Quasi-Static Electric Field Measurements Made with the Plasma Diagnostics Package in Free Flight during SPACELAB-2.
As part of the Spacelab-2 mission the Plasma Diagnostics Package (PDP) was released from the shuttle as a free flying satellite. The shuttle performed maneuvers around the PDP in order that the ionospheric plasma around the shuttle might be studied. One objective of the PDP was to measure quasi-static electric fields in the vicinity of the shuttle. During most of the free flight, the measured electric field was comparable to the induced electric field due to the orbital motion of the spacecraft. The difference between the measured field and the motional field was typically on the order of the uncertainty of measurement. At certain times, when the shuttle thrusters were operating, decreases in the motional electric field by 10% to 20% were observed. The decreases are explained by the generation of an Alfven wave from pickup current. An estimate of the electric field associated with Alfven wave excitation agrees with the decreases observed at times of thruster firings. The Alfven wave model predicts that large changes in the electric field should occur only at times of large neutral gas releases from the shuttle. The decreases in the electric field occur in the region of the thruster plume, as well as along the magnetic flux tubes passing through the plume. During times when an electron beam was ejected from the shuttle, large signals were also recorded. These large signals were probably not due to ambient electric fields, but can be attributed to three causes: differences in fluxes of streaming electrons to the two probes due to shadowing by the PDP chassis, depressions in the plasma density caused by the PDP wake, and spatial gradients in the fluxes of energetic electrons reaching the probes. Energetic electrons were found in a region 20 m wide and up to at least 170 m downstream from the electron beam. At 80 or more meters downstream from the beam, the energetic electrons had a preferential direction of motion opposite to the beam injection direction.
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- Physics: Fluid and Plasma; Engineering: Aerospace