Observations of multiple ion beams in the magnetotail: Evidence for a double proton population

On day 143, 1978, 2230–2330 UT, the ISEE 2 plasma experiment detected the multiple population frequently found in the mantle and low-latitude boundary layer regions of the magnetotail. This population consisted at this time of ionospheric O+ and H+ mixed with solar wind plasma (typically H+ and He++). A detailed analysis of the low-energy population indicates that it can be further resolved into two distinguishable proton components. The ratio between the oxygen and the proton energies implies that one of the proton populations is of ionospheric origin and flows tailward together with the ionospheric oxygen. The angular distribution of the other proton population and of the He++ suggests a higher-altitude origin, the presence of He++ ions suggesting that this is the solar wind proton component.