ON THE ROLE OF AURORAL ELECTRIC-FIELDS IN THE FORMATION OF LOW ALTITUDE SPORADIC-E AND SUDDEN SODIUM LAYERS

The characteristics of metallic and molecular ion sporadic-E (E(s)) layers, formed by the action of strong electric fields at auroral latitudes, are examined using computer simulations. It is found that, for electric fields directed between northward and westward (northern hemisphere), thin metallic ion layers (< 2 km thick) can be formed above about 105 km altitude. For electric fields directed from westward, through southward, to south-eastward, slightly thicker (4-6 km thick) metallic ion layers can form between 90 and 105 km altitudes. Thin layers of molecular ions can be formed by electric fields directed between north and west if the ion density is low. Examples of E(s) layers observed by the EISCAT radar, together with simultaneous observations of electric fields and ion drifts are presented which show good agreement with the simulations. The relationship between the lower-altitude E(s) layers and sudden sodium layers (SSLs) is discussed leading to an explanation of some of the characteristics of SSLs at high latitude. A possible involvement of smoke particles in the formation of both E(s) layers and SSLs is proposed.