Solar Energetic Particle Events and Forbush Decreases Driven by the Same Solar Sources

The characteristics of Forbush decreases (FDs) and solar energetic particle (SEP) events driven by the same solar source (i.e., coronal mass ejection and associated solar flare) are investigated. The part of the solar disk (04 degrees E-35 degrees W) in which most of the solar events lead both to an FD and SEP event on Earth was chosen. SEPs for different energies (E > 10 MeV, E > 100 MeV, and Ground Level Enhancements) and with different flux thresholds were considered independently. The obtained results were compared with the control group of FDs that had solar sources within the same longitudinal zone but were not accompanied by any SEPs. It is shown that coronal mass ejections (CMEs) followed by SEPs have a very high probability of creating a large FD in the Earth's orbit and to further cause a geomagnetic storm. It is also found that the accelerative and modulating efficiencies of powerful solar events are well correlated; this can be explained mostly by high speeds of the corresponding CMEs.