SABRE and magnetometer studies of substorm associated pulsations
This thesis describes an experimental investigation of the geomagnetic pulsations (Pi2) associated with the onset of magnetospheric substorms undertaken with the Sweden And Britain Radar Aurora Experiment (SABRE) in conjunction with a network of ground-based magnetometers. The signatures observed on the two measuring systems are compared and have been interpreted in terms of the substorm current wedge and the substorm enhanced electrojet regions. The polarisation of the SABRE Pi2 pulsations is also investigated. The excellent spatial resolution of the SABRE radar has enabled the complex system of field-aligned and ionospheric currents associated with substorm onset and Pi2 pulsations to be examined in great detail. A southward turning of the flow often observed at substorm onset may be related to the earthward surge of plasma at onset. The subsequent westward movement of these regions may be related to the westward travelling surge. This movement, and a less frequent eastward movement, is also observed in studies of the SABRE wave spectral power. Four intervals of Pi2 activity have been selected for detailed study and the observed features related to different theoretical and observational models for the generation of these waves. The SABRE signature depends on the position of the viewing area relative to the substorm enhanced electrojet. The results are consistent with a field line resonance in one interval and the interaction of a pair of field-aligned currents in another. The highly complex structure of the substorm current systems in the magnetotail is apparent in the ionospheric radar data from the remaining two intervals. The high spatial resolution attained in the present observations far exceeds that achieved with other techniques. Moreover, the radar monitors the actual ionospheric currents directly allowing an enhanced understanding of the underlying production mechanisms for the Pi2 pulsation events.