Solar Proton Events and Forbush Decreases with the Same Sources

Abstract Characteristics of Forbush decreases (FDs) and solar proton events (SPEs) caused by the same solar sources (coronal mass ejection and associated solar flare) are compared. A range of heliolongitudes (E04–W35) is chosen in which flares are associated with both FDs and SPEs near the Earth. SPEs with different energies (E > 10, >100 MeV) and flux thresholds, and ground level enhancements (GLEs) are considered independently. Results are compared to a control group of solar flares that have solar sources within the same heliolongitudinal zone but do not produce any SPEs. It is shown that coronal mass ejections (CMEs) followed by solar proton events are likely to cause a strong FD in the Earth’s orbit and a geomagnetic storm. The accelerative and modulating efficiencies of such solar phenomena are interrelated, which is mainly explained by the high speeds of CMEs. These results can be used in practice to improve forecasts of geomagnetic storms and Forbush decreases.