Cluster observations of fast magnetosonic waves in the terrestrial foreshock

[1] Cluster observations of ULF waves in the terrestrial foreshock are presented. Four spacecraft timing techniques on magnetic field measurements are used with plasma bulk velocity measurements to determine wave properties in the plasma rest frame. Based on the propagation speed and polarisation, the waves are found to be fast magnetosonic. The analysis is complemented by a direct examination of the correlation between plasma and field perturbations; consequently, an unambiguous and comprehensive identification of the wave is achieved. The advantages of the four spacecraft analysis are discussed and in particular, calculations of the propagation direction are compared to single spacecraft minimum variance techniques. The ratio of intermediate to minimum eigenvalues is used to characterise the degree to which the wave is circularly polarised. The results of a preliminary statistical survey indicate that if this ratio is greater than 10, minimum variance techniques agree closely with the four spacecraft technique.