Density response of the mesospheric sodium layer to gravity wave perturbations

Abstract

Lidar observations of the mesospheric sodium layer often reveal wavelike features moving through the layer. It is often assumed that these features are a layer density response to gravity waves. Chiu and Ching [1978] described the approximate form of the linear response of atmospheric layers to gravity waves. In this paper, their results are used to predict the response of the sodium layer to gravity waves. These simulations are compared with experimental observations and a good correlation is found between the two. Because of the thickness of the sodium layer and the density gradients found in it, a linear model of the layer response is not always adequate to describe gravity wave ‐ sodium layer interactions. Inclusion of nonlinearities in the layer response is briefly discussed. Experimental data is seen to contain features consistent with the predicted nonlinearities.

Mesopause region temperature structure observed by sodium resonance lidar

“…As pointed out in the introduction, this phase reversal is to be expected and would result from the reversal in the vertical gradient of Na mixing ratio above the layer peak. Although such wave modulation does, indeed, appear to occur (and has been pointed out in much earlier studies, see, for example, Shelton et al, 1980;Gardner and Shelton, 1985;Batista et al, 1985), it does not fully explain our observations. In particular, we note that only positive temperature gradients are correlated with the gradient in Na concentration.…”

“…The behaviour of such waves and their interactions with the atmospheric constituents has been the subject of many studies, both experimental and theoretical (Chiu and Ching, 1978;Shelton et al, 1980;Gardner and Shelton, 1985;Batista et al, 1985;Yang et al, 2008b). Lidar measurements of the vertical distribution of meteor metals such as sodium have been used to study tides (Batista et al, 1985;Fricke-Begemann and Höffner, 2005) and gravity waves (Shelton et al, 1980;Yang et al, 2008a) in the 80-100 km region. Simultaneous measurements of temperature and sodium concentration (Gardner et al, 2005) have shown, as expected, a high degree of correlation between the time variations of these two parameters, the correlation being positive on the bottomside of the layer and negative on the topside.…”

Investigation of a mesospheric gravity wave ducting event using coordinated sodium lidar and Mesospheric Temperature Mapper measurements at ALOMAR, Norway (69°N)

“…Sodium density perturbations due to GWs have previously been simulated [Swenson and Gardener, 1998;Shelton et al, 1980] using a relation derived by Chiu and Ching [1978]. Sodium density perturbations can be derived from the sodium continuity equation and are given by…”

On nonlinear response of minor species with a layered structure to gravity waves

“…The density response, which contains higher harmonics of the gravity-wave frequency, indicates the possible presence of nonlinearity. Measurements of sodium number density reveal nonlinearities [Shelton et al, 1980], and thus the linear treatment of the minor species response does not seem to be adequate for correctly interpreting such measurements. By nonlinearity, we mean that the density response is much larger than the unperturbed density or that the density response contains terms with higher harmonics of gravity waves.…”