Whistlers recorded at Siple, Antarctica (76¿S, 84¿W), show sudden changes in nose frequency (fn) in response to a magnetospheric sudden impulse (si) on June 9, 1973. These changes are partly due to changes in local magnetic field strength and partly due to the motion of the duct under the influence of induced electric field. This example is used to illustrate how ground-based VLF radio techniques can be used to monitor the dynamic response of the magnetosphere to shocks and discontinuities in the solar wind.