Lidar observations of atmospheric sodium at an equatorial location

Lidar measurements of upper atmospheric sodium were made at Alcântara (2°20′ S, 44°30′ W), Brazil, on 8 days during the month of November 1996. These are the first sodium measurements to be reported from an almost equatorial location. The average sodium distribution at Alcântara was found to be very similar to the annual mean for São José dos Campos (23° S), but about 1 km higher than the November mean value of 92 km for the latter site. Only one, weak, sporadic sodium layer was observed during more than 26 h of observations, an occurrence frequency much less than that seen at low latitude sites in the northern hemisphere. A survey of the observations of sporadic layers at various locations suggests that both magnetic dip and declination angles might influence their rate of occurrence.