[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

*櫻井 敬久¹、鈴木 颯一郎¹、川村 容明¹、門叶 冬樹¹、武山 美麗¹、森谷 透¹、乾 恵美子¹、菊地 聡¹、三宅 芙沙²、門倉 昭³、小財 正義⁴、佐藤 夏雄³、杉原 奈央子⁵、Gunnlaugur Bjornsson ⁶、Ruffolo David ⁷、Mitthumsiri Warit⁷、Macatangay Ronald ⁸ (1.山形大学、2.名古屋大学、3.国立極地研究所、4.情報・システム研究機構 デ ータサイエンス共同利用基盤施設、5.東京大学大気海洋研究所、6.アイスランド大学、7.マヒドー大学、8.タイ国立宇宙研究所)

キーワード：宇宙線生成核種　Be-7、第25太陽活動期、宇宙線

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.