Global Ionospheric Current System Associated With Penetration Electric Field and New Mechanism for the Generation of Dayside Westward Electric Field at Low Latitudes During Northward IMF

It is well known that penetration and shielding electric fields are related to the southward and northward turnings of the interplanetary magnetic field (IMF). The shielding electric field is assumed to be caused by the region‐2 field‐aligned currents. We have analyzed observations of equatorial ionospheric plasma drifts measured by the Jicamarca incoherent scatter radar and global ground magnetic perturbations in an event when the IMF fluctuates between southward and northward. The vertical plasma drifts/zonal electric fields in the dayside equatorial ionosphere are well correlated with the IMF reorientations, indicating that the disturbance electric fields in the equatorial ionosphere are caused by the IMF. Magnetometer data are used to reconstruct the global ionospheric currents and convection. The DP 2 current system consists of a large vortex in the afternoon‐evening sector and a much smaller vortex in the dawn sector, and the ionospheric currents associated with the dayside penetration electric field extend into the postmidnight sector. The downward plasma drift and southward magnetic perturbations at equatorial latitudes during northward IMF are correlated with the reverse convection in the polar cap, indicating that the polar electric field associated with the reverse convection is an important source of the westward electric field in the dayside equatorial region. A new mechanism is proposed to explain the generation of westward electric field in the dayside equatorial ionosphere during northward IMF.