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As the current solar cycle 25 progresses and moves towards solar maxima, solar activity is increasing and extreme space weather events are taking place. Two severe geomagnetic storms accompanied by two large Forbush decreases in galactic cosmic ray intensity were recorded in March and May, 2024. More precisely, on 24 March 2024, a G4 (according to the NOAA Space Weather Scale for Geomagnetic Storms) geomagnetic storm was registered, with the corresponding geomagnetic indices Kp and Dst equal to 8 and −130 nT, respectively. On the same day, the majority of ground-based neutron monitor stations recorded an unusual Forbush decrease. This event stands out from a typical Forbush decrease because of its high amplitude decrease phase and rapid recovery phase, i.e., 15% decrease and an extremely rapid recovery of 10% within 1.5 h, as recorded at the Oulu neutron monitor station. Furthermore, on 10–13 May 2024, an unusual G5 geomagnetic storm (geomagnetic indices Kp = 9 and Dst = −412 nT) was registered (the last G5 storm had been observed in 2003). In addition, the polar neutron monitor stations recorded a Ground Level Enhancement (GLE74) during the recovery phase of a large Forbush decrease of 15%, which started on 10 May 2024. In this study, a detailed analysis of these two severe events in regard to the accompanying solar activity, interplanetary conditions and solar energetic particle events is provided. Moreover, the results of the NKUA “GLE Alert++ system”, the NKUA/IZMIRAN “FD Precursory Signals” method and the NKUA “ap Prediction tool” concerning these events are presented.

Forbush decreases are usually characterized by increased values of cosmic ray anisotropy. The precursory signs, i.e., pre-increases and especially pre-decreases of the cosmic ray intensity, are highly anisotropic phenomena that ordinarily forewarn of such events. Two Cosmic Ray Groups from the National and Kapodistrian University of Athens (NKUA) and the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN) have been investigating the existence of precursory signs preceding Forbush decreases in relation to different solar phenomena, interplanetary parameters, and geomagnetic conditions. In this study, large Forbush decreases (magnitude > 5%) accompanied by geomagnetic storms (i.e., geomagnetic index Dst < −100 nT and 5 ≤ Kp-index ≤ 9) and characterized by an equatorial anisotropy 1 h before the onset of the event (Axyb, %) less than 0.8% were examined regarding precursors. In total, 50 events with the aforementioned features were selected and analyzed from the IZMIRAN’s Forbush Effects and Interplanetary Disturbances database concerning the time period from 1969 until 2023. The Ring of Stations method, which depicts the cosmic ray variations for various asymptotic longitudes in relation to time, was applied on each event. The results revealed that clear signs of pre-decreases were not present for the majority of the events. Since particularly strong events were considered, most of them still showed some precursory signs, albeit mainly weak. Despite this, the value of Axyb = 0.8% proves to be a good threshold for the manual selection of FDs with well-expressed precursors.

The paper considers the modeling of proton transport through the Earth’s atmosphere during several SEP events (12 August 1989, 23 March 1991, and 8 November 2000), as well as during the GLE73 event. Solar sources and interplanetary medium conditions during these events are described in detail. Calculations are carried out using own model implemented with GEANT4. As the main results, quantitative estimates of the calculated ambient dose equivalent for altitudes in a wide range (also including civil aircraft flight altitudes of 10–11 km) for the geomagnetic cutoff rigidity values Rc = 0.13 GV are given.

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.

The dependence of Interplanetary Coronal Mass Ejections’ (ICMEs) transit speed on the corresponding Coronal Mass Ejections’ (CMEs) initial speed is investigated. It is shown that the transit speed and transit time depend not only on the CME’s initial speed, but also on the longitude of the solar source. The longitudinal dependence of the expected transit speeds and times are obtained from the analysis of 288 CMEs, associated with solar flares, observed from 1995 to 2020. A model, estimating the transit and maximum speeds, as well as the time of arrival of an ICME to Earth, based on the initial CME speed and the longitude of the associated solar flare has been created. It is shown that taking into account the longitude of the solar source in addition to the initial CME speed significantly improves the quality of the model, especially for events in the central part of the solar disk (E10°–W10°). The simplicity of the described model makes it accessible to a wide range of users and provides opportunities for further improvement as the statistics and the number of input parameters increase.

Interplanetary coronal mass ejections (ICMEs) are the counterparts of coronal mass ejections (CMEs) that extend in the interplanetary (IP) space and interact with the underlying solar wind (SW). ICMEs and their corresponding shocks can sweep out galactic cosmic rays (GCRs) and thus modulate their intensity, resulting in non-recurrent Forbush decreases (FDs). In this work, we selected all FDs that were associated with a sudden storm commencement (SSC) at Earth, and a solar driver (e.g., CME) was clearly identified as the ICME’s source. We introduce and employ the tH parameter, which is the time delay (in hours) of the maximum strength of the interplanetary magnetic field from the FD onset (as is marked via the SSC), and consequently derive three groups of FD events (i.e., the early, medium, and late ones). For each of these we examine the mean characteristics of the FDs and the associated IP variations per group, as well as the resulting correlations. In addition, we demonstrate the outputs of a superposed epoch analysis, which led to an average time profile of the resulting FDs and the corresponding IP variations, per group. Finally, we interpret our results based on the theoretical expectations for the FD phenomenon. We find that both the shock sheath and the ejecta are necessary for deep GCR depressions and that the FD amplitude (A0) is larger for faster-propagating ICMEs. Additionally, we note the importance of the turbulent shock-sheath region across all groups. Finally, we present empirical relations connecting A0 to SW properties. [ABSTRACT FROM AUTHOR]