Date of Award:
Doctor of Philosophy (PhD)
Electrical and Computer Engineering
Charged particle chaos and its collective effects in different magnetic geometries are investigated in a sequence of various numerical experiments. The fields generated by the particles as a result of interaction with the background electric and magnetic fields is not accounted for in the simulation. An X-line is first used to describe the geometry of the magnetotail prior to magnetic reconnection and a study of the behavior of charged particles is done from a microscopic viewpoint. Another important geometry in the magnetotail prior to substorm onset is Bifurcated Current Sheet. The same analysis is done for this configuration. The existence of at least one positive Lyapunov exponent shows that the motion of the particles is chaotic. By using statistical mechanics, the macroscopic properties of this chaotic motion are studied. Due to particles being charged, an electric field (perpendicular to the magnetic field in weak magnetic field region) accelerates the particles on average. Finite average velocity in the direction of electric field gives rise to an effective resistivity even in a collisionless regime such as solar corona and the magnetotail. Starting from initial velocities that are chosen randomly from a uniform distribution, the evolution of these distributions tends to a Maxwellian by the end of the simulation that is somewhat analogous to collisions in a Lorentz gas model. The effective resistivity due to such collisions is estimated. Ohmic heating is found to occur as a result of such an effective resistivity. Such collisions due to collective particle effects are essentially a different mechanism from classical collision notion. These experiments are done for two types of ions found in the plasma sheet prior to substorm onset, viz., protons and oxygen ions. Observational evidence of oxygen ions in the central plasma sheet, which flow out along open field lines from the ionosphere, were also simulated in the same manner. Oxygen ions have been found to influence the bifurcation of the current sheet and are also important in reconnection and other nonohmic instabilities, such as Kelvin Helmholtz instability, due to their mass. It is found that acceleration in X-line scales with the mass of ion species and the resistivity remains constant for different electric field strengths. In a Bifurcated Current Sheet, the acceleration scales with the square of mass of ion species and the resistivity scales with the electric field. Also, the overall resistivity values found in a Bifurcated Current Sheet are an order of magnitude lower than that found in an X-line.
Andriyas, Tushar, "Particle Dynamics and Resistivity Characteristics in Bifurcated Current Sheets" (2013). All Graduate Theses and Dissertations. 1513.
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