[P-EM11] Space Weather and Space Climate

[PEM11-P18] Concentration of cosmogenic nuclide Be-7 in the surface atmosphere at the beginning of the 25th solar cycle

*Hirohisa Sakurai¹, Soichiroro Suzuki¹, Youmei Kawamura¹, Fuyuki Tokanai¹, Mirei Takeyama¹, Toru Moriya¹, Emiko Inui¹, Satoshi Kikuchi¹, Fusa Miyake², Akira Kadokura³, Masayoshi Kozai⁴, Natsuo Sato³, Naoko Sugihara⁵, Bjornsson Gunnlaugur⁶, David Ruffolo⁷, Warit Mitthumsiri⁷, Ronald Macatangay⁸ (1.Yamagata University, 2.Nagoya University ISEE, 3.NIPR, 4.ROIS-DS, 5.Atmosphere and Ocean Research Institute,The University of Tokyo, 6.Iceland University, 7.Mahidol University, 8.National Astronomical Research Institute of Thailand)

Keywords:cosmogenic nuclide Be-7, the 25th solar cycle, cosmic rays

The Be-7 concentration (BEC) in the surface atmosphere is correlated with cosmic rays because Be-7 is produced by the interaction of cosmic rays with nitrogen and/or oxygen in the atmosphere and falls with aerosols. Because cosmic rays reaching Earth are modulated by solar activity as they travel through the heliosphere, BEC variations include modulation profiles such as 27-day variations and 11-year solar cycles. Daily Be-7 concentrations in the surface atmosphere have been continuously observed in Yamagata, Japan (38°N) since 2000. To investigate the influence of latitude on the 11-year solar cycle, we installed a daily observation system of Be-7 concentrations at Husafell, Iceland (64°N). The BEC has been observed continuously since September 2003 as well as those in Japan. Furthermore, we have set up a daily observation system for Be-7 concentration in Bangkok, Thailand (13.7°N), and have been continuously observing BEC since June 2014. The BECs of Iceland, Japan, and Thailand averaged 2.2 mBq/m3, 6.1 mBq/m3, and 3.7 mBq/m3, respectively, at the start of the 25th solar cycle in 2019, representing the effect of latitude. The reduction rates from 2019 to 2022 were 18%, 13%, and 38% at Iceland, Japan, and Thailand, respectively, while the neutron monitor rate in Oulu was 4.4%. Although this decrease represents part of the 11-year solar modulation, the larger rate of BEC decrease than in the neutron monitoring data suggests the influence of a much lower Be-7 production threshold of about 10 MeV. However, the latitudinal effect of BEC includes both the latitudinal effect of cosmic rays due to geomagnetic cut-off and the latitudinal variation of the troposphere. Therefore, we investigated the trajectory of the air mass reaching the sampling point and the Be-7 production rate along the route, and calculated the BEC of the sampling point. We describe the observed data on Be-7 concentrations by considering the latitudinal effect on Be-7 production rates in the atmosphere at the beginning of the 25th solar cycle.