Measurement of the radial density gradient of cosmic ray in the heliosphere by the GRAPES-3 experiment |
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| Institution: | 1. Faculty of Engineering, Aichi Institute of Technology, Toyota City 470-0392, Japan;2. The GRAPES-3 Experiment, Cosmic Ray Laboratory, Ooty 643 001, India;3. Tata Institute of Fundamental Research, Mumbai 400 005, India;4. Graduate School of Science, Osaka City University, Osaka 558-8585, Japan;5. College of Engineering, Chubu University, Kasugai, Aichi 487-8501, Japan;1. Dept. of Physics and Astronomy, University of California, Irvine, United States;2. Dept. of Physics and Astronomy, University of Kansas, United States;3. Dept. of Electrical Engineering and Computer Science, University of California, Irvine, United States;4. Lawrence Berkeley National Laboratory, United States;5. Instrumentation Design Lab, University of Kansas, United States;6. Moscow Physics and Engineering Institute, United States;7. Dept. of Physics, University of Washington, United States;1. School of Animal Biotechnology, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, Punjab, India;2. Department of Animal Genetics and Breeding, Guru Angad Dev Veterinary and Animal Sciences University, Ludhiana, Punjab, India;1. Departamento de Física, Universidade Estadual da Paraíba, 58429-500 Campina Grande, PB, Brazil;2. Departamento de Física, Universidade Federal da Paraíba, João Pessoa, PB, Brazil;3. Departamento de Astronomia, Observatório Nacional, 20921-400 Rio de Janeiro, RJ, Brazil;1. Department of Physics, Graduate School of Science, Kyoto University, Kyoto, Japan;2. SFTA Department, Physics Section, University of Siena and INFN, Siena, Italy |
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| Abstract: | A radial anisotropy in the flux of cosmic rays in heliosphere was theoretically predicted by Parker and others within the framework of the diffusion–convection mechanism. The solar wind is responsible for sweeping out the galactic cosmic rays, creating a radial density gradient within the heliosphere. This gradient coupled with the interplanetary magnetic field induces a flow of charged particles perpendicular to the ecliptic plane which was measured and correctly explained by Swinson, and is hereafter referred as ‘Swinson flow’. The large area GRAPES-3 tracking muon telescope offers a powerful probe to measure the Swinson flow and the underlying radial density gradient of the galactic cosmic rays at a relatively high rigidity of ∼100 GV. The GRAPES-3 data collected over a period of six years (2000–2005) were analyzed and the amplitude of the Swinson flow was estimated to be (0.0644 ± 0.0008)% of cosmic ray flux which was an ∼80σ effect. The phase of the maximum flow was at a sidereal time of (17.70 ± 0.05) h which was 18 min earlier than the expected value of 18 h. This small 18 min phase difference had a significance of ∼6σ indicating the inherent precision of the GRAPES-3 measurement. The radial density gradient of the galactic cosmic rays at a median rigidity of 77 GV was found to be 0.65% AU−1. |
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| Keywords: | Cosmic rays Heliosphere Solar wind Convection Diffusion |
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