Detection of alpha particles in auroral phenomena

Abstract

A coordinated experiment with rocket-borne particle detectors and ground-based photometers has shown that the ratio of the differential flux of alpha particles with Ea ∼ 200 kev to protons with Ep ∼ 100 kev was approximately 1.6% in an aurora on February 9, 1967. This value is consistent with ratios observed at that time by spacecraft in the solar wind but is not consistent with the ratio He++/H+ of thermal ion densities measured between the ionosphere and an altitude of 20,000 km. These data argue that the energetic auroral ions were originally constituents of the solar wind and were accelerated at altitudes above 20,000 km by a mechanism that gives an energization proportional to the charge of the ion. The measured proton energy spectrum between a few hundred ev and 650 kev was used to predict the Hβ optical intensity; calculated limits of 35–105 rayleighs were in good agreement with the measured value of 46 R. An experimental upper limit of 1 R for the λ5876 He I intensity is not inconsistent with the alpha-particle flux. Theoretical implications of these several results are discussed.