Ionospheric superstorms: Polarization terminator effects in the Atlantic sector

A combination of the effects of stormtime penetration electric fields, the reduced magnetic field strength in the South Atlantic magnetic anomaly (SAA), and the geographic distortion of the magnetic field in the Atlantic sector influence the characteristics of polarization electric fields which form in the E region conductivity-gradient region at the sunset terminator. These effects lead to a strong localized enhancement of total electron content (TEC) at dusk at low-mid latitudes in the American sector during ionospheric superstorms. The E region electric fields map along field lines to the F region and into the opposite hemisphere. We define the polarization terminator (PT) to be the ensemble of points at a given altitude above the E region for which the shadow height at either end of the magnetic field line equals 100 km. Electric fields associated with polarization charge build-up in the conductivity-gradient region along the PT are directed perpendicular to the magnetic field and increase in magnitude as the PT is approached from the dayside. The configuration of the magnetic field in the Atlantic sector due to the offset of the poles and declination effects near the SAA creates a preferred longitude/Universal Time sector (western Atlantic/21 UT) for the build-up of enhanced disturbance time TEC at lower mid latitudes. During strong storms, DMSP observations reveal that the plasma in the equatorial ionization anomaly (EIA) crests moves westward in lockstep with the PT. Electric fields at the PT sweep up the plasmas of the EIA crests and redistribute it into magnetically-conjugate regions at mid latitudes. This effect is strongest in the western Atlantic sector for northern hemisphere summer conditions.