Citation: Belov, A.; Shlyk, N.; Abunina, M.; Belova, E.; Abunin, A.; Papaioannou, A. Solar Energetic Particle Events and Forbush Decreases Driven by the Same Solar Sources. Universe 2022, 8, 403. https://doi.org/10.3390/ universe8080403 Academic Editor: Ruisheng Zheng Received: 4 July 2022 Accepted: 27 July 2022 Published: 1 August 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). universe Article Solar Energetic Particle Events and Forbush Decreases Driven by the Same Solar Sources Anatoly Belov 1 , Nataly Shlyk 1 , Maria Abunina 1 , Elena Belova 1 , Artem Abunin 1 and Athanasios Papaioannou 2, * 1 Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of Russian Academy of Sciences (IZMIRAN), Kaluzhskoe hw., 4, Troitsk, 108840 Moscow, Russia; abelov@izmiran.ru (A.B.); nshlyk@izmiran.ru (N.S.); abunina@izmiran.ru (M.A.); lbelova@izmiran.ru (E.B.); abunin@izmiran.ru (A.A.) 2 Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), National Observatory of Athens, I. Metaxa & Vas. Pavlou St., 15236 Penteli, Greece * Correspondence: atpapaio@astro.noa.gr; Tel.: +30-210-810-9182 Abstract: 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 ◦ E–35 ◦ 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. Keywords: ground level enhancements; Forbush decreases; solar flares; coronal mass ejections 1. Introduction The possibility of accelerating charged particles on the Sun to high energies was first described many years ago (for example, in Forbush [1], Dorman and Miroshnichenko [2], Shea and Smart [3]). Initially, it was assumed that proton flux increases (solar energetic particle or SEP events) were associated exclusively with powerful solar flares (Dorman and Miroshnichenko [2], Duggal [4]). After regular observation of coronal mass ejections (CMEs), many scientists began to associate proton acceleration with shock waves from the corresponding interplanetary CMEs (ICMEs; for example, Gosling [5], Reames [6]). A lot of papers suggest that gradual SEP events occur as a result of particles’ diffusive acceleration on CME-induced coronal and interplanetary shock waves, and impulsive SEPs have been explained by acceleration during magnetic reconnection in solar flares (Zhang et al. [7] and references therein). There are many studies demonstrating good agreement between the magnitude of proton enhancements and both the CME speed and the magnitude of accompanying X-ray flares (e.g., [8–16]). With the development of ground- based detectors and satellite electronics, it became possible to confidently detect and isolate SEPs of different energy ranges, and now several catalogs of SEPs with hundreds of events compiled by different research groups are available for analysis, for example: the NOAA SEP event list ftp://ftp.swpc.noaa.gov/pub/indices/SPE.txt Major SEP Events https: //cdaw.gsfc.nasa.gov/CME_list/sepe/ (1997–2017); and SEP catalogs described in separate scientific articles, for example, “Solar CR GLEs in 1976–2006” [17], “Fe-rich SEP events 1995–2013”(https://link.springer.com/article/10.1007/s11207-014-0547-1/tables/2,[18]), and “55–80 MeV proton events in 1996–2016”(https://www.swsc-journal.org/articles/ Universe 2022, 8, 403. https://doi.org/10.3390/universe8080403 https://www.mdpi.com/journal/universe