Multisatellite studies of ULF waves

Multisatellite studies of ULF waves are reviewed, with a special emphasis on compressional Pc 5 waves. The observations of the radial extent, azimuthal wave number and the field-aligned standing wave structure of the waves are described. When combined with plasma data, the waves appear to be consistent with drift-mirror waves originating from the pressure anisotropy of the ring current plasma. The most recent observations revealed some features not considered in previous theories of the drift-mirror wave: antisymmetric standing wave structure and rather small pressure anisotropy that can drive the waves to instability. Recent theoretical work shows that these observations can be explained if the coupling of the drift-mirror wave to the shear Alfven wave is properly treated in a realistic magnetic field geometry.