Ion temperature in the upper atmosphere

Abstract

The problem of ion temperature in the ionosphere is analyzed in terms of an energy budget that includes heating by the ambient electron gas, cooling by the different gases of the neutral atmosphere, and thermal conduction along the lines of geomagnetic force. With the introduction of the ion temperature fractional separation X, the time-dependent equation for ion temperature excluding transport effects is reduced to a form that removes the explicit dependence upon the neutral gas temperature and separates the effects of the electron and neutral gases. Adopting the appropriate aeronomic conditions, it is shown with different models of the neutral atmosphere that the ion temperature has a transitional behavior, increasing from the neutral gas temperature near 250 km toward the electron temperature above 700 km. While the ion thermal transport does not significantly affect ion temperatures below about 500 km at midgeomagnetic latitudes, it plays an increasingly important role above this height, acting to keep the high-altitude ion temperature significantly less than the electron temperature. In addition, as a result of ion cooling in He and H, the ion temperature can decrease with altitude in the transition atmospheric regions between O and H dominance. The time constant for changes in ion temperature resulting from changes in aeronomic parameters is evaluated and found to be less than 10 minutes for altitudes below 700 km; above this altitude, conduction cooling must be considered. Finally, the theoretical expressions derived here are compared with recent ionospheric Thomson scatter data.