A research group including Professor Kazuki Munakata (Physics) presented the results of their research into the origin of cosmic ray fluctuations associated with solar activity.
2023/5/26
An international collaborative research group, including Project Professor Munakata Kazuki of the Physics Course, has revealed that a physical process known as "particle drift" plays a major role in the "push back" (Note 1).
This international collaborative research project promotes cosmic ray observations using the CALorimetric Electron Telescope (CALET) onboard the International Space Station, and the results of this research were jointly announced by Waseda University, Ibaraki National College of Technology, and Shinshu University (see the details page below for details).
Cosmic rays are radiation (mainly positively charged protons) that travel through space, and those with particularly high energy are born in our galaxy and travel to Earth. The amount of cosmic rays observed on Earth decreases when the number of sunspots increases and solar activity becomes active, and increases when solar activity declines. A wind of plasma called the solar wind is constantly spewing outward from the Sun, and this wind pulls the Sun's magnetic field out into space. Since charged cosmic rays move by wrapping around the magnetic field, the cosmic rays are pushed back outward by the solar wind. However, the physical process that causes this "push back" was not well understood.
Therefore, the research group focused on the fact that the direction of particle drift is reversed even if the particle charge is positive or negative, and simultaneously observed the amount of negatively charged electrons and positively charged protons with CLAET (cosmic rays contain a few percent of electrons). This allows us to verify the drift effect without being affected by changes in solar activity that differ from one activity cycle to another. The blue and red circles in the lower figure of Figure 1 below represent the time changes in the observed amount of electrons and protons, respectively. The observation period was about six years. From this figure, we can see that during the solar minimum in 2020, the amount of electrons shows a sharp peak, while the amount of protons shows a gentle peak. We attempted to reproduce this observational data using a model of the cosmic ray transport process that takes into account the particle drift effect. The blue and red lines in the figure are the reproduction results by the model. We can see that the observed fluctuations in the amount of electrons and protons are reproduced simultaneously. This result is evidence that the drift effect plays a major role in the fluctuations in the amount of cosmic rays during each solar activity cycle, and is the first result in the world to be obtained from observations by CALET. Recent research has revealed that changes in cosmic radiation levels due to solar activity also have various effects on the Earth's environment. The results of this study are attracting attention as they pave the way for future research in this field.
The results of this research were published in the May 25th issue of Physical Review Letters.
*Note 1 Particle drift:
Charged particles wrap around magnetic field lines and make a rotating motion, but if the strength of the magnetic field varies from place to place or if the magnetic field lines are curved, the center of the rotating motion will gradually shift (drift) in a direction perpendicular to the magnetic field lines. Cosmic rays also drift to Earth near the Sun, but as the time and distance of the drifting motion increases, they gradually lose energy in the expanding solar wind and their amount decreases. It is thought that when solar activity increases, the magnetic field drawn from the Sun by the solar wind becomes stronger, resulting in longer drift times and distances and a decrease in the amount of cosmic rays. The direction of particle drift reverses depending on the positive/negative charge of the particle and the direction of the magnetic field.
This international collaborative research project promotes cosmic ray observations using the CALorimetric Electron Telescope (CALET) onboard the International Space Station, and the results of this research were jointly announced by Waseda University, Ibaraki National College of Technology, and Shinshu University (see the details page below for details).
Cosmic rays are radiation (mainly positively charged protons) that travel through space, and those with particularly high energy are born in our galaxy and travel to Earth. The amount of cosmic rays observed on Earth decreases when the number of sunspots increases and solar activity becomes active, and increases when solar activity declines. A wind of plasma called the solar wind is constantly spewing outward from the Sun, and this wind pulls the Sun's magnetic field out into space. Since charged cosmic rays move by wrapping around the magnetic field, the cosmic rays are pushed back outward by the solar wind. However, the physical process that causes this "push back" was not well understood.
Therefore, the research group focused on the fact that the direction of particle drift is reversed even if the particle charge is positive or negative, and simultaneously observed the amount of negatively charged electrons and positively charged protons with CLAET (cosmic rays contain a few percent of electrons). This allows us to verify the drift effect without being affected by changes in solar activity that differ from one activity cycle to another. The blue and red circles in the lower figure of Figure 1 below represent the time changes in the observed amount of electrons and protons, respectively. The observation period was about six years. From this figure, we can see that during the solar minimum in 2020, the amount of electrons shows a sharp peak, while the amount of protons shows a gentle peak. We attempted to reproduce this observational data using a model of the cosmic ray transport process that takes into account the particle drift effect. The blue and red lines in the figure are the reproduction results by the model. We can see that the observed fluctuations in the amount of electrons and protons are reproduced simultaneously. This result is evidence that the drift effect plays a major role in the fluctuations in the amount of cosmic rays during each solar activity cycle, and is the first result in the world to be obtained from observations by CALET. Recent research has revealed that changes in cosmic radiation levels due to solar activity also have various effects on the Earth's environment. The results of this study are attracting attention as they pave the way for future research in this field.
The results of this research were published in the May 25th issue of Physical Review Letters.
*Note 1 Particle drift:
Charged particles wrap around magnetic field lines and make a rotating motion, but if the strength of the magnetic field varies from place to place or if the magnetic field lines are curved, the center of the rotating motion will gradually shift (drift) in a direction perpendicular to the magnetic field lines. Cosmic rays also drift to Earth near the Sun, but as the time and distance of the drifting motion increases, they gradually lose energy in the expanding solar wind and their amount decreases. It is thought that when solar activity increases, the magnetic field drawn from the Sun by the solar wind becomes stronger, resulting in longer drift times and distances and a decrease in the amount of cosmic rays. The direction of particle drift reverses depending on the positive/negative charge of the particle and the direction of the magnetic field.
Figure 1. Changes in the amount of cosmic ray electrons (blue circles) and protons (red circles) observed by CALET (bottom (b)). The horizontal axis is the year, and the blue and red lines are the calculation results of the amount of electrons and protons using a model that takes into account the particle drift effect. The red and blue shaded areas indicate the range of uncertainty in the calculation results. Top (a) shows the number of sunspots (black line) and the tilt angle of the magnetic neutral plane of the solar magnetic field (blue dots) over the same period. Reprinted with some editing from a figure published in Physical Review Letters.
Presented Papers
Publication:Physical Review Letters
Title:Charge-Sign Dependent Cosmic-Ray Modulation Observed with the Calorimetric Electron Telescope on the International Space Station
Authors:
Akiko Miyake (Ibaraki National College of Technology), Kazuki Munakata (Shinshu University), Yosui Akaike (Waseda University), et al. (CALET Collaboration)
DOI:10.1103/PhysRevLett.130.211001
URL:https://link.aps.org/doi/10.1103/PhysRevLett.130.211001
Publication date (local time):Thursday, May 25, 2023
Details page (Waseda University)⇒https://www.waseda.jp/top/news/90562
Paper page (THE PHYSICAL REVIEW LETTERS) ⇒https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.211001
＜Other related links＞
Ibaraki National College of Technology ⇒https://www.ibaraki-ct.ac.jp/ JAXA Institute of Space and Astronautical Science ⇒https://www.isas.jaxa.jp/
Publication:Physical Review Letters
Title:Charge-Sign Dependent Cosmic-Ray Modulation Observed with the Calorimetric Electron Telescope on the International Space Station
Authors:
Akiko Miyake (Ibaraki National College of Technology), Kazuki Munakata (Shinshu University), Yosui Akaike (Waseda University), et al. (CALET Collaboration)
DOI:10.1103/PhysRevLett.130.211001
URL:https://link.aps.org/doi/10.1103/PhysRevLett.130.211001
Publication date (local time):Thursday, May 25, 2023
Details page (Waseda University)⇒https://www.waseda.jp/top/news/90562
Paper page (THE PHYSICAL REVIEW LETTERS) ⇒https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.130.211001
＜Other related links＞
Ibaraki National College of Technology ⇒https://www.ibaraki-ct.ac.jp/ JAXA Institute of Space and Astronautical Science ⇒https://www.isas.jaxa.jp/