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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. [ABSTRACT FROM AUTHOR]

Numerical modeling of primary cosmic ray protons' transport through the Earth's atmosphere was performed for the energy spectra of solar energetic particle events (SEPs). Several events in the last three solar cycles were considered. A comparative analysis of the characteristics of coronal mass ejections and primary proton fluxes was carried out. The main results were quantitative estimates of the calculated atmospheric ionization count rate for a wide range of altitudes (from sea level up to 98 km). The difference in the influence of solar protons on the Earth's atmosphere is considered for seven SEPs divided into three groups with similar solar sources (X-flare magnitude and coordinates) but with different characteristics of accelerated particle fluxes. The data obtained in this work are very important for future studies of radio wave propagation, atmospheric chemistry and climate change. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The rigidity dependence of all Forbush decreases (FDs) recorded from 1995 to 2015 has been determined using neutron monitor (NM) and Solar and Heliospheric Observatory (SOHO) (EPHIN) spacecraft data, covering the energy (rigidity) range from ∼433 MeV (1 GV) to 9.10 GeV (10 GV). We analyzed a total of 421 events and determined the spectrum in rigidity with an inverse power-law fit. As a result, the mean spectral index was identified to be 〈γF〉 = 0.46 ± 0.02. The majority (∼66%) of the FDs have γF within the range 0.3–0.7. The remaining one-third of the events (∼33%) have either (very) soft or hard FD spectra, with the latter being more common than the former. Significant variations of γF occur within almost every FD event. During the initial FD decay phase the spectrum becomes gradually harder, in contrast to the recovery phase, when it becomes softer. Additionally, low energies (rigidities) seem to be better suited for studying the fine structure of interplanetary disturbances (primarily interplanetary coronal mass ejections) that lead to FDs. In particular, FDs recorded by the EPHIN instrument on SOHO better capture a two-step structure than FDs observed by NMs. Finally, the ejecta of an ICME, especially when identified as a magnetic cloud, often leads to abrupt changes in the slope of γF. [ABSTRACT FROM AUTHOR]

Forbush Effects and Interplanetary Disturbances database created and maintained at IZMIRAN has been used to compare properties of Forbush decreases (FDs) observed during solar cycles 23-24, maximums of the cycles and the minimum between the cycles. The database provided 1677 isolated FDs since 1996 to 2015; 350 FDs caused by high speed streams from coronal holes (recurrent events) and 207 FDs caused by interplanetary disturbances from coronal mass ejections (sporadic events). The last two groups were selected with big reliability within the group of isolated FDs. Each FD was described by 6 parameters: FD magnitude, decrease rate, duration of main phase, maximum values of cosmic ray anisotropy and solar wind velocity and interplanetary magnetic field intensity. Statistical methods (distribution analyses, calculation of medians, interquartile ranges and means with 95% confidence intervals) were used to compare parameters of recurrent and sporadic FDs in the different phases of solar activity. The results revealed that sporadic FDs prevailed in the maximums of the cycles, recurrent FDs – in the minimum between the cycles. FD parameters (magnitude, decrease rate, anisotropy) are larger for sporadic events than for recurrent ones, especially in the maximums of the cycles. FD magnitude is greater in the maximums than in the minimum for sporadic events and it changes weakly for recurrent ones. Solar wind velocity is in average greater for recurrent events than for sporadic ones; it is larger for recurrent FDs in the minimum and for sporadic FDs – in the maximums. Magnetic field is stronger for sporadic FDs than for recurrent ones in the maximums and it is approximately equal for both types of events in the minimum. The duration of FD main phase is less in the maximums for both types of events; sporadic FDs develop significantly faster than recurrent ones in the maximum of cycle 23. [ABSTRACT FROM AUTHOR]