dc.description.abstract | In this dissertation, we focus on two studies for fast shocks: 1) a new model for generating the observed cold O+ beam in the geomagnetic tail lobe, and 2) a new method for analysis of interplanetary shocks. These two studies are described briefly below:
(1) The model for generating the observed cold O+ beam:
Observations of cold O+ beams (COBs) in the lobe/mantle region of Earth’’s magnetotail showed that O+ ions originating from the ionosphere can stream into the distant tail (40 – 200 RE). These O+ ions have a high parallel streaming energy (~ 1 – 20 keV) and low perpendicular thermal energy (~ 70 – 210 eV) in the distant tail. Here we propose that the non-adiabatic shock heating of O+ ions in the polar magnetosphere and the subsequent adiabatic evolution of ion velocity can lead to the occurrence of COBs in the tail lobe. The heating and acceleration of heavy O+ ions by fast shocks are studied by a theoretical analysis and hybrid simulations. It is found that after the passing of fast shock, heavy ions gain a large perpendicular thermal energy. After heating, the adiabatic evolution in the tail lobe can transfer a major part of perpendicular thermal energy to the observed parallel streaming energy. We have found that weak fast shocks in the polar magnetosphere with Alfvén Mach number MA ~ 1.05 – 1.2 can lead to the observed streaming energy associated with COBs. Weak fast shocks in the magnetosphere can be generated by the interaction of interplanetary shocks/discontinuities with Earth’’s magnetosphere. Escaping ionospheric O+ ions can gain enough energy from shock heating to account for observations of COBs.
(2) The method for analysis of interplanetary shocks:
Here we present a novel procedure for shock fitting of the one-fluid anisotropic Rankine-Hugoniot relations and of the time difference between two spacecraft observations in the case of small He2+ slippage. A Monte Carlo calculation and a minimization technique are used. The observed variables including the up- and downstream magnetic fields, plasma densities, plasma betas, plasma anisotropies, W (the difference between the down- and upstream velocities, W=V2-V1), and delta_t (the time difference between two spacecraft observations) are used in our procedure where V is defined as the center of mass velocity of plasmas. A loss function based on a difference between the calculated and the observed values is defined, and the best-fit solution is found by searching for the minimum loss function value. For shocks that cannot be fitted well, we introduce two new parameters in the modified RH relations, one in the normal momentum flux and the other in the energy flux equations. These two parameters are interpreted as the equivalent “normal momentum” and “heat” fluxes needed in the RH relations. Their amounts can be estimated from our procedure. Several synthetic shocks are given to verify our procedure. We also apply this procedure to two interplanetary shocks observed by both the WIND and Geotail spacecraft. The results demonstrate that our method works for both the synthetic and the real shocks. We have shown that our method can provide accurate shock normal estimations for perpendicular and parallel shocks as well.
Given that our model is based on the RH relations that do not include the effect of He2+ slippage, it can only be applied to the cases with an ignorable slippage pressure tensor. We found that when the slippage pressure is large, the magnetic coplanarity property may not be valid. We have investigated the pressure tensor due to alpha particle slippage using the WIND spacecraft data. It is found that in general the slippage pressure is small in comparison with the thermal pressure of the system and can be ignored. Thus, our model can be applied to most interplanetary shocks observed near the ecliptic plane. | en_US |