Foreshock transients cavities and hot flow anomalies

Many pictures of the foreshock assume a dynamical equilibrium. However, the bow shock - foreshock system is highly non-linear, with many complicated feedback mechanisms, and it is by no means clear that changes to the upstream conditions, particularly the magnetic field orientation, cause proportional changes to large-scale foreshock structure. Nor are the system response time scales particularly clear. Work has been published examining the effects of brief connection to the foreshock and the way in which these connections can highly perturb the solar wind - known as foreshock cavities (Sibeck et al., 2002). Also, if an interplanetary discontinuity intersects the bow shock, a hot flow anomaly may be generated - causing a massive (in the context of shock dynamics) disruption to solar wind flow upstream of the shock (Schwartz et al., 2000). Identifying the structure, dynamics and evolution of hot flow anomalies is problem highly suited to multi-spacecraft observations.

Since its launch in 2000, data from the Cluster mission has been used to address many of these questions. Here, we attempt to gather together the main results that have been derived from Cluster thus far. In particular, results from Cluster have contributed to our understanding of:

■ the morphology of particle and wave boundaries in the foreshock,

■ the nature of upstream waves and the origin of cyrating ions,

■ the three-dimensional structure of hot-flow anomalies.

The remainder of this chapter is divided according to these three topics. In Section 2.2, work relating to the large scale structure of the foreshock and its boundaries is presented. In Section 2.3, work relating to the 'microphysics' of the fore-shock, the basic wave particle interactions, is presented. In Section 2.4, work relating to hot flow anomalies (HFAs) is presented. The chapter is summarised in Section 2.5.

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