Auroral helium precipitation

We have adapted the metal foil sampling technique, which has been used to measure helium, neon, and argon fluxes in the solar wind, to the problem of measuring the fluxes of these gases in the auroral primary radiation. Aluminum and platinum foils have been flown into two bright auroras and have been recovered. The foils have been analyzed for helium and neon isotopes with a mass spectrometer; so far we have only detected 4He, but with a more sensitive mass spectrometer it should be possible to detect ³He. In the first flight the precipitating flux of 4He with particle energies above about 1 kev was approximately 1 × 106 cm−2 sec−1, and the backscattered flux was smaller by about a factor of 10. In the second flight the aurora was less bright, and the 4He fluxes were lower by a factor of about 2. A rough analysis suggests that the mean energy of the incident particles was >3 kev. These fluxes are consistent with measurements of the ion fluxes observed at high altitudes in the auroral zone and in the magnetosphere, if it is assumed that the helium contributes a few per cent of the total flux.