We analyze Polar Ultraviolet Imager (UVI) observations of auroral transient events (ATEs) in the dayside Northern Hemisphere. During 5 winter months in 1996 and 1997, we found 31 prenoon ATEs but only 13 afternoon events. Prenoon and afternoon event characteristics differ. Prenoon ATEs generally appear as bright spots of auroral luminosity in the area from 0800 to 1000 magnetic local time (MLT) and 74.5¿ and 76.5¿ corrected geomagnetic latitude (CGL). Bright aurorae then quickly expand westward and poleward, accompanied by high-latitude magnetic impulsive events (MIE) and traveling convection vortices (TCV). Afternoon ATEs usually appear as a sudden intensification of aurorae in the area from 1400 to 1600 MLT and 75.5¿ to 78.5¿ CGL. Within 15--20 min the bright band of luminosity extends eastward to reach 2000--2100 MLT at 70¿--72¿ CGL. Although midlatitude and low-latitude ground magnetograms in the evening sector record increases in the horizontal component of the magnetic field, no corresponding features occur at stations in the morning sector. Afternoon ATEs correspond to abrupt changes in the interplanetary magnetic field (IMF) orientation, but not to significant variations of the solar wind dynamic pressure, indicating that the auroral transient events occur as part of the magnetospheric response to abrupt changes in the foreshock geometry. ¿ 2001 American Geophysical Union