In this thesis, the plasma flows in the high-latitude nightside ionosphere are investigated. Firstly, the interaction between the solar wind and the Earth’s magnetic field is reviewed with particular attention to the formation of the magnetosphere and the resulting magnetospheric convection. The ionosphere is briefly described with a discussion of magnetosphere- ionosphere coupling and auroras. Secondly, the question ‘what is a magnetospheric substorm ?’ is addressed, together with a review of the current substorm models. There are two main research topics contained in the thesis, investigating the variation of flows in the ionosphere during magnetospheric substorms using data from radars, magnetometer chains and satellites. The first uses EISCAT radar observations to study the ionospheric convection during substorms occurring on a contracted auroral oval on two successive days. Supporting data from the PACE radar, magnetometer chains and geostationary satellites are compared to the EISCAT observations. On 4 October 1989 a weak substorm resulted in the convection reversal boundary observed by EISCAT contracting rapidly poleward, causing a band of elevated ion temperatures. A similar event was observed on the following day with the unusual and significant difference that the flow structure in the dawn cell comprised an anti-sunward flow band between two sunward bands. Both of the sunward bands were contracting poleward, collocated with enhanced ion temperatures. The voltage associated with the anti-sunward flow between the two convection reversals reached a maximum of 13 kV during the substorm expansion phase, which suggests that it was associated with the reconnection of open flux in the geomagnetic tail which had mimicked “viscous-like” momentum transfer across the magnetopause. The poleward band of sunward flow is found to be associated with lobe reconnection due to the inferred Bz > 0. The second research topic is another multipoint study to investigate the nature of the ionospheric flows with a varying IMF direction and magnitude, using UV auroral images taken from the Freja satellite to orientate the ground-based substorm cycle observations and flow boundary motions. Variations in the flow and currents poleward of both the main electrojet and the auroral activity in the midnight sector during the expansion phase are reported. It is found that the poleward edge of the auroral oval lay equatorward of the convection reversal boundary in both the growth and expansion phases and that the evolution of the flows was consistent with the expanding/contracting polar cap convection model. It is found from both of the substorm studies that the ionospheric flows have influences from magnetopause coupling and substorm behaviour. It is the combination of these flows which leads to an understanding of the complex and time dependent nature of the flows in the ionosphere.