Helium isotopes in an aurora

Aluminum and platinum foils were flown into two bright auroras and subsequently recovered. They were then analyzed for traces of trapped noble gases in a sensitive high-resolution mass spectrometer. In this paper we report the detection of He-3 and confirm measurements of the He-4 flux reported in a previous paper. The He-4:He-3 ratio in the first (brighter) aurora was 2950 plus or minus 250. This ratio is only slightly higher than the average solar wind ratio of 2350 and since the atmospheric ratio is grossly different (250 times larger), establishes the solar wind as the principal source of auroral helium, at least in the aurora concerned. The result supports earlier suggestions that auroral precipitation is one of the main sources of the He-3 in the terrestrial atmosphere. Current estimates of He-3 sources and loss rates are found to be consistent with a balanced atmospheric He-3 budget.