Time-of-year/time–of-day (F–UT) plots for transpolar voltage and polar cap flux for 2001–2002, inclusive from the DMSP satellites. Data are sorted into 24 equal-width bins in F and 16 equal width bins of UT using the central time of the polar cap traversal. The top panels (a–d) are for passes of the northern polar cap, the lower panels (e–h) are for passes of the southern polar cap. (a and e) Show the transpolar voltage Φ_PC; (b and f) show the polar cap flux, W_PC; (c and g) show the simultaneous am index value (linearly interpolated from the three-hourly values to the time of the centre of the polar cap crossing) and (e and h) shows the number of data points, n_bin in each F–UT bin.