World first: Probe of Solar Magnetic Field Intensity Using High-energy Cosmic Rays and the "Cosmic-ray Shadow" of the Sun
The followings are the findings from research by Yoshiaki Nakamura, in the third year of the doctoral program at the Interdisciplinary Graduate School of Science and Technology, that were published in Physical Review Letters, the journal of the American Institute of Physics. In this research, a Japan-China joint experiment conducted on the Tibet Plateau in China, in which Professor Kazuoki Munakata, and Associate Professor Chihiro Kato of the Faculty of Science participated, revealed the solar magnetic field intensity using the Sun's shadow of high-energy cosmic rays for the first time in the world.
Because high-energy cosmic rays (cosmic radiation) coming from space to Earth travel almost in a straight line between the Sun and Earth, cosmic rays coming from behind the Sun are blocked by the Sun. Therefore, the number of cosmic rays coming from the direction of the Sun is smaller if observed on Earth (figure 1). This is the Sun’s shadow. The preceding studies indicated that the smaller number of cosmic rays, and the center position of the Sun’s shadow, are affected by the solar magnetic field. This is because cosmic rays are electrically charged particles composed mainly of protons, and the direction of motion is curved when cosmic rays are moving in the solar magnetic field. Solar magnetic field intensity can be revealed by closer examination of the Sun’s shadow. Presently the solar magnetic field has been scrutinized using satellites. However observation with these tools can only clarify the magnetic field at one point in the vast Universe. There is no method to obtain information about solar magnetic fields of wider areas. The Sun’s shadow provides a new observation method to complement this aspect.
The direction of motion of cosmic rays, particles with a positive charge, is slightly curved when they move in a magnetic field. The direction is decided by the direction of the magnetic lines. In the space between the Sun and Earth, a magnetic sector in which solar magnetic lines going away from the Sun (away), and a magnetic sector in which the solar magnetic line is going toward the Sun (toward), are observed alternately in accordance with the Sun’s rotation. Therefore, the cosmic rays moving in such areas are curved in a reversed manner. As a result, the center position of the Sun’s shadow seen from Earth is observed a little northward or southward depending on the area the cosmic ray has passed through (figure 1). Yoshiaki Nakamura obtained the value of the difference from the observation data in a quantitative way, and revealed that the absolute value for the difference is, as expected from the orbit of charged particles curved by the magnetic field, inversely proportional to the cosmic rays (figure 2). He also discovered that the absolute value of the observed difference is about one and a half times as large as the expected value obtained from simulations, and as a result, revealed that the solar magnetic field model, widely used today, underestimates the magnetic field intensity between the Sun and Earth. This is a world first achievement successfully proving that the Sun’s shadow can be used to estimate solar magnetic field intensities in a quantitative way.
He concluded that the reason for simulations underestimating the magnetic field intensity is the underestimation of the open magnetic flux, which is pulled out into interplanetary space by the solar wind. Open magnetic flux is known as an important physical parameter that governs the number of cosmic rays observed near Earth. The conclusion of this paper has a significant impact on the academic world also from this aspect.
The research team will further improve experimental equipment to increase the accuracy of observations, and will continue their detailed observation of the Sun’s shadow.
* The followings are the institutes participating in the Japan-China joint experiment (in the order written in the paper).
1.Department of Physics, Hirosaki University
2.Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences
3.National Astronomical Observatories, Chinese Academy of Sciences
4.Department of Mathematics and Physics, Tibet University
5.Department of Physics, Hebei Normal University
6.Department of Physics, Shandong University
7.Institute of Modern Physics, SouthWest Jiaotong University
8.Faculty of Engineering, Kanagawa University
9.Faculty of Education, Utsunomiya University
10.Department of Physics, Konan University
11.Research Institute for Science and Engineering, Waseda University
12.Faculty of Engineering, Yokohama National University
13.Department of Physics, Shinshu University
14.Institute for Cosmic Ray Research, University of Tokyo
15.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency
16.National Center for Space Weather, China Meteorological Administration
17.School of Information Science and Engineering, Shangdong Agricultural University
19.National Institute of Informatics
20.Sakushin Gakuin University
21.College of Science, China University Of Petroleum
22.Tokyo Metropolitan College of Industrial Technology
23.Max-Planck-Institut für Physik
24.Escuela de Ciencias Físicas y Nanotechnologia, Yachay Tech
25.College of Industrial Technology, Nihon University
26.Shonan Institute of Technology
27.Japan Atomic Energy Agency