Geomagnetic negative sudden impulse due to a magnetic cloud observed on May 13, 1995

A geomagnetic negative sudden impulse (SI-) observed on May 13, 1995, was e xamined using magnetic field data from ETS VI and GOES 7 in the magnetosphe re and ground geomagnetic observation networks. The SI- was caused by a sud den decrease ill the solar wind dynamic pressure at the front boundary of a magnetic cloud embedded in a density enhancement region. The amplitude and the fall time at Kakioka geomagnetic observatory (magnetic latitude 26.9 d egrees, magnetic longitude 208.3 degrees) were 26 nT and 10 min, respective ly. Although SI- has been considered to be the mirror image of geomagnetic positive sudden impulse (SI+) or sudden commencement (SC), we found the pol arization distribution of the SI- consistent with that of SC. We suggest th at the contribution from the longitudinal movement of a twin-vortex ionosph eric current system is dominant to produce the polarization of SC and SI-. We also discuss the relationship between the angle at which discontinuities in the solar wind impinge upon the magnetosphere and the geomagnetic respo nse.