When the magnetosphere is compressed by a sudden change in the solar wind dynamic pressure, the horizontal component of the Earth's magnetic field is increased at low latitudes. Often there is an overshoot associated with this increase in the field, but not always. The overshoot does not appear to be due to induced currents in the interior of the Earth or in the ionosphere. Rather, its magnitude appears to be controlled by both the strength of the ring current, and by local time. We speculate that the overshoot in the horizontal component is due to an overshoot in the compression of the magnetosphere and that when the ring current is strong the compressional wave is damped and the magnetosphere is not set into oscillation by the compressional wave. ¿ American Geophysical Union 1993