Based on the equatorial vertical ion drift measured by DMSP and cross polar cap potential (φcpc) from AMIE output during 2001 to 2003, this paper investigates the relationship of φcpc and its temporal variation rate (△φcpc) with the disturbed ion velocity (△Vx) which is the difference between the disturbed days (Kp≥4) and quiet days (Kp<2). The statistical analysis shows: (1) The △Vx correlates better with △φcpc than with φcpc, indicating that the electric field penetration is more easily to occur when solar wind input rapidly varies with time. (2) The optimal delay time of electric field penetration from the high-latitude mag- netosphere to equatorial ionosphere has local time dependence which is longer on the nightside than on the dayside. It may be due to more complicated electrodynamic process on the nightside. (3) With the linear relationship between △φcpc and △Vx, it is obtained that the penetration efficiency is about 4.5%-13.9% at day and 31%-42% at night, coinciding well with former studies.
