[P-EM10] Space Weather and Space Climate

[PEM10-11] Top 57 Major Forbush Decrease Events Observed by the Nagoya Muon Detector (1976–2024)

Hayashi Yuki^1,2、Kataoka Ryuho^2,3、*Kato Chihiro¹、Kadokura Akira^2,4、Kozai Masayoshi⁴、Iwai Kazumasa⁵、Munakata Kazuoki¹ (1.Shinshu University、2.National Institute of Polar Research、3.SOKENDAI、4.Polar Environment Data Science Center, Joint Support-Center for Data Science Research, ROIS、5.ISEE, Nagoya University)

キーワード：Cosmic Rays、Forbush Decrease、Magnetic Storm

Forbush decrease events, which are instigated by coronal mass ejections (CMEs) within the inner heliosphere, play important roll for profound disturbances in the Earth's magnetic field. This study presents a systematic ranking of the most significant Forbush decrease events, using 48 years of muon count rate data from Nagoya, Japan (1976–2024). The ground-based muon detector employed in this research demonstrates sensitivity to high-rigidity protons, specifically those approximately 60 GV, whereas traditional neutron monitors capture Forbush decreases in a lower rigidity range, typically below 15 GV. Our analysis reveals that the recent Forbush decrease events that occurred in May and October 2024 rank among the top 15 most significant events documented over the past four decades, which are supported by observations from both neutron monitors and the Nagoya muon detector. Notably, the frequency of such major events tends to increase during the declining phase of the solar cycle, suggesting the likelihood of additional significant events as the next solar minimum approaches. We posit that the amplitude of Forbush decreases can enhance our comprehension of space weather and space climate dynamics. Furthermore, the systematic ranking of major Forbush decrease events may facilitate rapid assessments of space weather phenomen.