Satellite measurements of auroral alpha particles

Energetic He++ ions were observed precipitating into the night side auroral zone on a satellite pass over the southern hemisphere on May 16, 1972, during a magnetic storm. The He++ spectrum was peaked near 1 keV/nucleon and was quite similar to that of the simultaneously observed protons when it was plotted versus velocity. The averaged alpha-to-proton ratio varied from 5.6±0.7% in a latitudinal interval centered at L = 7.3 to 2.9±0.5% in an interval centered at L=5.0. It is concluded that the He++ ions and protons originated in the solar wind and were acted upon within the magnetosphere by adiabatic processes and that no substantial electrostatic acceleration occurred.