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.

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.

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.

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 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.

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.

In this paper, we study the development of Forbush decreases associated with coronal mass ejections from active regions accompanied by solar flares and filament eruptions from non-active regions using the database of Forbush effects and interplanetary disturbances created at IZMIRAN. We compared the development of two types of Forbush decreases during solar cycles 23–24, the maxima of these cycles, and the minimum between them. Using statistical methods, we studied the distributions of time intervals from the beginning of the Forbush decrease to registration: the minimum cosmic ray density, the maximum hourly decrease in density, the maximum cosmic ray anisotropy, the maximum solar wind velocity, the maximum strength of the interplanetary magnetic field, and the minimum of the Dst index. The difference in the development of two types of Forbush decreases was compared when the interplanetary disturbance contains or does not contain a magnetic cloud near the Earth. The results showed that flare-associated events develop faster than filament-associated events, even at close values of the solar wind parameters. The difference in the development of two types of Forbush decreases is more noticeable in the case of the presence of a magnetic cloud near the Earth’s orbit. The largest difference between the time parameters in the two types of events is observed for the time of registration of the maximum intensity of the interplanetary magnetic field. The main phase of the two types of Forbush decreases is the same at the solar cycle 23 maximum and longer for filament-associated events at the cycle 24 maximum and 23–24 minimum. Considering all time parameters, the difference in the development of the two types of Forbush decreases is more noticeable at the maximum of cycle 23 and at the minimum of cycle 23–24 than at the maximum of cycle 24.

In this paper, we study the similarities and differences of Forbush decreases in solar cycles 23 and24. Groups of events associated with various types of solar sources were analyzed: coronal mass ejections fromactive regions accompanied by solar flares (CME1 group), filament ejections outside active regions (CME2group), and high-velocity streams from coronal holes (CH group). The distributions and relations of variousparameters were studied: the amplitude of Forbush decreases, the maximum values of the hourly decrease inthe cosmic ray density, the equatorial cosmic ray anisotropy, the solar wind velocity, and the magnetic fieldstrength, as well as the values of the solar wind velocity and the magnetic field strength 1 hour before the onsetof the Forbush decrease during the event. The results showed that the number of events, parameter values,and their relations depend on the phase and cycle of solar activity. In the 24th cycle, the number of events inthe CME1 group decreased, did not change in CME2, and increased in CH. The values of the parametersand the difference between them in various groups of events are higher in cycle 23, which is characterized bygreater asymmetry and long “tails” of distributions. The magnitude of the Forbush decreases in the CME1group in cycle 23 depends more strongly on the solar wind velocity while, in cycle 24, on the magnetic fieldstrength, as in the CME2 group in both solar cycles. Multiple linear regression describes the dependences ofthe parameters of Forbush reductions in the CME1 and CME2 groups in the 23rd cycle and in the CME1group in the 24th cycle well.

According to the data of Moscow neutron monitor, using harmonic analysis, the characteristics of the solar-diurnal anisotropy of cosmic rays on quiet days have been obtained for a long period from 1965 to 2020. It has been established that the average diurnal variation of cosmic rays at the Moscow neutron monitor is almost completely described by two harmonics of the solar-diurnal anisotropy and does not contain signs of other influences. A comparison with the average daily characteristics of the equatorial component of cosmic rays vector anisotropy, obtained from the data of the worldwide neutron monitor network using the global survey method showed a good agreement between the results of the two methods. From a comparison of local and global results, estimates were obtained for the coupling coefficients of the first harmonic of the cosmic ray anisotropy for the Moscow neutron monitor, and a new experimental method for calculating the coupling coefficients of individual detectors was proposed. The limitations of the local method, as well as the possibility of continuing and expanding this study, have been discussed and justified.

— In this paper we study the statistical relations between geomagnetic indices and the characteristics of cosmic rays and interplanetary disturbances for Forbush decreases associated with (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. For sporadic Forbush decreases, the dependence of geomagnetic indices on cosmic ray and solar wind parameters in the presence or absence of a magnetic cloud is compared using statistical methods. The results show that (a) the highest geoeffectiveness is characteristic of interplanetary disturbances associated with solar matter eruptions from active regions in the presence of a magnetic cloud (and the lowest one for recurrent disturbances), (b) sporadic and recurrent events differ not only in the magnitude of geomagnetic indices and the southern component of the magnetic field but also in the nature of the relationship between them, (c) the geoeffectiveness of transient solar wind disturbances depends more strongly on the presence or absence of a magnetic cloud than on the type of solar source, and (d) for interplanetary disturbances associated with filament eruptions outside active regions, in the presence of a magnetic cloud, the geoeffectiveness depends only on the southern component of the magnetic field, while for other types of disturbances it depends on other solar wind parameters.

In this paper, we study the similarities and differences of Forbush decreases in solar cycles 23 and 24. Groups of events associated with various types of solar sources were analyzed: coronal mass ejections from active regions accompanied by solar flares (CME1 group), filament ejections outside active regions (CME2 group), and high-velocity streams from coronal holes (CH group). The distributions and relations of various parameters were studied: the amplitude of Forbush decreases, the maximum values of the hourly decrease in the cosmic ray density, the equatorial cosmic ray anisotropy, the solar wind velocity, and the magnetic field strength, as well as the values of the solar wind velocity and the magnetic field strength 1 hour before the onset of the Forbush decrease during the event. The results showed that the number of events, parameter values, and their relations depend on the phase and cycle of solar activity. In the 24th cycle, the number of events in the CME1 group decreased, did not change in CME2, and increased in CH. The values of the parameters and the difference between them in various groups of events are higher in cycle 23, which is characterized by greater asymmetry and long “tails” of distributions. The magnitude of the Forbush decreases in the CME1 group in cycle 23 depends more strongly on the solar wind velocity while, in cycle 24, on the magnetic field strength, as in the CME2 group in both solar cycles. Multiple linear regression describes the dependences of the parameters of Forbush decreases in the CME1 and CME2 groups in the 23rd cycle and in the CME1 group in the 24th cycle well.

In this paper, Forbush decreases (FDs) from 1997 to 2020 associated with coronal mass ejections from active and non-active regions are compared between themselves and to FDs caused by high-speed streams from coronal holes. The two types of sporadic FDs are also compared when corresponding solar wind (SW) disturbances contain, or do not contain, magnetic clouds (MCs) near Earth. Cosmic ray density and anisotropy variations, SW speed, interplanetary magnetic field (IMF) strength, and geomagnetic indices have been examined using statistical methods. The results reveal that these parameters are larger for FDs associated with active region (AR) ejections and have highly skewed distributions for both types of sporadic events. In the same ranges of SW parameters, FD magnitude is larger for flare-associated events; more efficient modulation occurs in FDs associated with AR ejections. Differences between FDs associated with AR and non-AR ejections are more pronounced when an MC is registered. For IMF strength and geomagnetic indices, differences between the distributions depend more upon MC presence or absence than on the type of solar source. Correlation of IMF strength and SW speed differs slightly between FDs caused by AR and non-AR ejections regardless of the presence or absence of an MC, akin to the partial correlation between FD magnitude and IMF strength. Difference between the speeds of disturbed and background SW is larger for FDs associated with AR ejections especially when an MC is registered; the interaction region of different-speed SW streams occurs more frequently in interplanetary disturbances induced by AR ejections.

The effect of snow accumulating around mountain, mid-latitude, and high-latitude neutron monitors of the global network on the quality of their data is estimated. A correction for the snow effect according to the reference station method is proposed. In addition, this method is used to evaluate the effective thickness of the snow cover for the observation points. The different available methods of correcting for the effect of snow on the results of monitoring are compared, and a new method based on the results of the analysis of data from the global network of neutron monitors is proposed.

Forbush decreases occurring from 1997 to 2017 (1055 events in total) have been analyzed with the use of a database of Forbush effects and interplanetary disturbances built and currently maintained by the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation of the Russian Academy of Sciences (IZMIRAN). Based on statistical methods, we compared the temporal evolution of Forbush decreases in events of four types: (1) those associated with coronal mass ejections from active regions and accompanied by solar flares; (2) those induced by interplanetary disturbances due to filament eruptions from areas beyond active regions; (3) those caused by high-speed streams from coronal holes; and (4) those associated with two or more sources of different types of disturbances. In the comparison, we used the following time parameters of the development of Forbush decreases: the time intervals from the event onset to the detection of the minimal density of cosmic rays, the maximal hourly decrease in the density of cosmic rays, the maximal equatorial anisotropy of cosmic rays, the maximal speed of the solar wind, the maximal strength of the interplanetary magnetic field, and the minimal Dst index. Analysis of the distributions of the time parameters and their intercorrelation has shown that there are substantial differences between the evolution scenarios of Forbush decreases in the four sets of examined events.

ABSTRACTThe paper discusses changes in various characteristics of the solar wind, interplanetary magnetic field, geomagnetic activity, and cosmic rays during the registration of paired interacting solar wind disturbances on the Earth using the data base of Forbush effects and interplanetary disturbances – FEID. The cases of pair interaction are considered for 1995–2020: (i) successive coronal mass ejections; (ii) coronal mass ejections and high-speed streams from coronal holes; (iii) successive high-speed streams from coronal holes. It is shown that for the first events from a pair, the times for reaching the maximum values of the interplanetary magnetic field and solar wind velocity are significantly reduced, and the amplitudes of the recorded Forbush decreases decline. It is also found that the presence of interaction enriches the second event at the expense of the resources of the first, increasing geomagnetic efficiency and the degree of cosmic ray modulation for the second event in comparison with isolated events, which is especially pronounced for a pair of interacting coronal mass ejections. The existence of the described effects can be explained by the observed increase in the interplanetary magnetic field magnitude in the second events due to the presence of interaction.

The Forbush decreases for the period from 1997 to 2020 were studied based on data from the database on Forbush effects and interplanetary disturbances created and maintained at the Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation (IZMIRAN). Using statistical methods, we compared the Forbush decreases associated with coronal mass ejections from active regions of the Sun accompanied by solar flares; filament ejections outside active regions; high-speed streams from coronal holes; and several sources. The difference between Forbush decreases was related to coronal mass ejections when magnetic clouds in the interplanetary disturbances near the Earth were observed or not observed. It has been shown that the distributions of most of parameters are asymmetric for the sporadic Forbush decreases; for the recurrent Forbush decreases, they are nearly symmetric. The strongest correlations between the parameters of Forbush decreases and interplanetary disturbances are observed in the group of coronal ejections from active regions that are accompanied by solar flares and have a structure of magnetic cloud.

ABSTRACT 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.

An analysis is performed of a reduction in the intensity of cosmic rays during the minimum of solar cycle 24 in April–May, 2019. It is assumed that the observed slow decline and recovery of the cosmic ray flux consisted of a series of small Forbush decreases.

Analysis of the variation in galactic cosmic rays and changes in various characteristics of Forbush effects associated with the influence of interacting solar wind disturbances on the Earth (disturbances with a time interval between registration of less than 50 h) is presented based on the database of Forbush effects and interplanetary disturbances developed at the Institute of Terrestrial Magnetism, Ionosphere, and Radio Wave Propagation. Cases of the pair interaction of high-speed streams from coronal holes and coronal mass ejections in 1995–2019 are considered, and the behavior of the solar wind parameters, the interplanetary magnetic field, and the cosmic ray variations for two types of interacting solar wind disturbances (the interaction of successive coronal mass ejections and the interaction of a coronal mass ejection with a high-speed coronal hole stream) are analyzed. It is found that the average times of the onset of the Forbush effect minimum and the record of the maximum solar wind velocity and the absolute value of the interplanetary magnetic field decrease for the first of a pair of interacting events, i.e., the second events of the pair do not allow the first to develop fully. It is also found that the presence of interaction enriches the second event at the expense of the resources of the first. This increases its geomagnetic efficiency and the degree of cosmic ray modulation in comparison with isolated events.