Wave-associated sporadic neutral layers in the upper atmosphere

Sporadic neutral layers, with thicknesses between a few hundred meters and several kilometers, are observed by lidar in the same height range as ionospheric sporadic E. Ns layers were first observed in sodium, 20 years ago, and more recently have also been seen in potassium, iron and calcium. As in the case of Es there is a strong link with atmospheric waves, and a number of studies have shown evidence for a possible connection with tidal oscillations in the lower thermosphere. Recent observations at São José dos Campos, Brazil, have shown that sporadic sodium layers are frequently observed to occur at the heights of peaks in sodium concentration corresponding to a propagating atmospheric wave. In cases where the vertical wavelength of the propagating wave is short, as many as three complete oscillations can sometimes be seen, and the sporadic layer always occurs at the highest wave maximum, typically in the range 95-100 km. In the case of long-lived events the Ns layer is observed to accompany the downward phase propagation of the atmospheric wave. On the basis of these observations a case is made for the recombination of sodium ions as the source of the observed sporadic layers