We made a study of the characteristics of geomagnetic negative sudden impulses (SI-s) identified in the midlatitude geomagnetic SYM indices and the causative structures in the solar wind using data from the Wind and ACE spacecraft. A total of 28 SI-s with an amplitude larger than 20 nT in the H component SYM index were found over the period 1995 through 1999, with 50% of them occurring in conjunction with a positive sudden impulse, SI+ (i.e., SI pair). In the SI pairs the amplitude of SI- was almost always larger than that of the preceding SI+. We attempted for the first time a classification of structures in the solar wind associated with SI-s. It is found that reverse shocks are not responsible for SI-s. Instead, SI-s are associated with varied structures such as tangential discontinuities at high-low speed stream interfaces, front boundaries of interplanetary magnetic clouds, and trailing edges of heliospheric plasma sheets. There is no preferential association of SI-s in our sample with any particular type of solar wind structure. We investigated statistically the polarization characteristics of SI-s at high latitude. The sense of the polarization in the auroral zone tended to be clockwise in the afternoon and counterclockwise in the morning. The rotational sense reversed in the polar cap. The latitudinal reversal occurred in the range from 65¿ to 80¿. Thus the polarization distribution of SI- is not opposite to but is consistent with that of SI+. 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-.