Foreshock density cavitons

From global hybrid simulations (kinetic ions, fluid electrons) we have found the existence of foreshock density cavitons immersed in regions permeated by ULF waves (Omidi, 2007, Blanco-Cano et al., 2008). These cavitons show large depressions in magnetic field magnitude and density, and are bounded by regions with enhanced field and density. Foreshock cavitons share some characteristics with previously reported isolated foreshock cavities, but in contrats to the cavities, are always found surrounded by a sea of ULF waves. Recently, Sibeck et al. (2008) compared observations of isolated foreshock cavities with hybrid simulation results for a day with small cone angle and concluded that isolated foreshock cavities are associated with changes in the IMF orientation which causes the spacecraft to go in and out of the foreshock. In other words, isolated foreshock cavities should be interpreted in terms of transient encounters with the foreshock compressional boundary and not as distinct regions inside the foreshock. In this work we study the charactericstics of observed foreshock cavitons using Cluster magnetic field and plasma data and compare them with observed crossings of the foreshock compressional boundary for various IMF orientations.