Empirical simulations for the am annual‐diurnal activity

The relationships between solar wind parameters and geomagnetic indices are inherently complex. More often than not, the annual‐diurnal factor of these indices is neglected, likely for two main reasons. First, it is not crucial against the high variability of interplanetary parameters. Second, it remains partially unexplained. This is the case for the dependence of geomagnetic activity on the angle Ψ between the geomagnetic dipole axis and the Earth‐Sun line. It also depends on the southward IMF‐magnetosphere reconnection process, in which the role of the GSEQ B z contribution to GSM B z has been so far neglected in annual‐diurnal studies. Using IMF data from 1963 to 2002, we obtain the long due GSM Bz statistical model, which has the ability to reconcile earlier conflicting results. It is then combined with the Ψ dependence to simulate the well‐known am subauroral geomagnetic index. Annual‐diurnal simulations confirm that Ψ is the main source of semiannual activity with a near 60% contribution to am . We further attribute less than 20% to the so‐called Russell and McPherron effect, which stems from GSEQ B y, and the complement to the Earth's variable heliographic latitude factor.