A numerical model of the electrodynamics of plasma within the contaminant gas cloud of the Space Shuttle Orbiter at low Earth orbit

This paper presents results from a two‐dimensional, finite‐difference model used to solve for the time evolution of low beta plasma within the neutral contaminant cloud in the vicinity of space platforms in low earth orbit. The model of the ambient and contaminant plasma dynamics takes into account the effects of the geomagnetic field, electric fields, background ionosphere, ion‐neutral collisions, chemistry, and both Pederson and Hall currents. Net ionization and charge exchange source terms are included in the fluid equations to study electrodynamic effects of chemistry within a moving neutral cloud in the low earth orbit ionosphere. The model is then used with complete water cloud chemistry to simulate the known outgassing situation of the space shuttle Orbiter. A comparison is made of the model results with plasma observations made during daytime on OSS‐1/STS‐3 mission. The reported density enhancements of the OSS‐1 mission are unattainable with normal photoionization and charge exchange rates of simple water cloud chemistry used in the two‐dimensional model. The enhanced densities are only attained by a generic chemistry model if a net ionization rate 1000 times higher than the photoionization rate of water is used. It is also shown that significant plasma buildup at the front of the contaminant neutral cloud can occur due to momentum transfer from the neutral outgas cloud to the plasma through elastic collisions and charge exchange. The currents caused by elastic and reactive collisions result in the generation of a small polarization electric field within the outgas cloud. The polarization electric field, in combination with the geomagnetic field, causes the plasma to drift with the contaminant neutral cloud but at a slower speed in the stationary reference frame. The slow drifting is consistent with OSS‐1 plasma probe interpretations where a high relative speed between the Orbiter and ions is inferred. Also, if there is a fast net ionization process within the outgas cloud, the drifting plasma allows for nonlinear buildup of plasma density with respect to the net ionization rate.