村上 豪

Abstract Interplanetary coronal mass ejections (ICMEs) cause “Forbush decreases” (FDs), which are local decreases in background galactic cosmic rays (GCRs). Even though FDs can be observed with simple particle instruments, their amplitude and shape provide physical profiles of passing ICMEs. However, in some cases, previous statistical studies of the heliocentric distance dependence of FD changes associated with ICME propagation have found no strong correlation. We need the criteria for evaluating the relationship between ICME structure and FDs, necessary for the FD’s statistical analysis. This study investigates the effect of the evolution and interactions of ICMEs on FD profiles in the inner solar system using multipoint comparisons. We focus on multipoint ICME observations by Solar Orbiter, BepiColombo, and near-Earth spacecraft from 2022 March 10 to 16, when these spacecraft were ideally located for studying the radial and longitudinal evolution of ICMEs and accompanying FDs. We compared GCR variations with the multiple in situ data and ICME model, clarifying the correspondence between the evolution of each ICME structure in the radial and azimuthal directions and the depth and gradients of the FD. The radial comparison revealed decreases in FD intensities and gradients associated with the expansion of the ICME. The longitudinal difference found in FD intensity indicates longitudinal variations of the ICME’s shielding effect. These results suggest that accurate multipoint FD comparisons require determining the relationship between the observer’s position and the inner structure of the passing ICMEs.

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