The morphology of the bulge region of the plasmasphere


  • C. R. Chappell,

  • K. K. Harris,

  • G. W. Sharp


The concentration of H+ ions as a function of L has been measured by the ion mass spectrometer on OGO 5. The data of June through November 1968 are presented. These data cover the local time region from 1400 to 2200 LT, which encompasses the bulge region of the plasmasphere. H+ ion density versus L profiles show that the plasmapause position in this bulge region is located between approximately L = 4 and 9 and is generally found at increasingly larger L values as the magnetic activity decreases. In almost all cases, the density profiles through the bulge region exhibit a 1/R4 dependence with radial distance and have a steep density gradient at the outer edge of the bulge. The plasmasphere is observed to fill itself out to large L values during quiet magnetic periods and to have its plasma peeled away from its outer parts during times of moderate to high activity. One aspect of this peeling process is the occasional apparent detachment and sunward movement of a portion of the bulge region. A very extensive depletion of plasma is seen to accompany large magnetic storms. During unsteady and moderate to high magnetic activity periods, the bulge profiles usually contain large fluctuations in density at or immediately outside the plasmapause. Many characteristics of the observed profiles are consistent with the profiles predicted by using a magnetospheric convection model, which treats the bulge region as part of the main flow pattern of the plasmasphere. In this model the changes in the plasmasphere profiles and boundaries are dependent on a modulation of the convection electric field across the magnetospheric tail.