A study of the coupling of the solar wind to temporal fluctuations in high--latitude ground magnetic fields is presented. The fluctuation measure considered is the absolute value of the north-south horizontal field time derivative (∣dBx/dt∣) averaged over a 30--minute interval. The fluctuation level is predicted using a nonlinear mapping of solar wind plasma and field measurements. The predictability of ground magnetic field fluctuations is shown to depend on both local time and latitude. For lower polar cap magnetometer locations, the coupling is highest at the local times that have the highest average fluctuation level and lowest at the local times that have the lowest average fluctuation level. The highest level of coupling is at auroral-zone latitudes in the post-midnight sector. The relative influence of different solar wind inputs is shown to be highly dependent on spatial location.