Wave-particle interaction in the terrestrial ion foreshock:: new results from Cluster

Different types of backstreaming ion distributions have been reported in the region upstream from the Earth's bow shock and magnetically connected to it (ion foreshock): field-aligned beams (FABs), gyrating ion and diffuse ion distributions. Contrary to the first type, the two others are always associated with ULF waves. Among them, gyrating ions with well-defined pitch-angle and gyrophase organization around the local magnetic field have been frequently observed in association with large amplitude quasi-monochromatic right-hand mode waves. These waves reveal the existence of coherent wave-particle interaction which is an efficient process to dissipate the energy of the particles reflected at the collisionless bow shock. It has been shown recently from a large data set from multi-spacecraft observations by Cluster that the gyrophase-bunched ion distributions are mainly produced by such a process from cyclotron-resonant FABs observed just both at the edge of the gyrating ions region and the boundary of ULF waves.