Models and numerical simulation of velocity distribution of pickup ions

Large manned spacecraft release and carry up a huge amounts of water in its orbital motion. Some of the neutral water molecules undergo charge exchange with the ambient ionospheric plasma and become what are called pick-up ions at Low Earth Orbit (LEO) altitudes. The purpose of this dissertation is to model those ions and predicts their effect on a space vehicle mounted ion probe so that a comparison between the model and measured data can be made.;The velocity distribution of the pickup ions ( H2O+ ), which are created by charge exchange between released neutral water molecules from a space shuttle orbiter and the ambient ionospheric plasma at LEO altitudes, is systematically studied. The factors which affect the velocity distribution of the pickup ions are discussed. A partial differential equation of the pickup ion's distribution function in the frame fixed on the earth is derived based on the analysis of the pickup ion's source, loss, and collisions with other particles. The trajectories of the pickup ions in the frame moving with the space orbiter are analyzed. The influence of collisions between the pickup ions and other particles on the number of the ions collected by a payload ion probe is discussed and the probability of the collision in the path of the pickup ions from their creation place to an instrument mounted on the orbiter is studied.;Three integral models are developed to calculated the velocity distribution of the pickup ions in the frame moving with the orbiter. The first one is for the pickup ions created in very cold neutral water gas, the second one is for the pickup ions created in warm neutral water gas and the third one is for the pickup ions created in very hot neutral water gas. The algorithms are discussed to numerically calculate the velocity distribution of the pickup ions in the frame fixed on the orbiter with the three models.;To compare the models with the data from the Spherical Retarding Potential Analyzer (SRPA) mounted on the orbital space shuttle, a model and an algorithm are developed to calculate the velocity magnitude distribution of the pickup ions in the frame moving with the orbiter. The method to numerically simulate a payload SRPA output is studied and a method to estimate the ratio of the pickup ions in the total ions collected by the payload SRPA is obtained. The comparison between the simulation result and actual measurement data from the SRPA on mission TSS-1R and earlier missions on which the SRPA was flown shows that the models and algorithms obtained in the dissertation are correct for the situation of a large space platform at LEO altitudes and useful in analyzing the data of the ion probes mounted on similar spacecraft. With the models obtained in the dissertation, the influence of some factors to the velocity magnitude distribution of the ions collected by the payload SRPA and the to the telemetered output signal of the payload SRPA are analyzed. These factors include the angle between the magnetic induction vector of the geomagnetic field and the orbital velocity vector of the orbiter, the density of the released water source of the orbiter, some selected attitude angles of the orbiter and the payload position of the payload SRPA.