Study of Cusp Structure and Wave Activity in Cusp Regions

Several cusp crossing events observed by Cluster spacecraft are analyzed to study the structure of the cusp region and wave activity in the region. Signatures of magnetic field and plasma parameters show that during a cusp crossing event, the spacecraft encounter plasmas from the following regions: (1) high latitude magnetopause boundary layer, which separate the magnetosheath and the magnetosphere; (2) lobe/cusp boundary layer; (3) the cusp proper; and (4) a closed field line region where keV electrons and enhanced energetic particles (30 - 1500 keV for ions and 20 - 400 keV for electrons) are observed. Along the above Cluster orbits, the fourth region is usually observed between the magnetopause boundary layer and the cusp. These observations provide an overall picture of the cusp structure along the Cluster trajectories. Magnetic field and plasma fluctuations (ULF and VLF waves) are mostly observed in the magnetopause boundary layer and in the cusp and lobe/cusp boundary layer, accompanying strong plasma flow/beam bursts. Calculation of the wave Poyting flux and kinetic energy flux will be presented to examined the energy flow in these regions. Preliminary results show the Poynting flux in the cusp is toward the ionosphere, indicating that ULF wave activity in the cusp is one of the sources of energy that transferred into the magnetosphere.