The possibility of using ULF (1-20 mHz) waves to diagnose the structure of the magnetosphere has recently given new impetus to the study of Pc 4,5 magnetic pulsations. In this paper we demonstrate that the frequency spectrum of dayside Pc 4,5 pulsations near noon may be significantly altered in association with the onset of a magnetospheric substorm near midnight. The response time for the dayside Pc pulsations to a substorm onset can be as short as 2-3 min, suggesting information transfer across the magnetosphere at velocities of the order of the Alfv¿n speed. The characteristic response of the dayside pulsations is a marked increase in the dominant frequency at stations inside the dayside auroral oval. Our results taken together with the observations of dayside auroras by Eather et al. [1979> suggest that substorm onsets are accompanied by a sudden inward motion of the center of the partial ring current. We propose that this ring current motion causes changes in the magnetic field in the equatorial plane of the magnetosphere that result in changes in the Alfv¿n velocity on field lines where the magnetic pulsations are observed. Possible mechanisms for the generation of Pc 4,5 pulsations are discussed in the light of the observations reported in this paper.