Special features of the September 24-27, 1998 storm during high solar winddynamic pressure and northward interplanetary magnetic field

The geomagnetic storm on September 24 - 27, 1998, was initiated by a sudden compression of the magnetosphere in response to the solar wind dynamic pre ssure pulse. Simultaneous with the pressure increase, the interplanetary ma gnetic field (IMF) became strongly northward. Several unexpected magnetosph eric responses to this sudden impulse were observed. First, for similar to 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 origi nal size. The second unusual observation associated with the sudden impulse is the global magnetic perturbation measured by low-latitude magnetic stat ions. The field shows an asymmetric increase in the axial component (parall el to the dipole axis) with the strongest enhancement measured on the night side and at local magnetic noon the perturbation is small or slightly nega tive, This is very unusual since sudden compressions are generally measured by low-latitude stations to have the largest enhancement of the field on t he dayside. The main phase of the geomagnetic storm begins in the second ho ur on September 25, 1998, following the southward turning of the IMF. The a uroral oval becomes thin and moves equatorward increasing the polar cap are a by more than a factor of 3. The theoretical analysis presented in this pa per suggests that the response to the sudden impulse is an electrodynamic e ffect produced by a "transition" current system in response to the northwar d turning of the IMF.