Comparisons of spectrally resolved nightglow emission locally simulated with space and ground level observations

11 A mesospheric model of the airglow emission is developed to recover the night variations observed 12 at ground level. The model is based on a 1D vertical photochemical model, including the photodis- 13 sociation and heating processes. The spectral radiation is calculated at high altitude and propa- 14 gated through the atmosphere to the ground. We also include short scale vertical dynamic such 15 as turbulences and the molecular di ﬀ usion. Simulations reveal realistic emissions when compared 16 with space observations. In addition, we estimate the impact of changes associated with parame- 17 terized atmospheric tides. The comparison with observations is performed over high altitude and 18 ground level. We confront the model outputs at high altitude with satellite observations (SABER 19 and GOMOS) and the simulations propagated at ground level are compared to local measurements 20 campaigns performed in France and India. Biases between observed and simulated radiances and 21 volume emission rates are suspected to be due to the impact of gravity waves or the large scale 22 dynamic.


Introduction 24
The night airglow is the radiation emitted over a wide spectrum originating from the chemical pling an OH*-model with a chemistry-transport model. 55 However, these studies do not spectrally resolve the emission as they only produce global -or 56 transition specific-volume emission rate (VER). In order to simulate the emission spectrum at 57 the various altitudes concerned, it is mandatory to include in the model the various excited states 58 implicated in the emission as reactive species. Very few models able to simulate the full spectrum 59 observed at high altitude were developed and none are available for ground-based analyses. Moreels noting that these models listed above are not implemented with a radiative transfer model, required 70 in order to propagate the spectrum simulated at high altitude down to the ground. 71 The objective of this study consist in simulating nightglow that can be observed at ground level. 72 Therefore a local photochemical model was developed based on the most up-to-date coefficients. 73 On the contrary to other models, various excited states along with a radiative transfer module are in-74 cluded in order to obtain the OH spectral emission emitted at high altitude, and propagate it down to 75 the ground through interaction with the neutral atmosphere for comparison to local measurements.

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To include the various dynamical processes on the 1D model, temperature and wind fluctuations have been operated down to 0.05 nm in the UV region to take into account the Lyman-α line, the 121 Schumann-Runge and the Huggins bands. Above 3 µm, data are derived from Thekaekara (1974).

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The solar zenith angle is calculated using the Chapman function (Smith and Smith, 1972)    The mesopause is subject to strong energy exchanges (Mlynczak, 1997). The importance of the 134 heating has been noticed by Mlynczak (2000). We consider in the model the solar heating, the 135 chemical heating and also the radiation cooling (by CO 2 ). The solar irradiance is absorbed by the states is converted into kinetic energy. We apply here the formulation from Brasseur and Solomon 139 (2005) that expresses the difference of absorbed solar radiation at a specific layer i, between two 140 vertical levels: with dT/dt the heating rate, i.e. the variation of the temperature T with time t at the layer i, the 143 solar zenith angle Z, the density ρ, the calorific capacity C P , and I(z, λ) the incident solar intensity With k r the rate of the considered reaction, the density of the reactants considered ρ(1, 2) and air ρ, 152 the reaction enthalpy H, the Boltzmann constant k b and the Avogadro number N A .

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The radiative cooling tallies with the CO 2 infrared radiation around 15 µm. We use here Thermodynamic Equilibrium) and non-LTE effect at high altitude.

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These heating rates, encompassing the solar heating, the chemical heating and the radiative cool-  Since we aim to compare the model results with ground based observations, we compute the fully 164 resolved spectrum of the airglow. The intensity of an emission line is written according to: Where I ( j ,ν → j ,ν ) is the transition intensity between the rovibrational state ( j , ν ) and ( j , ν ), j and

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The VER for a specific vibrational transition is given by: It is also worthily to mention that because of the low temperature at this altitude, the local thermal 177 emission of the atmosphere is spectrally located in the mid and far infrared and does not interfere 178 with the emission.

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The radiative transfer equation is written hereafter: With L(τ, Ω), the radiance for the optical depth τ, which propagates in the direction Ω. J th , J ds , 187 J dm and J glow are the different sources functions, respectively from the thermal emission, the sim-188 ple scattering, the multiple scattering and the nightglow emission. The expressions of the various 189 sources follow:  The wind advection presents two components, the vertical drift velocity, calculated with the 211 molecular diffusion coefficient, and the tidal wind, which is described in the next paragraph. We 212 use here a semi-Lagrangian scheme to resolve the advection.    but is higher than the mean profile peak. The consistency is increased for the 2.0 µm peak altitude.

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The difference in the profile-to-profile comparison is useful to highlight the limits of the model.

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To understand this, the temperature profiles are displayed in Figure 5   Therefore, changes in temperature and concentrations will lead to changes in emission. For ex-303 ample, a local increase in density can induce a local increase in the observed VER. A change in 304 temperature will imply changes in chemical rates and therefore in OH excited states sources. In this 305 particular case where the simulated VER is lower than observed, we assume that the GW increases 306 the temperature as seen in Figure 5 (c) and modify the density, leading to changes in the chemistry 307 of the nightglow production. Not shown here, the oxygen profile is also larger for the observation 308