Magnetosphere-ionosphere coupling at high latitude during periods of Northward IMF

International audienceDuring periods of northward IMF, auroral features are observed above the polar caps. These polar cap arcs are the signature of magnetosphere-ionosphere coupling processes occurring at high latitudes. Recent observations from the Cluster satellites at high altitude (5-7 RE) and from spacecraft imagers give new insight on these processes. Polar cap arcs can occur on a variety of scales, from large-scale arcs called 'transpolar arcs' or 'theta aurora' to small-scale isolated arcs. There is strong evidence that large-scale transpolar arcs are located on field lines that have been closed by magnetotail reconnection but remain embedded within the lobes. This is further confirmed by recent observations from the IMAGE and Cluster satellites. Small scale structures consisting in a succession of current sheets with opposite polarity are also observed above the polar caps during periods of northward IMF. Cluster and TIMED observations reveal that they are associated with thin polar cap arcs. The upward current is carried by upflowing ionospheric ions and precipitating electrons accelerated by quasi-static electric fields. The precipitating electrons are cold and likely of solar wind origin. However, for half of the events, plasmasheet-like isotropic ions are detected simultaneously. Consequently the magnetic field lines do not neatly fit into either a closed and open configuration. A detailed review of these observations is proposed, which are subsequently used to constrain ideas on magnetosphere-ionosphere interactions and on magnetospheric configuration at high latitudes during prolonged periods of northward IMF