O+-O COLLISION FREQUENCY IN HIGH-SPEED FLOWS

Throughout much of the terrestrial thermosphere and ionosphere, the mo tions of the neutral and ionized constituents are closely coupled and relative velocities are small, of the order of 100 m s(-1) or less. Th is is particularly true at midlatitudes to low latitudes where typical velocities in the neutral gas due to tidal forcing are only 20-50 m s (-1). However, the solar wind-magnetosphere interaction drives a large -scale convection pattern in the polar ionosphere. When the rapid adju stment of the plasma to changes in the solar wind is combined with the slower response of the more massive neutral gas, large relative veloc ities on the order of 1 km s(-1) can exist for substantial lengths of time. This will be more common during periods of high geomagnetic acti vity, as a result of the greater number of magnetic substorms and othe r particle precipitation events. When a significant relative velocity is present, the calculation of interaction parameters of the two gases passing through each other, such as collision frequency, must include that velocity. These effects are usually neglected when interpreting wind and ion drift observations. We show how the collision frequency i s affected by a directed relative velocity between any two gases inter acting with a power law or exponential potential energy curves. The di rected velocity increases the collision frequency at all temperatures for most ion-neutral interactions. For certain power law potentials, s uch as the charge quadrupole, the collision frequency is decreased. We present an analytic solution for the high-speed collision integral us ing the resonance charge exchange cross section.