THE DENSITY MINIMUM AT THE EARTHS MAGNETIC EQUATOR

Observations of the density structure in the plasmapause region reveal the existence of a local minimum in the total electron density at the magnetic equator. Data from the plasma wave instrument and ion mass spectrometer on the Dynamics Explorer (DE) 1 satellite are used to study this phenomenon. The density depletion typically extends from +/- 5-degrees to +20-degrees in latitude and is found at altitudes from 2 to 5 R(E). Density depletions of 10-70% are found in regions where the off-equator density ranges from 10 to 1000 cm-3. This density structure is associated with equator crossings where the thermal plasma has been heated over normal plasmasphere values. The heated plasma is the equatorially trapped plasma previously reported from DE 1 and the SCATHA satellite. Within the plasmasphere, the drop in total (electron) density corresponds to a decrease in the cold-ion density, in both H-1 and He-1. There is a rough pressure balance provided by the warm tail of the distribution, which is a few percent by density but 1-2 orders of magnitude higher in temperature.