Eastward electric field enhancement and geomagnetic positive bay in the dayside low‐latitude ionosphere caused by magnetospheric substorms during sawtooth events

Abstract

Substorms cause significant disturbances in the magnetosphere and ionosphere. Although many substorm‐induced phenomena have been relatively well understood, a serious controversy arises as to how the magnetospheric and ionospheric convection responds to the expansion onset. In this paper, we present the observations of the low‐latitude ionospheric electric field and geomagnetic field at substorm onsets during sawtooth events when the interplanetary magnetic field (IMF) remains steady southward, so the ionospheric disturbances are attributed solely to the substorm effects. In the substorm events we have analyzed, the dayside equatorial ionospheric electric field shows an eastward enhancement at the onset, and the geomagnetic field northward component at middle and low latitudes shows an increase (positive bay) on both the dayside and nightside. We suggest that the magnetospheric convection is enhanced at substorm onset and that the enhanced magnetospheric convection electric field penetrates to the low‐latitude ionosphere. The penetration electric field driven by substorm onset causes the eastward ionospheric electric field enhancement and geomagnetic positive bay on the dayside, and the substorm current wedge causes the geomagnetic positive bay on the nightside. Our findings are significantly different from the results of most previous studies and provide new insight into the substorm dynamic process.