Multi-point CLUSTER observations of VLF saucers in the auroral region

Cluster has been exploring the magnetosphere for over 10 years, offering new insights into the physics and phenomenology of this highly dynamic plasma environment. Since 2008, the orbit has evolved such that the spacecraft regularly pass over the auroral zones at altitudes of 5-10,000 km. We utilized these unique multi-point measurements made in the auroral regions between 2008 and 2011 to study this region. We focused on the study of electrostatic whistler waves, the so-called VLF saucers which have been frequently observed by auroral spacecraft. It is well-known that the frequency-time dependence of VLF saucers is due to their dispersion relation. However multi-point CLUSTER observations provide unique information on the spatio-temporal variability of VLF saucers source regions and allow their detailed characterization. It is shown that saucer source regions are localized on downward current auroral field lines. Their limited spatial extension is an indirect evidence that the auroral acceleration region itself is highly structured along and across the magnetic field.