LIDAR OBSERVATIONS OF A LARGE HIGH-ALTITUDE SPORADIC NA LAYER DURING ACTIVE AURORA

A large sporadic Na layer was observed near 109 km during active auror a at Poker Flat, Alaska, on the night of 12-13 January 1996. This spor adic layer is significantly denser and broader than previously reporte d thermospheric sporadic layers;having a peak concentration equal to t he peak concentration of the background Na layer, a FWHM of 4.7 km, an d a column abundance of 40% of the background Na layer. The appearence of the Na-s layer coincided with a significant OH temperature gradien t across the sky but did not coincide with the presence of a discrete are overhead nor a distinct signature in the local magnetometer data. The presence of such a large amount of neutral Na at these altitudes c ontradicts the current models of Na-s layer formation. The observation s are discussed in the light of other observations, current models, la yered auroral phenomena, and meteoric ablation.