Effects of continuous solar wind pressure variations on the long-lasting penetration of the interplanetary electric field during southward interplanetary magnetic field

It is believed that the interplanetary electric field (IEF) can penetrate into the mid- or low-latitude ionosphere. For the penetration of the IEF, the relationship between the solar wind pressure and the penetrated electric field in the equatorial ionosphere is yet an unsolved topic. With observations of the IEF, solar wind pressure obtained from the Wind satellite, and the geomagnetic field on four magnetometers from high latitudes to magnetic equator, this paper has present an event of a long-lasting penetration of the IEF into the dayside equatorial ionosphere during the main phase of a magnetic storm on April 17, 1999. In the event, variations of the geomagnetic field both at high latitudes and at magnetic equator show coherence with those of the solar wind pressure, indicating that a series of enhancements of solar wind pressures can drive those coherent increases of the penetrated electric field in dayside equatorial ionosphere during stable and southward interplanetary magnetic field (IMF). Therefore, not only the dawn-dusk component of the IEF, but also the solar wind pressure can strongly control the long-lasting penetration of the magnetospheric electric field into low-latitude or equatorial ionosphere during southward IMF. In addition, our result suggests that a combination of equatorial and low-latitude ground magnetograms may be used to monitor the solar wind dynamic pressure variation not only in the northward IMF, but also in the southward IMF.