14:15 〜 14:30
[PCG19-03] Effects of Hot Oxygen Corona on the Ion Escape from Venus-like Planets
キーワード：金星、酸素コロナ、大気散逸、MHDシミュレーション
Since Venus has no significant planetary magnetic field, the fast-flowing solar wind plasma interacts directly with its ionosphere and upper atmosphere. The extended oxygen corona in the exosphere, as well as thermal atomic oxygen in the thermosphere, is a source of ion pickup loss. This ion loss is considered to be the dominant mechanism of atmospheric escape on present-day Venus. Therefore, to understand the atmospheric evolution of Venus-like exoplanets, it is crucial to assess the contribution of the hot oxygen corona to ion escape.
The hot oxygen corona of present-day Venus has been studied for many years by spacecraft observations and numerical simulations for many years (e.g., Nagy et al., 1981; Gröller et al., 2010). However, the structure of its upper atmosphere is strongly influenced by X-ray and extreme ultraviolet (XUV) radiation from the host star. In the past, the Sun is thought to have emitted XUV radiation tens of times more intense than today. Close-in exoplanets in the habitable zones (HZs) of M dwarfs are also expected to experience extreme XUV fluxes. This could mean that the effects of hot oxygen corona on the ion escape could be different from those of present-day Venus.
In this study, we investigated the effects of hot oxygen corona on the ion escape under different XUV environments and stellar wind conditions. We developed a Monte Carlo code to calculate 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). The hot oxygen density was used as input for the multi-species MHD simulation model REPPU-Planets (Terada et al., 2009; Sakata et al., 2022). We assumed a Venus-like atmospheric composition depending on the stellar XUV flux as the input thermosphere based on Kulikov et al. (2007).
Our model successfully reproduces the observed hot oxygen corona observed by the Pioneer Venus Orbiter when using present-day Venus atmospheric conditions as input. The results show that the pressure balance between the ionospheric pressure and the stellar wind dynamic pressure plays a key role in the escape rate dependence on stellar wind conditions. The contribution of hot oxygen corona to the total escape rate decreases under high XUV radiation or low-density stellar wind. This is due to enhanced thermospheric heating, which causes the thermospheric component to dominate over the non-thermal component, and reduced stellar wind interactions, which reduces ionization processes via charge exchange and electron impact ionization. Our results suggest that the hot oxygen corona plays a crucial role in ion escape for close-in exoplanets orbiting inactive M dwarfs
The hot oxygen corona of present-day Venus has been studied for many years by spacecraft observations and numerical simulations for many years (e.g., Nagy et al., 1981; Gröller et al., 2010). However, the structure of its upper atmosphere is strongly influenced by X-ray and extreme ultraviolet (XUV) radiation from the host star. In the past, the Sun is thought to have emitted XUV radiation tens of times more intense than today. Close-in exoplanets in the habitable zones (HZs) of M dwarfs are also expected to experience extreme XUV fluxes. This could mean that the effects of hot oxygen corona on the ion escape could be different from those of present-day Venus.
In this study, we investigated the effects of hot oxygen corona on the ion escape under different XUV environments and stellar wind conditions. We developed a Monte Carlo code to calculate 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). The hot oxygen density was used as input for the multi-species MHD simulation model REPPU-Planets (Terada et al., 2009; Sakata et al., 2022). We assumed a Venus-like atmospheric composition depending on the stellar XUV flux as the input thermosphere based on Kulikov et al. (2007).
Our model successfully reproduces the observed hot oxygen corona observed by the Pioneer Venus Orbiter when using present-day Venus atmospheric conditions as input. The results show that the pressure balance between the ionospheric pressure and the stellar wind dynamic pressure plays a key role in the escape rate dependence on stellar wind conditions. The contribution of hot oxygen corona to the total escape rate decreases under high XUV radiation or low-density stellar wind. This is due to enhanced thermospheric heating, which causes the thermospheric component to dominate over the non-thermal component, and reduced stellar wind interactions, which reduces ionization processes via charge exchange and electron impact ionization. Our results suggest that the hot oxygen corona plays a crucial role in ion escape for close-in exoplanets orbiting inactive M dwarfs