Mass composition of substorm‐related energetic ion dispersion events

Abstract

The Lockheed ion mass spectrometer flown onboard the SCATHA (P78‐2) spacecraft is used to study the mass composition of two ion dispersion events. The energy‐dispersed ions are observed over the full energy range of the instrument (0.1–32 keV/q) in the noon‐dusk local time sector. On one of the days, March 22 (day 81) 1979, the dispersing ions are first observed following an isolated substorm. A long period of low magnetic activity is present prior to the substorm on this day, and a decrease in Dst is observed following the first observation of the dispersing ions. On the second day studied, June 7 (day 158), 1979, the correlation between ground magnetic activity and initial observation of dispersing ions is not so clear, since the dispersion follows a period of high magnetic activity. Both dispersion events follow a marked reduction in the preexisting near‐geosynchronous plasma near the noon sector. The mass composition shows that although there are similarities in the dispersion for both protons and oxygen, there are also distinct differences. Both species show a definite dispersion ridge, but the protons also have additional fluxes at energies greater than the dispersion ridge energy. It is proposed that the composition changes are attributed to localized injection of ionospheric plasma in the dusk‐midnight sector, with proton‐rich plasma sheet ions convecting past the spacecraft from larger radial distances. By using the ability to scan in pitch angle, it is noted that there are both pitch angle and mass dependences to the arrival times of the dispersing ions. Protons at 30° pitch angles arrive at the spacecraft before oxygen ions at the same pitch angle.