Polar cap particle acceleration: Electron dynamics associated with ion outflows

During periods of northward Interplanetary Magnetic field (IMF), Cluster observed outflowing ion beams at altitudes of 5-7 terrestrial radii above the polar cap. These ions, accelerated at energies of the order of 200 eV to 1 keV, form energy structures with typical inverted-V shapes as already observed in auroral zone. The associated electron distribution shows two different behaviours. During ion outflow events, downgoing electrons are accelerated at energies of the order of 200 - 300 eV. The simultaneous acceleration of the ions in the upward direction and of the electrons in the downward direction is interpreted as the effect of a quasi-static potential drop along polar cap magnetic field lines. This potential structure is not localized near the topside of the ionosphere as generally inferred in auroral zone from lower altitude satellites. It rather extends to higher altitudes than Cluster orbit (more than 5 Earth radii for the considered events). Its life time may exceed half an hour which implies that the polar ionosphere represents a significant particle source for the magnetosphere. Between the ion outflow structures, upward electron beams are accelerated at low energies (30-100 eV). These low-energy beams carry current densities of the order 10 nA/m², corresponding to a few μA/m² at ionospheric altitudes, which balance the currents carried by the surrounding regions involving ion outflows. During northward IMF periods, the polar cap current system appears to be structured in successive current sheets of opposite polarities.