The role of electric fields in sporadic E layer formation over low latitudes under quiet and magnetic storm conditions

Sporadic E layers are formed dominantly by wind shear mechanism, but their formation and dynamics are driven also by ionospheric electric fields. Investigation of low latitude sporadic E layers under quiet conditions shows that Es layer formation during post sunset hours can be disrupted or enhanced depending upon the vertical structure of the vertical electric field arising from sunset electrodynamic processes. During magnetic storms the formation and disruption of these layers are also strongly controlled by vertical Hall electric field induced by the zonal magnetospheric electric fields that penetrates to equatorial/low latitude ionosphere. Observational results on storm time Es layer responses in the Brazilian and Indian-Asian longitudes are compared. An under-shielding prompt penetration electric field (PPEF) of westward polarity that dominate the night side ionosphere, or an over-shielding electric field also of westward polarity in the evening sector can cause formation of sporadic E layers near 100 km, while an eastward polarity electric field, (under-shielding/over-shielding depending upon local time) can lead to disruption of an Es layer in progress. Ionization convergence/divergence leading to the Es layer formation/disruption is driven by a vertical Hall electric field, induced by the primary zonal PPEF, in the presence of storm associated enhanced ratio of field line integrated Hall to Pedersen conductivity (Sigma(H))/(Sigma(P)). A downward polarity of the Hall electric field leads to Es layer formation, while an upward polarity causes the Es layer disruption. An enhancement in the Sigma(H)/E-P ratio can result from E layer conductivity enhancement due to energetic particle precipitation peculiar to the longitude of the South Atlantic Magnetic Anomaly (SAMA) and/or from a drastic reduction in integrated Pederson conductivity in the form of reduced foF2 that is observed in all longitudes. (C) 2013 Elsevier Ltd. All rights reserved.