Cluster Observations of Foreshock Bubbles and Their Boundary Shocks

Foreshock Bubbles (FBs) are transient structures in the Earth's foreshock region, which are diamagnetic cavities formed by hot ion concentration around interplanetary magnetic field discontinuities and have significant compressional boundary shocks contributed to particle acceleration. We present here Cluster observations of FB events from January 2002 to April 2007 that each was encountered by all four spacecraft in order to accurately determine the parameters of its boundary shock. Statistical distributions show that the majorities of the FB boundary shocks are supercritical and steep with large magnetic compression ratios and are in quasi-perpendicular direction to their upstream magnetic field. The magnetic compression ratios of FB boundary shocks are roughly correlated positively with their shock normal angles. Additionally, the magnetic compression ratios enhance with increasing upstream incident velocities, which is interpreted as a manifestation of diamagnetic Hall current generation inside the boundary. These results along with the conclusions given in previous numerical simulations and laboratorial experiments suggest a fast formation of a sharp boundary shock by the Larmor coupling between the super-thermal ions and magnetized ambient plasma in a hot plasma expanding process.