村上 豪

Abstract BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission. Graphical Abstract

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

Remote sensing with ultraviolet wavelength (UV) are one of powerful probes to uncover dynamic behaviors of the planetary environment. The Hisaki satellite was an earth orbiting extreme ultraviolet (EUV) spectroscope dedicated for observing solar system planets. Thanks to its long-term monitoring capability, Hisaki had carried out unprecedented continuous observation of Io plasma torus, Jovian aurora, and Mars and Venus upper atmospheres from 2013 to 2023. One of notable phenomena observed by Hisaki is significant enhancements of neutral gas from presumed activation of volcanic activity on Io. Hisaki revealed, for the first time, 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.After the end of the Hisaki mission, we have proposed the next UV space telescope, LAPYUTA (Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly). 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. LAPYUTA will not only provide a UV monitoring platform like Hisaki but also have a high spatial resolution and high sensitivity to uncover stability of Io’s atmosphere, water plumes that gushes from the subsurface ocean of icy moons, and spatio-temporal aspects of Jupiter's giant UV aurora. Primary goal of the LAPYUTA mission other than the Jovian system includes atmospheric evolution of Venus and Mars, characterization of exoplanet atmosphere, galaxy formation, and time-domain astronomy.

Abstract Although primarily a housekeeping instrument for measuring ambient radiation, the Solar Particle Monitor (SPM) onboard BepiColombo can measure high‐energy particles, making it useful for observing phenomena such as galactic cosmic rays and Solar Energetic Particles (SEPs). However, it only records time‐series data of particle energy loss and counts, which requires characterization by radiation simulation for scientific analysis. In this study, a physical model of the SPM was constructed using the “Geant4” radiation simulation toolkit to investigate its response to charged particles. The probability density functions were derived from the response functions to indicate the proportion of particles in each energy range among the SPM counts. Finally, we inverse‐calculated the flux from the counts in the corresponding energy ranges. We applied this method to data from the terrestrial radiation belt and SEPs in March 2022. The results agreed with the empirical radiation belt model and another instrument onboard BepiColombo, demonstrating the validity of the method. This study highlights the potential for scientific applications of housekeeping instruments and suggests the broader use of similar methods on other missions for expanding inner heliosphere multi‐point exploration.

Abstract C⁺ emission is generated by electron impact, dissociative ionization, photoionization, and resonant scattering with carbon-related atoms, molecules, and ions in the Martian ionosphere and thermosphere. The contribution of each mechanism to the emission, however, has not been elucidated due to the difficulty of observation and the fact that a part of the emission cross section is unclear. The current paper isolates the C⁺ emission mechanism using remote-sensing and in situ observations on board Mars Atmosphere and Volatile EvolutioN. Both electron impact and dissociative ionization/photoionization contribute to C⁺ emission below 150 km altitude when the CO density is high, but only dissociative ionization/photoionization contributes to the emission for the low CO density case, while only dissociative ionization/photoionization dominates the emission at altitudes between 150 and 165 km for both CO density cases. It is difficult to estimate the total flux of suprathermal electrons in the ionosphere from remote-sensing observations of C⁺ emission because the contribution of electron impact to C⁺ emission is small. In contrast, C-atom remote-sensing observations might provide a better understanding of the total flux of suprathermal electrons in the ionosphere than C⁺ emission, and global ultraviolet observations could be utilized as a tool for monitoring the ionosphere. The total flux of suprathermal electrons estimated from C-atom emission may be utilized to isolate the contribution of each C⁺ emission process to the brightness more accurately. This suggests that the C⁺ and C-atom emissions might be tracers of spatiotemporal variations in the Martian ionosphere and thermosphere.