LIDAR, RADAR AND AIRGLOW OBSERVATIONS OF A PROMINENT SPORADIC NA SPORADIC-E LAYER EVENT AT ARECIBO DURING AIDA-89

Sporadic Na (Na(s)) 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(s)) 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 cm-3. 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 E(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(s) 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.