09:15 〜 09:30
[PCG21-02] Effects of XUV radiation on hot oxygen corona of Venus-like planets
キーワード：金星、酸素コロナ
Since Venus has no substantial planetary magnetic field, the fast-flowing solar wind plasma interacts directly with its ionosphere and upper atmosphere. Venus' extended oxygen corona in the exosphere, as well as thermal atomic oxygen in the thermosphere, is a source of the ion loss. Ionized oxygen loss is thought to be the main mechanism of atmospheric escape at current Venus. Therefore, to understand the atmospheric evolution of a Venus-like planet, it is necessary to understand how much hot oxygen corona contributes to the ion escape.
Venus' exosphere has been studied based on spacecraft observations and numerical simulations for many years (e.g., Nagy et al., 1981; Gröller et al., 2010). These studies focus on current Venus. However, the structure of its upper atmosphere is strongly influenced by X-ray and extreme ultraviolet (XUV) radiation from the host star (Sun). In the past, the Sun is thought to be emitted XUV radiation that was tens of times stronger than it is today. This could mean that the structure of the corona of ancient Venus was different from current Venus.
In this study, we investigated the density distribution of the hot oxygen corona of Venus-like atmospheres under different XUV environments to evaluate the role of the hot oxygen corona in the atmospheric escape. We have developed a Monte Carlo code for calculating the hot oxygen transport in the thermosphere. The hot oxygen density above the exobase is also calculated by using Liouville's equation (Schunk and Nagy, 2009). We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere based on Nakayama et al. (2022). The stellar XUV flux is set between 1 and 50 times the current Venus value.
We found a good agreement of the model results, which use the atmospheric composition of current Venus as input to the transport code, with the observations of the hot oxygen corona by the Pioneer Venus Orbiter. We also found that the contribution of the oxygen corona can be ignored under strong XUV conditions because of the enhanced thermospheric temperature. This is because the thermosphere expands due to the temperature increase, and the production of hot oxygen decreases near the exobase altitude. In the presentation, effects of XUV radiation on the structure of the hot oxygen corona and ion escape will be also reported in detail.
Venus' exosphere has been studied based on spacecraft observations and numerical simulations for many years (e.g., Nagy et al., 1981; Gröller et al., 2010). These studies focus on current Venus. However, the structure of its upper atmosphere is strongly influenced by X-ray and extreme ultraviolet (XUV) radiation from the host star (Sun). In the past, the Sun is thought to be emitted XUV radiation that was tens of times stronger than it is today. This could mean that the structure of the corona of ancient Venus was different from current Venus.
In this study, we investigated the density distribution of the hot oxygen corona of Venus-like atmospheres under different XUV environments to evaluate the role of the hot oxygen corona in the atmospheric escape. We have developed a Monte Carlo code for calculating the hot oxygen transport in the thermosphere. The hot oxygen density above the exobase is also calculated by using Liouville's equation (Schunk and Nagy, 2009). We assumed a Venus-like atmospheric composition that depends on the stellar XUV flux as the input neutral atmosphere based on Nakayama et al. (2022). The stellar XUV flux is set between 1 and 50 times the current Venus value.
We found a good agreement of the model results, which use the atmospheric composition of current Venus as input to the transport code, with the observations of the hot oxygen corona by the Pioneer Venus Orbiter. We also found that the contribution of the oxygen corona can be ignored under strong XUV conditions because of the enhanced thermospheric temperature. This is because the thermosphere expands due to the temperature increase, and the production of hot oxygen decreases near the exobase altitude. In the presentation, effects of XUV radiation on the structure of the hot oxygen corona and ion escape will be also reported in detail.