Lidar, radar and airglow observations of a prominent sporadic Na/sporadic E layer event at Arecibo during AIDA-89

Sporadic Na (Na,) layer events were frequently identified during 160 h of lidar observations at Arecibo in January, March and April 1989. Most were accompanied by sporadic E (E.) layers. The most spectacular Na_s E_s, event occurred on the night of 30-31 March when both the Na and electron abundances between 90 and 100 km increased by approximately 700% during a period of 2.25 h starting at 2100 LST. The maximum Na density was almost 42,000 cm³. The vertical and temporal structure of the Na and electron densities were remarkably similar during the event. The ratio of the average Na enhancement to the electron density varied from a maximum of 3.5 Na atoms/electron at 98 km to about 0.5 Na atoms/ electron below 94 km. Between 93 and 97 km the electron enhancement preceded the Na enhancement by 15-30 min. Above 97 km and below 93 km the Na and electron density variations were in phase. The data suggest that the E_s, layer triggered the release of Na from a reservoir, but the E_s layer was not the source of the major Na_s layer. Two minor Na_s layers were observed between 101 and 107 km after midnight LST which were also accompanied by intense _s layers and enhancements of the O(¹S) emission intensities. The abundances of these high altitude Na_s layers were less than 1% of the electron abundances. These Na, layers appear to be caused by the conversion of Na in the E_s layer to Na through a set of clustering reactions involving N₂ CO₂ and H₂O.