In this paper, the X propagation delay of the solar wind is calculated. We take the case in Fig. 1a as an example. During the period of interest, the ACE satellite was located at 232 RE upstream in the solar wind, and the solar wind velocity was 500 kms1. The solar wind would take 46 min to propagate from the position of ACE to the bow shock and an additional 10 min to travel through the magnetosheath, so the total propagation delay was 56 min. Then we compare whether the calculated propagation delay is consistent with the sudden changes in the measured electric fields. In Fig. 1a, the IMF/IEF data are shifted by 56 min. After being shifted, the minimum IEF Ey is at 1457 UT and exactly at the time of the minimum equatorial ionospheric electric field. Although the X propagation delay alone may have some uncertainty, the matching of the shifted minimum IEF Ey with the minimum ionospheric electric field provides an accurate determination of the solar wind propagation time. This method is used for all cases. In the following, we will only present the shifted solar wind data and will not give the details of the solar wind propagation delay.