Field-aligned acceleration above the polar caps during prolonged periods of Northward IMF: 2. Polar electron precipitation

The Cluster spacecraft reveal the presence of successive current sheets of opposite polarity above the polar caps during extended periods of northward IMF. At Cluster altitude (5-7 RE), the upward part of this current system consists of ion beams accelerated by quasi-static electric fields and associated with precipitating electrons. They are surrounded by low energy upflowing electron beams carrying a downward current. In this paper, we focus on the precipitating electrons systematically observed by Cluster during the events of ionospheric ion outflows above the polar cap and contributing to the upward currents. Energetic events are occasionally observed, but usually the precipitating electrons are accelerated at weak energies, around 100 - 300 eV. This electron acceleration is interpreted similarly to the simultaneous ion acceleration as the effect of an electrostatic electric field along magnetic field lines. This implies the existence of a potential structure located above CLUSTER altitude, i.e. typically above 5-7 RE, which accelerates the magnetospheric electrons towards the polar cap , in addition of the low-altitude potential structure responsible for the ion outflows escaping the ionosphere. We present statistics on the characteristics of these precipitating electron structures, only observed during time periods of northward IMF. We discuss the source regions and the mechanisms possibly at their origin.