Title: The terrestrial foreshock: new results from Cluster

Jonathan Eastwood

NRC/ RRA
Code 696, Electrodynamics Branch
Laboratory for Extraterrestrial Physics


Abstract

Of all the ways in which a collisionless plasma shock differs from its 
neutral fluid counterpart, one of the most striking is the fact that at a 
collisionless shock, particles are observed to stream from the shock back 
into the upstream region. The combination of the backstreaming particles 
together with the upstream core plasma distribution is subject to a number 
of instabilities that lead to the generation of waves and subsequent 
wave-particle interactions. Here we concentrate on ion driven wave 
phenomena as observed by the multi-spacecraft Cluster mission.

The Cluster mission, a constellation of four identical spacecraft in polar 
orbit around the Earth, is designed to resolve spatial and temporal 
variations in plasma properties in the near-Earth space environment. 
Cluster and its associated analysis techniques are explained, and reviewed 
in light of operational experience. The instrumentation is briefly 
described, concentrating in particular on the flux-gate magnetometers, 
from which the majority of the results presented here are derived.

In particular, observations of ultra-low-frequency (~30s period) foreshock 
waves are presented and analyzed using various multi-spacecraft analysis 
techniques. Wave modes are identified, and different wave production 
mechanisms (beam interactions) are considered; the beam resonance 
conditions are investigated experimentally. The problem of oblique wave 
propagation is also examined, requiring careful consideration of the 
relative motions of waves and particles in the foreshock. Finally, we 
consider the open questions relating to shock/foreshock physics, in 
particular concentrating on time-dependence introduced by variations in 
the upstream conditions.