Solar activities, such as CME(Coronal Mass Ejection), cause geomagnetic storm that is a temporary disturbance of the Earth's magnetosphere. Geomagnetic storms can affect GPS positioning, radio communication, and power transmission system. Solar explosions also emit radiation, which can affect satellite failures, radiation exposure to aircraft crew, and space activity. Therefore, it is important to understand space weather phenomena and their impact on the Earth.
Space weather research by continuous observation of cosmic rays on the ground is mainly conducted using observation data from neutron monitors and multi-directional muon detectors. Since the phenomenon of space weather is on a short-term, days-long scale, it is effective to investigate changes in the flow of cosmic rays for several hours, which requires a total sky monitor of cosmic rays. In the muon detector, the global muon detector network (GMDN) has been observing space weather phenomena since 2006, and in the neutron monitor, the Spaceship Earth project constitutes a similar observation network and the role of the all-sky monitor. Until now, observations by neutron monitors and muon detectors have been performed independently, and progress has been made in space weather research.
In February 2018, Professor Chihiro Kato of Shinshu University took the lead in acquiring simultaneous observations of the neutron monitor and muon detector at Syowa Station in the Antarctic in order to acquire bridging data of observations by the neutron monitor and muon detector. In the polar regions, unlike low latitude regions on the earth, it is possible to observe cosmic rays coming from the same direction with a neutron monitor and a muon detector due to the weaker deflection by the geomagnetism. This is the reason why Syowa Station was selected as the observation point.
Syowa muon detector and neutron monitor observed small fluctuation in CR count like a Forbush decrease on 2018.8. The research group including researchers from Shinshu University and the National Polar Research Institute found curious cosmic-ray density variation on this event by analyzing GMDN data.
On the CME event, a huge amount of coronal material released with a bundle of the solar magnetic field, called Magnetic Flux Rope (MFR), into the interplanetary space. MFR moves through interplanetary space as expanding. CR density is low inside of it because it is originally coronal material. When the Earth enters the MFR, CR counts on the ground decreases. This is called Forbush Decrease.
Normally, when MFR arrives on Earth, CR density observed at the ground level decreases rapidly, and then turns to increase recovering to the original level while the Earth is in the MFR. On this event, however, the CR exceeds the original level before the Earth exits the MFR.
This event attracts interest from researchers because 1) The solar activity is currently near the minimum and the scale of the event itself is small, 2) It causes a disproportionately large geomagnetic storm, and 3) There is high-speed solar wind catching up the MFR expected to interact with it.
By analysis of the GMDN and solar plasma data, it is concluded that the high-speed solar wind causes the unusual enhancement of the CR density by compressing the rear part of the MFR locally.
Cosmic ray observation data is closely related not only to space weather research but also to atmospheric phenomena such as sudden stratospheric temperature rise and is expected to be used in a wide range of fields in the future. The cosmic ray observation data at Syowa Station, including the phenomenon in August 2018, which was the subject of this research, is published on the website and updated daily: http://polaris.nipr.ac.jp/~cosmicrays/
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Acknowledgments
The authors are grateful to Center for Antarctic Programs at NIPR (National Institute of Polar Research), JARE59 team, and Shirase crew for installing the system to Syowa Station. This research project is supported by NIPR, ICRR (Institute of Cosmic Ray Research, Tokyo University), ISEE (Institute for Space-Earth Environmental Research, Nagoya University), University of Delaware, and Shinshu University. Some of the scientific data of this research project has begun to be published at http://polaris.nipr.ac.jp/~cosmicrays/ supported by ROIS-DS-JOINT2018. The Bartol Research Institute neutron monitor program is supported by the United States National Science Foundation under grants PLR-1245939 and PLR-1341562, and by Department of Physics and Astronomy and the Bartol Research Institute, the University of Delaware. The neutron monitor data from Thule are provided by the University of Delaware Department of Physics and Astronomy and the Bartol Research Institute. The neutron monitor data from South Pole and the South Pole Bares are provided by the University of Wisconsin, River Falls. We acknowledge the NMDB database (http://www.nmdb.eu), founded under the European Union's FP7 program (contract no. 213007) for providing data. The editor thanks two anonymous reviewers for their assistance in evaluating this paper.
Journal
Journal of Space Weather and Space Climate