Scatter plot of daily means of the transpolar voltage Φ_PC, as determined for 2001–2002 by the DMSP satellites and normalised for the satellite track to an ideal 06–18 MLT path using the procedure of Lockwood et al. (2009), as a function of the means of the am index for the same day. The error bars are plus and minus one standard error in the means. The mauve line is the best-fit 3rd order polynomial fit of Φ_PC and the grey area is bounded by the 2-sigma uncertainty level in the fit.