Observations in the plasma sheet during substorm activity

This thesis presents a statistical study of the plasma sheet location and thickness of the plasma sheet boundary layer, including the effect of magnetic and solar wind conditions. Next are presented two case studies of in-situ particle data from the Polar spacecraft as it entered and passed through the near-Earth plasma sheet during substorm activity. These data sets are compared with in-situ magnetic field measurements, from the Magnetic Fields Experiment (MFE), and remote observations of the aurora from the Ultra-Violet Imager (UVI). The latter, in conjunction with ground magnetograms, allow us to place the in-situ plasma flows in the overall context of the substorm phases.;In the first case study the spacecraft entered the plasma sheet boundary layer (PSBL) and observed field-aligned ion beams, which were connected with field-aligned signature in MFE. When Polar entered the PSBL its footprint mapped to the poleward edge of an auroral double oval. Then, a decrease in the number and energy of the ions occurred, which coincided with the spacecraft footprint entering the central low-luminosity region of the UV double oval. After the spacecraft entered the CPS an expansion phase onset occurred, and Polar observed large ion count rate increases and depolarisation of the magnetic field.;When Polar entered the PSBL in the second case study, which again coincided with entry into the auroral oval, there was evidence of pseudobreakups. Then the PSBL retreated over the spacecraft, which was mirrored in the ionosphere by equatorward motion of the auroral oval. Polar entered to the PSBL and rapidly entered the CPS when a substorm expansion phase onset occurred. Due to the large amount of substorm activity during these case studies, emphasis is placed on the use of both remote observations of the aurora and ground-based data to place the in-situ measurements in context.