Results
2.1. Observations and data analysis
We use the solar wind data measured by the ACE satellite to derive the interplanetary electric field by assuming that the plasma motion is determined by ExB drift. The time shift of the solar wind data is determined as follows. We first use the satellite position and solar wind velocity to calculate the possible propagation delay, and then compare the changes in the interplanetary and ionospheric electric fields. There are two different methods to calculate the solar wind propagation time. One is the X propagation time with a constant solar wind velocity, and the other is the new method recently developed by Weimer et al. (2002, 2003) with better accuracy for large IMF clock angle