Effects of nonmigrating diurnal tides on the Na layer in the mesosphere and lower thermosphere

Neutral metal layers such as Na, Mg, and Fe occur in the Earth's mesosphere and lower thermosphere (80-120 km) region due to the ablation of cosmic dust. These layers provide important tracers of chemical and dynamical processes within this region. Nonmigrating diurnal tides are persistent global oscillations in atmospheric fields (e.g., wind, temperature, and density) with a period of 24 hours and nonsynchronous propagation with the sun. A complex combination of tidal forcing, chemistry, and photochemistry drives the diurnal cycle of these meteoric atoms. However, the mechanism behind their diurnal variation is not yet fully understood.

The influence of nonmigrating diurnal tides on Na layer variability in the mesosphere and lower thermosphere regions is investigated for the first time using data from the Optical Spectrograph and InfraRed Imaging System (OSIRIS) on the Odin satellite and Specified Dynamics Whole Atmosphere Community Climate Model (SD-WACCM) with metal chemistry. The Na density from OSIRIS exhibits a clear longitudinal variation indicative of the presence of tidal components. Similar variability is seen in the SD-WACCM result. Analysis shows a significant relationship between the nonmigrating diurnal tides in Na density and the corresponding temperature tidal signal. Below 90 km, the three nonmigrating diurnal tidal components in Na density show a significant positive correlation with the temperature tides. Conversely, the phase mainly indicates a negative correlation above 95 km. Around the metal layer peak, the response of the Na density to a 1 K change in tidal temperature is estimated to be 120 cm⁻³.