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Title: Distinct Pattern of Solar Modulation of Galactic Cosmic Rays above a High Geomagnetic Cutoff Rigidity
Authors: Pierre Simon Mangeard
John Clem
Paul Evenson
Roger Pyle
Warit Mitthumsiri
David Ruffolo
Alejandro Sáiz
Tanin Nutaro
Ubon Rajathanee University
Mahidol University
Bartol Research Institute
Pyle Consulting Group, Inc.
Keywords: Earth and Planetary Sciences;Physics and Astronomy
Issue Date: 1-May-2018
Citation: Astrophysical Journal. Vol.858, No.1 (2018)
Abstract: © 2018. The American Astronomical Society. All rights reserved.. Solar modulation refers to Galactic cosmic-ray variations with the ∼11 yr sunspot cycle and ∼22 yr solar magnetic cycle and is relevant to the space radiation environment and effects on Earth's atmosphere. Its complicated dependence on solar and heliospheric conditions is only roughly understood and has been empirically modeled in terms of a single modulation parameter. Most analyses of solar modulation use neutron monitor (NM) data from locations with relatively low geomagnetic cutoff rigidity, i.e., the threshold for cosmic rays to penetrate Earth's magnetic field. The Princess Sirindhorn Neutron Monitor at Doi Inthanon, Thailand, has the world's highest cutoff rigidity (≈17 GV) where observations span a complete solar modulation cycle (since late 2007). The pattern of solar modulation at Doi Inthanon during 2011-2014 was qualitatively very different from that at a low geomagnetic cutoff and is not well described by the same modulation parameter. At other times, NM count rates from Doi Inthanon and McMurdo, Antarctica (cutoff ∼1 GV), were linearly correlated and confirm the observation from latitude surveys in the previous solar cycle that the slope of the correlation changes with solar magnetic polarity. Low solar magnetic tilt angles (<40° at negative polarity) were well correlated with variations at both NM stations, as predicted by drift models. At a higher tilt angle, the Doi Inthanon count rate is well correlated with the interplanetary magnetic field, which is consistent with an increase in diffusion at high rigidity short-circuiting the effects of drifts and the heliospheric current sheet.
ISSN: 15384357
Appears in Collections:Scopus 2018

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