Statistical reality of globally nonsimultaneous Forbush decrease events

Abstract

The Forbush decrease (FD) event, a sudden decrease of galactic cosmic ray intensity measured by neutron monitor (NM), is known as a globally simultaneous phenomenon. However, some of them do not occur simultaneously in universal time. On the basis of the difference of main phase onset time distributions of two kinds of FD events, Oh et al. (2008) suggested that the global simultaneity of FD events depends on the solar wind physical parameters and propagation direction in three‐dimensional heliosphere around the Earth. In order to get the hypothesis approved with a higher confidence level, the FD event data set is extended to 218 events with a longer period from 1971 to 2006 using Oulu, Inuvik, and Magadan station data. In addition, the probability of the same distribution of those two different classes of FD events is calculated in each NM station's view. All three NM stations confirm the probability of different distributions of FD main phase onset times of globally simultaneous and nonsimultaneous with a confidence level of 99%, compared with 94% of previous study. The statistics in this study may support the hypothesis that the simultaneous FD events occur when stronger magnetic barriers pass by the Earth through the central part of the magnetic barriers and in contrast that the nonsimultaneous FD events occur only if the less strong magnetic barriers pass the Earth on the dusk side of the magnetosphere.