Effect of the planet shine on the corona: Application to the Venusian hot oxygen

Abstract

We present a new derivation of the hot oxygen density in the Venusian upper exosphere from Pioneer Venus Orbiter ultraviolet spectrometer, considering the effect of the solar photons backscattered by the oxygen atoms below ~ 300 km (Venus shine), not included in past analysis. The Venus shine increases the volume emissivity of the hot oxygen corona by ~60% compared to the direct illumination and the O I 130.4 nm brightness measured by PVO with a hot oxygen is reproduced with adensity reduced by 1.6 compared to the density derived without the Venus shine.

Introduction

At the surface of Venus, the atmosphere is predominantly composed of CO₂ but, above ~150 km,  O, generated through the photodissociation of CO₂ [1] and dissociative recombination of O₂⁺ becomes dominant [2]. Solar EUV photons not only dissociate neutral molecules in the thermosphere but also ionize them, driving the formation of Venus’ ionosphere. The dominant ion in the Venusian ionosphere is O₂⁺ produced from reactions between CO₂⁺ and O [3]. The major sink of O₂⁺ is dissociative recombination, which produced two energetic atoms. These energetic atoms were first observed in the exosphere of Venus by the PVO ultraviolet spectrometer (UVS) between ~400 – 1600 km [4]. However, these values were derived considering only the direct solar illumination of the oxygen corona above 400 km as the source of O I 1304 emission not the photons resonantly backscattered by the cold oxygen atoms (Venus shine) [5]. In this work, we include the Venus shine.

Model

We use a radiative transfer model, used in the past to study the Martian hot oxygen corona [6]. This model considers spherically symmetric cold and hot oxygen densities and use a Monte Carlo approach to compute the O I 1304 resonant scattering. Test particles, representative of the solar photons in the 130.4 nm triplet [7] are followed inside the Venusian thermosphere and exosphere. The spectral emission volume rate ε(r,λ,i) at the position r and wavelength λ of each line i of the triplet is the sum of five terms:

ε(r,λ,i)=ε_0,c(r,λ,i)+ε_m,c(r,λ,i)+ε_0,h(r,λ,i)+ε_s,h(r,λ,i)+ε_m,h(r,λ,i)

The two first terms are the single scattering and multiple scattering terms of the cold oxygen. The three last terms are the single scattering (photons scattered for the first time by a hot oxygen atom and not before), “shine term” (photons scattered for the first time by a hot oxygen atom but scattered one or several times by a cold oxygen before) and multiple scattering terms (photons scattered several times by a hot oxygen atom) for the hot oxygen.

Results and discussion

The shine term of the hot population, using the density profiles from [4] and normalized by the g-factor is not negligible compared to the single scattering term on the dayside below 2000 km but decreases faster with altitude (Fig. 1)

Fig. 1 (left) Altitude profile for the single-scattering (blue), multiple-scattering (red), and shine from the cold oxygen (green) volume emission rate (normalized by the g-factor) at SZA=0° of the hot oxygen. The hot oxygen density is indicated by the black line. (right) Variations of the same volume emission rates with the solar zenith angle for an altitude near 600 km.

 

Above ~400 km, the volume emission rate of the cold population is negligible and the brightness I(z) along the line of sight can be simplified by

I(z) ≈∑∫∫[ε_0,h(r,λ,i)+ε_s,h(r,λ,i)]dsdλ ≈g_exc∫[1+G_c(s)]n_hot(s)ds,

where G_c is the ratio between the “shine” term and the single scattering term, and n_hot(s) the density of hot oxygen.

The simulated brightness using the hot oxygen density derived by [4] with and without the shine term are compared to the observed brightness, showing that when the shine term is considered, the hot oxygen density derived by [4] is not in agreement with the observations. This hot density must be reduced by ~ 1.6 to agree with the observations when the shine term is considered (Fig. 2)

Figure 2. Simulated brightness vertical profile, when the shine term is neglected (solid green line) and included (solid red line) obtained with the hot oxygen density from [3]. The simulated profile including the shine term but with a hot oxygen density divided by 1.6 is also represented (red dashed line). The PVO-UVS data fit from [4] is represented by the black diamonds and the fit from [5] is represented by the black triangles.

This reduced density underscores the need to revisit previous models of hot oxygen corona [8, 9, 10]. Indeed, as shown by [8], the inclusion of non-elastic collisions can lead to a more efficient thermalization. These authors found that the simulated hot oxygen was underestimated compared to the PVO observations [3]. The new inferred density, including the Venus shine, could help to reconcile this simulation with the observation.

Conclusion

We revised the hot oxygen density in the Venusian upper atmosphere derived from Pioneer Venus Orbiter by using an updated radiative transfer model including the Venusian shine emission. This correction increases the excitation frequency of the hot atoms. Then, the oxygen density needed to reproduce the observations is reduced by 1.6 compared to the past derived density. This reduction requires reassessing key assumptions in exospheric models, particularly the roles of inelastic collisions of the hot oxygen with the atmosphere. Indeed, these collisions would increase the thermalization of the hot oxygen and then reduce their density that could match better the new derived density.

Acknowledgements

JYC is supported by the Programme National de Planétologie (PNP, France) of CNRS-INSU co-funded by CNES and Programme National Soleil Terre (PNST, France) of CNRS-INSU co-funded by CNES and CEA

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