Estimated (a) north–south polarised electric field (E _x ) time series; (b) east–west polarised electric field (E _y ) time series decimated to a sample rate of 32 s for 3 sites in Scotland during the Halloween 2003 magnetic storm obtained through frequency-domain multiplication of site-dependent impedance tensors with magnetic time series from ESK observatory. Subscript “W” denotes use of impedances modified to take into account horizontal magnetic field gradients embodied in the perturbation tensor. Other time-series traces are for measured impedances. Peak electric field fluctuations occur at the same time for both E _x and E _y polarisations.