Descending layer variability over Arecibo

Descending layers of ionization over Arecibo exhibit very diverse behavior from night to night that does not appear to be strongly correlated to geoma gnetic activity, solar forcing, or average semidiurnal tidal winds. On some nights, three or more distinct layers are observed to form near 170 km ove r timescales of similar to2 hours. Rather than descending smoothly over per iods of several hours, these layers stall, abruptly disappear, or even reve rse direction in the midst of their descent. The time scales for their disa ppearance are examined and compared to loss rates arising from diffusion an d recombination. Diffusion alone is found to be too slow to account for the observations, but recombination is fast enough provided that the convergen t wind shear that forms the layer is relatively weak coincident with their disappearance. The continuity equation is solved in conjunction with a time sequence of radar profiles to estimate the vertical drift and horizontal n eutral wind consistent with the observed behavior. The resultant wind field is northward, has an average speed of similar to 80 m s(-1), and varies si gnificantly neat the altitude where the layers are observed. These inferred winds are consistent with the presence of the observed layers, and their m agnitudes as obtained from the classical continuity and momentum equations are reasonable for this altitude range.