Oblique propagation of 30 s period fast magnetosonic foreshock waves:: A Cluster case study -: art. no. L04804

It is accepted that the 30 s period Ultra-Low-Frequency waves observed in the terrestrial foreshock are generated by backstreaming ions through the ion-ion right hand instability. However, such waves are generally observed to propagate obliquely to the field, in direct contrast to theoretical predictions of parallel propagation. A case study, based on data from Cluster, examining the oblique propagation of ULF waves when the IMF cone angle is large is presented. Errors associated with the analysis are considered. It is found that although the waves propagate obliquely to the magnetic field, they are principally confined to the plane defined by the magnetic field and the x GSE direction. The observed systematic deviation by similar to20degrees of the wave k vector in this plane is explained in terms of non-local wave refraction processes, requiring careful consideration of the relative motion between the backstreaming ions and the upstream waves.