Ogo 5 measurements of the plasmasphere during observations of stable auroral red arcs

Ambient H+ ion concentration was measured in the plasmasphere during two intense magnetic storms that were accompanied by occurrences of stable auroral red (SAR) arcs. The H+ ion density profiles were measured by the ion mass spectrometer on Ogo 5 during the periods October 29 to November 7, and October 10 to 17, 1968. Correlations of the H+ ion densities in the plasmasphere were made with the Dst and Kp magnetic indices and with ground observations of the 6300-A SAR arcs. This study shows that the SAR arcs are observed in the rapid recovery phase of the storm in the 10–20 hours following injection of ring current particles, and that the plasmasphere is drastically reduced in size during the storm, with the SAR arc located in L value near the position of the plasmapause. The arc is found to occur in the ionosphere at the base of flux tubes that have ambient H+ ion densities of 100–1000 ions/cm3 near the magnetic equatorial plane.