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We identify and investigate cosmic ray Forbush decreases (FDs) observed in the E > 50 MeV data from the Helios 1 and 2 spacecraft, spanning from 1974–1985 and covering heliocentric distances in the range 0.28–1 au. A Helios FD catalogue is compiled, including the characteristics of the cosmic ray variations, as well as those of the solar wind (SW) and the interplanetary magnetic field (IMF) of the associated interplanetary disturbances. An extended statistical study considers the radial dependence of the FD magnitude, the SW velocity, and the IMF intensity in these disturbances. It is found that the Helios FD sizes at different distances from the Sun are determined by the parameters of the interplanetary disturbances. In particular, the FD magnitudes observed at Helios, as well as near Earth, correlate well with VB, which is the product of the maximum SW velocity and the IMF intensity when normalized by the average values of these parameters at the radial distance of the observations. However, we found that, on average, the Helios FD sizes are statistically independent of the radial distance in the range of 0.28–1 au.

A statistical study of interplanetary disturbances with different positions of the interplanetary magnetic field maximum that generate Forbush decreases is performed. It is shown that the most effective interplanetary disturbances are those with the position of the field maximum 6–15 h after the start of the event.

The behaviour of the solar wind (SW) proton temperature and velocity and their relationship during Forbush decreases (FDs) associated with various types of solar source – coronal mass ejections (CMEs) and coronal holes (CHs) – have been studied. Analysis of cosmic ray variations, SW temperature, velocity, density, plasma beta, and magnetic field (from 1965–2019) is carried out using three databases: the OMNI database, Variations of Cosmic Rays database (IZMIRAN) and Forbush Effects & Interplanetary Disturbances database (IZMIRAN). Comparison of the observed SW temperature (T) and velocity (V) for the undisturbed SW allows us to derive a formula for the expected SW temperature (Texp, the temperature given by a T–V formula, if V is the observed SW speed). The results reveal a power-law T–V dependence with a steeper slope for low speeds (V < 425 km s−1, exponent = 3.29 ± 0.02) and flatter slope for high speeds (V > 425 km s−1, exponent = 2.25 ± 0.02). A study of changes in the T–V dependence over the last five solar cycles finds that this dependence varies with solar activity. The calculated temperature index KT = T/Texp can be used as an indicator of interplanetary and solar sources of FDs. It usually has abnormally large values in interaction regions of different-speed SW streams and abnormally low values inside magnetic clouds (MCs). The results obtained help us to identify the different kinds of interplanetary disturbance: interplanetary CMEs, sheaths, MCs, corotating interaction regions, high-speed streams from CHs, and mixed events.

The behavior of high-energy magnetospheric electrons with energies of >2 MeV in geostationary orbit during solar cycles 22–24 is considered. It is shown that the most increases in electrons occurs during the declining phase of solar activity, when a greater number of geoeffective coronal holes are observed. The 2003–2015 data show there is generally a correspondence between the behavior of the number of increases in electrons and the area of geoeffective coronal holes.

For over 60 years, neutron monitors have remained the main standard, stable instrument for the measurement of cosmic rays with energies from 400 MeV to hundreds of GeV. In order to obtain sufficiently complete information about the distribution of cosmic rays outside the magnetosphere, it is necessary to have a network of detectors that are fairly evenly spaced around the globe. One of the most useful methods to obtain the properties of the angular distribution of cosmic rays without decomposition into harmonics is the ring of station method. It allows one to obtain the instantaneous (more precisely, hourly) longitudinal distribution of the intensity of cosmic rays without its modeling. The main goal of this work is to expand the use of the ring of station method, a convenient and useful method for the study of cosmic ray variations.

Specific features of the behavior of streams of relativistic magnetospheric electrons at energies higher than 2 MeV on geostationary orbits during different types of interplanetary disturbances are studied. More than 30 years (1986–2019) of observations of these particles onboard the GOES satellites is analyzed. It is shown that plasma coronal ejections and high-speed streams from coronal holes differently influence the behavior of high-energy electrons. High-speed streams from coronal holes are more effective In formation of electron flux rises than solar plasma ejections.

With the 'Proceedings of the 1st virtual symposium on cosmic ray studies with neutron detectors' launches the new open access series 'Cosmic ray studies with neutron detectors'. The volume comprises the papers presented at the online meeting held in July 2020. The contributions show that neutron detectors on the ground provide significant results for studying the interaction of galactic cosmic rays with magnetic fields in the heliosphere, for accelerating energetic particles, and for a growing number of applications, including geophysics and space weather. The easily accessible databases around the project 'Real-Time database for high resolution Neutron Monitor measurements' (NMDB) make the original data readily available to a large user community.

An overview on the presentations at the first virtual symposium on cosmic ray studies with neutron detectors is given. The meeting was held online in July 2020. Neutron detectors on ground are shown to provide significant contributions to research on interactions of galactic cosmic rays with magnetic fields in the Heliosphere and on the acceleration of energetic particles, as well as to a growing range of applications, including geophysics and space weather. The advent of easily accessible databases makes original data easily available to a large user community. The present overview outlines and introduces the more detailed articles contained in the proceedings.

The study of space-weather effects and more specifically Forbush decreases of the cosmic-ray intensity depends on space and ground measurements. Very often Forbush decreases and geomagnetic storms are accompanied by pre-increases and/or pre-decreases manifested in cosmic-ray behavior, known as precursory signs. These cosmic-ray intensity variations do not coincide with the shock arrival but begin well before (up to 24 hours) the onset of the main event. In this study a group of large Forbush decreases with amplitude ≥ 4% was examined for precursors. According to the helio-longitude of the solar source, the events were separated into three categories: western ( $21^{\circ} \leq $ helio-longitude ≤ 60∘), eastern ( $-60^{\circ} \leq $ helio-longitude $\leq -21^{\circ}$ ), and central ( $-20^{\circ} \leq $ helio-longitude $\leq 20^{\circ}$ ). The selected events cover 1967 – 2017. The analysis of the Forbush decreases and the plotting of the asymptotic longitudinal cosmic-ray distribution diagrams were based on the “Global Survey Method” and the “Ring of Stations” method, respectively. Data on solar flares, solar-wind speed, interplanetary magnetic field, and geomagnetic indices (Kp and Dst) were also used. The results show the clear signs of precursors in a significant number of events.

In this investigation the different features and characteristics of Forbush decreases, with emphasis on large For- bush decreases (≥4%) and their association to solar sources, are being examined. According to the heliolongitude of the solar source, the events under study were separated into three subcategories: western (21º ≤ heliolongitude ≤ 60º), eastern (-60º ≤ heliolongitude ≤ -21º) and central (-20º ≤ heliolongitude ≤ 20º). The selected events cover the time period 1967 - 2017. The ‘Global Survey Method’ was used for analyzing the Forbush decreases, along with data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. In ad - dition, the superimposed epoch method was applied in order to plot the time profiles for the aforementioned group of events. This detailed analysis reveals interesting results concerning the features of cosmic ray decreases in re- gard to the heliolongitude of the solar sources. Moreover, it is also shown that large Forbush decreases, regardless of the heliolongitude of the solar source, are accompanied by increased geomagnetic activity and increased aniso- tropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases.

One of the factors responsible for the wide variety of Forbush decreases is the different solar sources related to them. In this investigation the different features and characteristics of Forbush decreases, with emphasis on large Forbush decreases and their association with solar sources, are examined. Initially, a wider selection of events from the Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation of the Russian Academy of Sciences Forbush decreases database served as a starting point for this study, which was then narrowed down to a group of large Forbush decreases. According to the helio-longitude of the solar source, the events under study were separated into three subcategories: western ( $21^{\circ} \leq \mbox{helio-longitude} \leq 60^{\circ}$ ), eastern ( $-60^{\circ} \leq \mbox{helio-longitude} \leq -21^{\circ}$ ), and central ( $-20^{\circ} \leq \mbox{helio-longitude} \leq 20^{\circ}$ ). The selected events cover the period 1967 – 2017. The “Global Survey Method” was used for analyzing the aforementioned Forbush decreases, along with data on solar flares, solar-wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field. The superimposed epoch method was applied to display the temporal profiles for the selected events. This detailed analysis reveals interesting results concerning the features of cosmic-ray decreases in relation to the helio-longitude of the solar sources. Specifically, Forbush decreases related to central or eastern solar sources are more often observed, have a greater magnitude, and present a slower development than Forbush decreases related to western sources, which are rarer, have a smaller magnitude, and have a shorter lifespan. Nevertheless, regardless of the helio-longitude of the solar source, large Forbush decreases are accompanied by increased geomagnetic activity and increased anisotropy, including anisotropy before the events, which can serve as a typical precursor of Forbush decreases.

For over 60 years, neutron monitors have been the main standard and high precision detectors for measuring cosmic rays with energy from 400 MeV to hundreds GeV. In order to obtain sufficiently complete information about the distribution of cosmic rays outside the magnetosphere, it is necessary to have a network of detectors spaced evenly enough around the globe. The ring of stations method is one of the most useful methods for studying the properties of the angular distribution of cosmic rays without expressing the cosmic ray intensity in terms of spherical harmonics. The method allows one to get the hourly longitude distribution of the cosmic ray intensity without modeling. The main objective of this work is to expand the use of the ring of stations method, as it is a convenient and useful method of studying cosmic ray variation. Using the ring of stations method, it is possible to study specific angular distributions of cosmic ray variation that are described poorly by the sum of the first spherical harmonics. The ring of stations method is primarily used to study Forbush decreases. Detailed descriptions of Forbush decrease investigation by the ring of stations method are presented in this study. The application of the method to the study of the precursors of Forbush decreases and cosmic rays behavior inside the solar wind disturbances is shown.

In this paper, we compare Forbush decreases associated with solar wind disturbances from two types of solar sources: coronal mass ejections (sporadic Forbush decreases) and coronal holes (recurrent Forbush decreases). We used Forbush Effects and Interplanetary Disturbances database created and maintained by IZMIRAN and associated a certain amount of Forbush decreases in this database with interplanetary disturbances from coronal mass ejections (207 cases) as well as high speed streams from coronal holes (350 cases). Large number of events allows to apply statistical methods (distribution analysis, correlation analysis, multiple linear regression analysis) to compare those two groups of events. The results reveal that Forbush decreases associated with solar wind disturbances from the two types of solar sources differ significantly in their statistical characteristics: distributions, descriptive statistics, correlation and multiple regression coefficients.

The IZMIRAN database of Forbush effects and interplanetary disturbances has been used to study long-term changes in the number and magnitude of Forbush effects in the last six solar cycles (1957–2016) for cosmic rays of rigidity of 10 GV. Solar activity cycles have been shown to be well expressed in data of Forbush effects, especially in large magnitude events that almost disappear in minima. The changes in the distribution of Forbush effects and the decrease in their average values from solar activity maximum to minimum are explained by the predominance of cosmic-ray variations due to the action of coronal holes at low activity. It should be noted that the current cycle involves fewer and generally weaker Forbush effects than in the previous five cycles. For each month, an FD index combining the magnitude and number of Forbush effects and convenient for studying long-term variations has been proposed and calculated.

One of the variants of the global survey method developed and used for many years at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences is described. Data from the world network of neutron monitors for every hour from July 1957 to the present has been processed by this method. A consistent continuous series of hourly characteristics of variation of the density and vector anisotropy of cosmic rays with a rigidity of 10 GV is obtained. A database of Forbush decreases in galactic cosmic rays caused by large-scale disturbances of the interplanetary medium for more than half a century has been created based on this series. The capabilities of the database make it possible to perform a correlation analysis of various parameters of the space environment (characteristics of the Sun, solar wind, and interplanetary magnetic field) with the parameters of cosmic rays and to study their interrelationships in the solar–terrestrial space. The features of reception coefficients for different stations are considered, which allows the transition from variations according to ground measurements to variations of primary cosmic rays. The advantages and disadvantages of this variant of the global survey method and the opportunities for its development and improvement are assessed. The developed method makes it possible to minimize the problems of the network of neutron monitors and to make significant use of its advantages.

The IZMIRAN database of Forbush effects and interplanetary disturbances was used to study features of the action of high-speed solar wind streams from coronal holes on cosmic rays. Three hundred and fifty Forbush effects created by coronal holes without other actions were distinguished. The mean values and distributions have been found for different characteristics of events from this group and compared with all Forbush effects and Forbush effects caused by coronal ejections. Despite the great differences in high-speed streams from coronal holes, this group turned out to be more compact and uniform as compared to events related to coronal ejections. Regression dependences and correlation relations between different parameters of events for the studied groups have been obtained. It has been shown that Forbush effects caused by coronal ejections depend considerably more strongly on the characteristics of interplanetary disturbances as compared to Forbush effects related to coronal holes. This suggests a significant difference between the modulation mechanisms of Forbush effects of different types and corroborates earlier conclusions based on indirect data.

The effects interplanetary disturbances induced by coronal plasma ejections and the high-velocity solar wind from coronal holes have on high-energy (E > 2 MeV) magnetospheric electrons are investigated. Experimental data collected over 30 years of observations (1986–2017) are analyzed. Coronal plasma ejections and high-velocity outflows from coronal holes are shown to have different effects on the behavior of high-energy electrons: the latter create stronger electron fluxes at geostationary orbits than the former.

Forbush decrease (FD) parameters (magnitude, rate of decrease, anisotropy, and duration of the main phase) are compared for solar cycles 19–24 and in the maxima of cycles 23 and 24 and the minimum between them. The Forbush Effects and Interplanetary Disturbances database created and maintained at IZMIRAN allows statistical means to be used for data analysis. The results obtained in this work show that the relation between FDs and solar cyclicity is observed for all FD parameters; most FDs in the maximum of cycle 23 were caused by interplanetary disturbances associated with coronal mass ejections, while most of FDs in the minimum between cycles 23 and 24 were caused by high-speed streams from coronal holes.

Since 1998, the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN) has had an operating heliogeophysical service—the Center for Space Weather Forecasts. This center transfers the results of basic research in solar–terrestrial physics into daily forecasting of various space weather parameters for various lead times. The forecasts are promptly available to interested consumers. This article describes the center and the main types of forecasts it provides: solar and geomagnetic activity, magnetospheric electron fluxes, and probabilities of proton increases. The challenges associated with the forecasting of effects of coronal mass ejections and coronal holes are discussed. Verification data are provided for the center’s forecasts.

We consider the behavior of anisotropy and density of galactic cosmic rays in the first hours of Forbush effects from 1957 to 2014 initiated by the arrival of a shock wave. It has been shown that, as early as the event commencement, the first harmonic of anisotropy tends to increase substantially and its direction changes significantly. The more is powerful the interplanetary disturbance, the greater are the changes. Based on changes in some parameters of anisotropy and density, we can estimate the heliolongitude of the disturbance source, as well as the further development of the Forbush effect and geomagnetic activity.

Variations in the cosmic-ray vector anisotropy observed on Earth are closely connected with the state of the near-Earth interplanetary medium. Hourly characteristics of vector anisotropy for the period 1957–2013, which were obtained by the global survey method from the data of the worldwide network of neutron monitors, make it possible to study the relationship between the cosmic-ray anisotropy and solar wind parameters. In the present work, we have studied the connection between the equatorial component of anisotropy of cosmic rays with a rigidity of 10 GV and the following parameters: velocity and density of the solar wind; density of the interplanetary magnetic field; and cosmic-ray density variations, in which the spatial gradient of cosmic rays in the interplanetary medium is manifested. The characteristics of cosmic-ray anisotropy at various combinations of the interplanetary medium parameters are compared. The possibility of diagnosing the solar wind state from data on the cosmic-ray anisotropy is discussed.

On the approach to minimum of Solar Cycle 24, on 26 August 2018, an unexpectedly strong geomagnetic storm (GMS) suddenly occurred. Its Dst index reached -174 nT, that is the third of the most intense storms during the cycle. The analysis showed that it was initiated by a two-step long filament eruption, which occurred on 20 August in the central sector of the solar disk. The eruptions were accompanied by two large-scale divergent ribbons and dimmings of a considerable size and were followed by relatively weak but evident Earth-directed coronal mass ejections. In the inner corona, their estimated speed was very low of about 200-360 km/s. The respective interplanetary transients apparently propagated between two high-speed solar wind streams originated from a two-component coronal hole and therefore their expansion was limited. The resulting ejecta arrived at the Earth only on 25 August and brought an unexpectedly strong field of Bt ~ 18.2 nT with a predominantly negative Bz component of almost the same strength. The geospace storm also manifested itself in the form of a peculiar Forbush decrease (FD). Its magnitude was about 1.5%, which is rather small for the observed G3-class GMS. The main unusual feature of the event is that large positive bursts with an enhancement up to 3% above the pre-event level were recorded on the FD background. We argue that these bursts were mainly caused by an unusually large and changeable cosmic ray anisotropy combined with lowering of the geomagnetic cutoff rigidity in the perturbed Earth's magnetosphere under conditions of the cycle minimum., 19 pages, 8 figures, Solar Physics

In spite of the fact that the current Solar Cycle 24 is close to its end now, it is a less active Solar Cycle, during its time period (2008 – 2016) and a lot of Forbush decreases of cosmic ray intensity with rigidity 10 GV and amplitude greater than 2% were recorded by the ground-based neutron monitors. Among these events, the ones associated with sudden geomagnetic storm commencements (SSCs) and presenting a first harmonic of cosmic ray anisotropy greater than 0.8% were examined. Cosmic ray data recorded at the neutron monitor stations were obtained from the European high resolution neutron monitor database, while the Forbush decreases, accompanied by their characteristics were accessed from the updated Database of the Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (IZMIRAN). Solar, interplanetary and geomagnetic characteristic parameters of each event separately have been studied in detail and analyzed. It was shown through the usage of the “ring of neutron monitor stations” method that, in some cases, precursory signals before the main phase of the event appeared. After an extended study of the Forbush decreases precursors during the examined period, the appearance of pre-decreases and/or pre-increases of the cosmic ray intensity before the beginning of the events, acting as precursory signals, were identified in almost half of the cases studied. In combination with other parameters, their common features are discussed, with the purpose of monitoring and possibly forecasting the space-weather conditions.

Forbush decreases (FDs) in galactic cosmic rays (GCRs) have been recorded by neutron monitors (NMs) at Earth for more than 60 years. For the past five years, with the establishment of the Radiation Assessment Detector (RAD) onboard the Mars Science Laboratory (MSL) rover Curiosity at Mars, it is possible to continuously detect, for the first time, FDs at another planet: Mars. In this work, we have compiled a catalog of 424 FDs at Mars using RAD dose rate data, from 2012 to 2016. Furthermore, we applied, for the first time, a comparative statistical analysis of the FDs measured at Mars, by RAD, and at Earth, by NMs, for the same time span. A carefully chosen sample of FDs at Earth and at Mars, driven by the same ICME, led to a significant correlation ($cc=0.71$) and a linear regression between the sizes of the FDs at the different observing points at the respective energies at Mars and Earth. We show that the amplitude of the FD at Mars ($A_{\mathrm{M}}$), for an energy of $E>150~\mbox{MeV}$, is higher by a factor of 1.5 – 2 compared to the size of the FD at Earth ($A_{\mathrm{E}}$), for a definite rigidity of 10 GV. Finally, almost identical regressions were obtained for both Earth and Mars as concerns the dependence of the maximum hourly decrease of the CR density ($\mathit{DM}_{\mathrm{in}}$) to the size of the FD.

In this work, forecasting results of the daily Ap geomagnetic index by different Space Weather Prediction Centers (SWPCs) covering the time period from October 2014 up to July 2020, are considered. A three-day forecast of this index obtained from the space weather prediction centers, is analyzed. Standard forecast verification measures, descriptive statistics with correlation coefficients, and error analysis between forecasts and observations have been performed to evaluate the quality or the skill of the predictions. In particular, an error analysis using fit performance metrics, such as the mean average error (MAE) and the root mean squared error (RMSE) among others, as well as the threshold performance metrics, such as the Hansen-Kuipers, Gilbert and Heidke skill scores, the probability of detection (POD), the probability of false detection (POFD) and the area under the curve in receiving the operating characteristic (ROC) plot among others, are calculated. For the Ap geomagnetic index predictions during Day-0, the Pearson's correlation coefficient is ranging between the values of 0.57 and 0.79, while during the Day-2 it is decreased to the range from 0.37 to 0.44. In conclusion, the majority of the used SWPCs perform quite accurately on the conditions of Space Weather in active as well as in quiet periods showing a reliable effort for predicting geomagnetic storms.

The daily fluence is selected as the main characteristic of the behavior of electrons with E > 2 MeV measured by GOES satellites in geostationary orbits, since this characteristic is closely associated with malfunctions of satellite electronic equipment. It is shown that increases in the flux of high-energy magnetospheric electrons are associated with considerable interplanetary and magnetospheric perturbations, but lag behind them by 1–3 days. A greatly increased solar wind speed is observed as early as 3 days before the electron flux starts to grow, with a maximum being reached by the onset of growth. It is shown that the electron fluence is weakly associated with the level of geomagnetic activity on the same day but correlates to the А р index of geomagnetic activity observed 2–3 days earlier. The fluence of high-energy magnetospheric electrons is closely associated with the solar wind’s speed, especially with its value measured 2 days earlier.

Variations in the cosmic ray density during the initial phase of the Forbush effect during the first hours after the arrival of the interplanetary shock wave have been studied with the use of data on variations in the cosmic ray density with a rigidity of 10 GV obtained by the global survey method by the world network of neutron monitors in 1957‒2012. It is found that behavior of this parameter after the arrival of the shock wave demonstrates high variability. A small (~1/5 of total number), though distinct, group of Forbush effects, in which the density of the cosmic ray increases (not decreases) after the arrival of the shock wave, is defined. As a whole, the initial variation in cosmic ray density is correlated with the Forbush effect magnitude and the strength of the associated geomagnetic disturbance.

We perform a principal component analysis (PCA) on a set of six solar variables (i.e. width/size ( $s$ ) and velocity ( $u$ ) of a coronal mass ejection, logarithm of the solar flare (SF) magnitude ( $\log\mathit{SXRs}$ ), SF longitude ( $\mathit{lon}$ ), duration ( $\mathit{DT}$ ), and rise time ( $\mathit{RT}$ )). We classify the solar energetic particle (SEP) event radiation impact (in terms of the National Oceanic and Atmospheric Administration scales) with respect to the characteristics of their parent solar events. We further attempt to infer the possible prediction of SEP events. In our analysis, we use 126 SEP events with complete solar information, from 1997 to 2013. Each SEP event is a vector in six dimensions (corresponding to the six solar variables used in this work). The PCA transforms the input vectors into a set of orthogonal components. By mapping the characteristics of the parent solar events, a new base defined by these components led to the classification of the SEP events. We furthermore applied logistic regression analysis with single, as well as multiple explanatory variables, in order to develop a new index ( $I$ ) for the nowcasting (short-term forecasting) of SEP events. We tested several different schemes for $I$ and validated our findings with the implementation of categorical scores (probability of detection (POD) and false-alarm rate (FAR)). We present and interpret the obtained scores, and discuss the strengths and weaknesses of the different implementations. We show that $I$ holds prognosis potential for SEP events. The maximum POD achieved is 77.78% and the relative FAR is 40.96%.

The global survey method (GSM) technique unites simultaneous ground-level observations of cosmic rays in different locations and allows us to obtain the main characteristics of cosmic-ray variations outside of the atmosphere and magnetosphere of Earth. This technique has been developed and applied in numerous studies over many years by the Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation (IZMIRAN). We here describe the IZMIRAN version of the GSM in detail. With this technique, the hourly data of the world-wide neutron-monitor network from July 1957 until December 2016 were processed, and further processing is enabled upon the receipt of new data. The result is a database of homogeneous and continuous hourly characteristics of the density variations (an isotropic part of the intensity) and the 3D vector of the cosmic-ray anisotropy. It includes all of the effects that could be identified in galactic cosmic-ray variations that were caused by large-scale disturbances of the interplanetary medium in more than 50 years. These results in turn became the basis for a database on Forbush effects and interplanetary disturbances. This database allows correlating various space-environment parameters (the characteristics of the Sun, the solar wind, et cetera) with cosmic-ray parameters and studying their interrelations. We also present features of the coupling coefficients for different neutron monitors that enable us to make a connection from ground-level measurements to primary cosmic-ray variations outside the atmosphere and the magnetosphere. We discuss the strengths and weaknesses of the current version of the GSM as well as further possible developments and improvements. The method developed allows us to minimize the problems of the neutron-monitor network, which are typical for experimental physics, and to considerably enhance its advantages.

We study the Forbush decreases in cosmic-ray intensity from January 2008 to December 2013, covering the minimum between Solar Cycles 23 and 24 and the ascending phase of Cycle 24. We performed a statistical analysis of 617 events and concentrated on three of the most important ones. We used the IZMIRAN database of Forbush effects obtained by processing the data of the worldwide neutron monitor network using the global survey method. The first event occurred on 18 February 2011 with a ${\sim}\,5~\%$ decrease of cosmic rays with 10 GV rigidity, the second on 8 March 2012 with an amplitude of ${\sim}\,12~\%$ , and the third on 14 July 2012 with an amplitude of ${\sim}\,6~\%$ . For these three events, we also studied the events that occurred on the Sun and the way that these affected the interplanetary space, and finally provoked the decreases of the galactic cosmic rays near Earth. We found that each neutron monitor records these decreases, which depend on the cut-off rigidity of the station. We carried out a statistical analysis of the amplitude of the cosmic-ray decreases with solar and geomagnetic parameters.

We investigate the characteristics of Galactic cosmic rays in events associated with magnetic clouds that reach Earth. A mathematical model, capable of describing the distribution of the cosmic-ray density in a magnetic cloud is considered. We show that in most cases the behavior of the cosmic-ray density within magnetic clouds at 1 AU can be described accurately by a parabolic function of the distance to the center of the magnetic cloud measured in gyroradii. As expected, the majority of magnetic clouds modulate cosmic rays, resulting in a reduction of their density. However, there is a group of events (about one fifth of the total sample) in which the density of cosmic rays in a magnetic cloud increases. Furthermore, the extremum (a minimum or a maximum) of the cosmic-ray density is found closer to the cloud center and not at its edges. We consider a number of the factors contributing to the model and estimate the effect of each factor.

Coronal mass ejections (CMEs) and their interplanetary counterparts (interplanetary coronal mass ejections, ICMEs) are responsible for large solar energetic particle events and severe geomagnetic storms. They can modulate the intensity of Galactic cosmic rays, resulting in non-recurrent Forbush decreases (FDs). We investigate the connection between CME manifestations and FDs. We used specially processed data from the worldwide neutron monitor network to pinpoint the characteristics of the recorded FDs together with CME-related data from the detailed online catalog based upon the Solar and Heliospheric Observatory (SOHO)/Large Angle and Spectrometric Coronagraph (LASCO) data. We report on the correlations of the FD magnitude to the CME initial speed, the ICME transit speed, and the maximum solar wind speed. Comparisons between the features of CMEs (mass, width, velocity) and the characteristics of FDs are also discussed. FD features for halo, partial halo, and non-halo CMEs are presented and discussed.

It is shown that the behavior of the cosmic ray density in a magnetic cloud near the Earth can for most events be described by a simple parabolic dependence on a distance to the cloud’s center, measured in gyro radii. Most magnetic clouds modulate cosmic rays, lowering their density. However, there are events (approximately 1/5) in which the cosmic ray density rises in a magnetic cloud. Events with the positive effect usually have a maximum in the leading part of the cloud. Several factors are considered that contribute to a model description, and these contributions are estimated.

The distinguished directions, dependent on the solar wind velocity and IMF line position, exist in the interplanetary space, which results in the nonuniform distribution of phases and the amplitude-phase interrelation of the first cosmic ray anisotropy harmonic. The characteristics of the first anisotropy harmonics, determined for each hour using the global survey method based on the worldwide neutron monitor network from 1957 to 2010, were used to study long-period variations in the cosmic ray anisotropy. The longitudinal distributions of the cosmic ray vector anisotropy and the interrelation between the anisotropy amplitude and phase have been obtained for each year in this time interval. The results evidently demonstrate the anisotropy variations caused by the solar magnetic and activity cycles. The anisotropy distributions at different solar wind velocities have also been studied. Periods with a specific cosmic ray anisotropy behavior are distinguished and discussed. The obtained cosmic ray anisotropy variations agree with the convection-diffusion anisotropy model.

As suggested in many studies the pre-increases or pre-decreases of the cosmic ray intensity (known as precursors), which usually precede a Forbush decrease, could serve as a useful tool for studying space weather effects. The events in this study were chosen based on two criteria. Firstly, the heliolongitude of the solar flare associated with each cosmic ray intensity decrease was in the 50° –70°W sector and, secondly, the values of the geomagnetic activity index, Kp max, were ≥ 5. Twenty five events were selected from 1967 to 2006. We have used data on solar flares, solar wind speed, geomagnetic indices (Kp and Dst), and interplanetary magnetic field in our detailed analysis. The asymptotic longitudinal cosmic ray distribution diagrams were plotted using the “Ring of Stations” method for all the events. The results reveal clear signs of precursors in 60 % of selected events.

All significant events in galactic cosmic rays for the last 55 years have been collected in a Forbush effect database created at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation (hereinafter, IZMIRAN) based on data from the global network of neutron monitors. The solar sources of ∼800 of these events have been identified. These events were divided into five groups with respect to the heliolongitudes of the associated X-ray solar flares, and typical behavior of their characteristics such as cosmic ray density and anisotropy, was studied independently for each group. The Forbush effect characteristics, which are the most dependent on the source heliolongitude, have been identified.

For a comprehensive study of the Forbush effects and their relation to solar and geomagnetic activity, a database of transient phenomena in cosmic rays and the interplanetary medium has been created, which is continuously updated with data on new events. Based on these data, we study the dependence of the Forbush effects on various internal and external parameters, as well as select different groups of events. In this paper, we consider recurrent (caused by high-speed solar wind streams from coronal holes) and sporadic (associated with coronal mass ejections) events. We investigate groups of events with a sudden and gradual onset. We show that the resulting dependencies of the Forbush effects (on the parameters of interplanetary disturbances, geomagnetic activity indices, etc.) are substantially different for the above-mentioned groups. Most likely, these differences are caused by different sources of solar wind disturbances.