Quantitative Analysis of the Influence of Solar Cycle Variations on Timing, Number and Magnitude of Forbush Events at Moscow and Oulu Stations

Although investigations of short-term time-intensity changes, such as Forbush decreases (FDs) and cosmic ray (CR) diurnal anisotropies, and long-term CR flux variations began decades ago, the influence of the 11-year solar cycle variations on FDs is rarely examined in detail. This study presents the quantitative implications of the 11-year CR cycle on FDs. The rigorous analyses performed show that the 11-year solar cycle variations significantly influence the timing, number, and magnitude of FD event characteristics. Long-term variations can induce a significant number of spurious FDs in raw CR data. These false FDs can lead to false alarms in space weather predictions and FD-based research. Since these spurious FDs are ubiquitous, as evidenced by the results, manual or semi-automated case event analysis of FDs may be inefficient. Thus, many existing catalogs of FDs created without accounting for the 11-year solar cycle variation may be flawed. The efficiency and speed of the computer subroutines implemented in this work to account for the long-term CR variations may be useful in predicting space weather hazards. However, these conclusions should be interpreted with caution, as the influence of short-term CR modulation − specifically anisotropies and geomagnetic field variations − has not been accounted for. Until these unwanted signals are removed, the FD catalog developed in this work remains preliminary.