村上 豪

IntroductionOne of the outstanding questions regarding Venus is whether the planet once retained a significant amount of water. Observations of hydrogen atoms provide critical insights into atmospheric escape processes. Previous studies using Venus Express/SPICAV indicate that the Venusian hydrogen atmosphere consists of two distinct components characterized by different scale heights: a hot component and a cold component [1]. The hot hydrogen component primarily arises from charge exchange reactions and momentum transfer between cold hydrogen atoms and ionospheric ions [2]. Conversely, the cold component originates from the dissociation of sulfuric acid in the lower atmosphere. It is well known that, due to the absence of an intrinsic magnetic field, Venusian atmosphere interacts directly with the solar wind. However, it remains unclear whether the Venusian hydrogen corona dynamically responds to variations in solar wind conditions.ObservationTo address this question, we analyzed variations in global hydrogen column densities derived from the brightness of resonantly scattered Ly-α (121.6 nm) and Ly-β (102.6 nm) emissions observed by Hisaki[3-5], solar wind velocities and densities measured by ASPERA-4 on Venus Express[6], and solar UV irradiance at Ly-α and Ly-β wavelengths obtained from the Flare Irradiance Spectral Model (FISM) for Planets[7]. The analysis periods spanned March 9 to April 3, 2014 (Period1), and April 25 to May 23, 2014 (Period2). High-speed solar wind events were confirmed during Period1 but not during Period2.ResultWe derived variations in hydrogen column density at altitudes above approximately 310 km and 90 km from the observed Ly-α and Ly-β airglow brightness. Figure 1 shows that after the arrival of high-speed solar wind originating from a corotating interaction region (CIR) in Period1, the hydrogen column density derived from Ly-α increased by approximately 18% within a few days and subsequently remained nearly constant for several weeks. In contrast, the hydrogen column density derived from Ly-β remained relatively stable throughout the same period. Differences between Ly-α and Ly-β brightness suggest an increase in hydrogen atom abundance at higher altitudes during high-speed solar wind events. In Period 2, when no significant increase in both solar wind velocity and density was observed, there was no clear indication of the arrival of a corotating interaction region. During this period, the hydrogen column density remained nearly constant for both Ly-α and Ly-β.Figure1 (a and b)Times series of column densities of Venusian hydrogen atoms derived from Ly-α and Ly-β observed by Hisaki respectively. The red line indicates the 1-day moving average. (c and d) Solar wind velocity and density respectively observed by Venus Express. DiscussionA possible explanation for the observed ~18% variation in Ly-α emission is an increase in high altitude hot hydrogen abundance due to charge exchange reactions and momentum transfer between neutral hydrogen and ionospheric ions. By considering charge exchange between cold hydrogen and ionospheric ions as a production process, and charge exchange between hot hydrogen and the solar wind as a loss process, we estimated the reaction timescales and found consistency with the observed variation. Alternative explanations include an increase in low-altitude cold hydrogen abundance or a rise in hydrogen temperature. These findings provide important implications for understanding non-thermal hydrogen escape mechanisms, thus contributing significantly to our knowledge of the atmospheric evolution of Venus. [1] Chaufray, J. Y., et al., Icarus, 217, 2, 767, 2012[2] Hodges, R. R., and E. L. Breig, Journal of Geophysical Research: Space Physics, 96, 7697, 1991[3] Yoshikawa, I., et al., Space Science Reviews, 184, 237, 2014[4] Yoshioka, K., et al., Planetary and Space Science, 85, 250, 2013[5] Yamazaki, A., et al., Space Science Reviews, 184, 259, 2014[6] Barabash, S., et al., Planetary and Space Science, 55, 12, 1772, 2007[7] Chamberlin, P. C., et al., Space Weather, 6., S05001, 2008

Hisaki is an earth orbiting extreme ultraviolet spectroscope dedicated for observing solar system planets. Thanks to its monitoring capability, Hisaki has carried out unprecedented continuous observation of Io plasma torus, Jovian aurora, and Mars and Venus upper atmosphere since December 2013. One of notable phenomena observed by Hisaki is significant enhancements of neutral gas (sodium and oxygen) from Io occurred in the spring of 2015. Hisaki revealed that not only the plasma source, but transport, heating, and loss processes of magnetospheric plasma were influenced by the variation in the neutral source input. The presentation will include related topics from recent Hisaki publication. Since the autumn of 2016, the Juno spacecraft was in the orbit around Jupiter. Hisaki monitored activities of Jovian aurora and the plasma torus in the Juno era. These datasets will provide opportunities to compare in-situ observation by Juno with the global view by Hisaki. JAXA approved the Hisaki mission period by the end of March 2023. As a future remote observation platform, we are going to propose a UV space telescope, LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly), a Japanese-leading mission using heritages of UV instruments for planetary science (e.g., Hisaki) and space telescope techniques for astronomy. One of goals of this mission is dynamics of our solar system planets and moons as the most quantifiable archetypes of extraterrestrial habitable environments in the universe. Water plume that gushes from the subsurface ocean of Galilean moons and tenuous atmosphere which is generated by bombardment of energetic charged particles to the surface are primary targets of LAPYUTA. As the plume activity and the atmosphere are not stable, continuous monitoring with high spatial resolution is essential. The icy moon's plume and ambient space will be deeply explored with the spacecraft by NASA's and ESA's icy moon missions in 2020s-2030s. The complementary remote sensing by LAPYUTA will visualize their global structure and temporal dynamics.

2021年3月から4月にかけて，あかつきと水星探査機BepiColomboが地球から見て太陽のほぼ反対側を同時に通過する貴重な機会があり，これを活かして2 機が協調して太陽コロナの電波掩蔽観測を実施した．太陽観測衛星ひのでによる太陽表面の観測も同時に行い，コロナ研究のためのユニークなデータセットが得られた．