Statistical study on sporadic sodium layers (SSLs) based on diurnal sodium lidar observations at Beijing, China (40.5 °N, 116 °E)

Sporadic sodium layers (SSLs) have been observed all over the globe and always has been of great concern. However, most researches on SSLs are based on only nocturnal observations due to strong daytime solar background noise, thus there is still insufficient understanding of the diurnal variation of SSLs. In this paper, a statistical analysis of SSLs observed by a diurnal sodium (Na) lidar at Beijing (40.5 °N, 116 °E) from April 2014 to December 2016 has been reported. A total of 54 and 87 events that peaked during daytime and nighttime, respectively, are identified. The average occurrence interval for daytime observations is ~36 h per SSL, which is slightly longer than the value of ~29 h per SSL for nighttime observations. Both daytime and nighttime SSLs show evident seasonal variations with summer SSLs occurring much more frequently and tending to appear at higher altitude with lower peak density while larger strength factor. The seasonal dependence of SSL occurrence is consistent with that of sporadic E (Es) occurrence, however, their diurnal variations are found to be totally different. The SSLs are observed more frequently around 03–04 LT during nighttime and 16–17 LT during daytime when Es layers appear at lower altitudes (below 110 km), while have a lowest occurrence rate around midday when Es layers occurrence rate peak. The average peak altitude of SSLs during the second half of the night is lower compared to that during the first half of the night. Around midday, the average peak width of SSLs is narrower and peak density tends to be smaller when compared to those at other times. The possible explanations for the diurnal and seasonal variations of SSLs characteristics are discussed, and the key role of the evolution of Es layer height modulated by semidiurnal tide in the local time variation of SSL is highlighted.