Ionospheric Storm Fronts at Low and Mid Latitudes
J. C. Foster
MIT Haystack Observatory, Westford, Massachusetts, USA
In the early phases of large geomagnetic storms the low and mid-latitude ionosphere are greatly perturbed. Discrete storm fronts of enhanced ionospheric plasma at low and mid latitudes are produced in a two-stage process which spans the inner and outer regions of the ionosphere-magnetosphere system. The particular configuration of the magnetic field at low latitudes in the Atlantic sector creates a preferred longitude/Universal Time sector (western Atlantic/ 21 UT) for the build-up of significantly enhanced TEC on field lines inside the dusk plasmapause. At the plasmasphere boundary layer (PBL), the sub-auroral polarization stream electric field (SAPS) forms as pressure gradients at the inner edge of the magnetospheric ring current drive Region-2 field-aligned currents into the evening-sector ionosphere. Large poleward-directed electric fields at ionospheric heights are set up to drive closure currents across the low-conductivity region equatorward of the auroral electron precipitation. The inward extent of the SAPS overlaps and erodes the outer plasmasphere and mid-latitude ionosphere, drawing out extended plumes of storm enhanced density (SED) which span the dusk sector, transporting ionospheric and outer-plasmaspheric material to the noontime cusp and onto polar cap field lines.
At low latitudes, a combination of stormtime penetration electric fields, the effect of the reduced magnetic field strength in the South Atlantic magnetic anomaly, and the geographic distortion of the magnetic field in the Atlantic sector contribute to a unique set of characteristics of the low-latitude polarization electric fields at the sunset terminator. At dusk, polarization electric field effects begin at a given point in the ionosphere when the E region at either end of the magnetic field line through that point begins to go into darkness. We define the polarization terminator (PT) to be the locus of points at a given altitude for which the E-region shadow height at either end of the magnetic field line equals 100 km. Electric fields associated with the charge build-up in the E-region conductivity-gradient region due to the effects of winds or penetration electric fields are directed perpendicular to the PT and increase in magnitude as the PT is approached from the dayside. In the Atlantic sector, the electric fields at low latitudes along the PT have significant poleward as well as eastward components resulting in a westward and poleward redistribution of the plasmas at the crests of the equatorial ionization anomalies. This low-latitude plasma is swept onto field lines in the plasmasphere boundary layer which are overlapped by the SAPS electric field. The redistribution of the low-latitude ionosphere by the dusk sector PT creates an enhanced source population of high-TED plasma which feeds into the high total content storm enhanced density (SED) plumes observed during strong storms in the American sector.