Horizontal and vertical structure of the major sporadic sodium layer events observed during ALOHA‐90

Sporadic Na (Na s ) layers have been observed frequently at low‐ and high‐altitude sites primarily in the Northern Hemisphere. Na s layers are very dense and very thin (∼1 km FWHM) and can form over periods ranging from a few minutes to several hours. Radar and lidar observations by our group at Arecibo have shown conclusively that Na s layers are linked to the formation of sporadic E (E s ) layers. Large Na s events were observed on several flights during ALOHA‐90 to extend over horizontal distances of approximately 1800 km. Atmospheric tides and gravity waves were observed to have an effect on the vertical motion of these layers. We describe the 22 March event which was observed from Maui (21°N) to near the equator (5°N) and the 27 March event which was observed from Maui (156°W) to almost the International Date Line (∼172°W).