The transport of turbulence in large-scale in homogeneous flows with application to the solar wind
Date of Award
Doctor of Philosophy (PhD)
Gary P. Zank
Jakobus le Roux
Solar wind., Space plasmas., Heliosphere (Astrophysics)
Turbulence is an ubiquitous process in the magnetized solar wind plasma, and is thought to be integral to multiple important phenomena in the solar wind, such as coronal and solar wind heating, acceleration of the solar wind, the transport of energetic particles, and so forth. To explain these phenomena, it is necessary to understand the transport of low-frequency turbulence in the solar wind. This thesis investigates several sets of turbulence transport model equations in the context of turbulence in different regions of the heliosphere. This includes investigating turbulence in sub-Alfvenic and super-Alfvenic flows. Solar wind properties vary with solar cycle, and hence the solar cycle may affect solar wind turbulence. This thesis investigates the temporal solar wind - its variable velocity and the variability in the source of turbulence, and studies the dependence of solar wind turbulence with solar cycle beyond 1 astronomical unit (AU). We find that the temporal solar wind introduces a periodic variability, particularly beyond ~ 10 AU, in the fluctuating magnetic energy density, the correlation length, and the solar wind temperature. The variability is insufficient to account for the full observed variability in these quantities, but we find that the time-dependent solutions trace the steady-state solutions quite well, suggesting that the steady-state models are a reasonable first approximations. Shocks are intrinsic to the solar wind. This thesis studies the interaction of turbulence with simplified parallel and perpendicular shock waves. The generation of turbulent kinetic and magnetic energy by the shock is investigated together with the related residual energy and cross helicity. We find that the shock is responsible for generating backward propagating modes. The transport of solar wind turbulence in the supersonic solar wind has been an active area of study over the past decade. We apply the most general and detailed model of turbulence transport [the six coupled equation model of Zank et al. (2012a)] to the super-Alfvenic solar wind flow. We compute theoretically various turbulence transport quantities, such as the energy in forward and backward propagating modes, the cross helicity and residual energy, and the correlation lengths, and then compare them with Helios 2, Ulysses, and Voyager 2 observations. We find that theoretical results are in good agreement with observations. More importantly, we calculate the residual energy, and the correlation lengths corresponding to forward and backward propagating modes and the residual energy throughout the heliosphere for the first time. We show theoretically and observationally that backward propagating modes are generated within 1 AU by shear driving due to fast and slow streams. Turbulence in sub-Alfvenic flows is more complicated than turbulence in super-Alfvenic flows because of the important role that the Alfven speed plays. To study turbulence in the sub-Alfvenic regime, we introduce several sets of simplified equations that correspond to either zero or non-zero cross helicity limits of the general Zank et al. (2012a) model equations. These sets of equations are solved numerically and compared to analytical solutions obtained from an analysis of the related autonomous system.
Adhikari, Laxman, "The transport of turbulence in large-scale in homogeneous flows with application to the solar wind" (2015). Dissertations. 86.