An evaluation of the evidence for ion recombination as a source of sporadic neutral layers in the lower thermosphere

Sporadic layers of metal atoms (Ns), occurring in the same height range as ionospheric sporadic-E layers, were first detected by lidar some 20 years ago. Ns layers have typical thicknesses of a few hundred meters to a few km, peak atom concentrations several times that of the ambient background layer, and are sometimes seen to grow and decay over time scales as short as a few minutes. Layers have been detected in Na, Fe, K and Ca, but it seems likely that they exist in other meteoric metals such as Mg. Despite a great deal of excellent experimental work over the past decade, the source of Ns layers is still an open question. Mechanisms suggested include direct meteor deposition, release from aerosol particles, chemical reduction of appropriate metal compounds, redistribution of existing atoms, and recombination of ions. The last-named of these mechanisms, although capable of explaining many of the observed characteristics of Ns layers, including their strong correlation with Es, has generally been rejected in the past, at least in the case of Na, because mass spectrometer measurements of Na+ have mostly shown concentrations too small to explain the observed sporadic sodium layers. However, recent laboratory measurements of the relevant recombination processes, and a re-evaluation of the rocket-borne mass-spectrometer measurements, suggest that ion recombination is in fact the strongest contender. (C) 1999 COSPAR. Published by Elsevier Science Ltd.