Climatology, Long-Term Trend and Solar Response of Na Density Based on 28 Years (1990–2017) of Midlatitude Mesopause Na Lidar Observation

The climatology of earth's Na density over Fort Collins, CO (41°N, 105°W) based on nocturnal Na lidar observations between 1990 and 1999 was reported by She et al. (2000, https://doi.org/10.1029/2000gl003825). Based on a continued 28-year data set between 1990 and 2017 with the latter part observed over Logan, UT (42N, 112W), we update the seasonal variations between 80 and 110 km. This data set is also used to deduce long-term responses of Na density (profile) between 75 and 110 km, showing a positive linear trend between 75 and 93 km (with maximum ∼2.87 × 10⁸ m⁻³/decade at 87 km); it turns negative before approaching zero at 110 km (with minimum ∼−2.96 × 10⁷ m⁻³/decade at 100 km). The associated solar response is also positive for the altitude range in question (with maximum ∼5.20 × 10⁶ m⁻³/SFU at 91 km). We also derived the 28-year mean Na layer column abundance, centroid altitude, and root mean square width to be 3.92 ± 2.14 10¹³ m⁻², 91.3 ± 1.0 km, and 4.62 ± 0.56 km, respectively, and deduced long-term trend and solar cycle responses of column abundance and centroid altitude, respectively to be 7.81 ± 1.63%/decade and 16.9 ± 2.8%/100SFU, and −355 ± 35 m/decade and −1.94 ± 0.69 m/SFU. We explained conceptually how positive long-term responses in Na density led to positive responses in column abundance and negative responses in centroid altitude.