Forbush Decreases (FD) are transient, sudden reductions of cosmic ray (CR) intensity lasting a few days, to a week. Such events are observed globally using ground neutron monitors (NMs). Most studies of FD events indicate that an FD event is observed simultaneously at NM stations located all over the Earth. However, using statistical analysis, previous researchers verified that while FD events could occur simultaneously, in some cases, FD events could occur non-simultaneously. Previous studies confirmed the statistical reality of non-simultaneous FD events and the mechanism by which they occur, using data from high-latitude and middle-latitude NM stations. In this study, we used long-term data (1971-2006) from middle-latitude NM stations (Irkutsk, Climax, and Jungfraujoch) to enhance statistical reliability. According to the results from this analysis, the variation of cosmic ray intensity during the main phase, is larger (statistically significant) for simultaneous FD events, than for non-simultaneous ones. Moreover, the distribution of main-phase-onset time shows differences that are statistically significant. While the onset times for the simultaneous FDs are distributed evenly over 24- hour intervals (day and night), those of non-simultaneous FDs are mostly distributed over 12-hour intervals, in daytime. Thus, the existence of the two kinds of FD events, according to differences in their statistical properties, were verified based on data from middle-latitude NM stations.

1. INTRODUCTION
 2. DATA AND SELECTION OF FD
 3. RESULTS
 4. SUMMARY
 ACKNOWLEDGEMENTS
 REFERENCES

• Seongsuk Lee(Department of Astronomy and Space Science, Chungnam National University, Daejeon 305-764, Korea)
• Suyeon Oh(Department of Earth Science Education, Chonnam National University, Gwangju 500-757, Korea)
• Yu Yi(Department of Astronomy and Space Science, Chungnam National University, Daejeon 305-764, Korea) Corresponding author
• Paul Evenson(Bartol Research Institute and Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA)
• Geonhwa Jee(Korea Polar Research Institute, Incheon 406-940, Korea)
• Hwajin Choi(Korea Polar Research Institute, Incheon 406-940, Korea)