Date of Award
Doctor of Philosophy (PhD)
Jakobus le Roux
Solar wind., Plasma astrophysics., Space plasmas.
In this dissertation, we study the physics of locally-born interstellar pickup proton diffusive shock acceleration (DSA) at the nearly perpendicular solar wind termination shock (SWTS) using a focused transport model. We are particularly interested in how the statistical characteristics of the interplanetary magnetic field (IMF) near SWTS, and of the SWTS compression ratio itself can modify the effi- ciency of pickup ion injection, transport across the SWTS, and DSA itself. Guided by Voyager 2 observations, the IMF spiral angle and strength statistical variations are modeled with a truncated q-Gaussian and a log-normal distribution, respectively, to capture the non-Gaussian, intermittent nature of the fluctuating IMF at the SWTS. The magnetic field angle fluctuations, which are used to model perpendicular diffusion of pickup protons across the SWTS in real time in the focused transport model, play a key role in enabling efficient injection and rapid DSA when these particles mostly follow field lines. Theoretical variation of the field-line-random-walk statistics sheds interesting new light on the role of wandering field lines on injection into DSA and the DSA process itself. Our simulations suggest that both the shape (q-value) and the standard deviation (σ-value) of the q-Gaussian probability density function (PDF) ivsignificantly affect the injection speed, pitch-angle anisotropy, spatial distribution and the efficiency of DSA of pickup protons at the SWTS. For example, increasing q and especially reducing σ enhances the DSA rate. Likewise variation in the statistics of the magnetic field magnitude, which are modeled to induce statistical fluctuations in the pitch-angle scattering rate and thus in parallel diffusion, and in the SWTS com- pression ratio were found to significantly impact the characteristics of pickup ion DSA at the SWTS. Finally, the limitations of standard focused transport theory at nearly perpendicular shocks, such as the SWTS, are discussed. The derivation of a more gen- eral focused transport equation that address these limitation by retaining gyrophase information is presented. Interesting new aspects of the generalized focused transport equation, such as slow gyrophase variations, and additional nonuniform magnetic field effects in the generalized magnetic mirroring/focusing term, are discussed.
Ye, Junye, "A focused transport approach to interstellar pickup ion acceleration at the solar wind termination shock : the role of a fluctuating magnetic field and termination shock structure" (2016). Dissertations. 119.