Date of Award

Summer 9-2003

Document Type

Thesis - Open Access

Degree Name

Master of Science in Space Science

Department

Physical Sciences

Committee Chair

Michael P. Hickey

Committee Member

John Olivero

Committee Member

Peter W. Erdman

Committee Member

Irfan Azeem

Abstract

Atmospheric gravity waves (GWs) perturb minor species involved in the chemical reactions of airglow emissions in the mesopause region of the earth's atmosphere. The so-called 'Cancellation Factor' (CF) is defined as a transfer function relating the amplitude of airglow brightness fluctuation to the amplitude of GW-induced fluctuation in temperature [Swenson and Gardner, 1998]. This transfer factor can be used to determine GW fluxes and the forcing effects of GWs on the mean state through airglow observations, because GW fluxes are proportional to the square of GW amplitude.

Numerical models [Walterscheid et al., 1987; Schubert et al., 1991] have previously shown that the airglow relative brightness fluctuation can be much larger than the brightness-weighted relative temperature fluctuation (that is, Krassovsky's ratio is much greater than 1). Analytical expressions of the CF in the OH nightglow were derived by Swenson and Gardner [1998] and later used by Swenson and Liu [1998]. We introduce the full-wave model [Hickey et al., 1997, 1998] describing GW propagation in a non-isothermal, windy, and viscous atmosphere (combined with the chemical reaction scheme for the OH (8, 3) Meinel emission) to derive fluctuations in the OH nightglow from which an equivalent CF is calculated. Extensive comparisons between our CF and that of Swenson and colleagues show under what atmospheric conditions and which range of GW parameters the CF would be expected to provide a good measure of GW amplitude.

This thesis consists of four chapters that deal with the calculations and comparisons of the CFs in the OH nightglow from both the analytical and numerical models under various atmospheric conditions.

In the first chapter the general subject of internal GWs is introduced for the non-specialist of this field. It reviews the historical theory and observation of atmospheric GWs, and also emphasizes the role of atmospheric GWs in producing the reversal of global temperature gradients at the mesopause. At the end of this chapter, the motivation for calculating the CF is introduced.

In the second chapter numerical models of GW-driven fluctuations in the OH nightglow are described in detailed in three developing stages. The Walterscheid et al. [1987] model incorporated a five-reaction photochemical scheme and the complete dynamics of linearized acoustic GWs in an isothermal and motionless atmosphere, but only calculated Krassovsky's ratio for an infinitesimally thin airglow emission layer. Hickey's [1988] model was extended to include the dynamical effects of internal GWs propagating in a viscous, thermally conducting, and rotating (though windless) isothermal atmosphere. The model of Schubert, Walterscheid & Hickey [1991] investigated how the characteristics of the OH nighglow from an extended emission region were modified by eddy momentum and eddy thermal diffusivities. In the rest of the second chapter the full-wave model [Hickey et al., 1997, 1998] along with the chemical reaction scheme for the OH (8, 3) Meinel emission as well as the analytical model of Swenson and Gardner [1998] are introduced.

The third chapter commences with a comparison of the CFs derived from the analytical model of Swenson and Gardner [1998] with the CFs calculated with the full-wave model numerically. Much of the work involves the development of computer programs and the plots of data outputs. The analysis and discussion begin with the assumption of an ideal atmosphere, which is isothermal, quasiadiabatic, and motionless, and later continue to that of a more realistic atmosphere (non-isothermal, dissipative, and with meridional and zonal winds). In the case including the influence of mean winds, we employ wind profiles representative of December 15 and GWs traveling in the eastward direction. These comparisons allow us to determine the accuracy of the calculations and the validity of the assumptions used in the analytically derived CF of Swenson & Gardner [1998].

In the last chapter we summarize the advantage and disadvantage in both approaches. The more accurate calculation of the CF in the OH nightglow under a more realistic atmosphere provides a better understanding of GW effects on the mesospheric dynamics. The CF can be used by optical experimenters to relate their airglow observations to GW energy and momentum fluxes in the stated altitude region.

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