A geomagnetic negative sudden impulse (SI-) observed on May 13, 1995, was examined using magnetic field data from ETS VI and GOES 7 in the magnetosphere and ground geomagnetic observation networks. The SI- was caused by a sudden decrease in 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¿, magnetic longitude 208.3¿) were 26 nT and 10 min, respectively. Although SI- has been considered to be the mirror image of geomagnetic positive sudden impulse (SI+) or sudden commencement (SC), we found the polarization distribution of the SI- consistent with that of SC. We suggest that the contribution from the longitudinal movement of a twin-vortex ionospheric 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 response. ¿ 2000 American Geophysical Union