H-ACCELERATION IN THE MIDALTITUDE IONOSPHERE( ION)

H+ ion acceleration by electromagnetic ion cyclotron waves in the Eart h's mid-altitude (300 to 2000 km) ionosphere is investigated. Below ab out 800 km, the ionospheric density is sufficiently high to induce sig nificant collision rates between thermal particles and accelerated Hions. The dominant collisions are associated with ion-ion Coulomb inte ractions and with charge exchange reactions of the type H+ + A --> H A(+). These reactions are able to substantially modify the space and velocity distribution of the accelerated H+ ions. Using Monte-Carlo si mulations, it is shown that the combination of collisions and ion cycl otron acceleration produces energetic H+ ion distributions whose chara cteristics (flux, average energy and pitch angle) are in quantitative agreement with satellite observations. The distributions of the energe tic H atoms which are produced by charge exchange and which might be u sed for remote sensing of such H+ ion acceleration processes, are also presented.