Cluster observations of a cusp diamagnetic cavity: Structure, size, and dynamics

We have analyzed Cluster magnetic field and plasma data during high‐altitude cusp crossing and compared them with high‐resolution MHD simulations. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as the IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side of the cusp because the reconnection site had changed location. We found evidence of magnetic reconnection both during northward and southward IMF conditions. The Cluster separation was ∼5000 km, enabling for the first time measurements both inside the DMC and surrounding boundaries that allowed us to construct the structure of the DMC and put the observations of ion pitch angle distributions in context of local reconnection topology and gradients of the boundaries. The cavity is characterized by strong magnetic field fluctuations and high‐energy particles. At the magnetosheath boundary the high‐energy particle fluxes reduced by several orders of magnitude. Throughout the magnetosheath, the high‐energy proton fluxes remained low except during brief intervals when sc4 and sc1 dropped back into the cavity due to changes in solar wind dynamic pressure. However, the high‐energy O+ fluxes did not drop as much in the magnetosheath and were mostly at 60°–120° pitch angles, indicative of a trapped population in the DMC which is observed in the magnetosheath due to a large gyroradius. Significant fluxes of protons and ionized oxygen were also observed escaping from the diamagnetic cavity antiparallel to the magnetic field in a time scale more consistent with the local DMC source than with a reflected bow shock source.