Observed, conductivity-corrected and modelled UT variations for the (red) an, (blue) as, and (black) am indices. (a) shows the values, an, as, and am linearly interpolated from three hourly observations to 1-hour resolution and then averaged for the 24 1-hour UT values for all data (for 1959–2019, inclusive). (b) shows the conductivity-corrected values an_cc = an/P_ΣN, as_cc = as/P_ΣS and am_cc = (an_cc + as_cc)/2. (c) shows the modelled values, an_m, as_m and am_m = (an_m + as_m)/2 (d) compares the conductivity corrected values (solid lines) and the modelled values (dot-dash lines) on the same plot.