Bicubic-spline interpolated (Smith & Wessel, 1990) maximum peak-to-peak electric-field magnitudes at UT 01:25 during the 8th September 2017 magnetic storm for (a) E _x and (b) E _y polarisation using measured impedances ( Z _ij ) and measured magnetic fields from RAN (Fig. 1); (c) E _x and (d) E _y polarisation with estimated electric fields replaced with measured storm-time electric fields (Simpson & Bahr, 2020b) at sites ACH, ELC, CAI, TAN, MAR, LEA and RAN (Fig. 1). Black stars indicate impedance-tensor site locations, circled stars in panel (c) and (d) indicate sites at which measured electric fields have been substituted for the estimated fields used in panels (a) and (b).