Solar Magnetic Polarity Effect on Neutron Monitor Count Rates: Comparing Latitude Surveys and Antarctic Stations

K. Poopakun* (1), W. Nuntiyakul (1), S. Khamphakdee (1), A. Seripienlert (2), D. Ruffolo (3), P. Evenson (4), P. Jiang (5), P. Chuanraksasat (2), K. Munakata (6), M. L. Duldig (7), J. E. Humble (7), J. Madsen (8), B. Soonthornthum (2), and S. Komonjinda (1)
(1) Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
(2) National Astronomical Research Institute of Thailand (NARIT), Chiang Mai 50180, Thailand
(3) Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
(4) Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
(5) Polar Research Institute of China, Pudong, Shanghai 200136, China
(6) Physics Department, Shinshu University, Matsumoto, Nagano 390-8621, Japan
(7) School of Natural Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia
(8) Wisconsin IceCube Particle Astrophysics Center, University of Wisconsin-Madison, WI 53703, USA

The Galactic cosmic ray spectrum manifests subtle variations over the 22-year solar magnetic cycle in addition to more pronounced variations over the 11-year sunspot cycle. We conducted numerous latitude surveys by operating a neutron monitor onboard icebreakers that traveled across a wide range of geomagnetic cutoff rigidity. Here we revisit our previous work to study spectral changes using 13 annual latitude surveys from 1994 to 2007 using neutron monitor data from Mawson instead of McMurdo (closed in 2017) to allow extension to more recent latitude surveys. We confirm linear trends between count rates at different geomagnetic cutoff rigidity and changes in slope before and after the polarity reversal in 2000. We performed two more latitude surveys (in 2019 and 2020) with a monitor similar to the 3NM64 in the previous surveys but without lead rings around the central tube; a so-called “semi-leaded neutron monitor”. We found similar results for the relationship between the count rate of the semi-leaded neutron monitor and that of the Mawson neutron monitor in Antarctica. We confirm the “crossover” in spectra measured near solar minima during epochs of opposite solar magnetic polarity using recent latitude surveys and verify the absence of crossover for the same solar magnetic polarity. Thus we confirm both the change in the linear relationship and the crossover as effects of solar magnetic polarity on the cosmic ray spectrum resulting from solar modulation. Partially supported by grant RTA6280002 from Thailand Science Resesarch and Innovation.

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