Upper-atmosphere helium ions

An experimental measurement of the ion concentration in the upper ionosphere by Hale is examined in light of Nicolet's recent suggestion that helium is an important constituent of the high atmosphere. An isothermal region of helium ions at a temperature of 1600°K is found above approximately 1200 km. Consideration of the loss mechanism for helium ions by ion-atom interchange with nitrogen molecules indicates that the helium ions are removed in the neighborhood of 370 km and that the rate constant for the ion-atom interchange process is of the order of 10−12 cm3 sec−1. It is concluded that a layer of helium ions at least 2000 km thick (the heliosphere) separates the ionosphere from the protonosphere.