Radial dependence of foreshock cavities: a case study

D G Sibeck,K Kudela,J Safrankova,T Mukai,Z Nemecek

doi:10.5194/angeo-22-4143-2004

Abstract

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 associated with bursts of energetic ions in the foreshock. Consistent with theoretical predictions, the pressure of the energetic 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. Consequently, the cavities can only be observed immediately upstream from the bow shock. A comparison of conditions upstream from the pre- and post-noon bow shock demonstrates that foreshock cavities introduce perturbations into the oncoming 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 magnetopause. Key words. Interplanetary physics (energetic particles; planetary bow shocks) – Magnetospheric physics (solar wind-magnetosphere interactions)

Highlights

Kinetic processes occurring in the immediate vicinity of the Earth’s bow shock can introduce considerable structure into the solar wind shortly prior to its interaction with the Earth’s magnetosphere
The relatively rare hot flow anomalies (Schwartz et al, 1985) and much more common foreshock cavities (e.g. Sibeck et al, 2002) represent prominent examples. The latter structures form on bundles of magnetic field lines connected to the bow shock that are embedded in regions of magnetic field disconnected from the bow shock
Other studies indicate that fluxes increase as Bn decreases (Mitchell and Roelof, 1983), where Bn is the angle between individual interplanetary magnetic field (IMF) lines and the normal to the bow shock at the point of intersection

Summary

Introduction

Kinetic processes occurring in the immediate vicinity of the Earth’s bow shock can introduce considerable structure into the solar wind shortly prior to its interaction with the Earth’s magnetosphere. Other studies indicate that fluxes increase as Bn decreases (Mitchell and Roelof, 1983), where Bn is the angle between individual IMF lines and the normal to the bow shock at the point of intersection Based on these predictions, foreshock cavities should be most prominent immediately upstream from the pre-noon bow shock during typical periods of spiral IMF orientation. The spacecraft outside the pre-noon bow shock observed completely different energetic ion bursts, foreshock cavities, and density and magnetic field strength variations. To consider the radial dependence of the properties describ- most prominent peaks in the energetic ion flux corresponded ing foreshock cavities, we will present simultaneous Geo- to density and magnetic field strength minima (e.g. at 16:07, tail, Interball-1, IMP-8, and Wind solar wind plasma, mag- 16:22, 16:26, 16:29, 17:06, 17:19, 17:26, and 17:54 UT). From Geotail, these minima can be interpreted as foreshock we will present LEP plasma density observations with 12.2- cavities excavated by the ion bursts

19 April 1996

Findings

Summary and conclusion