Substorms are known to cause geomagnetically induced currents (GIC) in power transmission lines through variations in the ground magnetic field. An improved knowl- edge and understanding of how the different phases of sub- storms affect the ground magnetic field will ultimately help to better understand how GIC arise. Although usually as- sociated with high latitude power transmission networks, GIC potentially pose a risk to mid latitude networks such as the UK’s National Grid. Using a list of substorm expan- sion phase onsets derived from auroral observations by the IMAGE-FUV satellite, this study examines 553 individual onsets. In order to cover mid latitudes, ground magnetome- ter data from the UK Sub-Auroral Magnetometer Network (SAMNET) are exploited. These high time resolution (5 s) data are used to study the ground magnetic field for an hour after onset, in particular the time derivative of the horizon- tal magnetic field, H . The data covers the period from 2000 to 2003 (just after solar maximum). Results are compared with a previous study of magnetic field variations at higher latitudes, using data with a much lower (1 min) cadence dur- ing substorms identified from geomagnetic indices during a period just after solar minimum.