Equatorward expansion of the westward electrojet during magnetically disturbed periods

[ 1] The auroral electrojet (AE) indices have widely been used in various fields of solar terrestrial physics since their introduction to the community. Recently, it has been reported that the AE indices do not, at times, properly monitor the auroral electrojets because as magnetic activity increases, they expand equatorward beyond the standard AE network, resulting in a serious underestimation of the auroral electrojet intensity. It is particularly the case during severe geomagnetic storms. To determine quantitatively the equatorial expansion of the auroral electrojets, we examined an extensive database obtained from the Alaska, International Monitor for Auroral Geomagnetic Effects ( IMAGE), and Canadian Auroral Network for the OPEN Program Unified Study chain of magnetometers. These chains of magnetometers enable us to determine the latitude where the maximum current density of the auroral electrojet flows. It is generally understood that the center of the auroral electrojet tends to migrate equatorward with an increase in magnetic activity. We note, however, that there seems to be a lower limit particularly of the westward electrojet, similar to60 in corrected geomagnetic latitude, regardless of magnetic activity levels. The relative location of the westward electrojet with respect to the global auroral image taken from the Polar satellite is also examined. Contrary to the generally accepted notion, the auroral electrojets are found to be most intense not in the region of bright auroral luminosity but slightly poleward of it in less luminous region. The current center seems to be the region where both ionospheric conductivity and electric field become significantly high.