Low‐energy ion distribution functions on a magnetically quiet day at geostationary altitude (L = 7)

Abstract

By using averaged data from ATS 6, ion energy and pitch angle distribution functions were examined for a magnetically quiet day (July 18, 1974). The data showed that for both field‐aligned and perpendicular fluxes, the populations had a mixture of characteristic energies. It was found that over three different energy bands in the range 3–600 eV the distribution functions could be fairly well approximated by Maxwellian distributions with temperatures in the ranges 3–10 eV, 30–50 eV, and approximately 70 eV in energy bands of 3–30 eV, 30–140 eV, and 140–600 eV, respectively. Pitch angle distributions were found to vary with the local time; strong field‐aligned particle fluxes were measured in the midnight and afternoon sectors, minor field‐aligned components persisted to some extent at all times, especially at low energies (E < 10 eV). The pitch angle distributions and energy distributions were used to compute total ion density, assuming that the proton was the dominant ion species. In the morning sector (0300–0600 LT) a change of field‐aligned fluxes to pancake distributions peaking at pitch angles near 90° was seen. By using the assumption that the plasma was corotating with the satellite, we have examined pitch angle scattering mechanisms responsible for the observed transformation of pitch angle distribution. It was found that a magnetic noise of power spectral density b²≲10−3 γ²/Hz belonging to electromagnetic ion cyclotron mode (L mode) near the ion cyclotron frequency could be very effective in trapping the field‐aligned fluxes by pitch angle scattering.