A time-resolved model of the mesospheric Na layer: constraints on the meteor input function

Abstract

Abstract.A time-resolved model of the Na layer in the mesosphere/lower thermosphere region is described, where the continuity equations for the major sodium species Na, Na + and NaHCO 3 are solved explicity, and the other shortlived species are treated in steady-state. It is shown that the diurnal variation of the Na layer can only be modelled satisfactorily if sodium species are permanently removed below about 85 km, both through the dimerization of NaHCO 3 and the uptake of sodium species on meteoric smoke particles that are assumed to have formed from the recondensation of vaporized meteoroids. When the sensitivity of the Na layer to the meteoroid input function is considered, an inconsistent picture emerges. The ratio of the column abundance of Na + to Na is shown to increase strongly with the average meteoroid velocity, because the Na is injected at higher altitudes. Comparison with a limited set of Na + measurements indicates that the average meteoroid velocity is probably less than about 25 km s −1 , in agreement with velocity estimates from conventional meteor radars, and considerably slower than recent observations made by wide aperture incoherent scatter radars. The Na column abundance is shown to be very sensitive to the meteoroid mass input rate, and to the rate of vertical transport by eddy diffusion. Although the magnitude of the eddy diffusion coefficient in the 80-90 km region is uncertain, there is a consensus between recent models using parameterisations of gravity wave momentum deposition that the average value is less than 3×10 5 cm 2 s −1 . This requires that the global meteoric mass input rate is less than about 20 t d −1 , which is closest to estimates from incoherent scatter radar observations. Finally, the diurnal variation in the meteoroid input rate only slight perturbs the Na layer, because the residence time of Na in the layer is several days, and diurnal effects are effectively averaged out.

Satellite measurements of the global mesospheric sodium layer

“…In the mesosphere and lower thermosphere (MLT), Na + ions and sodium bicarbonate (NaHCO 3 ) are the major reservoir species above and below the atomic Na layer, respectively (Plane et al, 1999;Plane, 2004). Due to the long residence time (∼7 days) of ablated sodium in the region between 80 and 100 km (Plane, 2004), and the comparatively short lifetimes of the ratedetermining chemical reactions that convert sodium between atomic Na and these reservoirs, the chemistry reaches a photochemical steady-state on the timescale of vertical transport by eddy or molecular diffusion . Consequently, atmospheric chemistry plays a key role in determining the atomic Na distribution.…”

“…Consequently, atmospheric chemistry plays a key role in determining the atomic Na distribution. Na is converted to NaHCO 3 via the following sequence of reactions (Plane, 2004):…”

Sodium lidar‐observed gravity wave breaking followed by an upward propagation of sporadic sodium layer over Hefei, China

“…Below 90 km, the main chemical reaction that produces sodium atoms is NaHCO 3 + hv → Na + HCO 3 . The reaction rate coefficients are listed in Table 1 of Plane [2004]. The potential correlations between SSLs and the temperature structure have also been studied previously [Hansen and von Zahn, 1990;Zhou et al, 1993;Gardner et al, 1995;Delgado et al, 2012].…”

“…And a less pronounced link, or absence of a link, between Es and SSL is observed below 95 km [Hansen and Von Zahn, 1990]. Second, the complex SSL chemical process has been published [Cox and Plane, 1998;Collins et al, 2002;Plane, 2003Plane, , 2004 while SSLs and temperature have been observed simultaneously [Hansen and Von Zahn, 1990;Gardner et al, 1993Gardner et al, , 1995Batista et al, 2002]. Temperature-independent conjecture Zhou and Mathews, 1995] and modeling [Delgado et al, 2012] along with sporadic neutral metal atom layers have been studied and discussed.…”

Global ozone monitoring by occultation of stars: an overview of GOMOS measurements on ENVISAT

“…It has recently attracted a renewed interest, because, besides the scientific questions related to the origin and the vertical distribution of the layer, the mesospheric sodium is of major importance in astronomy, where it is used to produce an artificial star on which to base guidance for adaptive optics with a lidar illumination. Although extensive data exist about the mesospheric sodium layer measured from particular sites, very little is known about the geographical distribution (Plane, 2004). GOMOS has demonstrated a new method of studying the sodium layer, and has provided the first global mesospheric sodium climatology made so far .…”