Equatorially trapped plasma populations

Plasma observations made at 5.5RE. by the USCD detectors on the SCATHA satellite reveal the existence of a thermal plasma population trapped within a few degrees latitude of the magnetic equator, defined here as the minimum B surface. These measurements were restricted to the 1000 to 2000 LT sector by the spacecraft orbit. The ions show a higher degree of anisotropy than the electrons, with a FWHM of 10° to 25°, narrowing with increasing energy. The electron distribution shows a width of 20° to 60°, again narrowing with increasing energy. The 20‐ to 100‐eV ion fluxes typically show temperatures in the 20‐ to 50‐eV range, and densities of 1–10 cm−3. The electron population typically extends from 50 to 500 eV, with temperatures of 100‐200 eV and densities also in the 1‐ to 10‐cm−3 range. Lower energy field‐aligned populations are occasionally found in both ions and electrons at the same location. These could be sources for the warmer pancake plasma if heated primarily in the perpendicular direction. Equatorial noise in the 20‐ to 200‐Hz range is seen in the electric field data about 90% of the time in association with the trapped plasma, with an intensity corresponding to the strenth of the trapped fluxes.