FORMATION MECHANISMS FOR LOW-ALTITUDE E(S) AND THEIR RELATIONSHIP WITH NEUTRAL FE LAYERS - RESULTS FROM THE METAL CAMPAIGN

The METAL campaign was a multi-instrument campaign conducted in September-October 1991 that was designed to investigate the relationship between neutral and ionized metallic layers in the high-latitude lower ionosphere. Measurements included electron density profiles and electric fields from the EISCAT UHF incoherent scatter radar, ionosonde measurements of E(s) layers, neutral Fe profiles from lidar and rocket observations of winds (by chaff release), and measurements of plasma density and ion composition (by mass spectrometer). E(s) layers were observed on eight occasions. The measurements are here compared with simulations of E(s) formation by tidal wind shear and electric fields. The main features of the E(s) layers are found to be consistent with formation by tidal wind shear alone in one case and by the combined action of wind shear and electric fields in the remaining cases. On three occasions, layers were seen to descend rapidly, in one case to below 90-km altitude, and the rate of descent was found to be consistent with the expected action of the observed electric fields. Sporadic neutral Fe layers were observed on seven occasions, including four cases in close correlation with E(s). However, the neutral layers were generally not at exactly the same altitude as the E(s), nor did they follow the same altitude changes. The results indicate that both wind shears and electric fields are important for the formation of thin layers and the vertical transport of metal ions in the high-latitude lower thermosphere. They show that neutral Fe(s) layers, although they may be formed in close association with E(s) layers, can persist and migrate independently of E(s) layers. This latter observation suggests that partial reionization of neutral layers by intense auroral particle precipitation may explain some observations of multiple E(s) layers.