Schematic of the Russell-McPherron (R-M) effect. Part (a) illustrates Earth in its orbit at the June and December solstices and the March and September equinoxes, showing the orientation of Earth’s rotational axis in red and defining the ecliptic plane (shaded yellow) and the GSE reference frame. Also shown is the GSEQ reference frame (defined in Fig. 4); (b), (c) and (d) are views looking toward Earth from the Sun (i.e., in the −X_GSEQ direction) and show the Z_GSEQ and Y_GSEQ axes and the projections of and Earth’s magnetic axis (in blue): precesses around the Z_GSEQ axis once per year and precesses around every 24 h. The cone angles of these annual and daily precessions are δ_a = 23.5° and δ_a ≈ 9.6°, respectively. (Note that δ_a of 9.6° is an approximate figure because Earth’s field in not a geocentric dipole; for example, in 2007 the geomagnetic pole was 6.05° from the rotation pole in the Northern hemisphere but 25.55° from it in the Southern hemisphere; (b) is the view for the March equinox, (c) for the June and December solstices and (d) is for the September equinox. The GSM (Geocentric Solar Magnetospheric) reference frame is, like GSE and GSEQ, a right handed set and GSEQ, GSE and GSM share the same X axis, but the Z_GSM and Y_GSM axes are rotated through an angle β_GSEQ (that varies with Universal Time, UT, and time-of-year, F) such that Z_GSM is aligned with the projection of onto the Z-Y plane. The idea of the R-M effect is that geomagnetic activity is driven by a coupling function that depends of the southward interplanetary magnetic field (IMF) in the GSM fame, for example, the “half-wave rectified” southward component B_S in the GSM frame (where B_S = −[B_Z]_GSM for [B_Z]_GSM < 0 and B_S = 0 for [B_Z]_GSM ≥ 0) but the dominant IMF reaching Earth lies in the GSEQ X-Y plane (so [B_z]_GSEQ = 0). The resulting F-UT pattern of B_S for |[B_Y]_GSEQ| = 1 nT is set by sin(β_GSEQ) and is shown in the upper panel of part (e) and daily means as a function of F are given in the lower panel. The F variation (currently, 2019) due to R-M peaks around 4 April (F = 0.257) and October 7 (F = 0.769) whereas the equinoxes are March 20 (F = 0.216) and September 22 (F = 0.726).