Ionospheric effects due to interplanetary neutral helium

Interstellar neutral helium streams into the heliosphere and is focused strongly by the solar gravitation in the anti-apex direction of the streaming motion relative to the sun. Due to this effect relatively high interplanetary helium concentrations are predicted for a distance of about 1 AU from the sun near the ecliptic plane. The earth moves through this helium bulge in winter. The velocity of the helium flux is added to the orbital velocity of the earth. The particle stream is estimated to be above 107 particles/cm2/sec during winter and 106 during summer. As the helium in neutral, it passes through the magnetosphere undisturbed. The energy of each atom is sufficient to ionize several neutral atoms of terrestrial atmosphere, but due to elastic collissions which compete very effectively with ionizing collisions only about 0.23 ionizations per incoming helium atom actually occur. This ionization source must be taken into account for the winter ionization balance, especially at night in the height region above 200 km. For the He+ ion balance this source is of importance even during daylight hours over the whole year