DIURNAL OSCILLATION OF DST - A MANIFESTATION OF THE RUSSELL-MCPHERRONEFFECT

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.