New Cluster results on foreshock field-aligned beams

We examine in detail the reduced particle distributions of Field-Aligned Beams observed upstream of the bow shock using Cluster spacecraft. Our study shows that that both the reduced parallel and perpendicular distribution forms are strongly geometry dependent. Above a certain critical value of thetaBn, the angle between the IMF and the shock normal at the particle source point, the reduced distribution distributions are remarkably fitted by a Maxwellian. When the angle decreases, leading to smaller beam velocities, a high energy tail in the distribution appears. When the tail is present, the peak of the distribution remains Maxwellian. The more the high energy tail is developed, the more the beam speed (or equivalently thetaBn) is small. Moreover, the examination of the angular distribution indicates that only particles having beam speed close to the beam speed are field-aligned; for higher speed [tail of the distributions], particles propagate at a significant angle with respect to the magnetic field; this angle is energy-dependent. We have examined the origin of these skewed distributions and favour a production mechanism at the shock surface.