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 en-
ergetic 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 fore-
shock cavity location with the cone angle between the solar wind flow and
magnetic field. At low (high) cone angles foreshock cavities are observed out-
side (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 multi-
spacecraft nature to constrain the cavity extent. Associated particle pop-
ulations 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.