Scatter plot of daily means of the am index as a function of the corresponding daily mean transpolar voltage, Φ_PC, as determined for 2001–2002 by DMSP satellites (principally F13) and normalised for the satellite track to an ideal 06–18 MLT path using the procedure of Lockwood et al. (2009). The error bars are plus and minus one standard error in the means. The mauve line is the best-fit 3rd order polynomial fit and the grey area is bounded by the 2-sigma uncertainty level in that fit. The fitting procedure is given in Appendix to this paper, along with polynomial expressions for the best fit and the uncertainty band edges. These can be used to estimate the am level associated with a given Φ_PC: Appendix also gives the corresponding 3rd order polynomials that allow computation of the Φ_PC value associated with a given am value.