Structured variations of the plasmapause: Evidence of a corotating plasma tail

Direct measurements of the pole-to-pole distributions of topside thermal protons obtained with the Ogo-4 ion composition experiment, near local midnight, reveal sharply structured variations of the plasmasphere boundary. With increasing L values the proton density distributions may exhibit a pronounced inner trough, in which the ambient H+ concentrations decrease by an order of magnitude near L=2 and subsequently recover to midlatitude concentration levels before a more pronounced and persistent trough is encountered at higher L positions. The time evolution of the trough boundaries observed during a sample of five consecutive satellite orbits following the peak of the magnetic storm on September 21, 1967, suggests that the observed structure results from a plasma tail or elongation of the plasmasphere that tends to corotate with the earth. The empirical model deduced indicates that at the time of these observations the plasma tail has a width of about 0.8 L at the base, extending to an extremely thin region of 0.05 L or less near the tip, and with an azimuthal extent of approximately 85° longitude. The general features of the observed plasma tail structure are in accord with the time evolution of the plasmapause predicted theoretically, assuming that the magnitude of the thermal plasma convective flow from the magnetosphere tail varies directly with the magnetic index, Kp.