Observations of warm plasma in the dayside plasma trough at geosynchronous orbit

Authors:
  • JHJ. L. Horwitz
    J. L. Horwitz
  • CCC. R. Chappell
    C. R. Chappell
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Abstract
Positive ions in the energy range E = 3–50 eV have been measured on the dayside at geosynchronous orbit with the University of California at San Diego auroral particles experiment on ATS 6. It is found that the near‐equatorial plasma trough contains a warm plasma component with a relatively stable temperature κT ≃11 eV and a low “isotropic component” of density (determined from fluxes near α=90°) typically in the range 0.1–0.4 ion/cc. The pitch angle distributions of these warm ions are typically field‐aligned, indicating an ionospheric origin, and have pitch angle half widths in the range 20°–40°. Pancake (flux peaks at α = 90°) and conical (sharp peaks at α = 10°–40°) distributions are also observed. Abrupt enhancements of the plasma density to 1–10 ions/cc, or more, are seen in the late afternoon and last from several minutes to a few hours. These regions of enhanced density are typically cooler than the ambient trough plasma, are often associated with pancake pitch angle distributions, and are believed to be plasma clouds convected to the spacecraft from the vicinity of the dusk plasmaspheric bulge region.

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