The geomagnetic storm on September 24--27, 1998, was initiated by a sudden compression of the magnetosphere in response to the solar wind dynamic pressure pulse. Simultaneous with the pressure increase, the interplanetary magnetic field (IMF) became strongly northward. Several unexpected magnetospheric responses to this sudden impulse were observed. First, for ~30 min following the sudden impulse, the entire auroral oval became active and thick, while the polar cap area decreased to less than 1/2 of its original size. The second unusual observation associated with the sudden impulse is the global magnetic perturbation measured by low-latitude magnetic stations. The field shows an asymmetric increase in the axial component (parallel to the dipole axis) with the strongest enhancement measured on the night side and at local magnetic noon the perturbation is small or slightly negative. This is very unusual since sudden compressions are generally measured by low-latitude stations to have the largest enhancement of the field on the dayside. The main phase of the geomagnetic storm begins in the second hour on September 25, 1998, following the southward turning of the IMF. The auroral oval becomes thin and moves equatorward increasing the polar cap area by more than a factor of 3. The theoretical analysis presented in this paper suggests that the response to the sudden impulse is an electrodynamic effect produced by a transition current system in response to the northward turning of the IMF. ¿ 2001 American Geophysical Union