Abstract
Abstract. An auroral flux tube is modelled from the magnetospheric equator to the ionosphere using Vlasov simulations. Starting from an initial state, the evolution of the plasma on the flux tube is followed in time. It is found that when applying a voltage between the ends of the flux tube, about two thirds of the potential drop is concentrated in a thin double layer at approximately one Earth radius altitude. The remaining part is situated in an extended region 1–2 Earth radii above the double layer. Waves on the ion timescale develop above the double layer, and they move toward higher altitude at approximately the ion acoustic speed. These waves are seen both in the electric field and as perturbations of the ion and electron distributions, indicative of an instability. Electrons of magnetospheric origin become trapped between the magnetic mirror and the double layer during its formation. At low altitude, waves on electron timescales appear and are seen to be non-uniformly distributed in space. The temporal evolution of the potential profile and the total voltage affect the double layer altitude, which decreases with an increasing field aligned potential drop. A current–voltage relationship is found by running several simulations with different voltages over the system, and it agrees with the Knight relation reasonably well.
Highlights
Introduction point and the local quantitiesV (z) and B(z) alone. Chiu and Schulz (1978) studied the electrostatic potential as a functionElectric fields parallel to the magnetic fieldHarye kdnroowlnotgo yex-andof the spatial coordinate along thHe myadgrnoetlioc gfieyldaanndd, asist in ation othf eauaruorroarlalelzeoctnreo,nas.ndTrtahnesyvecrosnetreilbeuctteriEctoafietrhltdehsaacStcehyliegsrh-temshuamviinnggnaostmataixoinmaruymstiastet,hsaht odwVe/ddtBhaE>t tah0eratcnhodnSddi2tyVios/ndtfeBorm2U≤(z0).altitude result in parallel electric fields as a conseqSuecnciee nofcesPublished by Copernicus Publications on behalf of the European Geosciences Union
Static solutions to Vlasov’s equation have been found for auroral flux tubes (Ergun et al, 2000) covering several Earth radii, and Vlasov simulations of double layers have been performed in shorter simulation regions (Main et al, 2006)
Driftkinetic simulations of auroral field lines above 1.5 RE altitude showed that trapping of warm plasma sheet electron populations by shear Alfven waves can prevent wave damping, allowing acceleration of auroral electrons (Watt and Rankin, 2009)
Summary
The drift speed and the length of the arc determine the transit time for the drifting plasma, and that sets a limit to the timescales that can be modelled, or for a given timescale it sets the limit on what drift speeds can be accepted. From these geometrical considerations we conclude that the model is always valid for the centre of the arc where the perpendicular electric field component is zero, and where there is a perpendicular electric field component it is valid in the drifting frame of reference as long as the drift speed is not too large
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