Low-latitude ionospheric electric and magnetic field disturbances in response to solar wind pressure enhancements

We study the characteristics of the low-latitude ionospheric electric field and geomagnetic field in response to a sudden enhancement of the solar wind pressure. When the magnetosphere is compressed by an interplanetary shock, a significant enhancement in the dayside equatorial ionospheric ion vertical velocity occurs over 30-40 min and is measured by the Jicamarca incoherent scatter radar. A similar enhancement occurs in the high-latitude ionospheric convection and is detected by the SuperDRAN HF radars. The simultaneous enhancements of the ionospheric ion velocity at high and low latitudes provide strong evidence of the occurrence of penetration electric fields produced by solar wind pressure enhancements. The geomagnetic field first increases rapidly over 2-3 min and then falls over 30-40 min to an asymptotic value. The enhanced magnetic field occurs from the subauroral region to the equator at all local times. The time scale of 30-40 min is much longer than the conventional preliminary and main impulses of geomagnetic response to interplanetary shocks. There are two possible mechanisms that may be responsible for the generation of the enhanced ionospheric electric and magnetic fields. One mechanism is that the solar wind shock causes an over-compression of the magnetosphere, and the other is that the field-aligned and ionospheric currents driven by the solar wind shock cause the enhancements of the ionospheric electric and magnetic fields. However, neither of the mechanisms appears to be able to provide a complete explanation of all observed features.