Electrodynamics of the polar cusp ionosphere

Satellite measurements of magnetic field and ion drift components above auroral structures observed from the ground permits detailed investigations of auroral electrodynamics in the cusp ionosphere. Information on Birkeland currents, Poynting flux, height-integrated Pedersen conductivity and Joule heat dissipation rate has been derived. Intermediate scale (10's of km) structures of enhanced electron precipitation with peak energy fluxes ∼ 5 ergs/cm2sec (5.10-3 W/m2) and average energy ∼ 0.1 keV were associated with strong, red-dominated auroral forms (I 630.0 nm≅ 5 kR) with typical spectral ratio I 630.0/nm/I557.7 nm ≅ 5. Within these forms strong northward electric fields with peak values of ∼ 200 mV/m and Birkeland current densities ∼ 10 μA/m2 were observed. Rather complex patterns of Birkeland currents were connected to these structures. Both downward and upward directed Poynting fluxes associated with the Birkeland currents were inferred, at different latitudes within the cusp. The downward flux is dissipated as ionospheric Joule heating. Indications of additional dissipation and/or source mechanisms associated with the Poynting flux are observed in some limited regions. From correlated latitudinal variations in electric and magnetic field components, the Joule heat dissipation rate wihtin one of the auroral forms could be estimated. The dissipation rate in the center of this structure was found to be ∼ 5 times higher than the particle energy input rate. The satellite pass occurred within an interval (IMF Bz < 0) characterized by a series of short-lived (few minutes) poleward moving auroral structures, each appearing at the cusp equatorward boundary. The close latitudinal relationship between these auroral forms and the Birkeland current and ion drift structures, indicate that the latter features are of a similar transient nature, probably related to the transient injection of magnetosheath plasma elements into the dayside magnetospheric boundary layer