Large amplitude bidirectional anisotropy of cosmic-ray intensity observed in November, 2021

K. Munakata (1), M. Kozai (2), C. Kato (1), Y. Hayashi (1), R. Kataoka (3), A. Kadokura (2,3,4), M. Tokumaru (5), R. R. S. Mendonça (6), E. Echer (6), A. Dal Lago (6), M. Rockenbach (6), N. J. Schuch (7), J. V. Bageston (7), C. R. Braga (8), H. K. Al Jassar (9), M. M. Sharma (9), M. L. Duldig (10), J. E. Humble (10), I. Sabbah (11), P. Evenson (12), P.-S. Mangeard (12), T. Kuwabara (12), D. Ruffolo (13), A. Sáiz (13), W. Mitthumsiri (13), W. Nuntiyakul (14), and J. Kóta (15)
(1) Department of Physics, Shinshu University, Matsumoto, Nagano 390-8621, Japan
(2) Polar Environment Data Science Center, Joint Support-Center for Data Science Research, Research Organization of Information and Systems, Tachikawa, Tokyo 190-0014, Japan
(3) National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan
(4) Department of Polar Science, School of Multidisciplinary Sciences,The Graduate University for Advanced Studies, SOKENDAI, Tachikawa, Tokyo 190-8518, Japan
(5) Institute for Space-Earth Environmental Research, Nagoya University, Nagoya, Aichi 464-8601, Japan
(6) National Institute for Space Research (INPE), 12227-010 São José dos Campos, Brazil
(7) Southern Space Coordination, National Institute for Space Research, P.O. Box 5021 - 97110-970 - Santa Maria, RS - Brazil
(8) George Mason University, 4400 University Drive, Fairfax, Virginia 22030, USA
(9) Physics Department, Kuwait University, P.O. Box 5969 Safat, 13060, Kuwait
(10) School of Natural Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
(11) Department of Natural Sciences, College of Health Sciences, Public Authority of Applied Education and Training, Kuwait City 72853, Kuwait
(12) Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, 217 Sharp Laboratory, Newark, DE 19716, USA
(13) Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
(14) Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
(15) Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

We analyze the cosmic-ray variations during a significant Forbush decrease observed with world-wide networks of ground-based neutron monitors and muon detectors during November 3-5, 2021. Utilizing the difference between primary cosmic-ray rigidities monitored by neutron monitors and muon detectors, we deduce the rigidity spectra of the cosmic-ray density (or omnidirectional intensity) and the first- and second-order anisotropies separately, for each hour of data. A clear two-step decrease is seen in the cosmic-ray density with the first ~2 % decrease after the interplanetary shock arrival followed by the second ~5 % decrease inside the magnetic flux rope (MFR) at 15 GV. Most strikingly, a large bidirectional streaming along the magnetic field is observed in the MFR with a peak amplitude of ~5 % at 15 GV which is comparable to the total density decrease inside the MFR. The bidirectional streaming could be explained by adiabatic deceleration and/or focusing in the expanding MFR, which have stronger effects for pitch angles near 90 degree, or by selective entry of GCRs along a leg of the MFR. The peak anisotropy and density depression in the flux rope both decrease with increasing rigidity. The spectra vary dynamically indicating that the temporal variations of density and anisotropy appear different in neutron monitor and muon detector data.