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 in the morning sector of the auroral zone and associated low frequency electromagnetic waves. Data obtained during near radial passes of 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 0⁺ and H⁺ component, and 2) near the boundary between discrete and diffuse electron aurorae a drastic change in the ion characteristics is observed. The flux of energetic precipitated H⁺ ions is sharply reduced, which suggests the formation of an Alfvén layer. However, intense fluxes of precipitated H⁺, O⁺, and He⁺ ions with energies < 3 keV are observed equatorward of the Alfvén layer, in coincidence with the diffuse aurora and in association with quasi-monochromatic electromagnetic waves with frequencies around the proton gyrofrequency. As the characteristic convection and bounce times of the low energy upward flowing 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.