UCLA

In this dissertation, we discuss two fundamental processes in the inner heliosphere: magnetic reconnection and turbulence. Understanding the two processes is crucial for a more complete description of the solar atmosphere and solar wind. Our studies are confined within magnetohydrodynamic (MHD) and Hall-MHD regimes. We employ methods of linear stability analysis, numerical simulation, as well as satellite data analysis. The main results of our studies are summarized as follows. (1) Through linear stability calculation and linear MHD simulations, we confirm that the inhomogeneous flow inside a reconnecting current sheet stabilizes the tearing instability. The threshold Lundquist number $S_c$ for stabilization is on the order of $10^4$. (2) Through nonlinear Hall-MHD simulations, we show that during the ``recursive'' reconnection process driven by tearing instability, the generation of secondary plasmoids is suppressed as the system scale approaches the ion inertial length. (3) We calculate the linear dispersion relation of the tearing instability under a generalized three-dimensional configuration with Hall effect and guide field. We show that the largest linear growth rate is in general found in parallel modes rather than oblique modes, regardless of the guide field strength. (4) We carry out high-resolution two-dimensional MHD simulations, based on expanding-box-model (EBM), of Alfv en waves propagating inside large-scale solar wind streams. We show that the stream structures significantly impact the evolution of Alfv enic turbulence properties, such as the cross helicity and residual energy. (5) Through the EBM simulations in combination with a superposed-epoch analysis using OMNI data set, we show that, around heliospheric current sheets (HCSs), the Alfv enicity of the solar wind turbulence is destroyed. (6) We analyze Parker Solar Probe (PSP) data of the first four orbits and carry out a statistical study of the turbulence properties in the young solar wind. A radial evolution of various turbulence properties is observed in the data.