The extension of the neutral atomic Na layer into thermosphere (up to 170 km) was observed in low and high latitudes. This observation was made recently by the Na Resonance-Fluorscense Lidar in Cerro Pachón, Chile during Equinox while the geophysical mechanisms and implications of the formation are unknown.

Time and altitude distribution of a) vertical ion drift, b) vertical neutral velocity, c) density of Na+ and d) Na over ALO by WACCM-DART. The day time is from 11:05 UT to 22:10 UT, and night time is from 22:10 UT to 11:05 UT. The red line in d is the phase line of the semi-diurnal tide vertical wind over ALO.

In this study, we conduct a numerical simulation on the variations of Na and Na+ in the low latitude E and F region, where the contributions from electromagnetic force, neutral wind, diffusion and gravity are included. The simulation suggests: First, as the major reservoir of atomic Na in the E region, Na+ in the subtropics region of the geomagnetic equator is able to converge below 200 km at night due to the combined effect of the vertical component of E cross B drift and Coulomb drift. Second, fountain effect does not have much influence on the behavior of Na at night. Third, the possible explanation for the frequent generation of the thermospheric sodium layer during spring equinox in Cerro Pachón, Chile is owing to the large vertical dynamic transport modulated by tidal wave perturbations. However, the mechanism of such perturbation is still not clear at the moment.

Publication Name: JGR Space Physics

First HAO Author's Name: Nick Pedatella