17:15 〜 19:15
[PCG19-P03] Influence of the solar wind on the Venusian hydrogen exosphere observed by Hisaki and VEX/SPICAV
キーワード：金星、水素、ひさき、Venus Express
One of the big questions for Venus is whether Venus had a large amount of water in the past. While the Venusian current atmosphere is extremely dry, isotopic measurements of the D/H ratio suggest that Venus likely retained significant amounts of water in its past. Observations of hydrogen and escape from the Venusian upper atmosphere can provide constraints on these processes. Venus Express observations have shown the presence of hot components in the Venusian hydrogen corona, which plays an important role in hydrogen loss from Venus. The hot component is responsible for atmospheric escape through the ion-pickup process. It was suggested that charge exchange between the cold component and ionospheric or solar wind protons play a significant role in producing the hot component (Chaufray et al. 2012), the response of the hot component to the solar wind is not fully understood and observations are still necessary.
In this study, we investigate how the Venusian hydrogen atmosphere responds to the arrival of solar wind by analyzing Ly-α and Ly-β data obtained from Hisaki and SPICAV (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus) on Venus Express in March 2014. Hisaki observations were conducted from March 9 to April 3, 2014 (P1) and from April 25 to May 23, 2014 (P2). High-speed solar wind arrivals were confirmed in P1 but not in P2. SPICAV data corresponded to the Hisaki observation periods.
Hisaki observations show that when high-speed solar wind from a corotating interaction region (CIR) arrived, the hydrogen column density derived from Ly-α increased by approximately 10% over several days and then remained nearly constant for weeks. In contrast, the column density derived from Ly-β remained nearly unchanged during this period. We selected two SPICAV limb observations: one during the ~10% increase in hydrogen column density observed by Hisaki and another 10 days later. Since each observation had a limited altitude range, we extrapolated the altitude profiles of hydrogen Ly-α brightness for comparison. SPICAV data analysis indicates that between 1,000 km and 4,000 km, Ly-α airglow was up to ~2.5 times brighter on March 16 (immediately after the arrival of the high-speed solar wind) than on March 26. On the other hand, at altitudes above 4,000 km, the brightness was greater on March 26.
One possible explanation for these variations is an increased abundance of high-altitude hot hydrogen due to interactions between the Venusian upper atmosphere and the high-speed solar wind. This result may support the possibility that the ~10% brightening observed by Hisaki was caused by an increase in hot hydrogen with a larger scale height. Although Venus’s exosphere and thermosphere have generally been considered relatively insensitive to solar activity, our findings suggest that solar wind transients and interaction regions influence the production of the hot hydrogen component.
In this study, we investigate how the Venusian hydrogen atmosphere responds to the arrival of solar wind by analyzing Ly-α and Ly-β data obtained from Hisaki and SPICAV (Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Venus) on Venus Express in March 2014. Hisaki observations were conducted from March 9 to April 3, 2014 (P1) and from April 25 to May 23, 2014 (P2). High-speed solar wind arrivals were confirmed in P1 but not in P2. SPICAV data corresponded to the Hisaki observation periods.
Hisaki observations show that when high-speed solar wind from a corotating interaction region (CIR) arrived, the hydrogen column density derived from Ly-α increased by approximately 10% over several days and then remained nearly constant for weeks. In contrast, the column density derived from Ly-β remained nearly unchanged during this period. We selected two SPICAV limb observations: one during the ~10% increase in hydrogen column density observed by Hisaki and another 10 days later. Since each observation had a limited altitude range, we extrapolated the altitude profiles of hydrogen Ly-α brightness for comparison. SPICAV data analysis indicates that between 1,000 km and 4,000 km, Ly-α airglow was up to ~2.5 times brighter on March 16 (immediately after the arrival of the high-speed solar wind) than on March 26. On the other hand, at altitudes above 4,000 km, the brightness was greater on March 26.
One possible explanation for these variations is an increased abundance of high-altitude hot hydrogen due to interactions between the Venusian upper atmosphere and the high-speed solar wind. This result may support the possibility that the ~10% brightening observed by Hisaki was caused by an increase in hot hydrogen with a larger scale height. Although Venus’s exosphere and thermosphere have generally been considered relatively insensitive to solar activity, our findings suggest that solar wind transients and interaction regions influence the production of the hot hydrogen component.