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 ³He and confirm measurements of the ⁴He flux reported in a previous paper. The ⁴He∶³He ratio in the first (brighter) aurora was 2950 ± 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 ³He in the terrestrial atmosphere. Current estimates of ³He sources and loss rates are found to be consistent with a balanced atmospheric ³He budget.