A new, unique signature of the true cusp

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 th erefore a weak-field region with continuous contact with magnetosheath plas ma. Data from the recent Astrid-2 Swedish microsatellite are presented whic h show a new, unique signature (dubbed the "true cusp") during cusp crossin gs 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 m ade by the DE-2, UARS and DMSP-F10 satellites also show this same, unique s ignature although in most cases the spatial resolution was much less than t hat of the Astrid-2 MEDUSA spectrometer. The presence of concurrent feature s 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 coi ncidental structure among plasma instruments. The persistence of this featu re may lend new insight into the dynamics of the cusp and magnetospheric pa rticle entry.