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A strong Forbush decrease, i.e., suppression of the flux of galactic cosmic rays recorded on Earth, was observed by the global network of ground-based neutron monitors (NMs) on 24 – 25 March 2024. The decrease was very unusual as characterised by so rapid recovery that a false Ground-level enhancement (GLE) alarm was produced by the corresponding warning systems. Here we present the first comprehensive collection and analysis of the available data for this event. The event was highly anisotropic as exhibited in a 3-h spread of the deep-phase timing for different NMs. The anisotropy was focused nearly at the anti-sunward direction with a narrow cone of 20 – 30 °. The heliospheric situation leading to this unusual Forbush decrease was quite complex. An analysis of first look records was performed, considering the stations acceptance, taking into account the complex geomagnetic conditions. A leader fraction analysis indicates that the recovery phase of the event was rigidity-independent and had essentially the same spectral shape as the pre-event period. A summary of the solar-terrestrial phenomena is provided to assist in future work on modelling this complex event. [ABSTRACT FROM AUTHOR]

We analyze strong space weather disturbances during first ten days of September 2017, using the geomagnetic Dst index, parameters of normals to interplanetary shock fronts, direct measurements of interplanetary magnetic field, solar wind, and cosmic ray parameters. By applying spectral analysis methods to interplanetary medium data, we analyze MHD waves at the pre-front of two interplanetary shocks responsible for geomagnetic disturbances on September 6 and 7, 2017. The main results are as follows: the contribution of three branches of MHD waves (Alfvén, fast and slow magnetosonic) to the observed spectrum of the interplanetary magnetic field modulus has been established. We have confirmed the conclusion that the generation of Alfvén waves and fast magnetosonic waves is due to the presence of low-energy proton fluxes (Ep~1 MeV) at the pre-front of interplanetary shocks. We have also discovered a predominant contribution of slow magnetosonic waves to the observed spectrum of the interplanetary magnetic field modulus, but its reason is yet unknown. It is noted that different orientations of the normals to the interplanetary shock fronts and to the direction of the interplanetary magnetic field average vector on spacecraft located fairly close to each other may indicate waviness of the shock front structure.

This study investigated the role of solar wind disturbances on variations in cosmic ray intensity. To conduct the study, use was made of the data on cosmic rays from the SOPO, CLMX, and MOSC neutron stations, as well as solar wind speed data from 2000 to 2005. The source Forbush decrease (FD) dates were generated from onset journal publications, specifically the FD list of Dumbovi ́c et al., (2011). The manual method of FD selection was used to identify FDs. The FD dates, computed magnitudes, and solar wind speed data were recorded and presented. From the study, FDs were generated, some of which were of the same date as those in the source table, while others were not observed in the source table. FDs not observed in the source table were generated and catalogued by this research. It was also observed that the magnitude of FDs generally depends on the coordinates of the observing neutron stations. A strong correlation with a value of cc = 0.93 was observed between FDs of SOPO and MOSC stations, followed by FDs of CLMX and MOSC stations of value cc = 0.89, and lastly, FDs of SOPO and CLMX stations of value cc = 0.74. This implies that observed FDs from neutron stations are coordinate-dependent. The correlation between FD magnitudes and solar activities shows that solar wind had a high and significant correlation with FD magnitude to the tune of cc = 0.54. Based on the findings, the study concludes that solar wind disturbances play a crucial role in causing a sharp decrease in the intensity of cosmic rays known as Forbush decrease.

We utilized the Forbush decreases (magnitude > 1.5 % ) detected in cosmic ray neutron monitor data during continuous five solar cycles, viz., 20, 21, 22, 23 and 24 (1965 to 2019) and subjected them to wavelet analysis in order to obtain the possible periodicities in their occurrence. We also studied the periodicities separately during the odd and even solar activity cycles. In addition to solar activity, the solar magnetic polarity and its extension into the interplanetary space makes significant difference in the cosmic ray modulation in the helisphere, we have also applied the wavelet analysis procedure separately during positive (A > 0) and negative (A < 0) polarity states of the heliospheric magnetic fields. Observed periodicities in Forbush decreases have been discussed and compared with earlier detected periodicities in solar and geomagnetic activity indices, e.g., sunspot numbers, sunspot areas, sunspot groups, solar flares, coronal mass ejections, and various geomagnetic activity indices. Significant short-term periodic behaviour detected in the occurrence of Forbush decreases, which in general, corroborates the observed behaviour in solar (in particular, solar eruptive activity) and geomagnetic activity. Understanding the quasi-periodic process in magnetic field emergence from solar active regions and solar eruptive activity, as well as solar-terrestrial coupling and space weather effects, requires comparing the quasi-periodic behaviour between parameters representing solar and geomagnetic activity along with cosmic ray variability. [ABSTRACT FROM AUTHOR]

• Health Canada's Fixed Point Surveillance network can respond to cosmic radiation. • Forbush decrease events and the GLE69 event have been observed in the network. • The network response to these events is similar to the global muon/neutron detectors. • The network is readily available for real-time space weather monitoring. This work presents a feasibility study of utilizing Health Canada's terrestrial radiation monitoring network, the Fixed Point Surveillance (FPS) network, for space weather monitoring through demonstrating detections of Forbush decrease and ground level enhancement events. The network is currently comprised of more than eighty sodium iodide spectrometers distributed across Canada. It was designed for terrestrial radiation monitoring but is also capable of registering cosmic radiation in a high-energy channel. Data from fourteen FPS stations for the period from 2003 to 2018 were analyzed and compared with data obtained by other ground-level cosmic radiation monitoring systems. The level of atmospheric impacts on measurements can be well explained, and signatures of both long-term solar cycle variations and sporadic solar events have been detected in the FPS network. The Forbush decrease amplitudes in FPS were found to be comparable to those obtained in the global muon detector network but about 2–3 times lower than those recorded by the global neutron monitoring network. This study suggests that the 20 years of cosmic ray data from the FPS network can be used for climatological space weather studies. In addition, the network can be readily available for real-time space weather monitoring. [ABSTRACT FROM AUTHOR]

Diffusion and electromagnetic mechanisms determine the formation of sporadic Forbush decreases. The diffusion mechanism affects the Forbush decrease amplitude in the turbulent layer, and the part of the coronal mass ejection preceding the magnetic cloud, and its efficiency depends on the level of magnetic field turbulence. The electromagnetic mechanism works in a magnetic cloud, and its efficiency depends on the intensity of regular magnetic and electric fields. We analyze solar wind parameters and cosmic ray density, using the superposed epoch analysis. In 1996–2006, 23 strong Forbush decreases (amplitude >5 %) were detected. The average amplitude of 7 % is equally formed by both mechanisms. The events can be divided into 2 groups depending on the contribution of the mechanisms to Forbush decrease amplitude. Group 1 includes the strongest Forbush decreases (amplitude=8.5 %), formed by both diffusion and electromagnetic mechanisms. The diffusion mechanism forms 0.26 amplitude, and the electromagnetic mechanism is responsible for 0.74 one. In group 2, the averege amplitude Forbush decrease =5.7 %, the diffusion mechanism forms 0.79 of amplitude; and the electromagnetic one, 0.21. The spatial distributions of the mean values of the medium parameters in the region of disturbances in the groups differ. This difference can be explained by the fact that Forbush decrease amplitude in groups 1 and 2 are formed in the central and peripheral parts of coronal mass ejection respectively.

Observed galactic cosmic ray intensity can be subjected to a transient decrease. These so-called Forbush decreases are driven by coronal mass ejection induced shockwaves in the heliosphere. By combining in situ measurements by space borne instruments with ground-based cosmic ray observations, we investigate the relationship between solar energetic particle flux, various solar activity indices, and intensity measurements of cosmic rays during such an event. We present cross-correlation study done using proton flux data from the SOHO/ERNE instrument, as well as data collected during some of the strongest Forbush decreases over the last two completed solar cycles by the network of neutron monitor detectors and different solar observatories. We have demonstrated connection between the shape of solar energetic particles fluence spectra and selected coronal mass ejection and Forbush decrease parameters, indicating that power exponents used to model these fluence spectra could be valuable new parameters in similar analysis of mentioned phenomena. They appear to be better predictor variables of Forbush decrease magnitude in interplanetary magnetic field than coronal mass ejection velocities. [ABSTRACT FROM AUTHOR]

High-energy physics in the atmosphere (HEPA) has undergone an intense reformation in the last decade. Correlated measurements of particle fluxes modulated by strong atmospheric electric fields, simultaneous measurements of the disturbances of the near-surface electric fields and lightning location, and registration of various meteorological parameters on the Earth have led to a better understanding of the complex processes in the terrestrial atmosphere. The cooperation of cosmic rays and atmospheric physics has led to the development of models for the origin of particle bursts recorded on the Earth's surface, estimation of vertical and horizontal profiles of electric fields in the lower atmosphere, recovery of electron and gamma ray energy spectra, the muon deceleration effect, etc. The main goal of this review is to demonstrate how the measurements performed at the Aragats cosmic ray observatory led to new results in atmospheric physics. We monitored particle fluxes around the clock using synchronized networks of advanced sensors that recorded and stored multidimensional data in databases with open, fast, and reliable access. Visualization and statistical analysis of particle data from hundreds of measurement channels disclosed the structure and strength of the atmospheric electric fields and explained observed particle bursts. Consequent solving of direct and inverse problems of cosmic rays revealed the modulation effects that the atmospheric electric field has on cosmic ray fluxes. [ABSTRACT FROM AUTHOR]

The temporal variations of cosmic-ray intensity, measured by ground-based detectors at various latitudes, longitudes, and altitudes, are related to the geophysical and solar phenomena. The latter are interplanetary coronal mass ejections and fast solar wind from coronal holes, which cause interplanetary magnetic field (IMF) abrupt variations near Earth. Interacting with the magnetosphere, they cause worldwide sudden decreases (Forbush decreases, FDs) of intensity followed by gradual recovery. The amplitude of the flux depletion depends on the type and energy of the registered particle, which in turn depends on geographical coordinates and the detector's energy threshold and selective power. SEVAN particle detector network with nodes in Europe and Armenia selects three types of particles that demonstrate coherent depletion and recovery and correspond to different energy galactic protons interacting with disturbed magnetospheric plasmas. On November 3–4, 2021, an interplanetary coronal mass injection (ICME) hit the magnetosphere, sparking a strong G3-class geomagnetic storm and auroras as far south as California and New Mexico. All detectors of the SEVAN network have registered an (FD) of ≈5% depletion in a 1-min time series of count rates. Approaching the maximum solar activity cycle, large variations of the particle flux intensity were registered on February 27, March 23, 2023, and March 24, 2024. In this work, we present measurements of these FDs performed on mountain altitudes on Aragats (Armenia), Lomnicky Stit (Slovakia), Mileshovka (Czechia), and at sea level DESY (Hamburg, Germany). We compared FD measurements made by SEVAN detectors and neutron monitors located on Aragats and Lomnicky Stit and made a correlation analysis of FD registration at different locations. • Low energy CR, neutrons, and high energy muons measured by SEVAN detector are sensitive to solar modulation. • SEVAN and NM detectors demonstrate coherent depletion and recovery of cosmic ray fluxes. • SEVAN detectors at mountain altitudes registered a pre-FD increase in fluxes of high-energy muons. • FD coincides with large values of the interplanetary magnetic field B, and small values of geomagnetic field Bx. • SEVAN measurements confirm that FD depends highly on the disturbed IMF rather than its Bz component. [ABSTRACT FROM AUTHOR]

The Forbush effect is one of the striking examples of irregular variations in the cosmic ray (CR) flux. It is a heliospheric phenomenon that includes a decrease and a recovery of the CR intensity and small changes in the CR density and anisotropy before the start of the main decrease. They were first noted as the effects of decreasing the intensity of cosmic rays, coinciding with geomagnetic storms. The increased interest is observed in recent years and it is primarily because their variables are strongly related to processes in solar-terrestrial physics. Forbush effects convey significant information regarding interplanetary disturbances. In the present work, fifty-three Forbush decrease (FD) events were categorized into four types according to their profile shape. From 1990 to 2020, cosmic ray intensity data from five neutron monitor stations (two subpolar and three mid-latitude) were used. The superposed epoch technique has been used to calculate the vectorial average of the observed diurnal vectors for every day of the four types of FD. It can be found that the subpolar stations (McMurdo and Thule) showed different behavior than the other stations. The solar-diurnal variation at the Thule and McMurdo subpolar stations is comparable on some days to the variation at mid-latitude stations, indicating that CR anisotropy on these days is not the only (and sometimes not the main) source of the solar-diurnal variation. The contribution of the solar-diurnal anisotropy to the diurnal variation at subpolar stations is small, much less than at other neutron monitors.

We analyzed the behavior of Cosmic-Ray (CR) intensity during five geomagnetic events over 9-day periods of varying disturbance, using ground-based CR measurements from the Bartol Research Institute neutron-monitor program that occurred on 25th October – 2nd November 2003, 23rd – 31st July 2004, 13th – 21st May 2005, 13th – 21st Mar 2015, and 19th – 27th June 2015. A nine-day time frame gives us a reasonable time interval for data analysis before, during, and after the main event(s). These events have been deliberately chosen, intending to track Forbush Decreases (FDs) at high-latitude stations following geomagnetic storms of different intensity and duration from the perspectives of their origin and geoeffectiveness. FDs are observed when the magnetic fields entangled in and around a Coronal Mass Ejection (CME) exert a deflecting effect on galactic cosmic radiation, resulting in a sudden reduction in their intensities. The results revealed that the CR intensity dropped by 4 – 17% in the chosen events. The FDs examined were not typical, with multistage decrement during the event period. Furthermore, we have used the Discrete Wavelet Transform (DWT) technique to detect a singularity on the CR intensity at the stations described. The first three decomposition levels have proved sufficient to isolate the transients in CR intensity in conjunction with varying nature and intensities, ranging from intense to superintense geomagnetic storms to a high-intensity long-duration continuous auroral electrojet activity (HILDCAA) event. Finally, the findings of the detrended crosscorrelation technique showed a good association of percentage decrease in cosmic-ray intensity with the Dst index during the process. No noticeable lag has been found between the parameters discussed, which indicates a strong correlation between the IMF-Bz and the Dst index and the FD. [ABSTRACT FROM AUTHOR]

We analyze the time-variation of cosmic ray intensity during the periods when large and sudden disturbances occur and last for several days. We consider periods when there are sudden decreases of ≥5% in the galactic cosmic ray (GCR) intensity records. For the analysis, in addition to cosmic-ray intensity data, we utilize several solar wind plasma and field parameters. Time variation of changes in GCR intensity have been compared with simultaneous changes in solar wind plasma and field parameters, in order to gain insight about the physical processes responsible for large-amplitude cosmic ray disturbances. We utilize not only data about changes in the magnitude of some of the plasma and magnetic field parameters but also the parameters which provide information about the turbulent or quite (non-turbulent) nature of the parameters. Moreover, we also utilize the magnitude of a number of plasma and field parameters and their various products, during cosmic ray decrease of various magnitudes. The magnitudes of various plasma and field parameters and their various products including some newly tried products are subjected to correlation analysis with the magnitude of cosmic-ray decreases. We identify plasma and field parameters and their products which best correlates with cosmic-ray decreases. We provide an empirical relation that can be useful to estimate the amplitude of Forbush decreases using interplanetary plasma and field observations. Our results provide further insight about the physical processes playing important role during large cosmic ray disturbances in the heliosphere. Our results further demonstrate that large-amplitude Forbush decreases in GCR intensity occur due to passage of enhanced and turbulent magnetic field structure, consistent with the model that Forbush decreases are mainly due to scattering of cosmic ray particles by enhanced turbulent magnetic field regions in the heliosphere. [ABSTRACT FROM AUTHOR]

High-energy physics in the atmosphere (HEPA) has undergone an intense reformation in the last decade. Correlated measurements of particle fluxes modulated by strong atmospheric electric fields, simultaneous measurements of the disturbances of the near-surface electric fields and lightning location, and registration of various meteorological parameters on the Earth have led to a better understanding of the complex processes in the terrestrial atmosphere. The cooperation of cosmic rays and atmospheric physics has led to the development of models for the origin of particle bursts recorded on the Earth’s surface, estimation of vertical and horizontal profiles of electric fields in the lower atmosphere, recovery of electron and gamma ray energy spectra, the muon deceleration effect, etc. The main goal of this review is to demonstrate how the measurements performed at the Aragats cosmic ray observatory led to new results in atmospheric physics. We monitored particle fluxes around the clock using synchronized networks of advanced sensors that recorded and stored multidimensional data in databases with open, fast, and reliable access. Visualization and statistical analysis of particle data from hundreds of measurement channels disclosed the structure and strength of the atmospheric electric fields and explained observed particle bursts. Consequent solving of direct and inverse problems of cosmic rays revealed the modulation effects that the atmospheric electric field has on cosmic ray fluxes.

A comparison has been made between recurrent (associated with high-speed streams from coronal holes) and sporadic (caused by interplanetary coronal mass ejections (ICMEs)) Forbush decreases (FDs) in solar cycles 23 and 24 (as well as in the maxima of these cycles and the minimum between them). Forbush Effects and Interplanetary Disturbances database created and maintained in IZMIRAN provided a large number of events (about 1700 isolated FDs, among them 350 recurrent FDs, and 207 sporadic FDs selected with high reliability), which allowed us to apply statistical methods. The results revealed that sporadic FDs prevailed in the maxima 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 maxima of the cycles. FD magnitude is greater in the maxima than in the minimum for sporadic events, and it changes weakly for recurrent ones. The solar wind velocity is on average greater for recurrent events than for sporadic ones; it is larger for recurrent FDs in the minimum and for sporadic FDs in the maxima. The magnetic field is stronger for sporadic FDs than for recurrent ones in the maxima and it is approximately equal for both types of events in the minimum. The magnetic field of ICMEs is weaker in the current solar cycle than in the previous one. The duration of the FD main phase is less in the maxima for both types of events; sporadic FDs developed significantly faster than recurrent ones in the maximum of cycle 23.

Abstract The relation between low latitude lightning activity and Forbush decreases (FDs) of galactic cosmic rays was studied, with flash rates observed by the Lightning Image Sensor aboard the Tropical Rainfall Measuring Mission satellite and FDs selected from the events with decreases more than 2% measured by the Mexico City neutron monitor. The effect of spacecraft precession on Lightning Image Sensor data complicates the analysis, but this can be dealt with by the approach we use. Applying a superposed epoch analysis to all of the 70 FD cases, we found the low latitude lightning activity decreases by about 10% after the Forbush minimum, with a 95% confidence level tested by Monte Carlo simulation, and recovers 3 days later. The response is more significant in the Southern Hemisphere. Our result provides evidence for a proposed link between solar activity, cosmic rays, the global atmospheric electric circuit, and cloud microphysics.

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. [ABSTRACT FROM AUTHOR]

Forbush decreases (FDs), which are short‐term drops in the flux of galactic cosmic rays, are caused by the shielding from strong and/or turbulent magnetic structures in the solar wind, especially interplanetary coronal mass ejections (ICMEs) and their associated shocks, as well as corotating interaction regions. Such events can be observed at Earth, for example, using neutron monitors, and also at many other locations in the solar system, such as on the surface of Mars with the Radiation Assessment Detector instrument onboard Mars Science Laboratory. They are often used as a proxy for detecting the arrival of ICMEs or corotating interaction regions, especially when sufficient in situ solar wind measurements are not available. We compare the properties of FDs observed at Earth and Mars, focusing on events produced by ICMEs. We find that FDs at both locations show a correlation between their total amplitude and the maximum hourly decrease, but with different proportionality factors. We explain this difference using theoretical modeling approaches and suggest that it is related to the size increase of ICMEs, and in particular their sheath regions, en route from Earth to Mars. From the FD data, we can derive the sheath broadening factor to be between about 1.5 and 1.9, agreeing with our theoretical considerations. This factor is also in line with previous measurements of the sheath evolution closer to the Sun. Plain Language Summary: When eruptions from the Sun propagate through the interplanetary space, their strong and turbulent magnetic field deflects background cosmic ray particles nearby. This causes a temporary decrease of the flux of cosmic rays observed at locations that were passed by the eruption, a so‐called Forbush decrease. These decreases can be measured on Earth, and also by space missions around the solar system, and are often used to detect the arrival of solar eruptions, especially when no other direct measurements are available. We look at catalogs of Forbush decreases observed at Earth and Mars, which is 50% farther away from the Sun than Earth, and compare their properties to investigate whether, in addition to the arrival time, it is possible to derive more information about the eruptions from the observed Forbush decreases. We find that the relation of characteristic parameters describing the Forbush decrease changes between the two planets and that this can be explained by the broadening of the interplanetary structure erupted from the Sun during its propagation. The magnitude of this broadening derived from our data agrees with theoretical expectations and is in line with previous measurements of the evolution of solar eruptions at locations closer to the Sun. Key Points: We examine a large number of Forbush decreases (FDs) at Mars observed by MSL/RAD, comparing their properties with those observed at EarthThe maximum hourly decrease of the FDs is correlated with their amplitude, but with different linear regression slopes at Earth and MarsThis effect may be due to the broadening of the coronal mass ejection's sheath region, which we motivate with theoretical calculations [ABSTRACT FROM AUTHOR]

The geoeffectiveness of single and complex transient structures of the solar wind (SW), formed by coronal mass ejections (CMEs) and associated Forbush decreases (FDs) in the cosmic ray flux is considered. By the example of the period of 2010–2011, it is shown that complex SW structures resulting from the interaction of CMEs with high-speed streams from coronal holes or with other CMEs in the heliosphere more often lead to moderate and strong geomagnetic storms (Dst < –50 nT) in comparison with single CMEs. It is shown that the FD increases with the growth of the SW transient structure duration and the average magnitude of the interplanetary magnetic field of the transient and takes the highest values when the complex structures arrive at the Earth. [ABSTRACT FROM AUTHOR]

The relation between low latitude lightning activity and Forbush decreases (FDs) of galactic cosmic rays was studied, with flash rates observed by the Lightning Image Sensor aboard the Tropical Rainfall Measuring Mission satellite and FDs selected from the events with decreases more than 2% measured by the Mexico City neutron monitor. The effect of spacecraft precession on Lightning Image Sensor data complicates the analysis, but this can be dealt with by the approach we use. Applying a superposed epoch analysis to all of the 70 FD cases, we found the low latitude lightning activity decreases by about 10% after the Forbush minimum, with a 95% confidence level tested by Monte Carlo simulation, and recovers 3 days later. The response is more significant in the Southern Hemisphere. Our result provides evidence for a proposed link between solar activity, cosmic rays, the global atmospheric electric circuit, and cloud microphysics. Plain Language Summary: An analysis of satellite measurements of lightning flash rates shows a reduction of about 10% immediately following Forbush Decreases of the cosmic ray flux, as measured by neutron monitors. The lightning response is clearest at latitudes 19°–38°in the Southern Hemisphere. Various possible mechanisms are discussed. Key Points: Satellite observations show a reduction of low latitude lightning activity in response to cosmic ray FD eventsLow latitude lightning activity immediately decreases by about 10% after the key event day and lasts for about 3 daysThe response is more significant in the Southern Hemisphere [ABSTRACT FROM AUTHOR]

On the approach to minimum of Solar Cycle 24, on 26 August 2018, an unexpectedly strong geomagnetic storm (GMS) suddenly occurred. Its D st index reached − 174 nT , that is, the third most intense storm of 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 flare-like 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, about 200 – 360 km s − 1 . 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 Earth only on 25 August and brought an unexpectedly strong field of B t ≈ 18.2 nT with a predominantly negative B z 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 cycle minimum-like conditions. [ABSTRACT FROM AUTHOR]

It is well known today that a continuous stream of highly ionized plasma is emitted from the Sun's surface. This plasma is called the solar wind and consists of protons, electrons, and light nuclei. The solar wind pushes the solar magnetic field into interplanetary space to form the interplanetary magnetic field. The interplanetary magnetic field is a dynamical system that depends on the solar cycle and the Sun's rotation phase. Thus, the Solar System is a natural plasma physics laboratory with an enormous multitude of different effects showing the current state of the system. By recording cosmic-ray fluxes, one can study the behavior of the interplanetary magnetic field and obtain information about processes that occur both on the Sun's surface and throughout the Solar System. The main short-time variations in cosmic-ray intensity include the 27-day variations and the Forbush decreases. These variations are caused by complex solar plasma structures, which are generated by different processes on the Sun's surface and propagate through space in a wide range of velocities. Cosmic-ray fluxes recorded with the PAMELA magnetic spectrometer on board the Resurs DK1 satellite in 2006–2016 are used to show some examples of cosmic-ray variations. [ABSTRACT FROM AUTHOR]

The observable anisotropy of cosmic rays has first been decomposed into zonal harmonics and components of vector and tensor anisotropy. We examine Forbush decreases in cosmic rays that occurred in November 2001 and November 2004. It is shown that at the beginning of a Forbush decrease an antisunward convective current of cosmic rays predominates; and during the recovery phase, a sunward diffusive current of particles along the interplanetary magnetic field dominates. During the phase of intensity drop, short-time decreases in the second zonal harmonic take place. These decreases occur with abrupt changes of the interplanetary magnetic field intensity and solar wind speed. During the passage of large-scale solar wind disturbances, the tensor anisotropy behaves in a complicated way. To explain its behavior, a further detailed investigation is required. spherical harmonics of the angular CR-distribution are obtained using the global survey method. In each case, the average number of stations is equal to 32. The analysis shows that the temporal change of the isotropic component is caused by a magnetic mirror. For the first time, the zonal harmonics are reliably distinguished, and the existence of the antisymmetric diurnal CR-variation in a low energy range, which is oriented along IMF, is recognized. We compare our results with those obtained earlier.

Features and peculiarities of the cosmic ray intensity (CRI) and the geomagnetic activity, along with several solar plasma and interplanetary magnetic field, during the period 4–10 September 2017 are studied. The period was characterized by strong solar activity: Several solar flares occurred; several halo coronal mass ejections (CMEs) were ejected in space. In the near‐Earth interplanetary space, the CMEs driving shock(s) and sheath(s) were identified. At the Earth, strong Forbush decreases in CRI and geomagnetic storms were observed. Several large solar flares, one of them of very high X‐ray importance (X9.3) and three halo CMEs were detected in the solar atmosphere. Two shock‐associated interplanetary CMEs were observed during that interval in near‐Earth space; the latter and faster one arrived even as the ejecta of the earlier one was still crossing. Variations in interplanetary plasma and field parameters during, before, and after the Forbush decreases and geomagnetic storms that occurred during the considered period were examined. A detailed time‐lagged correlation analysis using data at three different temporal resolutions (hourly, 5‐min, and 1‐min) was also performed. Cross correlations of time series of CRI with geomagnetic activity during the period 4–10 September 2017 are computed. This cross‐correlation analysis between CRI variability (defined as the difference of the CRI count rate between the current and the previous time step) and the Dst indicates a delay of Dst by 3–4 hr. Key Points: Forbush decreases and geomagnetic storms in the interval 4–10 September 2017 are studiedSimultaneous high‐resolution data of cosmic rays, Dst index, and solar wind are analyzedTime lag correlation analysis is performed, and possibility of its use in space weather forecast is indicated [ABSTRACT FROM AUTHOR]

Abstract Applicability of our present setup for solar modulation studies in a shallow underground laboratory is tested on four prominent examples of Forbush decrease during solar cycle 24. Forbush decreases are of interest in space weather application and study of energy-dependent solar modulation, and they have been studied extensively. The characteristics of these events, as recorded by various neutron monitors and our detectors, were compared, and rigidity spectrum was found. Linear regression was performed to find power indices that correspond to each event. As expected, a steeper spectrum during more intense extreme solar events with strong X-flares shows a greater modulation of galactic cosmic rays. Presented comparative analysis illustrates the applicability of our setup for studies of solar modulation in the energy region exceeding the sensitivity of neutron monitors. [ABSTRACT FROM AUTHOR]

Highlights • Interplanetary mass ejections and high speed streams differ by modulation abilities. • Parameters distributions are more compact for Forbush decreases from coronal holes. • Partial correlation of geomagnetic and Forbush decrease parameters is insignificant. • Correlation of solar wind and recurrent Forbush decrease parameters is moderate. • Correlation of solar wind and sporadic Forbush decrease parameters is high. Abstract 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. [ABSTRACT FROM AUTHOR]

A mathematical model for observations from a muon hodoscope which takes into account the kinematic and geometric parameters of solar coronal mass ejections (CMEs) is proposed. Equations for the calculation of mutual motions of the elements of the Sun-CME-Earth-muon hodoscope system and parameters of solar CMEs for the plane case are formulated. A model for a sequence of observations from the muon hodoscope with possible Forbush decreases is developed. The application of the developed mathematical observation model to problems of estimating CME parameters is presented. [ABSTRACT FROM AUTHOR]

The problem of noise filtering in observations of 2D functions of muon flux intensity distributions from the URAGAN hodoscope is considered. A filtering method based on the construction of sliding approximate local 2D models is proposed. A filtering algorithm based on sliding local piecewise linear 2D models is developed. The results of testing the algorithm on model and experimental observations for problems of Forbush decrease detection are presented. Errors of the algorithm are estimated on the basis of statistical modeling. [ABSTRACT FROM AUTHOR]

Abstract: The propagation of 15 interplanetary coronal mass ejections (ICMEs) from Earth's orbit (1 AU) to Mars (∼1.5 AU) has been studied with their propagation speed estimated from both measurements and simulations. The enhancement of magnetic fields related to ICMEs and their shock fronts causes the so‐called Forbush decrease, which can be detected as a reduction of galactic cosmic rays measured on ground. We have used galactic cosmic ray (GCR) data from in situ measurements at Earth, from both STEREO A and STEREO B as well as GCR measurements by the Radiation Assessment Detector (RAD) instrument on board Mars Science Laboratory on the surface of Mars. A set of ICME events has been selected during the periods when Earth (or STEREO A or STEREO B) and Mars locations were nearly aligned on the same side of the Sun in the ecliptic plane (so‐called opposition phase). Such lineups allow us to estimate the ICMEs' transit times between 1 and 1.5 AU by estimating the delay time of the corresponding Forbush decreases measured at each location. We investigate the evolution of their propagation speeds before and after passing Earth's orbit and find that the deceleration of ICMEs due to their interaction with the ambient solar wind may continue beyond 1 AU. We also find a substantial variance of the speed evolution among different events revealing the dynamic and diverse nature of eruptive solar events. Furthermore, the results are compared to simulation data obtained from two CME propagation models, namely the Drag‐Based Model and ENLIL plus cone model. [ABSTRACT FROM AUTHOR]

The possibility of utilizing a shallow underground laboratory for the study of energy dependent solar modulation process is investigated. The laboratory is equipped with muon detectors at ground level and underground (25mwe), and with an underground asymmetric muon telescope to have a single site detection system sensitive to different median energies of primary cosmic-ray particles. The detector response functions to galactic cosmic rays are determined from Monte Carlo simulation of muon generation and propagation through the atmosphere and soil, based on CORSIKA and GEANT4 simulation packages. The present setup is suitable for studies of energy dependence of Forbush decreases and other transient or quasi-periodic cosmic-ray variations. [ABSTRACT FROM AUTHOR]