Quantitative calculations of helium ion escape fluxes from the polar ionospheres

Abstract

Recent experimental measurements of He+ outward fluxes have been obtained for winter and summer hemispheres. The observed fluxes indicate an average He+ escape flux of 2 × 107 cm−2 s−1 in the winter hemisphere and a factor of 10–20 lower in the summer hemisphere. Earlier theoretical calculations had yielded winter fluxes a factor of 4 lower than the measured values and summer fluxes a further factor of 20 below the winter fluxes. We have attempted to reduce this discrepancy between our earlier theoretical model and the experimental observations by improving our theoretical model in the following ways. The helium photo‐ionization cross sections used are accurate to 10%, the latest solar EUV fluxes measured by the Atmosphere Explorer satellites have been incorporated, and the most recent MSIS model of the neutral atmosphere is contained in the model. A range of conditions covering solar cycle, seasonal, and geomagnetic conditions were studied. The results show a maximum He+ escape flux of 1.4 × 107 cm−2 s−1 for solar maximum, winter, low magnetic activity conditions, which is within the scatter of the measured fluxes. The computed summer He+ escape flux is a factor of 20 lower than the winter value, a result which is in reasonable agreement with the summer experimental observations. Possible reasons for the slight discrepancy between theory and experiment in summer are discussed.