太陽系科学研究系
基本情報
経歴
5-
2026年4月 - 現在
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2017年4月 - 現在
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2014年4月 - 2017年3月
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2011年4月 - 2014年3月
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2008年4月 - 2011年3月
学歴
3-
2008年4月 - 2011年3月
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2006年4月 - 2008年3月
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2002年4月 - 2006年3月
主要な受賞
8-
2011年3月
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2011年3月
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2008年3月
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2007年9月
論文
160-
Journal of Geophysical Research: Space Physics 131(5) 2026年4月30日Abstract We studied a coronal mass ejection (CME) structure that propagated through the inner heliosphere in October 2021, which was observed by BepiColombo, Solar Orbiter, PSP, STEREO‐A, and DSCOVR spacecraft located at different radial distances and heliospheric longitudes. Notably, four of the five spacecraft detected a dip‐like signature in the magnetic field embedded within the CME. We investigated the cause of the dip‐like signature on the basis of both in situ solar wind data and solar surface data. We examined the flare‐triggering structure of the M1.6 flare, which probably caused the CME; however, there was no conclusive evidence found linking the flare to the dip‐like signature observed in the ejected CME. We applied the minimum variance analysis (MVA) to the in situ magnetic field data to determine whether the dip‐like signature could represent a planar magnetic structure (PMS) embedded in the CME‐driven shock sheath. Our MVA results confirmed that the magnetic field associated with the dip‐like structure lies along a plane perpendicular to the minimum variance direction, consistent with the characteristics of the PMS previously reported in the literature. Additionally, we examined the shock parameters and found that the CME‐driven shock developed more rapidly than usual, probably because of the passage of a stream interaction region (SIR) during the initial phase of its propagation. This fact suggests that the interaction between the CME and the SIR played a significant role in the formation of the PMS, resulting in the observation of dip‐like structure downstream at most of the spacecraft.
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Earth, Planets and Space 78(1) 2026年3月16日Abstract We analyze a unique solar energetic particle event observed simultaneously by the BepiColombo and STEREO-A spacecraft on March 30, 2022. The two spacecraft at heliocentric distances of 0.6 and 1.0 AU are expected to be aligned approximately along the same magnetic field line, providing a valuable opportunity to investigate particle transport processes in the inner heliosphere. Protons with energies above 1.0 MeV exhibit velocity dispersion during the rise phase, suggesting that the energetic particles are produced close to the Sun, possibly associated with a coronal mass ejection. In contrast, protons during the decay phase are characterized by long-lasting time profiles with longer time scales at 1.0 AU than at 0.6 AU, suggesting that the particles deviate from ballistic propagation. By assimilating these multi-spacecraft observation data into numerical simulations of the focused transport equation, for the first time, we estimate the mean free path parallel to the magnetic field as a time series. The inferred mean free path decreases over time and approaches around 0.5–1.0 AU at the STEREO-A location during the decay phase, suggesting an increasing influence of scattering on particle transport. This interpretation is qualitatively supported by independent STEREO-A observations that showed increasing magnetic field fluctuations, suggesting the connection between the particle transport and the local field fluctuations. However, only a fraction of these fluctuations is expected to contribute to particle scattering, which may be due to the multidimensional nature of magnetic field fluctuations. Graphical abstract
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2026年3月14日Life-environmentology, Astronomy, and PlanetarY Ultraviolet Telescope Assembly (LAPYUTA) is a future ultraviolet (UV) space telescope that is selected as a candidate for JAXA's 6th M-class mission. Launch is planned for the early 2030s. LAPYUTA will perform spectroscopic and imaging observations in the far-ultraviolet spectral range (110-190 nm) with a large effective area (>300 cm2) and a high spatial resolution (0.1 arcsec). LAPYUTA has the following four objectives: (1) atmospheres of solar system planets, (2) atmospheres of exoplanets around the habitable zone, (3) structures of present-day galaxies, and (4) synthesis process of heavy elements from observations of neutron star mergers. The key to addressing these scientific goals is the measurement of the physical state of hydrogen, oxygen, and carbon. These elements are common in the universe and are involved in understanding the structure and evolution of the universe at various spatial scales, from planets to stars to galaxies, and UV spectral measurement is adequate for measuring the physical state of the elements. LAPYUTA aims to achieve resolution and sensitivity in the far-UV wavelength range comparable to the Hubble Space Telescope (HST) while using JAXA’s small scientific satellite. The mission part consists of a Cassegrain telescope with a 60 cm aperture primary mirror, four focal plane instruments, a medium dispersion spectrograph (MRS), a high dispersion spectrograph (HRS), a UV slit imager (UVSI), and a wide-field fine guide sensor (FGS). To achieve a highly effective area and high angular resolution, we are developing three key technologies: UV mirror deposition, a large high-precision detector, and a pointing disturbance correction function, as well as studying the concept of the telescope structure. The key technologies for ultraviolet observations developed here will serve as a stepping stone for Japan's participation in the Habitable Worlds Observatory (HWO).
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The Astrophysical Journal 997(1) 2-2 2026年1月13日 査読有り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.
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NATURE COMMUNICATIONS 17(1) 2026年1月1日 査読有り
MISC
135-
Abstract EPSC-DPS Joint Meeting 2025 2025年7月9日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
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Proceedings of the SPIE 13093 2024年8月21日
共同研究・競争的資金等の研究課題
13-
日本学術振興会 科学研究費助成事業 2024年4月 - 2028年3月
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日本学術振興会 科学研究費助成事業 2023年4月 - 2026年3月
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日本学術振興会 科学研究費助成事業 国際共同研究加速基金(国際共同研究強化(B)) 2020年10月 - 2024年3月
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日本学術振興会 科学研究費助成事業 基盤研究(B) 2020年4月 - 2023年3月
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宇宙航空研究開発機構 搭載機器基礎開発研究費 2020年5月 - 2021年3月
