Upstream Transients at Planetary Bow Shocks

Kinetic processes occurring upstream of planetary bow shocks can greatly perturb the magnetosphere-striking solar wind. Collisionless shocks are highly non-linear systems with complicated feedback mechanisms. Transient events form part of the growing evidence that shocks are highly variable and far more complex than a large-scale quasi-static picture would suggest. In this thesis we survey Cluster magnetic field, thermal ion, and energetic particle observations upstream of Earth’s bow shock to investigate events known as foreshock cavities. Foreshock cavities exhibit depressions in magnetic field magnitude and thermal ion density, associated with enhanced fluxes of energetic ions. Two models of foreshock cavity formation exist and we attempt to discriminate between them. Cavities are found in all parts of the Cluster orbit upstream of the bow shock but preferentially in fast, moderate magnetic field strength solar wind streams. Localising foreshock cavity observations in a coordinate system organised by physical processes in the foreshock, reveals a systematic change in foreshock cavity location with the cone angle between the solar wind flow and magnetic field. At low (high) cone angles foreshock cavities are observed outside (inside) the expected boundary of the intermediate ion foreshock. This result favours a model of foreshock cavities as isolated bundles of magnetic flux preferentially filled with energetic particles. We present a case study locating an isolated foreshock cavity precisely with respect to measured bow shock position. We use Cluster’s multispacecraft nature to constrain the cavity extent. Associated particle populations are studied in detail. A second case study is shown to coincide with a transition from the deep ion foreshock to the electron foreshock. This event has some features expected from the new model of cavities as brief encounters with a spatial boundary in the global foreshock. Finally, we present the first observations of explosive events resembling Hot Flow Anomalies near Saturn’s bow shock