Oxygen airglow in the Venusian upper atmosphere: High-spectral resolution spectroscopy using WINERED

The upper atmosphere of Venus still leaves many unsolved questions. Even the thermal structure, a fundamental physical quantity of planetary atmospheres, is subject to limited observational knowledge and, moreover, there is a variation of several tens of Kelvins at altitudes above ~80 km in the results measured by Venus Express instruments and ground-based observations [e.g., 1]. This disagreement remains to be discussed whether it is indicative of actual spatial/temporal variability or whether it includes systematic biases from one instrument to another. The first and foremost challenge is to increase the number of more observational data.

To address this issue, we focus on oxygen (O₂) airglow emitted at an altitude of around 95 km in the Venusian atmosphere and derive the O₂ rotational temperature from its high-dispersion spectra to constrain the thermal structure at this altitude. Although observations of Venusian O₂ airglow itself have been conducted for many years [e.g., 2 - 7], most of the previous studies analyzed the spatial distribution of the emission intensity using spectrometers with relatively low or moderate spectral resolutions. There are only a limited number of studies in which the rotational temperature was derived with high-dispersion spectroscopy. Most of them were conducted with spectrometers at that time, which had a narrow instantaneous spectral coverage, and only a few lines of the O₂ airglow were acquired simultaneously.

A new and sensitive Venusian O₂ airglow observation became a reality with the recent installation of a new instrument, named WINERED, on the Magellan Clay 6.5-m telescope in the Las Campanas Observatory, Chile. WINERED is a cross-dispersed echelle spectrograph with a very high throughput [8]. It achieves high spectral resolution (resolving power of 28,000 or 70,000) with a sufficiently wide instantaneous wavelength coverage that captures the entire O₂ emission lines at the 1.27-micron band. This should significantly improve the precision and accuracy of the rotational temperature derivation. Needless to say, the high spatial resolution of the Magellan telescope and the high sensitivity of WINERED are beneficial for mapping the O₂ rotational temperature.

To assess the performance of Magellan/WINERED for Venusian O₂ observations with the aim of realizing long-term continuous observations in the future, we carried out pilot observations in June and November 2023 when Venus was in favorable positions for the airglow observation. It was confirmed that spectra of O₂ airglow emission can be obtained with sufficient quality for scientific analysis, although stray light contaminations from the dayside of Venus exist. The stray light from the dayside can be quantitatively evaluated by using the CO₂ absorption lines near 1.21 micron, a wavelength range where the emission from the Venus nightside should not be detected. This talk will discuss the results of these initial observations and our plan for future continued observations.

 

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