Enhanced electron density layers in the high-lattitude lower ionosphere

Narrow layers of enhanced electron density appearing at altitudes between 90 and 100 km are found to be of at least 3 distinct types. The first type consists of very narrow, short-lived, sporadic-E type layers, which are at least sometimes associated with similar layers in neutral sodium, and which may be composed of metallic ions gathered into thin layers by the action of vertical winds associated with long-period gravity waves. The second type comprises broader, short-lived layers which are seen in the magnetic evening and midnight sectors and which appear to be produced directly by precipitation of energetic electrons from the outer edge of the radiation belts. The third type comprises persistent daytime/early evening layers which seem to be composed of unusual ion species, at least some portion of which have a high mass (substantially greater than 30.5 a.m.u.) but with the bulk of the ions having a short lifetime (not longer than a few tens of minutes)