The circuit that couples the magnetospheric boundary to the ionosphere has a significant self inductance which, in combination with ionospheric resistivity, gives the circuit an appreciable inductive inertia. The inductive inertia attenuates the amplitude of fluctuation of the transpolar ionospheric voltage relative to fluctuations of the transpolar boundary voltage, which drives the circuit. We adopt a simple electric circuit analog of the tailward moving part of magnetospheric convection and combine it with statistical representations of the solar wind EMF to obtain a quantitative estimate for the average voltage attenuation at the ionospheric level. We find that in an average three hours interval-the period range in which most magnetospheric phenomena directly related to the transpolar voltage lie-fluctuation amplitudes in the ionosphere reach only about 50% of their boundary values. ¿ American Geophysical Union 1991