We introduce the inductive effects of polar cap motions towards and way
from the Sun into magnetospheric electrodynamics and show how this
explains observed Universal Time variations in hemispheric geomagnetic
indices. The large (and growing) hemispheric asymmetry in the offsets of
the geomagnetic (dip or eccentric dipole) poles from Earth’s rotational
axis means that the effect is not cancelled out in global indices. By
adding this effect to that of the Russell-McPherron effect on solar
wind-magnetosphere coupling, that of ionospheric conductivities, and
that of the solar wind dynamic pressure and dipole tilt on the
near-Earth tail lobe field and cross-tail current sheet, we are able to
model the persistent “equinoctial” time-of-day/time-of-year pattern
(with additional net Universal time variations) observed in the
an, as and am geomagnetic indices since 1959. We
discuss the implications for the longitudinal dependence of the effects
of extreme space weather events