News Release

CALorimetric Electron Telescope (CALET) captures charge-sign dependent cosmic ray modulation

The observations indicate that cosmic ray electron count rates are more strongly influenced by the solar modulation than that of protons

Peer-Reviewed Publication

Waseda University

CALorimetric Electron Telescope (CALET) captures charge-sign dependent cosmic ray modulation.

image: In a new study by the CALET collaboration, researchers from Japan demonstrate that cosmic ray electron count rates are significantly more affected than that of protons due to the drift effect on the long-term solar modulation. view more 

Credit: Yosui Akaike from Waseda University, Japan

The movement of cosmic ray particles across space, such as electrons and protons, is influenced by the Sun's magnetic field, causing fluctuations in the intensity of galactic cosmic rays (GCRs) reaching Earth in response to the solar cycle. During periods of low solar activity, such as the solar minimum, more GCRs have been observed to reach Earth compared to that for periods of high solar activity. This inverse correlation between the GCR-flux and solar activity is known as “solar modulation.”

Specifically, the intensity of GCRs on Earth is affected by the tilt angle of the heliospheric current sheet (HCS), a spiral surface separating the direction of oppositely directed magnetic field lines originating from the poles of the Sun. As the tilt angle of the HCS increases, the intensity of cosmic rays on Earth decreases. According to the drift model of GCR transport in the heliosphere, the negatively charged electrons in GCRs tend to travel along the HCS to reach Earth if the magnetic field is directed away from the Sun in the northern hemisphere, and towards the Sun in the southern hemisphere. In contrast, the positively charged protons reach Earth from the heliospheric polar region, suggesting that GCR electrons are more affected by solar modulation than the protons as they travel through the HCS to reach Earth.

While previous observations of cosmic ray particles made aboard space balloons and in space experiments show differences between the fluxes of positively and negatively charged GCR particles during the solar cycle, it is unclear whether the particle charge plays any role in the anticorrelation between GCR intensity and the tilt angle of the HCS. Now, in a recent observation of GCR charged particles made with the CALorimetric Electron Telescope (CALET) onboard the International Space Station’s “Kibo” Exposed Facility (EF) over a period of six years, researchers have revealed that this anticorrelation is, in fact, more prominent for electrons than for protons.

The study, published in Volume 130, Issue 21 of the Physical Review Letters journal on May 25, 2023, was co-led by three researchers from Japan, Associate Professor Yosui Akaike of the Waseda Research Institute for Science and Engineering (RISE) at Waseda University, Associate Professor Shoko Miyake of the National Institute of Technology (KOSEN) at Ibaraki College, and Professor Kazuoki Munakata of Shinshu University. It also included contributions from Professor Emeritus Shoji Torii from RISE. “Using CALET, we successfully observed a charge-sign dependent solar modulation of GCRs over six years,” says Akaike.

The researchers analyzed over 0.77 million electrons and 1.26 million protons collected in about 196 and 197 hours, respectively, over a six-year period from 2015 to 2021, which coincided with the end of solar cycle 24 and the beginning of solar cycle 25, the current solar cycle. The findings indicated that during the low activity state of the Sun towards the end of solar cycle 24, characterized by a reduction in the number of sunspots and HCS tilt angle, both electron and proton count rates were low but gradually increasing. This trend continued with the onset of solar cycle 25, reaching its peak in electron count rate six months after the beginning of the cycle in December 2019.

Thereafter, both electron and proton count rates gradually decreased as the Sun's activity and HCS tilt angle increased. Furthermore, the results showed that the variation in the count rates of electrons was significantly higher than that of protons during this period, suggesting that electrons are more susceptible to the effects of solar modulation, as predicted by the drift model.

“This is a clear signature of the drift effect dominating the long-term solar modulation of GCRs observed with a single detector,” highlights Akaike.

Overall, analyzing GCRs can shed important light on the composition of the universe and the acceleration mechanisms of high-energy particles observed in cosmic rays. Thus, the observations made by CALET could help better understand the space weather and its effects on the possibility of potential life on the Moon and other planets, like Mars.




Authors: O. Adriani,1, 2 Y. Akaike,3, 4 K. Asano,5 Y. Asaoka,5 E. Berti,2, 6 G. Bigongiari,7, 8 W.R. Binns,9 M. Bongi,1, 2 P. Brogi,7, 8 A. Bruno,10 J.H. Buckley,9 N. Cannady,11, 12, 13 G. Castellini,6 C. Checchia,7, 8 M.L. Cherry,14 G. Collazuol,15, 16 G.A. de Nolfo,10 K. Ebisawa,17 A. W. Ficklin,14 H. Fuke,17 S. Gonzi,1, 2, 6 T.G. Guzik,14T. Hams,11 K. Hibino,18 M. Ichimura,19 K. Ioka,20 W. Ishizaki,5 M.H. Israel,9 K. Kasahara,21 J. Kataoka,22R. Kataoka,23 Y. Katayose,24 C. Kato,25 N. Kawanaka,20 Y. Kawakubo,14 K. Kobayashi,3, 4 K. Kohri,26 H.S. Krawczynski,9 J.F. Krizmanic,12 P. Maestro,7, 8 P.S. Marrocchesi,7, 8 A.M. Messineo,8, 27 J.W. Mitchell,12 S. Miyake,28 A.A. Moiseev,12, 13, 29 M. Mori,30 N. Mori,2 H.M. Motz,31 K. Munakata,25 S. Nakahira,17 J. Nishimura,17 S. Okuno,18 J.F. Ormes,32 S. Ozawa,33 L. Pacini,2, 6 P. Papini,2 B.F. Rauch,9 S.B. Ricciarini,2, 6 K. Sakai,11, 12, 13 T. Sakamoto,34 M. Sasaki,12, 13, 29 Y. Shimizu,18 A. Shiomi,35 P. Spillantini,1 F. Stolzi,7, 8 S. Sugita,34 A. Sulaj,7, 8 M. Takita,5 T. Tamura,18 T. Terasawa,5 S. Torii,3 Y. Tsunesada,36, 37 Y. Uchihori,38 E. Vannuccini,2 J.P. Wefel,14 K. Yamaoka,39 S. Yanagita,40 A. Yoshida,34 K. Yoshida,21 and W. V. Zober9




1Department of Physics, University of Florence, Via Sansone, 1 - 50019, Sesto Fiorentino, Italy

2INFN Sezione di Firenze, Via Sansone, 1 - 50019, Sesto Fiorentino, Italy

3Waseda Research Institute for Science and Engineering, Waseda University, 17 Kikuicho, Shinjuku, Tokyo 162-0044, Japan

4JEM Utilization Center, Human Spaceflight Technology Directorate,

Japan Aerospace Exploration Agency, 2-1-1 Sengen, Tsukuba, Ibaraki 305-8505, Japan

5Institute for Cosmic Ray Research, The University of Tokyo, 5-1-5 Kashiwa-no-Ha, Kashiwa, Chiba 277-8582, Japan

6Institute of Applied Physics (IFAC), National Research Council (CNR),

Via Madonna del Piano, 10, 50019, Sesto Fiorentino, Italy

7Department of Physical Sciences, Earth and Environment,

University of Siena, via Roma 56, 53100 Siena, Italy

8INFN Sezione di Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3 - 56127 Pisa, Italy

9Department of Physics and McDonnell Center for the Space Sciences,

Washington University, One Brookings Drive, St. Louis, Missouri 63130-4899, USA

10Heliospheric Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

11Center for Space Sciences and Technology, University of Maryland,

Baltimore County, 1000 Hilltop Circle, Baltimore, Maryland 21250, USA

12Astroparticle Physics Laboratory, NASA/GSFC, Greenbelt, Maryland 20771, USA

13Center for Research and Exploration in Space Sciences and Technology, NASA/GSFC, Greenbelt, Maryland 20771, USA

14Department of Physics and Astronomy, Louisiana State University,

202 Nicholson Hall, Baton Rouge, Louisiana 70803, USA

15Department of Physics and Astronomy, University of Padova, Via Marzolo, 8, 35131 Padova, Italy

16INFN Sezione di Padova, Via Marzolo, 8, 35131 Padova, Italy

17Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo, Sagamihara, Kanagawa 252-5210, Japan

18Kanagawa University, 3-27-1 Rokkakubashi, Kanagawa, Yokohama, Kanagawa 221-8686, Japan

19Faculty of Science and Technology, Graduate School of Science and Technology, Hirosaki University, 3, Bunkyo, Hirosaki, Aomori 036-8561, Japan

20Yukawa Institute for Theoretical Physics, Kyoto University,

Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto, 606-8502, Japan

21Department of Electronic Information Systems, Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan

22School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

23National Institute of Polar Research, 10-3, Midori-cho, Tachikawa, Tokyo 190-8518, Japan

24Faculty of Engineering, Division of Intelligent Systems Engineering,

Yokohama National University, 79-5 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan

25Faculty of Science, Shinshu University, 3-1-1 Asahi, Matsumoto, Nagano 390-8621, Japan

26Institute of Particle and Nuclear Studies, High Energy Accelerator

Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan

27University of Pisa, Polo Fibonacci, Largo B. Pontecorvo, 3 - 56127 Pisa, Italy

28Department of Electrical and Electronic Systems Engineering,

National Institute of Technology (KOSEN), Ibaraki College,

866 Nakane, Hitachinaka, Ibaraki 312-8508, Japan

29Department of Astronomy, University of Maryland, College Park, Maryland 20742, USA

30Department of Physical Sciences, College of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan

31Faculty of Science and Engineering, Global Center for Science and Engineering,

Waseda University, 3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan

32Department of Physics and Astronomy, University of Denver, Physics Building, Room 211, 2112 East Wesley Avenue, Denver, Colorado 80208-6900, USA

33Quantum ICT Advanced Development Center, National Institute of Information and Communications Technology, 4-2-1 Nukui-Kitamachi, Koganei, Tokyo 184-8795, Japan

34College of Science and Engineering, Department of Physics and Mathematics, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo, Sagamihara, Kanagawa 252-5258, Japan

35College of Industrial Technology, Nihon University, 1-2-1 Izumi, Narashino, Chiba 275-8575, Japan

36Graduate School of Science, Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

37Nambu Yoichiro Institute for Theoretical and Experimental Physics,

Osaka Metropolitan University, Sugimoto, Sumiyoshi, Osaka 558-8585, Japan

38National Institutes for Quantum and Radiation Science and Technology, 4-9-1 Anagawa, Inage, Chiba 263-8555, Japan

39Nagoya University, Furo, Chikusa, Nagoya 464-8601, Japan

40College of Science, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan


About Waseda University

Located in the heart of Tokyo, Waseda University is a leading private research university that has long been dedicated to academic excellence, innovative research, and civic engagement at both the local and global levels since 1882. The University has produced many changemakers in its history, including nine prime ministers and many leaders in business, science and technology, literature, sports, and film. Waseda has strong collaborations with overseas research institutions and is committed to advancing cutting-edge research and developing leaders who can contribute to the resolution of complex, global social issues. The University has set a target of achieving a zero-carbon campus by 2032, in line with the Sustainable Development Goals (SDGs) adopted by the United Nations in 2015.

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About Researcher (Associate Professor) Yosui Akaike from Waseda University

Yosui Akaike is a Researcher (Associate Professor) at the Faculty of Science and Engineering at the Waseda Research Institute for Science and Engineering (RISE) at Waseda University in Japan. His research interests include experimental studies related to particle physics, nuclear physics, cosmic rays, and astrophysics. He has published 85 research papers so far, which have been cited over 900 times.


About Associate Professor Shoko Miyake

Shoko Miyake is an Associate Professor at the National Institute of Technology (KOSEN) at Ibaraki College in Japan. She completed her Ph.D. from the Graduate School of Science and Engineering at Ibaraki College. Her research areas include theoretical studies related to particle physics, nuclear physics, cosmic rays, and astrophysics. She has authored 37 research papers and undertaken six research projects.


About Professor Kazuoki Munakata

Kazuoki Munakata is a Professor Emeritus and Professor at the Faculty of Science at Shinshu University in Japan. He has published over 380 research articles, which have been cited around 3,800 times. His current research work includes the study of heliospheric physics and space weather via observations of high-energy galactic cosmic rays with space probes.

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