Positive ion distributions in the morning auroral zone: local acceleration and drift effects

We report on the typical structure of the large scale ion precipitation and field aligned currents in the morning sector of the auroral zone. Data obtained during near radial passes fo the AUREOL-3 satellite point to a distinction between two main precipitation regions: 1) In the poleward part of the auroral zone the latitudinal variation of the average energy (or temperature) of the precipitated ions (mainly H+) indicate that they are adiabatically accelerated in the outer magnetosphere. This ''high energy'' (≅ 3 to >20 keV) precipitation is usually associated with a low energy (E < 110 eV) upward flowing O+ (He+) component, and 2) near the boundary between discrete and diffuse electron aurorae a drastic change in the ion characteristics is observed. Here the flux of energetic precipitated H+ ions is sharply reduced, which results from the formation of Alfven layers. However, intense fluxes of precipitated H+, O+ (and He+) ions with energies < 2 keV are observed equatorward of the Alfven layers, in coincidence with the diffuse aurora. As the characteristic convection and bounce times of the low energy upward flowing heavy ion component are comparable (τ > 3 hours) we suggest that the precipitation of ionospheric ions inside the diffuse aurora results from convection and corotation of the ions accelerated to suprathermal energies at higher latitudes. The low energy H+ precipitation is interpreted as due to both a solar and to an ionospheric source