Annual Variations of Pre-Dawn Thermosphere-Ionosphere Na (TINa) Layers Observed by Lidar over Boulder and their Relationship to Sunrise and Tidal Winds Revealed by CTMT and ICON

Thermosphere-ionosphere metal (TIMt) layers (~110–200 km) were first discovered with high-sensitivity lidar observations in 2011 from Antarctica and then observed globally. Most of these TIMt layers exhibit intermittent occurrence until the surprising regular occurrence of TIMt layers in Na species (TINa) was reported in 2021 from Boulder where TINa layers occur before dawn and after dusk almost every night.
By analyzing 7 years of lidar data and characterizing 12 monthly composites of TINa layers, we have discovered that the peak phase of pre-dawn TINa layers (~110–150 km altitude) undergoes clear annual variations with the earliest occurrence in summer and latest in winter. TINa phase variations are closely correlated to the annual variations of sunrise. Researching the possible causes, we find from the Climatological Tidal Model of the Thermosphere (CTMT) (Oberheide et al., 2011) that the phases of semidiurnal westward-propagating tidal winds (SW2) undergo similar seasonal variations as sunrise and pre-dawn TINa. These neutral TINa layers occur where vertical ion convergence, computed from CTMT and from Ionospheric Connection Explorer (ICON) mission (Immel et al., 2018), is strong and accumulation of TINa+ ions is expected. Such results strongly support the formation mechanism (Na^++e^-→Na+hν) proposed previously and suggest that migrating tidal winds experience phase annual variations. These discoveries will help us better understand the sources and formation mechanisms behind TIMt layers, possibly providing new tracers to profile the neutral temperature and winds in the E to F regions. We believe that lidar observations of TIMt layers provide a unique way to study thermal tides in the “thermospheric gap” region of 100–200 km.