9:00 AM - 9:15 AM
[PCG21-01] Long-term observations of the EUV oxygen airglow in the upper atmosphere of Venus as observed by Hisaki
Keywords:Venus, Hisaki, Airglow
The upper atmosphere of Venus is mainly composed of atomic oxygen via photodissociation of CO2 and is significantly influenced by solar drivers, such as solar wind and solar radiation, leading to atmospheric escape. Recent studies, however, have demonstrated that the upper atmosphere is also affected by the lower atmospheric activities. The total amount of oxygen atoms in the Venusian thermosphere varies by several percent with a periodicity of approximately 4 days (Masunaga et al., 2015; 2017). Model calculations have suggested that this variation is caused by atmospheric gravity waves that propagates from the lower to the upper atmosphere (Nara et al., 2020).
In recent studies, many observations have shown that wind speeds and mixing ratios of SO2 and H2O in the Venusian cloud layer show long-term variations over hundreds of days to decades (Kouyama et al., 2013, Marcq et al., 2013, Lee et al., 2019, Encrenaz et al., 2023). Such long-term variations in the middle atmosphere could be coupled to the upper atmosphere, and similar time-scale variations could thus exist in the upper atmosphere. However, long-term variations in the upper atmosphere of Venus have not been investigated.
From 2013 to 2023, the Hisaki space telescope intermittently observed Venus, obtaining EUV spectra of its upper atmosphere. By utilizing this dataset, we have been able to monitor long-term variations in oxygen airglow at wavelengths of 1304 Å and 1356 Å, excited by photoelectron impact at an altitude of approximately 130 km.
We discovered that the brightness of the two airglow emissions generally varies with the solar UV radiation flux, except for the period between 2017 and 2021. During this period, both airglow emissions' brightness significantly increased, despite the solar activity being near its minimum. We attribute this anomaly to variations in oxygen abundance in the upper atmosphere during this timeframe. In this presentation, we will discuss possible scenarios to explain these results, based on previous studies relating the atmospheric dynamics and photochemical process that involves SO2 chemistry.
In recent studies, many observations have shown that wind speeds and mixing ratios of SO2 and H2O in the Venusian cloud layer show long-term variations over hundreds of days to decades (Kouyama et al., 2013, Marcq et al., 2013, Lee et al., 2019, Encrenaz et al., 2023). Such long-term variations in the middle atmosphere could be coupled to the upper atmosphere, and similar time-scale variations could thus exist in the upper atmosphere. However, long-term variations in the upper atmosphere of Venus have not been investigated.
From 2013 to 2023, the Hisaki space telescope intermittently observed Venus, obtaining EUV spectra of its upper atmosphere. By utilizing this dataset, we have been able to monitor long-term variations in oxygen airglow at wavelengths of 1304 Å and 1356 Å, excited by photoelectron impact at an altitude of approximately 130 km.
We discovered that the brightness of the two airglow emissions generally varies with the solar UV radiation flux, except for the period between 2017 and 2021. During this period, both airglow emissions' brightness significantly increased, despite the solar activity being near its minimum. We attribute this anomaly to variations in oxygen abundance in the upper atmosphere during this timeframe. In this presentation, we will discuss possible scenarios to explain these results, based on previous studies relating the atmospheric dynamics and photochemical process that involves SO2 chemistry.