Ion Kinetics at the Heliospheric Termination Shock
Pin Wu (BU)
Monday 30th November 2009, 12:00pm
Pratt conference room, 60 Garden Street
A number of interesting phenomena were revealed after the two Voyager spacecraft crossed the termination shock in 2004 and 2007, respectively. This dissertation presents theoretical and computational studies to address ion heating, energy dissipation and ion speed distributions at the quasi-perpendicular termination shock. In our model, the termination shock is distinguished by the presence of a significant fraction of pickup ions, interstellar atoms which have become ionized and subsequently accelerated up to solar wind flow speeds. Using the Los Alamos hybrid simulation code, we demonstrate that the heating of pickup ions is dependent on the phase of gyration about the local magnetic field when they encounter the termination shock. The temperature of solar wind ions is raised by a larger factor than that of the pickup ions because some solar wind ions are specularly reflected. An analytic model for energy partition is developed based on the Rankine-Hugoniot relations and a polytropic energy equation (the Multicomponent Rankine-Hugoniot model). The polytropic index is varied to improve agreement between the model and the simulations. When the pickup ion relative density is 0%, the polytropic index is 5/3. As the pickup ion relative density increases toward 40%, the polytropic index increases toward 2.2, suggesting a fundamental change in the character of the shock. We infer that the pickup ion relative density is about 25%, and that pickup ions gain the larger share (~90%) of the dissipated energy near the nose of the termination shock, consistent with Voyager 2 observations. Further, we explore the consequences of four different assumptions regarding upstream pickup ion velocities, each corresponding to a different thermalization level. The downstream ion speed distribution is found to be almost identical for each case, with two Maxwellian components providing a good fit. In addition, the downstream heated ion spectrum scales with the solar wind speed, with smaller spectral indexes corresponding to faster solar wind speeds, consistent with the Interstellar Boundary EXplorer (IBEX) inference. The IBEX team plans to use the constructed spectra to help interpret their measurements. Finally, we validate our simulations with numerical tests and observations.