While the level of auroral zone geomagnetic activity, as measured by the AE index, is determined by the value of the southward component, Bs, of the interplanetary magnetic field (IMF), the delay in the geomagnetic response is modified by the microscale fluctuations of the IMF vector B. The fluctuations are characterized by the square root &sgr;z of the variance of the solar magnetospheric (SM) z component of B. For extreme values of Bs(Bs>5 nT and Bs=0) the geomagnetic disturbances associated with small values of &sgr;z last longer than those associated with large &sgr;z. Conversely, for other values of Bs, the magnetosphere responds to the quasi-stationary interplanetary conditions with a shorter delay than to the time dependent solar wind conditions. Autocorrelations of the solar wind (SW), parameters evaluated for various time lags are always stronger when the initial value of &sgr;z is small than when it is large. Analysis of the autocorrelations and of the geomagnetic responses corresponding to them suggests that, for positive but not extremely large values of Bs, the temporal fluctuations of B trigger the internal magnetospheric (or the SW-magnetospheric coupling) processes which increase the observed time lags in the auroral zone geomagnetic activity. The value of &sgr;z is the SW parameter indicating whether the magnetosphere responds to SW conditions almost immediately (within less than ~1 hour) or whether the magnetospheric response is more gradual and is delayed for another ~1 to ~4 hours. The data analyzed in this study indicate the existence of magnetospheric processes delaying the geomagnetic response to the SW conditions for Bs=0 bur they do not allow us to draw conclusions on the effects of &sgr;z when Bs is extremely high or equal to zero. Future short-term forecasts of geomagnetic activity are likely to be based on the SW data obtained from an upwind located satellite and appropriate adjustments of the forecasts are in order to account for the effects of the fluctuations. ¿ American Geophysical Union 1991