in many astrophysical systems such as the solar corona and solar wind. Turbulent fluctuations of the magnetic field occur over a wide range of spatial scales, which are usually classified as the outer scale, magnetohydrodynamic (MHD) scale and kinetic scale (including ion and electron scales). The outer scale feeds energy into the turbulent cascade that is transferred through MHD scales without dissipation. At kinetic scales the fluctuations undergo a major transition: conservation of energy across scales breaks down, heating mechanisms start operating and the dispersion relation of fundamental wave modes change. In this dissertation we analyze emph{in situ} solar wind observations from Wind and Parker Solar Probe to characterize the physical mechanisms that operate in the turbulent cascade at the connection of MHD and kinetic scales.

1) We present the first statistical study on stochastic proton heating in the solar wind and identify the critical gyroscale turbulence amplitude when the first adiabatic invariant is violated and perpendicular heating takes places. Our results suggest that stochastic heating operates 76% of the time at 1 AU meaning that it has significant contribution to the non-adiabatic temperature profile of the solar wind.

2) The precise scale where MHD turbulence transitions into the kinetic range is a matter of considerable debate. Recent turbulence models suggested that current sheetlike structures form in the inertial range and get disrupted when the timescale of the tearing mode instability is shorter than the eddy turnover time. Our results suggest that these models can explain the ion-scale spectral break of the magnetic energy spectrum in 41% of the time. We also find that the disruption process may generate large amplitude ion-scale coherent structures.

3) Very little is known about the transition of proton velocity fluctuations from MHD to kinetic scales due to the scarcity of available measurements. We use a special operation mode of the Faraday Cup onboard Parker Solar Probe and develop a novel approach to study high frequency ($>1$ Hz) velocity fluctuations and their correlation with magnetic fields. Our results imply that the highly Alfv'{e}nic nature of the turbulence breaks down near the ion-scale spectral break potentially due to the demagnetization of protons and the onset of kinetic effects.