Characteristics of the sporadic sodium layers observed at 23 degree S

The mesospheric sodium data, obtained between 1975 and 1987 at Sao Jose dos Campos (23 degree S,46 degree W) with a laser radar, have been analyzed in order to identify the appearance of thin sporadic sodium layers. In this search, a total of 65 events were identified. The average height of the peaks is 95.0 km. The ratio of the maximum peak density to the average layer density is normally 2.5 to 3.0, but values as high as 7 have been observed in the most outstanding cases. The events last from a few minutes to several hours, although durations of 1-2 hours are more typical. The events occur more often during periods of large meteor showers, especially in August. The diurnal variation shows an increasing number of observed peaks from 1500 LT to midnight and remains almost constant from midnight to 0600 LT. In 52 out of 54 days for which sodium and ionsonde data are available there was an sporadic E layer nearly coincident with the sodium cloud. The coincidence is good for short-lived sporadic layers, but a substantial increase in sporadic E critical and blanketing frequencies normally precedes the long-lasting and broader ones. These results are compatible with the suggestion that the enhanced layers are produced by the wind shear distortion of sodium clouds originating in meteor deposition, but the authors cannot rule out the possibility of an ion conversion mechanism