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Abstract. The Spatio Temporal Analysis of Field Fluctuations (STAFF) experiment is one of the five experiments, which constitute the Cluster Wave Experiment Consortium (WEC). STAFF consists of a three-axis search coil magnetometer to measure magnetic fluctuations at frequencies up to 4 kHz, a waveform unit (up to either 10 Hz or 180 Hz) and a Spectrum Analyser (up to 4 kHz). The Spectrum Analyser combines the 3 magnetic components of the waves with the two electric components measured by the Electric Fields and Waves experiment (EFW) to calculate in real time the 5×5 Hermitian cross-spectral matrix at 27 frequencies distributed logarithmically in the frequency range 8 Hz to 4 kHz. The time resolution varies between 0.125 s and 4 s. The first results show the capabilities of the experiment, with examples in different regions of the magnetosphere-solar wind system that were encountered by Cluster at the beginning of its operational phase. First results obtained by the use of some of the tools that have been prepared specifically for the Cluster mission are described. The characterisation of the motion of the bow shock between successive crossings, using the reciprocal vector method, is given. The full characterisation of the waves analysed by the Spectrum Analyser, thanks to a dedicated program called PRASSADCO, is applied to some events; in particular a case of very confined electromagnetic waves in the vicinity of the equatorial region is presented and discussed.

Abstract. Extensive theoretical work has been performed on the equilibrium structure of tangential discontinuities (TDs) in collisionless plasmas. This paper reviews kinetic models based on steadystate solutions of the Vlasov equation. It is shown that most of the existing models are special cases of a generalized multi-species model. In this generalized model all particle populationsfrom both outer regions and from inside the layer -are described using a unique formalism for the velocity distribution functions. Because of their historical importance, the Harris and Sestero models are reviewed and deduced from the generalized model. The Lee and Kan model is also a special case of the generalized model. The generalized model, however, is also able to describe TDs with velocity shear and large angles of magnetic field rotation. Such a multi-species model with a large number of free parameters and different gradient scales illustrates many observable features of TDs, including their multiscale fine structure. Particular attention is paid to the magnetopause. Observed magnetopause crossings are simulated. The effects of the relative flow velocity and asymmetrical magnetic field profiles on the structure of the magnetopause and on its stability with respect to tearing perturbations are discussed. We also present calculations that demonstrate the potential of the generalized model in explaining the origin of discrete auroral arcs. Numerical simulations of solar wind TDs with heavy ions and a large spectrum of thicknesses are also feasible. This indicates that such a model is of fundamental importance for understanding the detailed structure of solar wind TDs, like those observed by the interplanetary spacecraft ULYSSES. The problems associated with the one-dimensional, time-independent Vlasov approach are discussed and a variational principle is suggested to reduce the arbitrariness resulting from the large number of free parameters.

Abstract. 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 Fregion, 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.

Abstract. 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 ( P ). Here we introduce an ionospheric feedback as the variation of P with the energy flux of precipitating magnetospheric electrons ( em ). 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 m , 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, , derived from standard kinetic models of the adiabatic motion of particles. The ionospheric electrostatic potential, i (and implicitely ) is determined from the current continuity equation in the ionosphere. We obtain values of of the order of kilovolt and of j || of the order of tens of µA/m 2 in thin regions of the order of several kilometers at 200 km altitude. The spatial scale is significantly smaller and the peak values of , j || and em are higher than in the case of a uniform P . 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 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 denCorrespondence to: M. M. Echim (marius.echim@oma.be) sity. For higher values of the LLBL electron temperature the model gives negative field-aligned potential drops in regions adjacent to upward field-aligned currents.

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Abstract. We consider sheared flows in magnetospheric boundary layers of tangential discontinuity type, forming a structure that is embedded in a large-scale convergent perpendicular electric field. We construct a kinetic model that couples the magnetospheric structure with the topside ionosphere. The contribution of magnetospheric electrons and ionospheric electrons and ions is taken into account into the current-voltage relationship derived for an electric potential monotonically decreasing with the altitude. The solution of the current continuity equation gives the distribution of the ionospheric potential consistent with the given magnetospheric electric potential. The model shows that a sheared magnetospheric flow generates current sheets corresponding to upward field-aligned currents, field-aligned potential drops and narrow bands of precipitating energy, as in discrete auroral arcs. Higher velocity magnetospheric sheared flows have the tendency to produce brighter and slightly broader arcs. An increase in arc luminosity is also associated with enhancements of magnetospheric plasma density, in which case the structures are narrower. Finally, the model predicts that an increase of the electron temperature of the magnetospheric flowing plasma corresponds to slightly wider arcs but does not modify their luminosity.

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