Simultaneous Measurements of the High-Energy Particle Properties at a Cusp Diamagnetic Cavity and Surrounding Magnetosheath: Evidence for Local Acceleration Mechanism?

On February 2003 four Cluster spacecraft had a tetrahedron separation of 5000 km when they encountered a large cusp diamagnetic cavity characterized by depressed and fluctuating magnetic field and increased fluxes of high energy (up to ~ 160 keV) electrons, protons and helium. The Cluster separation provided a unique opportunity to determine the large scale structure of the cavity as well as to simultaneously calculate properties of the high energy particles in the cavity and surrounding magnetosheath. The RAPID data indicates that most high energy particles in the cavity have pitch angles of 75-100 degrees, and the High Energy Particle (HEP) fluxes drop in the magnetosheath as a function of distance from the cavity boundary, which would be more supportive of the local acceleration mechanism. We have compared our Cluster observations with the test particle simulations in the high resolution MHD cusp model, which indicates that local acceleration at least to energies of ~ 60 keV is possible due to the 'quasi'-potential which is formed due to reconnection. Additional acceleration may arise from the non-MHD processes. But the story may not be so simple, as there are few HEP intervals with lower fluxes in the magnetosheath that have pitch-angles of 15-45 degrees that could originate from the quasi-parallel bow shock. We will also discuss the origin of the 'turbulence' in the cavity.