Simultaneity of Forbush decrease events observed at middle-latitude neutron monitors

Ground neutron monitors (NMs) sometimes observe a sudden reduction in galactic cosmic ray intensity—the so‐called Forbush decrease (FD) event. Such events are mainly associated with interplanetary coronal mass ejections passing around the Earth and corotating interaction regions in the heliosphere. Some FD events are observed globally, either simultaneously or nonsimultaneously, at different NM stations in the case that the simultaneity is determined by the overlapping of the FD main phase, with the period of the cosmic ray intensity decreasing before returning to a steady state. Previous studies have identified two types of FD events with statistically significant differences in the distributions of the main phase onset time. It has been hypothesized that simultaneous FD events occur when a strong magnetic cloud passes by the Earth through the central part of the magnetic barrier, whereas nonsimultaneous events occur if a weaker magnetic cloud passes on the duskside of the magnetosphere. However, the previous statistical analyses were performed using only data from high geomagnetic latitude NM stations in the Northern Hemisphere. To address this shortcoming and to further test the above hypothesis, we repeated the analysis using data from NM stations located at middle latitudes (Jungfraujoch, Irkutsk, and Climax), employing cutoff rigidities 3–6 GV for the last solar maximum period (1998–2002), spanning the same time period as Oh et al. (2008) that employed high‐latitude NM stations. The results of the present statistical analysis support the above hypothesis with high confidence levels. Key Points Simultaneity of FD events at middle latitude Confirm the existence of non‐simultaneous FD at middle latitude Suggest hypothesis explaining the simultaneity