On the simultaneity of Forbush decreases: The simultaneous effects of interplanetary parameters and geomagnetic activity indices

Forbush decreases (Fd) are transient, short-term reductions in the intensity of galactic cosmic rays that reach the Earth's surface. When this reduction is observed at multiple locations simultaneously, it is referred to as simultaneous Forbush decreases (SFd). Identifying Fd events in daily averaged cosmic ray (CR) raw data is always tedious, but the task has gone minimal through an algorithm (automated Fd detection). We deployed an automated Fd location algorithm on daily-averaged CR data from five neutron monitor stations covering 1998-2006. We identified 80 days with the most simultaneous events. While there exists extensive research on the subject using a case study approach, the current study is statistical. Whereas most of the previous investigations employed a small sample of Fds manually selected from a single CR station, large samples of Fds selected after disentangling the Sun's influence on CR data from multiple neutron monitors (NMs) are used. The connection between the Fds and many solar-terrestrial variables is tested. The beautiful and consistent results obtained between the space weather variables and Fds at the five NM stations call the attention of space weather researchers to the need for rigorous, detailed, and accurate cataloging of Fds. Solar cycle oscillation significantly impacts the amplitude and timing of Fds. Its influence should be removed before Fd selection.