KEIO RESEARCHERS INFORMATION SYSTEM

Career 【 Display / hide 】

Career 【 Display / hide 】

• 2012.04

  -

  2014.03

  Japan Society for the Promotion of Science , Research Fellowship (DC2)

• 2014.04

  -

  2014.06

  Hokkaido Universityu, Faculty of Science, Research Worker

• 2014.06

  -

  2016.09

  University of Kansas, Department of Physics and Astronomy, Postdoctoral Researcher

• 2016.10

  -

  2020.03

  University of Tokyo, Graduate School of Science, Project Researcher

• 2020.04

  -

  2025.03

  Tohoku University, Graduate School of Science, Assistant Professor

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Academic Background 【 Display / hide 】

Academic Background 【 Display / hide 】

• 2005.04

  -

  2009.03

  Hokkaido University, School of Science, Department of Earth Sciences

  University, Graduated

• 2009.04

  -

  2011.03

  Hokkaido University, Graduate School of Science, Department of Cosmosciences

  Graduate School, Completed, Master's course

• 2011.04

  -

  2014.03

  Hokkaido University, Graduate School of Science, Department of Cosmosciences

  Graduate School, Completed, Doctoral course

Academic Degrees 【 Display / hide 】

Academic Degrees 【 Display / hide 】

• Ph.D, Hokkaido University, Coursework, 2014.03

Research Areas 【 Display / hide 】

Research Areas 【 Display / hide 】

• Natural Science / Space and planetary sciences

Papers 【 Display / hide 】

Papers 【 Display / hide 】

• Effects of stellar XUV spectra on atmospheric escape from a Mars-like planet orbiting inactive low-mass stars

  Shotaro Sakai, Akifumi Nakayama, Kanako Seki, Naoki Terada, Hiroyuki Shinagawa, Ryoya Sakata, François Leblanc, David A Brain, Takashi Tanaka

  Monthly Notices of the Royal Astronomical Society 546 ( 4 ) stag156 2026.03

  Lead author, Corresponding author, Accepted

  • Access to Document (DOI)

• Dawn–Dusk Asymmetry of the Io Plasma Torus Derived from Io’s Auroral Footprints Observed by Juno-UVS

  Shinnosuke Satoh, Vincent Hue, Fuminori Tsuchiya, Shotaro Sakai, Yasumasa Kasaba, Hajime Kita, Masato Kagitani, Alessandro Moirano, Bertrand Bonfond, Hiroaki Misawa, Rikuto Yasuda

  The Planetary Science Journal 7 ( 34 )  2026.02

  Accepted

  • Access to Document (DOI)

• 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.

  • Access to Document (DOI)

• 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).

  • Access to Document (DOI)

• C⁺ 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⁺ 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.

  • Access to Document (DOI)

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

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

• New isotope obervations of Mars atmsopheric evolution: International collaboration of European and Japanese Mars exploration missions

  2022.10

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  2027.03

  国際共同研究加速基金(国際共同研究強化(B)), Coinvestigator(s)

• Influence of stellar spectra on the evolution of planetary atmospheres

  2022.04

  -

  2025.03

  基盤研究(C), Principal investigator

Courses Taught 【 Display / hide 】

Courses Taught 【 Display / hide 】

• ACADEMIC PROJECT(CLIMATE CHANGE AND DISASTER RISK GOVERNANCE)

  2026

• PROBABILITY

  2026

• SEMINAR A

  2026

• GRADUATION PROJECT 2

  2026

• THEORY OF ENVIRONMENT SENSING TECHNOLOGY

  2026

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