Cosmic Ray Intensity and Spectral Changes During 27-Day Variations Using Time-Delay Measurements from Antarctic Neutron Monitors

Pradiphat Muangha (1,2), David Ruffolo (1), Alejandro Sáiz (1), Chanoknan Banglieng (3), Paul Evenson (4), Surujhdeo Seunarine (5), Suyeon Oh (6), Jongil Jung (7), Marc L. Duldig (8), John E. Humble (8)
(1) Department of Physics, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
(2) National Astronomical Research Institute of Thailand (NARIT), Chiang Mai 50180, Thailand
(3) Division of Physics, Faculty of Science and Technology, Rajamangala University of Technology Thanyaburi, Pathum Thani 12110, Thailand
(4) Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA
(5) Department of Physics, University of Wisconsin, River Falls, WI 54022, USA
(6) Department of Earth Science Education, Chonnam National University, Gwangju 61186, South Korea
(7) Department of Astronomy, Space Science and Geology, Chungnam National University, Daejeon 34134, South Korea
(8) School of Natural Sciences, University of Tasmania, Hobart, Tasmania 7001, Australia

Using neutron time-delay measurements from Antarctic neutron monitors, we can extract the leader fraction, L, of neutron counts that do not follow a previous neutron count in the same counter tube due to the cosmic ray shower. L, the inverse of the neutron multiplicity, can indicate variations in the cosmic-ray spectral index over the rigidity range of the NM response function. In this presentation, a comparative analysis of L from four Antarctic NM stations at South Pole (SP), McMurdo (MC), and Jang Bogo (JB), and Mawson (MA) will be presented. We find that L is well correlated with the spectral index inferred using data from the Alpha Magnetic Spectrometer (AMS-02) aboard the International Space Station. The wavelet analysis of the count rate C, and heliosphere parameters show a strong 27-day periodicity due to high-speed solar wind streams (HSSs) and corotating interaction regions (CIRs) while L usually had a very weak variation.