On the left axis: mean monthly ULF power in the frequency range 2–7 mHz in IMF B _Z ; mean monthly ULF power on the ground at KEV in the magnetic field component B _H , with KEV power down-scaled by a factor of 250. On the right axis: mean monthly solar wind speed. Asterisks imply the four maximum times on the ground. The correlation between IMF and ground ULF wave power is 0.83 for the entire time span studied, 0.31 for the solar maximum, and 0.92 from that onward, i.e. for the declining phase and solar minimum.