Yihong Wu

Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)


Space Science

Committee Chair

Vladimir A Florinski

Committee Member

Jacob Heerikhuisen

Committee Member

Jakobus le Roux

Committee Member

Gary M. Webb

Committee Member

Jason Cassibry


Heliosphere (Astrophysics), Solar wind., Plasma astrophysics., Interstellar matter.


Pickup ions (PUIs) play a significant part in mediating the solar wind (SW) interaction with the interstellar medium. In this dissertation, I examine the details of spatial variation of the PUI velocity distribution function (VDF) in the SW by solving the PUI transport equation. I assume the PUI distribution is isotropic resulting from strong pitch-angle scattering by wave-particle interation. I take into account the effects of convection with the SW, adiabatic cooling, second-order Fermi process and production of PUIs. I analyze how PUIs transform across the heliospheric termination shock (TS) and obtain the PUI phase space distribution in the inner heliosheath including continuing velocity diffusion. A 3D model combining the MHD treatment of the background SW and neutrals with a kinetic treatment of PUIs throughout the heliosphere and the surrounding local interstellar medium (LISM) has been developed. It gives us the PUI and SW characteristics, such as phase space densities, spatial distribution maps, etc., from close to the sun to the heliopause. My simulated PUI spectra are compared with observations made by New Horizons, Ulysses, Voyager 1, 2 and Cassini, and a satisfactory agreement is demonstrated. Some specific features in the observations, for example, a cutoff of PUI VDF at v = V_SW in the reference frame of the SW, are well represented by the model. The compressed SW and PUIs behind the TS can create energetic neutral atoms (ENAs) via charge exchange. ENAs with energies high enough to overcome the outward flow speed can be detected back at Earth. I calculate ENA fluxes at 1 AU. Based on my PUI model, I compare my simulation results with the Interstellar Boundary EXplorer (IBEX) distributed ENA sky maps and the line of sight (LOS) spectra. This brings us closer to understanding the properties of distributed ENA flux observations.



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