  We use the am, an, as and the a-sigma geomagnetic indices to the explore a
previously overlooked factor in magnetospheric electrodynamics, namely the
inductive effect of diurnal motions of the Earth's magnetic poles toward and
away from the Sun caused by Earth's rotation. Because the offset of the
(eccentric dipole) geomagnetic pole from the rotational axis is roughly twice
as large in the southern hemisphere compared to the northern, the effects there
are predicted to be roughly twice the amplitude. Hemispheric differences have
previously been discussed in terms of polar ionospheric conductivities, effects
which we allow for by studying the dipole tilt effect on time-of-year
variations of the indices. The electric field induced in a geocentric frame is
shown to also be a significant factor and gives a modulation of the voltage
applied by the solar wind flow in the southern hemisphere of typically a 30%
diurnal modulation for disturbed intervals rising to 76% in quiet times. Motion
towards/away from the Sun reduces/enhances the directly-driven ionospheric
voltages and reduces/enhances the magnetic energy stored in the near-Earth
tail: 10% of the effect being directly-driven and 90% being in tail energy
storage/release. Combined with the effect of solar wind dynamic pressure and
dipole tilt on the pressure balance in the near-Earth tail, the effect provides
an excellent explanation of how the observed Russell-McPherron pattern in the
driving power input into the magnetosphere is converted into the equinoctial
pattern in average geomagnetic activity (after correction is made for dipole
tilt effects on ionospheric conductivity), added to a pronounced UT variation
with minimum at 02-10UT. In addition, we show that the predicted and observed
UT variations in average geomagnetic activity has implications for the
occurrence of the largest events that also show the nett UT variation.
