Applied Physics Laboratory, Johns Hopkins University, Laurel, Maryland

We present a case study of plasma, energetic particle, and magnetic field perturbations seen by Wind in the dawn foreshock of April 19, 1996. A comparison with the results of hybrid simulations confirms model predictions for a diamagnetic cavity flanked by regions of enhanced density and magnetic field strength, but filled with a suprathermal ion population. By contrast to previously reported hot flow anomalies, flow velocities within the foreshock cavities are nearly identical to those in the ambient solar wind, ion temperatures do not rise greatly, electron velocities do not increase at all, pressures are only slightly greater than those in the ambient solar wind, and the events do not lie centered upon interplanetary magnetic field tangential discontinuities intersection the bow shock. We attribute the cavities to the diamagnetic effects of ions Fermi accelerated within the foreshock. A review suggests that foreshock cavities are far more common than hot flow anomalies.

submitted, J. Geophys. Res., 2001