An Enhancement of Plasma Density by Neutral Gas Injection Observed in SEPAC Spacelab-1 Experiment

An enhancement of plasma density observed during a neutral gas injection in Space Experiments with Particle Accelerators: SEPAC by the Space Shuttle/Spacelab-1 is presented. When a plume of nitrogen gas was injected from the orbiter into space, a large amount of plasma was detected by an onboard plasma probe. The observed density often increased beyond the background plasma density and was strongly dependent on the attitude of the orbiter with respect to the velocity vector. This effect has been explained by a collisional interaction between the injected gas molecules and the ionospheric ions relatively drifting at the orbital speed.