Sakai, Shotaro

写真a

Affiliation

Faculty of Environment and Information Studies (Shonan Fujisawa)

Position

Senior Assistant Professor (Non-tenured)/Assistant Professor (Non-tenured)
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Papers 【 Display / hide 】

  • Magnetic field experiment at Phobos and in space around Mars by the Martian Moons eXploration (MMX) mission

    Ayako Matsuoka, Shoichiro Yokota, Naofumi Murata, Yuki Harada, Shun Imajo, Naoki Terada, Kunihiro Keika, Kei Masunaga, Shotaro Sakai, Hiromu Nakagawa, Kazushi Asamura, Satoshi Kasahara, Yoshifumi Saito

    Progress in Earth and Planetary Science (Springer Science and Business Media LLC)  12 ( 1 )  2025.08

    Accepted

    　View Summary

    Abstract

    The mass spectrum analyzer (MSA) is one of the instruments onboard MMX and observes the interaction between the Martian moons (Phobos and Deimos) and the solar wind as well as the material transport between Mars and its moons. MSA consists of an ion mass spectrum analyzer and a magnetometer. The objective of the magnetometer, MSA-MG, is to measure the magnetic field at the MMX position to trace the motion of the ions. We defined the requirements for the performance of the MSA-MG and designed the instrument to meet them. It is confirmed that MSA-MG as a unit has the required characteristics by the ground performance test and calibration. One of the essential calibration parameters, artificial bias in the data, must be determined by analyzing the flight data. To improve the accuracy of the determined bias, efforts to remove the magnetic noise from other components onboard MMX are essential.

  • Pre-flight performance of the ion energy mass spectrum analyzer for the Martian Moons eXploration (MMX) mission

    Shoichiro Yokota, Ayako Matsuoka, Naofumi Murata, Yoshifumi Saito, Kazushi Asamura, Satoshi Kasahara, Dominique Delcourt, Lina Z. Hadid, Naoki Terada, Kunihiro Keika, Yuki Harada, Hiromu Nakagawa, Kei Masunaga, Shotaro Sakai, Yoshifumi Futaana, Shun Imajo, Kanako Seki, Masaki N. Nishino, Yuki Kitamura

    Progress in Earth and Planetary Science (Springer Science and Business Media LLC)  12 ( 1 )  2025.07

    Accepted

    　View Summary

    Abstract

    An ion energy mass spectrum analyzer was developed for the Martian Moons eXploration (MMX) mission to measure the three-dimensional velocity distribution function and mass profile of low-energy ions around the Mars-Moon system. The hemispheric field-of-view (FOV) is acquired by a pair of angular scanning deflectors, and the energy/charge and mass/charge are determined for each ion by an electrostatic analyzer and a linear-electric-field (LEF) time-of-flight (TOF) analyzer, respectively, with an enhanced mass resolution of $$m/\Delta m\sim 100$$ . The ion analyzer, together with magnetometers, constitutes the mass spectrum analyzer (MSA), one of the scientific instruments on board the MMX spacecraft. This paper describes the instrumentation of the ion analyzer, and results of the performance tests of its flight model (FM).

  • C<sup>+</sup> 133.5 nm Emission Mechanisms on Mars Revealed by the MAVEN Observations

    Shotaro Sakai, Hiromu Nakagawa, Justin Deighan, Sonal K. Jain, Kei Masunaga, Fuminori Tsuchiya, Naoki Terada, Majd Mayyasi, Nicholas M. Schneider, David L. Mitchell, Christian Mazelle, Mehdi Benna, Robert J. Lillis, Go Murakami, Shannon M. Curry, Kanako Seki

    The Astrophysical Journal (American Astronomical Society)  977 ( 2 ) 226 - 226 2024.12

    Lead author, Corresponding author, Accepted,  ISSN  0004-637X

    　View Summary

    Abstract

    C<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> emission because the contribution of electron impact to C<sup>+</sup> 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<sup>+</sup> 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<sup>+</sup> emission process to the brightness more accurately. This suggests that the C<sup>+</sup> and C-atom emissions might be tracers of spatiotemporal variations in the Martian ionosphere and thermosphere.

  • Different Behavior of Density Perturbations Between Dayside and Nightside in the Martian Thermosphere and the Ionosphere Associated With Atmospheric Gravity Waves

    Hiromu Nakagawa, Scott L. England, Aishwarya Kumar, Mehdi Benna, Yuki Harada, Shotaro Sakai, Naoki Terada, Kanako Seki, Nao Yoshida

    Journal of Geophysical Research: Space Physics 129   e2024JA032988 2024.12

    Accepted

  • Comprehensive Comparison of Two Global Multis‐Species MHD Models of Mars

    Wenyi Sun, Ryoya Sakata, Yingjuan Ma, Kanako Seki, Christopher T. Russell, Naoki Terada, Shotaro Sakai, Hiroyuki Shinagawa, David Brain, Gabor Toth

    Earth and Space Science (American Geophysical Union (AGU))  11 ( 10 ) e2024EA003698 2024.10

    Accepted,  ISSN  2333-5084

    　View Summary

    Abstract

    Understanding the interaction between Mars and the solar wind is crucial for comprehending the atmospheric evolution and climate change on Mars. To gain a comprehensive understanding of the Martian plasma environment, global numerical simulations are essential in addition to spacecraft observations. However, there are still discrepancies among different simulation models. This study investigates how these discrepancies stem from the considered physical processes and numerical implementations. We compare two global multispecies MHD models: the “Sun model” based on the BATS‐R‐US code and the “Sakata model” based on a newly developed multifluid model MAESTRO. By employing the same typical upstream conditions and the same neutral atmosphere for current Mars, along with similar numerical implementations such as inner boundary conditions, we obtain simulation results that exhibit unprecedented agreement between the two models. The dayside results are nearly identical, especially along the subsolar line, indicating the robustness of MHD models to predict dayside interaction under given upstream conditions and ionosphere assumptions. The escape rates of planetary ions are also in good agreement. However, discrepancies remain in the terminator and nightside regions. Detailed numerical implementations, including inner boundary conditions, magnetic field divergence control methods, and radial resolutions, are shown to influence certain aspects of the results greatly, such as magnetotail configuration and ion diffusion.

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

  • Study on the atmospheric evolution of planets with Earth-like atmospheres

    2025.04
    -
    2028.03

    基盤研究(C), Principal investigator

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Courses Taught 【 Display / hide 】

  • THEORY OF ENVIRONMENT SENSING TECHNOLOGY

    2025

  • SEMINAR A

    2025

  • PROBABILITY

    2025

  • MASTER SEMINAR

    2025

  • INDEPENDENT RESEARCH

    2025

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