Radial dependence of foreshock cavities: a case study

Abstract. We present a case study of Geotail, Interball-1, IMP-8, and Wind observations of density and magnetic field strength cavities excavated by the enhanced pressures asso-ciated with bursts of energetic ions in the foreshock. Con-sistent with theoretical predictions, the pressure of the ener-getic ions diminishes rapidly with upstream distance due to a decrease in the flux of energetic ions and a transition from near-isotropic to streaming pitch angle distributions. Conse-quently, the cavities can only be observed immediately up-stream from the bow shock. A comparison of conditions up-stream from the pre- and post-noon bow shock demonstrates that foreshock cavities introduce perturbations into the on-coming solar wind flow with dimensions smaller than those of the magnetosphere. Dayside geosynchronous magnetic field strength variations observed by GOES-8 do not track the density variations seen by any of the spacecraft upstream from the bow shock in a one-to-one manner, indicating that none of these spacecraft observed the precise sequence of density variations that actually struck the subsolar magne-topause. Key words. Interplanetary physics (energetic particles; planetary bow shocks) – Magnetospheric physics (solar wind-magnetosphere interactions)