ALBERT

Description: From satellite data sampling the top ionosphere in the Northern Hemisphere we have identified strong eastward ion drifts, with speeds larger than 1 km/s, widths of 1°–2°, occurring at similar temporal and spatial locations as rapid westward ion drifts known as sub-auroral ion drifts (SAID). We have called these events "abnormal sub-auroral ion drifts" (ASAID). Two events observed in the 20:00–22:00 MLT interval are discussed: the first occurring on 21 September 2003 and the other on 12 October 2003. Tomographic reconstructions of the electron density in the F-region, based on satellite data, provided by the Scandinavian tomography chain, were also available. We have observed that ASAID are accompanied by upward flows with a speed of the same order as that of the zonal ion drift. They coincide with deep, narrow troughs in the total ion density, both at the altitude of the F15 DMSP satellite (850 km) and in the F-region of the ionosphere, but do not seem to be a feature of the convective transport. During the entire duration of ASAID the electron temperature is very high while, contrary to SAID, the ion temperature has no clear variation. Both events described in this paper end up turning into classical SAID. Satellite data indicate that the generator of ASAID could be located inside the plasmasphere close to the plasmapause and we suggest a possible mechanism for their formation.

Description: We discuss a model for the quasi-stationary coupling between magnetospheric sheared flows in the dusk sector and discrete auroral arcs, previously analyzed for the case of a uniform height-integrated Pedersen conductivity (ΣP). Here we introduce an ionospheric feedback as the variation of ΣP with the energy flux of precipitating magnetospheric electrons (εem). One key-component of the model is the kinetic description of the interface between the duskward LLBL and the plasma sheet that gives the profile of Φm, the magnetospheric electrostatic potential. The velocity shear in the dusk LLBL plays the role of a generator for the auroral circuit closing through Pedersen currents in the auroral ionosphere. The field-aligned current density, j||, and the energy flux of precipitating electrons are given by analytic functions of the field-aligned potential drop, ΔΦ, derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, Φi (and implicitely ΔΦ) is determined from the current continuity equation in the ionosphere. We obtain values of ΔΦ of the order of kilovolt and of j|| of the order of tens of μA/m2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of ΔΦ, j|| and εem are higher than in the case of a uniform ΣP. Effects on the postnoon/evening auroral arc electrodynamics due to variations of dusk LLBL and solar wind dynamic and kinetic pressure are discussed. In thin regions (of the order of kilometer) embedding the maximum of ΔΦ we evidence a non-linear regime of the current-voltage relationship. The model predicts also that visible arcs form when the velocity shear in LLBL is above a threshold value depending on the generator and ionospheric plasma properties. Brighter arcs are obtained for increased velocity shear in the LLBL; their spatial scale remains virtually unmodified. The field-aligned potential drop tends to decrease with increasing LLBL density. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.