Comparison of the Russell-McPherron and equinoctial patterns, derived using an eccentric dipole geomagnetic field for the year 2002. In the F-UT plot in part c, the colour contours give the absolute value of the dipole tilt angle, |ψ|, superposed on which are contours showing the IMF orientation factor used by P_α, namely A_θ = sin⁴(θ/2), where θ is the clock angle of the IMF in the GSM frame. These predictions are the average for an equal mix of [B_Y]_GSEQ = −|B| < 0 and [B_Y]_GSEQ = +|B| >0 and contours are shown for A_θ of 0.28 (in black) and 0.31 (in mauve). Parts a and b show the mean values of |ψ| over the range of F defined by the maximum extent in F of the mauve contour as a function of UT: Part a is for the March equinox and Part b for the September equinox. The area shaded pink is the UT extent of the peak defined by the mauve contour in c. The point of parts a and b is to show that the two equinoxes go through exactly the same sequence of variations in both A_θ and ψ with UT, but with a phase difference of 12 h.