Electrodynamics of the polar cusp ionosphere; A case study

By using continuous ground‐based observations of polar cusp auroras and electromagnetic parameters obtained from satellites in polar orbit, electrodynamic processes in the polar cusp have been investigated. HILAT satellite measurements of magnetic field, precipitating electrons and ion drift components above auroral structures observed from the ground permit detailed investigations of the cusp ionosphere. Information on Birkeland currents, Poynting flux, height integrated Pedersen conductivity and Joule heat dissipation rate has been derived. Similar latitudinal structures were observed from HILAT (electron precipitation/ion drift) and in the simultaneous photometer profiles obtained from the ground ∼200 km to the east of the satellite pass. Intermediate scale (tens of kilometers) structures of enhanced electron precipitation (peak energy flux ∼10 ergs/cm²s (0.01 W/m²) and average energy ∼0.1 keV), strong northward electric fields (peak values ∼200 mV/m), and Birkeland current densities ∼10 µA/m² were observed. Rather complex patterns of Birkeland currents were connected to these structures. From correlated latitudinal variations in electric and magnetic field components the downward directed Poynting flux and the ionospheric Joule heat dissipation rate within one of the auroral forms could be estimated. The dissipation rate in the center of this structure was found to be a factor of 2 higher than the electron energy input rate. The satellite pass occurred within an interval (interplanetary magnetic field (IMF) B z < 0) characterized by a series of poleward moving auroral structures (east‐west motion not resolved by the present technique), each appearing at the cusp equatorward boundary. The auroral forms were dominated by red oxygen emission (I630.0 nm/I557.7 nm ≃5 and I630.0 nm ≃5 kR), which is typical in the cusp region.