Positive Ion Densities from a Rocket Borne Hemispherical Langmuir Probe: 40 to 100 Km

摘要

The positive ion current to a rocket borne hemispherical electrostatic (Langmuir) probe is determined in terms of the ambient density for altitudes extending through the collision and free molecular flow regimes. In the free molecular flow regime (above 70 km.) the probe velocity is assumed to be sufficiently greater than the mean ion thermal velocity so that a cold plasma may be assumed. Charge trajectories to the probe are then calculated for small angle of attack, yielding a maximum impact parameter for a charge to strike the probe. An effective probe cross section is computed from this impact parameter. In the collision regime, below 60 km, the effects of convection due to supersonic neutral gas flow in the probe frame of reference, and of charge drift due to electrical mobility are examined. The basic features of the neutral gas flow behind the supersonic shock are analyzed leading to the conclusion that the flow velocity is great enough so that charge separation may be neglected except in a small region near the stagnation point. On this basis the total current to the probe is approximated by a simple mobility formula. The results are used to analyze positive ion currents from 40 to 100 km obtained during a P.C.A. event. (Author)