We analyze more than 17 years of OMNI data to statistically quantify the impact of IMF B-z fluctuations on AL by using higher-order moments in the AL-distribution as a proxy. For strongly southward interplanetary magnetic field (IMF), the AL distribution function is characterized by a decrease of the skewness, a shift of its peak from -30 nT to -200 nT, and a broadening of the distribution core. During northward IMF, the distribution of AL is characterized by a significant reduction of the standard deviation and weight in the tail. Following this characterization of AL for southward and northward IMF, we show that IMF fluctuations enhance the driving on timescales smaller than those of substorms by shifting the peak of the probability distribution function by more than 150 nT during southward IMF, and by narrowing the distribution function by a factor of 2 during northward IMF. For both southward and northward IMF, we demonstrate that high power fluctuations in B-z systematically result in a greater level of activity on timescales consistent with viscous processes. Our results provide additional quantitative evidence of the role of the solar wind fluctuations in geomagnetic activity. The methodology presented also provides a framework to characterize short timescale magnetospheric dynamics taking place on the order of viscous timescales tau