Split Langmuir probe measurements of current density and electric fields in an aurora

Abstract

A split Langmuir probe instrument was flown on a Black Brant VB sounding rocket from Fort Churchill into an auroral event at a time near local midnight on August 2, 1968. The split Langmuir probe consisted of two conducting plates that were separated by a thin insulator and that were simultaneously swept from negative to positive potentials with respect to the plasma. By measuring the current to each plate and the difference current between plates as a function of the sweep voltage, information was obtained on the plasma current density, bulk flow, electron temperature, and density. Perturbations due to wake effects, work functions, and probe contamination prevented complete measurement of all of these parameters. Upper limits on the ion current parallel to B in the frame of reference fixed to the earth of ±5 × 10−6 amp/m² and on the electron current parallel to B of ±8 × 10−4 amp/m² were obtained. The perpendicular electric field deduced from measurements of the ion bulk flow was in general agreement with independent results from a double probe electric field detector flown on the same rocket. The density and temperature measurements exhibited features consistent with expectations based on electric field, particle, and all-sky camera data. The upper limits on the parallel ion and electron currents were smaller than values required for the formation of free sheaths in a plasma of the measured density and temperature. Hence the parallel electric field of 20 mv/m reported on the same flight by Kelley, Mozer, and Fahleson (1971) must have been sustained by spatially diffuse regions of anomalous resistivity, and not by free sheaths.