Date of Award

2026

Abstract

The near-Sun solar wind provides a natural laboratory for studying the nonlinear and kinetic processes that govern the dynamics of collisionless astrophysical plasmas. This thesis investigates plasma turbulence and kinetic instabilities in the inner heliosphere using theoretical analysis, numerical modeling, and in situ observations.First, young solar wind observations collected by Parker Solar Probe (PSP) are used to examine the radial evolution of nonlinear solar wind turbulence by analyzing magnetic-field power spectra. I investigate the presence and radial evolution of the well-known 1/f spectral behavior in the energy-containing range during a single fast solar wind stream. Measurements close to the Sun are especially important because the solar wind has undergone less dynamical processing by nonlinear interactions, so the original characteristics of its turbulence spectrum can be directly assessed. The results show that this spectral feature is not simply inherited from the corona, but instead evolves dynamically as the solar wind expands away from the Sun. Second, I investigate the fast-magnetosonic/whistler (FM/W) instability driven by a drifting, temperature-anisotropic proton beam and derive approximate analytic instability threshold expressions. These expressions show how the combination of these two free energy sources expands the range of conditions where the instability can occur, such as how an anisotropic beam allows FM/W waves to become unstable at significantly lower beam drift speeds. In addition, a numerical code was developed to solve the full hot-plasma dispersion relation. The numerical results are consistent with and validate the derived analytic thresholds. Finally, recent PSP observations are used to examine the constraints on proton-beam drift speed and temperature anisotropy imposed by the parallel FM/W instability. Additionally, long-duration datasets from the Wind and Helios spacecraft are used to extend this investigation over a broader range of heliospheric distances. The distribution of proton-beam parameters from these observations is constrained by the analytic FM/W thresholds to regions of parameter space in which FM/W waves are either stable or have extremely low growth rates. This result provides support for the role of the FM/W instability in regulating beam dynamics in the solar wind.

Document Type

Dissertation

First Advisor

Benjamin D. G. Chandran

Second Advisor

Benjamin D. G. Chandran

Third Advisor

Joseph Dwyer

Department or Program

Physics

Degree Name

Doctor of Philosophy

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