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Abstract. Lidar observations of the mesospheric Na layer have revealed considerablediurnal variations, particularly on the bottom side of the layer, where morethan an order-of-magnitude increase in Na density has been observed below 80 kmafter sunrise. In this paper, multi-year Na lidar observations areutilized over a full diurnal cycle at Utah State University (USU) (41.8^∘ N,111.8^∘ W) and a global atmospheric model of Na with 0.5 kmvertical resolution in the mesosphere and lower thermosphere (WACCM-Na) to explorethe dramatic changes of Na density on the bottom side of the layer. Photolysis of the principal reservoir NaHCO₃ is shown to beprimarily responsible for the increase in Na after sunrise, amplified by theincreased rate of reaction of NaHCO₃ with atomic H, which is mainlyproduced from the photolysis of H₂O and the reaction of OH withO₃. This finding is further supported by Na lidar observation at USUduring the solar eclipse (>96 % totality) event on 21 August 2017, when a decrease and recovery of the Na density on thebottom side of the layer were observed. Lastly, the model simulation showsthat the Fe density below around 80 km increases more strongly and earlierthan observed Na changes during sunrise because of the considerably fasterphotolysis rate of its major reservoir of FeOH.

We report the first simultaneous lidar observations of thermosphere‐ionosphere sporadic nickel and Na (TISNi and TISNa) layers in altitudes ∼105–120 km over Yanqing (40.42°N, 116.02°E), Beijing. From two years of data spanning April 2019 to April 2020 and July 2020 to June 2021, TISNi layers in May and June possess high densities with a maximum of 818 cm⁻³on 17 May 2021, exceeding the density of main layer peak (∼85 km) by ∼4 times. They correlate with strong sporadic E layers observed nearby. TISNa layers occur at similar altitudes as TISNi with spatial‐temporal correlation coefficients of ∼1. The enrichment of Ni in TISNi is evident as the [TISNi]/[TISNa] column abundance ratios are ∼1, about 10 times the main layer [Ni]/[Na] ratios. These results are largely explained by neutralization of converged Ni⁺and Na⁺ions via recombination with electrons. Calculations show direct recombination dominating over dissociative recombination above ∼105 km.