A new, unique signature of the true cusp

Abstract. The "cusp proper" is generally understood to be the region enclosed by the outermost magnetospheric field lines as they map to low altitudes. It is therefore a weak-field region with continuous contact with magnetosheath plasma. Data from the recent Astrid-2 Swedish microsatellite are presented which show a new, unique signature (dubbed the "true cusp") during cusp crossings that can now be shown to be consistently present and with which one can redefine the physical meaning and topology of the cusp. Similar crossings made by the DE-2, UARS and DMSP-F10 satellites also show this same, unique signature although in most cases the spatial resolution was much less than that of the Astrid-2 MEDUSA spectrometer. The presence of concurrent features of the same scale size as the plasma in the energetic particle, field and wave power data shows that this is a real structural feature and not a coincidental structure among plasma instruments. The persistence of this feature may lend new insight into the dynamics of the cusp and magnetospheric particle entry.

Key words. Magnetospheric physics (magnetopause, cusp, arid boundary layers; magnetospheric configuration and dynamics)