Thermal Structure of Venus’s Upper Atmosphere (90–110 km) Derived from Radio Occultation Data

Understanding the dynamics of the Venusian atmosphere in the 90–110 km altitude range is critical for investigating the coupling processes between the neutral atmosphere and the lower ionosphere. Recent studies suggest that near-infrared heating due to CO₂ absorption at the subsolar point leads to significant temperature enhancements in this region ( Gilli et al., 2017). However, direct observations remain limited.

In this study, we present a new approach to estimate the neutral temperature in the atmospheric-ionospheric transition region of Venus. Assuming the atmospheric composition up to 110 km is primarily dominated by CO₂ and N₂, and that the atmosphere is in hydrostatic equilibrium, we retrieve temperature profiles from radio occultation data. As this shows a significant presence of plasma, its contributions to the radio signal are mitigated using dual-frequency measurements for Venus Express (VEX) data, while for single-frequency observations, a one-dimensional photochemical model (1D-PCM, Ambili et al. 2024) is employed to simulate and subtract the plasma effect. A boundary temperature of 200 K and 240 K at 110 km altitude is assumed based on previous model predictions (Navarro et al., 2021; Ponder et al., 2024), and the temperature profile is subsequently derived using the hydrostatic framework (Imamura et al., 2017).

Figure: The purple curve represents the result from the standard retrieval method, while the green and red curves correspond to outcomes from the newly proposed approach.

Our preliminary results indicate a temperature peak of approximately 330 K near 105 km altitude, with an uncertainty of ~50 K due to the choice of boundary conditions and frequency residual processing. Although the current method and results are early-stage, they show promising consistency with solar occultation observation by VEX (Mahieux et al. 2023), and further refinements will be presented at the conference.

Acknowledgment: This work was supported by the Japan Society for the Promotion of Science (JSPS). We thank Prof. B. Hauesler, the principal investigator of, Venus Express Radio Science payload, for archiving the frequency residual data sets. We also acknowledge the effort of the Akatsuki radio science team in Japan and India for making the dataset available to us.

 

References:

• Gilli, G., Lebonnois, S., González-Galindo, F., López-Valverde, M.A., Stolzenbach, A., Lefèvre, F., Chaufray, J.Y., Lott, F., 2017. Thermal structure of the upper atmosphere of Venus simulated by a ground-to-thermosphere GCM. Icarus 281, 55–72. http://dx.doi.org/10.1016/j.icarus.2016.09.016.

• Imamura, T., Ando, H., Tellmann, S., Pätzold, M., Häusler, B., Yamazaki, A., Sato, T.M., Noguchi, K., Futaana, Y., Oschlisniok, J. and Limaye, S., 2017. Initial performance of the radio occultation experiment in the Venus orbiter mission Akatsuki. Earth, Planets and Space, 69, pp.1-11.

• Ambili, K.M., Choudhary, R.K. and Tripathi, K.R., 2024. Exploring sensitivity: Unveiling the impact of input parameters on Venus ionosphere V2 layer characteristics. Icarus, 408, p.115839.

• Navarro, T., Gilli, G., Schubert, G., Lebonnois, S., Lefèvre, F. and Quirino, D., 2021. Venus’ upper atmosphere revealed by a GCM: I. Structure and variability of the circulation. Icarus, 366, p.114400.

• Ponder, B.M., Ridley, A.J., Bougher, S.W., Pawlowski, D. and Brecht, A., 2024. The Venus global ionosphere‐thermosphere model (V‐GITM): A coupled thermosphere and ionosphere formulation. Journal of Geophysical Research: Planets, 129(7), p.e2023JE008079.