Auroral helium precipitation

Abstract

Several measurements of the flux and charge state of auroral helium precipitation have been made and reported between 1967 and 1972. The primary purpose of these experiments has been to deduce the source location of auroral ions by comparison of auroral helium populations with those measured in the ionosphere and the solar wind. Accordingly, results of these experiments are reviewed and summarized, along with measurements of solar wind and ionospheric helium abundances. It is concluded that the available data suggest a solar wind origin of auroral ions and in addition place certain constraints on the acceleration mechanism. The problem of modification of the incident charge state of a helium beam by atmospheric charge exchange is discussed, and it is shown that auroral helium measurements should be made at altitudes above 400 km to insure that the measurements are a true indication of the original beam charge state composition. Attempts to measure optical emissions of auroral helium are also reviewed, with the conclusion that definitive reproducible detection of such emissions has not been accomplished to date.