Polar Cap Arcs
Understanding magnetosphere-ionosphere coupling during periods of northward IMF
The coupling between the magnetosphere and the ionosphere is a key issue in solar-terrestrial physics. It is well studied during active periods when the IMF is oriented southward, but the magnetosphere-ionosphere system is often thought to be inactive when the IMF is northward. However, observations over several decades have shown that under these conditions, optical emissions are detected either on the open magnetic field lines of the polar cap (the region of the ionosphere which is on field lines that map out into the magnetotail lobes) or on closed field lines that are embedded within the polar cap. These so-called polar cap arcs have been observed on both a local scale (from ground-based or low Earth orbit observations) and a global scale (from imagers in much higher orbits). As with auroral arcs within the auroral oval, polar cap arcs are indicators of the interaction between the magnetosphere and the ionosphere. They are the signature of poorly understood processes that couple the magnetosphere to the ionosphere when the IMF is northward; consequently observations of polar cap arcs are capable of revealing profound information about the topology of the magnetosphere during intervals of northward IMF. Furthermore, polar cap arcs are useful tools to study auroral conjugacy. Since the European Space Agency is currently evaluating the KuaFu B mission, the science objectives of which are centred on auroral conjugacy, the study of polar cap arcs is timely. Our proposal is to group scientists with diverse expertise in models and observations of polar cap arcs including observations from the ground (radars and imagers) and from low and high altitudes (imagers and in situ spacecraft data). We aim to identify and investigate key issues linked to polar cap arcs in order to give a better picture of magnetosphere-ionosphere interactions during periods of northward IMF. The IMF is northward 50% of the time. Polar cap arcs reveal an unexpected and poorly understood magnetospheric topology. This project will seek to explain this fundamental aspect of magnetospheric behaviour.