Low-Energy Electron Measurements and Spacecraft Potential: Vela 5 and Vela 6

Abstract

Low-energy electron measurements from 10 to 1000 eV made by the Los Alamos plasma probes aboard the VELA Satellites provide a direct measure of the spacecraft potential with respect to the surrounding plasma when this potential rises above the lowest measured energy level, and in addition, give an indirect indication when the potential is smaller.It has been found, as expected, that the spacecraft potential is a strong function of the incident electron flux from the ambient plasma and therefore depends strongly on the region in which the measurements are made(solar wind, magnetotail etc.). In addition, information concerning the dependence of spacecraft potential on photocurrent has been obtained from data gathered in the magnetotail and solar wind during eclipses of the Sun at the spacecraft. A preliminary evaluation of the data gives the following results: (1) In the high latitude magnetotail, when the electron flux from the ambient plasma is low, the potential is positive and can rise to over 70 V. (2) The measured relationship between the incident electron flux f e and the satellite floating potential φ can be approximated (for f e between 1 × 107 and 8 × 107 cm−2 s−1) by the relation f e = (f s /4) exp (−φ/φ 0); where f s (the photoelectron saturation flux) = 7.3 × 108 cm−2 s−1, and φ0 = 13.6 V. (3) The spacecraft potential in the solar wind in sunlight (where f e ≈ 3 × 108 cm−2 s−1) is positive and is usually between + 3 and + 5 V. (4) In the solar wind, but in darkness, the s/c potential is slightly negative — most probably between 0 and − 2 V. KeywordsSolar WindPlasma SheetElectron FluxAmbient PlasmaSolar Wind ElectronThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.