Abstract
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 (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.
Highlights
Discrete auroral arcs observed in the postnoon and evening sector are a rather common phenomenon
It has been suggested that auroral activity in these regions is associated with the Low Latitude Boundary Layer (LLBL), with direct solar wind – magnetosphere interaction (Bythrow et al, 1981; Evans, 1985; Lundin and Evans, 1985; Lundin et al, 1995)
According to recent statistical analysis based on particle spectra measured by the DMSP satellites and ground based data (Newell et al, 2004, 2005) the locus of bright postnoon auroral spots covers a region that maps to the LLBL and to the Boundary Plasma Sheet (BPS) or Plasma Sheet Boundary Layer (PSBL)
Summary
Discrete auroral arcs observed in the postnoon and evening sector are a rather common phenomenon. The field-aligned current density, j||, is a function of the field-aligned potential drop, = i− m; this relationship is known as the current-voltage relation (or Ohm’s law) for the auroral electric circuit It has been computed by several authors for a monotonically decreasing with altitude and assuming adiabatic motion of particles along the flux tube connecting the ionospheric load and the magnetospheric generator described by Maxwellian (Knight, 1973; Lemaire and Scherer, 1973; Chiu and Schulz, 1978), biMaxwellian (Fridman and Lemaire, 1980) or kappa (Pierrard, 1996) velocity distribution functions. We discuss numerical solutions of Eq (4) obtained for various profiles of the generator potential, m, corresponding to different values of plasma velocity, density and electron pressure at the inner edge of the LLBL
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