Browsing by Author "Wolf, Richard A"

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    Development and Application of Stochastic Methods for Radiation Belt Simulations
    (2015-09-04) Zheng, Liheng; Chan, Anthony A; Wolf, Richard A; Dugan, Brandon
    This thesis describes a method for modeling radiation belt electron diffusion, which solves the radiation belt Fokker-Planck equation using its equivalent stochastic differential equations, and presents applications of this method to investigating drift shell splitting effects on radiation belt electron phase space density. The theory of the stochastic differential equation method of solving Fokker-Planck equations is formulated in this thesis, in the context of the radiation belt electron diffusion problem, and is generalized to curvilinear coordinates to enable calculation of the electron phase space density as a function of adiabatic invariants M, K and L. Based on this theory, a three-dimensional radiation belt electron model in adiabatic invariant coordinates, named REM (for Radbelt Electron Model), is constructed and validated against both known results from other methods and spacecraft measurements. Mathematical derivations and the essential numerical algorithms that constitute REM are presented in this thesis. As the only model to date that can solve the fully three-dimensional diffusion problem, REM is used to study the effects of drift shell splitting, which gives rise to M-L and K-L off-diagonal terms in the radiation belt diffusion tensor. REM simulation results suggest that drift shell splitting reduces outer radiation belt electron phase space density enhancements during electron injection events. Plots of the phase space density sources, which are unique products of the stochastic differential equation method, and theoretical analysis further reveal that this reduction effect is caused by a change of the phase space location of the source to smaller $L$ shells, and has a limit corresponding to two-dimensional local diffusion on a curved surface in the (M,K,L) phase space.
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    Simulation of Dislocated Flux in Space Plasma Environments: Applications in Geospace Modeling and Ionosphere-Magnetosphere Coupling
    (2015-04-23) Schutza, Aaron Moore; Toffoletto, Frank R.; Wolf, Richard A; Ecklund, Karl M
    In this study, simulations of a dislocated flux tube are used to model oscillatory flow events and to explore possible ionospheric-magnetospheric coupling mechanisms. A numerical code called the Thin Filament Code (TFC) has been developed using a thin filament approximation to simulate flux tube motion in a stationary 2D background. Previous studies using similar magnetohydrodynamic thin filament models have been used to describe fast flow events and interchange oscillations in the Earth’s plasma sheet. A significantly extended numerical model is employed to explore additional applications. Simulation results include the time evolution of isolated flux tubes with a wide range of stationary background environments and boundary conditions defined by field aligned current systems. Simulations suggest that ionospheric disturbances can introduce waves that propagate to the magnetosphere triggering activity in the magnetotail. Oscillatory motion is simulated on a background model fitted to observation demonstrating new capabilities of the TFC.