During away sectors of the interplanetary magnetic field (IMF) around
autumnal equinox and toward IMF sectors around vernal equinox, Dst und
erwent an average 4.0 nT peak-to-peak diurnal oscillation during a 13-
year analysis period. Such an oscillation is predicted by the Russell-
McPherron effect. The same effect is also found in a case study showin
g Dst diurnally oscillating through a period of sustained, recurrent g
eomagnetic activity. The time of minimum Dst lags the time of maximum
energy coupling as given by the Russell-McPherron effect by between 0.
5 and 1.5 hours, which in the framework of the Russell-McPherron coupl
ing model gives a measure of the energy flow time from the solar wind
to the ring current. This lag seems to be significantly shorter than e
xpected for the Russell-McPherron effect used as a driver to an energy
input and decay model of ring current dynamics. The results suggest t
hat a mechanism besides ring current decay might operate on the diurna
l timescale, such as a flow of ring current energy into the ionosphere
via the region 2 currents.