Precursors (pre-decreases and/or pre-increases of the cosmic ray intensity) are observed preceding Forbush decreases. The large Forbush decreases (magnitude > 5%) accompanied by geomagnetic storms (i.e., Dst < -100 nT and 5 ≤ Kp-index ≤ 9) have been studied in regard to precursors. The threshold value of the equatorial anisotropy one hour before the onset of the event (Axyb, %) was set at 0.8%. In total, 59 events with Axyb > 0.8% (test group) and 68 events with Axyb < 0.8% (control group), from 1957–2023, were examined. The ›Forbush Effects and Interplanetary Disturbances‹ database was used for selecting events and analyzing the solar, interplanetary, and geomagnetic conditions during each event. The method ›Ring of Stations‹ was also used. The chosen threshold is well set at 0.8%, since precursors are clearly present in the test group and principally absent in the control group.

The Forbush effects (FEs) in cosmic rays associated with interplanetary disturbances caused by the disappearance of solar filaments (DSFs) outside active regions (ARs) are considered. In total, 481 FEs were detected for 1995–2023 using the database of Forbush Effects and Interplanetary Disturbances (FEID). The behavior of the cosmic ray density was calculated using the Global Survey Method (GSM). The distributions of the FE numbers depending on their duration and magnitude, as well as on the characteristics of the interplanetary and near-Earth medium, were obtained. It is found that the average duration of such FEs (33.4 ± 0.5 h) is almost the same as for events associated with CMEs from ARs, but the average magnitude is much smaller (0.83 ± 0.03%). It is also shown that coronal mass ejections (CMEs) caused by DSFs are often low-speed interplanetary disturbances (with an average maximum SW speed of 423.2 ± 3.5 km/s), the velocities of which are close to the speed of the background solar wind (SW). During FEs associated with CMEs after DSFs outside ARs, on average, unsettled geomagnetic activity is observed. Magnetic storms were recorded only in 19% of events. Lower values of FE magnitude and geomagnetic activity are associated with weakened magnetic fields and low speeds of such interplanetary disturbances.

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 in galactic cosmic rays (according to data from a network of neutron monitors) and accompanying geomagnetic disturbances over a long period from 1957 to 2022 have been identified and studied. Statistical relationships between various parameters of cosmic ray flux and geomagnetic activity indices are analyzed. It has been established that the magnitude of Forbush decreases depends nonlinearly on the class of geomagnetic storm. A moderate correlation is found between the extreme values of various geomagnetic activity indices (Ap, Kp, Dst) and the characteristics of cosmic rays. It is also shown that the simultaneous registration of extreme values of cosmic rays and geomagnetic activity parameters does not always occur but depends on the sign of the Bz-component of the interplanetary magnetic field in a particular event.

The behavior of the main parameters of the interplanetary medium, cosmic ray variations, and geomagnetic activity as magnetic clouds pass the Earth (466 events from 1967 to 2021) was studied. Time distributions of these parameters during magnetic clouds passage are considered. It is shown that the maximum values of the solar wind velocity, interplanetary magnetic field strength, and geomagnetic activity indices are more often recorded in the front part of the magnetic cloud, while the minimum values of the temperature index, density, and equatorial component of cosmic ray anisotropy can be observed in any part of the studied structure.

Based on a large amount of experimental material, the hourly values of the solar wind speed and proton temperature were compared; the expected proton temperature and temperature index (the ratio of the observed temperature to the expected one) were calculated. Using the Cosmic Ray Variations Database, from 1997 to 2022 low-temperature periods were identified (intervals lasting more 2 h, in which hourly values of the temperature index less than 0.5). The study investigated (a) statistical relationships between the parameters of low-temperature periods and the characteristics of Forbush decreases associated with different types of solar sources; (b) distributions of parameters of low-temperature periods for interplanetary disturbances containing or not containing a magnetic cloud. The results showed that with increasing duration of the low-temperature period, the proportion of events associated with ejections from active regions increases, while the proportion of recurrent events and events associated with ejections outside active regions decreases. The correlation of the parameters of low-temperature periods with the magnitude of Forbush decreases is weak; with the equatorial anisotropy of cosmic rays, moderate; and with the azimuthal anisotropy, significant. The solar wind speed and magnetic field strength correlate moderately with the temperature index, and the correlation of the range of these parameters with the duration of low-temperature periods is significant or strong.

An abnormal behavior of galactic cosmic rays in September 2014 – February 2015, manifested in a significant modulation of its flux with a period close to solar rotation, is studied. The state of the solar magnetic field, changes in the parameters of the solar wind and interplanetary magnetic field during the specified period are analyzed. The reasons for the occurrence of longitudinal asymmetry in the distribution of galactic cosmic rays in the inner heliosphere are discussed. It has been established that the period under study is divided into two parts with different physical conditions on the Sun. Conclusions have been drawn about the decisive joint influence of sporadic and recurrent events: repeatedly renewable “magnetic traps” created by successive coronal mass ejections from the same longitudinal zone and anomalously expanded polar coronal holes with an enhanced magnetic field.

Statistical relations between the geomagnetic Dst index, cosmic ray variations, and solar wind characteristics are compared for Forbush decreases associated with: (i) coronal mass ejections from active regions (AR-CMEs) accompanied by solar flares, (ii) filament eruptions outside active regions, (iii) corotating interaction regions (CIRs) caused by high-speed streams from coronal holes, (iv) mixed events induced by two or more solar sources. Relationships of geomagnetic indices and parameters of cosmic rays and the solar wind are also compared between sporadic events with or without magnetic clouds (MCs) and between Solar Cycles (SCs) 23 and 24. The results reveal that interplanetary disturbances originated by AR-CMEs associated with an MC are most geoeffective and cause powerful geomagnetic storms, while CIRs create only moderate and weak storms. Sporadic and recurrent events differ in values of the Dst index and southward component of the magnetic field, as well as in the relationship between them. For sporadic events, geomagnetic activity is more affected by the presence or absence of an MC than by the type of solar source. Interplanetary disturbances associated with AR-CMEs are more effective in SC 23 while those associated with other types of solar sources have approximately the same geoeffectiveness in both SCs.

An anomaly in the behavior of galactic cosmic rays in September 2014–February 2015 was studied, which manifested itself as significant modulation of their flux with a period close to the Sun’s rotation. The state of the solar magnetic field and changes in the parameters of the solar wind and interplanetary magnetic field during the specified period are analyzed. The reasons for the longitudinal asymmetry in the distribution of galactic cosmic rays in the inner heliosphere are discussed. It has been established that the studied period is divided into two parts with different physical conditions on the Sun. Conclusions are drawn on the decisive joint influence of sporadic and recurrent events: repeatedly renewed “magnetic traps” created by successive coronal mass ejections from the same longitudinal zone, and anomalously expanded polar coronal holes with an enhanced magnetic field.

The updated database of Forbush effects and interplanetary disturbances ( https://tools.izmiran.ru/feid ) is used for an extensive analysis of various characteristics of events caused by the influence of interacting solar wind disturbances on the near-Earth space. In particular, the cases of different combinations of the pair interaction of high-speed streams from coronal holes and coronal mass ejections over the long period from 1995 to 2022 are considered. Variations in the flux of galactic cosmic rays (with a rigidity of 10 GV) and changes in the parameters of the interplanetary medium and geomagnetic activity are described. It is shown that the degree of mutual influence depends on the time between the detection of neighboring events; with the most pronounced changes in various parameters for events whose interaction occurred before reaching Earth’s orbit. It is also established that in interacting solar wind disturbances not only the extremes of the parameters of cosmic rays, interplanetary medium, and geomagnetic activity but also their time profiles are subject to changes.

Forbush effects in galactic cosmic rays (according to the neutron monitor network data) and accompanying geomagnetic disturbances over a long period from 1957 to 2022 have been identified and studied. Statistical relationships between various parameters of cosmic ray flux variations and geomagnetic activity indices are analyzed. It has been established that the magnitude of Forbush effects depends nonlinearly on the class of geomagnetic storm. A moderate correlation (up to 0.67) was found between the extreme values of various geomagnetic activity indices (Ap, Kp, Dst) and cosmic ray characteristics. It is shown that the extreme values of cosmic ray and geomagnetic activity parameters are not always detected simultaneously: it depends on the sign of the Bz-component of the interplanetary magnetic field in a specific event.

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.

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.

This paper considers the modeling of the cosmic ray protons transport as well as the secondary component through the Earth atmosphere for periods corresponding to real events of solar energetic particles (SEP). We carried out an analysis of the primary proton flux spectral characteristics. The main work results are quantitative estimates of the calculated ionization rate for an altitude range from 0 to 98 km. Also, our work includes an estimation of the difference in the effect of solar protons on the Earth's atmosphere for SEP events with similar sources, but with different spectral energy characteristics of primary particle fluxes.

The article focuses on identifying and studying several large-scale solar-wind disturbances and associated Forbush effects in the first months of 2023. Variations of the cosmic-ray flux (with 10 GV rigidity) are obtained using the Global Survey Method with data from the global network of neutron monitors. The beginning of 2023 is characterized by a relatively large number of Forbush effects; the largest ones were recorded on 26 – 28 February, 15 – 16 March, 23 – 25 March, and 23 – 24 April. These events and their relationship with solar-wind parameters, geomagnetic activity, and associated solar sources are discussed in detail. In terms of the number and magnitude of interplanetary disturbances and corresponding cosmic-ray variations, February–April 2023 proves to be the first active period since the beginning of Solar Cycle 25.

The most interesting large-scale solar wind disturbances, which led to significant modulation of cosmic rays and registration of geomagnetic storms over the past two years, are studied. An analysis and a description of solar and interplanetary sources of these events are presented.

The study of precursors preceding Forbush decreases belongs to the applied side of space research and to a relatively new area of modern science, that of Space Weather. Moreover, it is a pioneering and innovative research field with interesting results. In the framework of the above, the Athens Cosmic Ray Group of the National and Kapodistrian University of Athens (NKUA) and the Cosmic Ray Group of the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences (IZMIRAN) have collaborated in investigating predecreases and/or preincreases of the cosmic-ray intensity before the development of a Forbush decrease, that could serve as precursory signs of the upcoming event and consequently play a significant role in the prediction of cosmic-ray and geomagnetic activity. In this work, the criterion of the increased anisotropy one hour before the onset of the event ( A_𝑥𝑦𝑏 , A_𝑥𝑦𝑏 ≥ 0.8

The article investigates the statistical relations between the values of geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases with a sudden and gradual commencement associated with different types of solar sources: (a) coronal mass ejections from active regions accompanied by solar flares; (b) filament eruptions outside active regions; (c) high-speed streams from coronal holes; and (d) multiple sources. Using statistical methods, we also compare the dependence of geomagnetic indices on cosmic ray and solar wind parameters for Forbush decreases in solar cycles 23 and 24. The results show that (a) interplanetary disturbances associated with coronal mass ejections from active regions cause mainly magnetic storms with a sudden commencement, (b) interplanetary disturbances associated with high-speed streams from coronal holes cause mainly storms with a gradual commencement, and (c) interplanetary disturbances associated with filament eruptions outside active regions cause equally likely storms with a sudden and gradual commencement. For sporadic Forbush decreases, the cosmic ray and geomagnetic activity parameters are, on average, larger for sudden commencement events; for recurrent Forbush decreases, the nature of the event commencement does not affect the magnitude of these parameters. For all types of solar sources, the disturbed solar wind parameters are, on average, larger in events with sudden commencement. The geoeffectiveness of interplanetary disturbances is significantly higher in cycle 23 for events associated with ejections from active regions; for other types of disturbances, the difference between cycles is weak.

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.

ABSTRACT In this paper, Forbush decrease (FD) profiles are compared for events associated with (i) coronal mass ejections from active regions accompanied by solar flares (AR CMEs), (ii) filament eruptions away from active regions (non-AR CMEs), and (iii) high-speed streams (HSSs) from coronal holes (CHs). FD profiles are described by time parameters that are delayed from an FD onset to the registration of maximum values of cosmic ray (CR) density variations, CR density hourly decrease, CR equatorial anisotropy, solar wind (SW) speed, interplanetary magnetic field (IMF) strength and minimum Dst index. Distributions of these parameters from 1997 to 2020 and within maxima and minima of the last solar cycles (SCs) were compared by statistical methods. The results obtained reveal that statistical properties of the time parameters depend both on the FD source and on the solar activity period. FDs associated with AR CMEs develop even at close values of SW parameters faster than those associated with non-AR CMEs and HSS from CHs. Differences between typical FD profiles for events associated with AR and non-AR CMEs are more significant when the interplanetary disturbance contains a magnetic cloud. The difference between FD profiles for events associated with AR and non-AR CMEs is less distinguishable within maximum SC 24 than within maximum SC 23. For FDs associated with HSS from CHs, the main phase durations and the time delays of maximal SW speed are longer within SC 23–24 minimum, while the time delays of maximal IMF strength differ insignificantly between 23–24 and 24–25 minima.