Special features of the September 24–27, 1998 storm during high solar wind dynamic pressure and northward interplanetary magnetic field

Abstract

Abstract. 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-latitu de 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 produce d by a "transition" current system in response to the northward turning of the IMF.

Nighttime enhancement of the amplitude of geomagnetic sudden commencements and its dependence on IMF-Bz

“…Clauer et al (2001) reported an SC event with the largest amplitude in nighttime which occurred during a northward IMF turning. They treated the SC as a very special and unexpected event.…”

“…Russell et al (1994) used the current system (1) and (2) in addition to the H -component reduction due to the en-hanced region-1 currents in the dayside in order to interpret the nighttime enhancement and daytime reduction of the SC amplitude for the southward IMF. Clauer et al (2001) proposed the enhanced region-1 current connected with the NBZ current system to explain the nighttime SC enhancement during the northward IMF turning.…”

Relationship between solar wind dynamic pressure and amplitude of geomagnetic sudden commencement (SC)

“…Russell et al (1994b) also studied 7 SCs at the same stations during the southward IMF. The normalized amplitude showed the maximum in midnight and the minimums near 6 and 16 h LT. Clauer et al (2001) found an SC with the largest amplitude in midnight during a strong northward IMF turning and analyzed it as a special event. Although there is a discrepancy on IMF dependence of nighttime amplitude, above description indicates that the LT variation of SC amplitude does not depend much on longitude except the South Atlantic anomaly region.…”

Auroral electrojets and boundaries of plasma domains in the magnetosphere during magnetically disturbed intervals

“…It is worth stressing that the longitudinal asymmetry pattern of the magnetic field reported by Clauer et al (2001) gives evidence of the PRC existence. Actually, ground-based measurements along the longitudinal chain of 19 mid-and low-latitude observatories showed that the magnetic field depression was not uniform around the Earth.…”