On kinetic dissipation in collisionless turbulent plasmas.

摘要

Plasma turbulence is a phenomenon that is present in astrophysical as well as terrestrial plasmas. The earth is embedded in a turbulent plasma, emitting from the sun, called the solar wind. It is important to understand the nature of this plasma in order to understand space weather. A critical unsolved problem is that of the source of dissipation in turbulent plasmas. It is believed to play a central role in the heating of the solar corona which in turn drives the solar wind. The solar wind itself is observed to be highly turbulent and hotter than predicted through adiabatic expansion models. Turbulence and its associated dissipation have been studied extensively through the use of MHD models. However, the solar wind and large regions of the solar corona have very low collisionality, which calls into question the use of simple viscosity and resistivity in most MHD models. A kinetic treatment is needed for a better understanding of turbulent dissipation. This thesis studies the dissipation of collisionless turbulence using direct numerical hybrid simulations of turbulent plasmas. Hybrid simulations use kinetic ions and fluid electrons. Having full kinetic ion physics, the dissipation in these simulations at the ion scales is self consistent and requires no assumptions. We study decaying as well as quasi steady state systems (driven magnetically). Initial studies of the Orszag-Tang vortex [Orszag, JFM, 1979] (which is an initial condition that quickly generates decaying strong turbulence) showed preferential perpendicular heating of protons (with T_perp /T_|| > 1). An energy budget analysis showed that in the turbulent regime, almost all the dissipation occurs through magnetic interactions. We study the energy budget of waves using the k - o spectra (energy in the wavenumber-frequency space). The k - o spectra of this study and subsequent studies of driven turbulent plasmas do not show any significant power in the linear wave modes of the system. This suggests that in the strong 2D limit, contrary to the conventional belief, waves do not appear to play an important role in the heating of plasma. We also study the onset of turbulence and heating of plasma as a function of the driving frequency. We find that the onset of turbulence has a critical dependence on the relative size of the driving time scales and the nonlinear time scales of the system. The driving time scale has to be longer than the nonlinear time of the system or the intrinsic nonlinear time associated with the driving function. For smaller driving time scales (or higher driving frequencies) we do not generate turbulence and do not heat the plasma. This setup has a resemblance to the generation of turbulence and heating of the plasma in the solar corona. The driving frequency corresponds to the frequency of driving because of the foot point motions of the field lines. Our results are consistent with Parker's picture for heating the corona (e.g. Parker, Planets Earth and Space, 2001). The time scale of the foot points has to be longer than the nonlinear time of the system in order to generate turbulence and heat the corona.