Forbush Decreases Research Articles

Rigidity dependences of the main characteristics of Forbush decreases

Forbush decrease is an effect in the cosmic rays physics which manifests itself as a sudden depression of the intensity of cosmic rays. Active phase of Forbush decrease lasts for several days and is often accompanied by a geomagnetic storm. The reason for such sudden depressions originates from the Sun, and is connected with a phenomenon of solar physics known as coronal mass ejections. Nowadays Forbush decreases are mostly studied by ground based detectors such as neutron monitors and muon hodoscopes. These detectors are characterized by small statistical fluctuations but register secondary cosmic rays which are created in interactions of primary cosmic rays with Earth atmosphere. This work is based on the data obtained by the satellite experiment PAMELA. The main characteristics of Forbush decreases are calculated for a fluxes of cosmic rays protons in the rigidity range 0.8 – 9 GV.

Study of Forbush decreases and lunar cycles effects in the thermal neutron flux registered near the Earth surface by means of “Neutron” setup data

The experimental data on thermal neutron flux registered by the “Neutron” setup (MEPhI, Moscow) on the Earth's surface for the period from May 2015 to February 2019 were analyzed. The effects of Forbush decrease (FD) and the lunar cycles were studied. The comparison with the results of FD studies in data of two other setups: the Moscow neutron monitor (MNM) and the muon hodoscope URAGAN (MEPhI, Moscow) was made. Comparison showed that the FD amplitudes of “Neutron” are comparable with those of MNM (on average, less by about 30%), and about 1.5 times more than for MH URAGAN. The counting rate recovery of “Neutron” detectors is much faster than for MNM and MH URAGAN.

Forbush decrease spectrum in a magnetic cloud in the 2004 July 27 event

Magnetic clouds affect the intensity of galactic cosmic rays. The diffusion mechanism is usually used as the formation mechanism for Forbush decrease (FD) in a magnetic cloud (MC). An FD is an observed decrease in the cosmic ray intensity. There is a new theory of FD formation, in which the mechanism is the loss of particle energy in the electromagnetic field of a magnetic cloud. The shape of the FD spectrum is calculated for a wide range of particle energies in the 2004 July 27 event. According to the measurements of ground-based neutron monitors and muon telescopes, synchronous changes in the FD amplitude in time indicate that the FD is formed in a magnetic cloud for all energies. However, the calculated FD spectrum differs from the obtained one from measurements. The reasons for the difference can be: 1) the mechanism of formation is not the electromagnetic one; 2) the method for determining the FD spectrum, using the notion of mean or median energies, needs additional studies.

Influence of Solar Disturbances on Galactic Cosmic Rays in the Solar Wind, Heliosheath, and Local Interstellar Medium: Advanced Composition Explorer, New Horizons, and Voyager Observations

Abstract We augment the heliospheric network of galactic cosmic ray (GCR) monitors using 2012–2017 penetrating radiation measurements from the New Horizons (NH) Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI), obtaining intensities of ≳75 MeV particles. The new, predominantly GCR observations provide critical links between the Sun and Voyager 2 and Voyager 1 (V2 and V1), in the heliosheath and local interstellar medium (LISM), respectively. We provide NH, Advanced Composition Explorer (ACE), V2, and V1 GCR observations, using them to track solar cycle variations and short-term Forbush decreases from the Sun to the LISM, and to examine the interaction that results in the surprising, previously reported V1 LISM anisotropy episodes. To investigate these episodes and the hitherto unexplained lagging of associated in situ shock features at V1, propagating disturbances seen at ACE, NH, and V2 were compared to V1. We conclude that the region where LISM magnetic field lines drape around the heliopause is likely critical for communicating solar disturbance signals upstream of the heliosheath to V1. We propose that the anisotropy-causing physical process that suppresses intensities at ∼90° pitch angles relies on GCRs escaping from a single compression in the draping region, not on GCRs trapped between two compressions. We also show that NH suprathermal and energetic particle data from PEPSSI are consistent with the interpretation that traveling shocks and corotating interaction region (CIR) remnants can be distinguished by the existence or lack of Forbush decreases, respectively, because turbulent magnetic fields at local shocks inhibit GCR transport while older CIR structures reaching the outer heliosphere do not.

Some Features of the Present-day Transition Period in Solar Activity

Various geophysical indices and their prognostic value were analyzed in (Obridko et al., 2013; Kirov et al., 2013, 2015, 2017; Georgieva et al., 2015, 2018). Two indices have been selected for the analysis (total annual Dst values, and duration of a storm). It is important to note that, unlike the other indices of solar and geophysical activity, the summary Dst index does not show violation of the Gnevyshev-Ohl rule. The analysis of annual mean Dst values reveals a clearly pronounced decrease in activity, at least since the 1980-ies. This allows us to suggest that we are at the descending branch of a secular cycle or on the threshold of a Grand Minimum. The decrease in solar activity is corroborated by the analysis of Forbush effects, solar wind speed, and intensity of the near-Earth magnetic field.

Solar wind temperature–velocity relationship over the last five solar cycles and Forbush decreases associated with different types of interplanetary disturbance

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.

Large Forbush Decreases and their Solar Sources: Features and Characteristics

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.

Automated detection of simultaneous/non-simultaneous Forbush decreases and the associated cosmic ray phenomena

The knowledge of the periodic diurnal anisotropy in the cosmic ray (CR) flux intensity as well as the enhanced anisotropy that might accompany Forbush events is not novel. Theoretical models and several methodological approaches such as Fourier techniques developed to predict the diurnal vectors started appearing in the literature since 1960s’. Consequently, the vectors have been successfully calculated on both annual, monthly and day to day basis. The amplitude and the phase of the unusually large diurnal wave trains that happen at the time of FDs have also been estimated by the method of Fourier analysis. However, direct application of Fourier transform algorithms to account for the enhanced CR diurnal anisotropies during the period of Forbush events is one of the open questions in CR research. Manual FD event identification technique, traditionally employed to select the short-term rapid depressions (Forbush decreases, FDs) in CR intensity is inefficient to handle the characteristic CR flux superposition tendencies. Identifications of FDs and the accompanying enhanced amplitude of the CR diurnal anisotropies are rarely investigated simultaneously. This poses a drawback on quantitative prediction of the impact of CR anisotropies on the detectable number of global and location dependent FDs as well as their magnitudes. A detailed and rigorous analysis involving simultaneous application of Fourier analysis to FDs and diurnal anisotropies, selections of FDs from Fourier transformed and raw CR data are attempted in the current work. The results are discussed with emphasis on simultaneity of FDs detected at different locations at Earth.

Behavior of the Speed and Temperature of the Solar Wind during Interplanetary Disturbances Creating Forbush Decreases

Forbush decreases of cosmic rays are caused by two types of solar sources: coronal holes and coronal mass ejections. In some cases, the identification of the solar source with the interplanetary disturbance causing the Forbush decrease is difficult and requires a thorough and detailed analysis of the solar-wind characteristics. In these cases, it is useful to compare the behavior of the proton temperature and speed of the solar wind. In this paper, this comparison is carried out based on a large amount of experimental material summarized in the IZMIRAN databases. The dependence of temperature on speed for a quiet solar wind is found to have a power character with a steeper spectrum in the low-speed range (V < 425 km/s; exponent index 3.37 ± 0.02) than in the high-speed range (V ≥ 425 km/s; exponent index 2.21 ± 0.02). Based on the obtained T–V dependence, the expected proton temperature Texp and the temperature index KT = Tobs/Texp (Tobs is the observed temperature) are calculated for every hour with available data on the solar-wind parameters from July 1965 to December 2018. This index is anomalously high in the regions of the interaction different speed solar-wind streams and is anomalously low inside magnetic clouds. This makes it possible to detect the Forbush decreases associated with these interplanetary structures and to identify their solar sources.

A New Method to Model Magnetic Cloud-driven Forbush Decreases: The 2016 August 2 Event

Abstract Interplanetary coronal mass ejections (ICMEs), generally containing magnetic clouds (MCs), are associated with galactic-cosmic ray (GCR) intensity depressions known as Forbush decreases (FDs). An ICME was observed at L1 between 2016 August 2 at 14:00 UT and August 3 at 03:00 UT. The MC region was identified and its magnetic configuration was retrieved by using the Grad–Shafranov (GS) reconstruction. A weak FD in the GCR count-rate was observed on 2016 August 2 by a particle detector on board the European Space Agency LISA Pathfinder mission. The spacecraft orbited around L1 and the particle detector allowed us to monitor the GCR intensity at energies above 70 MeV n −1. A 9% decrease in the cosmic-ray intensity was observed during the ICME passage. The first structure of the ICME caused a 6.4% sharp decrease, while the MC produced a 2.6% decrease. A suited full-orbit test-particle simulation was performed on the MC configuration obtained through the GS reconstruction. The FD amplitude and time profile obtained through the simulation show an excellent agreement with observations. The test-particle simulation allows us to derive the energy dependence of the MC-driven FD providing an estimate of the amplitude at different rigidities, here compared with several neutron monitor observations. This work points out the importance of the large-scale MC configuration in the interaction between GCRs and ICMEs and suggests that particle drifts have a primary role in modulating the GCR intensity within the MC under study and possibly in at least all slowly expanding ICMEs lacking a shock/sheath region.

Spectral Analysis of Forbush Decreases Using a New Yield Function

The Forbush decreases of the cosmic ray intensity observed on 24 December 2014 and on 8 September 2017 were chosen for cosmic ray spectral analysis. At first an analytical study of the solar and geomagnetic parameters of these events was carried out due to the fact that both are typical cosmic ray events. Hourly cosmic ray data of the neutron monitor stations obtained from the high-resolution neutron monitor database were used for calculating the cosmic ray density and anisotropy variations. Following the method of the coupling coefficients, the galactic cosmic ray spectral index was calculated using the technique of Wawrzynczak and Alania (Adv. Space Res. 45, 622, 2010). A newly presented yield function by Mishev, Usoskin, and Kovaltsov (J. Geophys. Res. 118, 2783, 2013) including a geometrical correction factor, already used in the spectral analysis of the cosmic ray ground level enhancements, was applied for the first time to the case of Forbush decreases. A comparison of these results during the events is performed by using two other coupling functions: the function presented in the work of Clem and Dorman (Space Sci. Rev. 93, 335, 2000) and the one in the work of Belov and Struminsky (Proc. 25th Int. Cosmic Ray Conf. 1, 201, 1997). The latter includes an extension for neutron monitor stations with rigidity $1~\mbox{GV} < R < 2.78~\mbox{GV}$ . The obtained spectral index and the calculated cosmic ray intensity in the heliosphere during the two Forbush decreases after the coupling by these three functions are presented and discussed.

A new approximate coupling function: The case of Forbush decreases

We attempt the derivation of a new coupling function between ground level and primary cosmic rays for the case of Forbush decreases. The calculations for the new function are based on basic Quantum Field Theory theoretical tools, something that has not been attempted till now in other widely used cosmic ray coupling functions. The use of Quantum Field Theory calculations in cosmic rays events in general, is expected to be a suitable frame of work since it describes well the high energy particle interactions which result in variations to the total number of the particles involved through annihilation or creation of particles. The newly computed function is tested to the case of two events of Forbush decreases, February 2011 and March 2012, using data from the high resolution neutron monitor database. Results for the primary particle intensity values obtained from this function are compared directly to the corresponding ones from the use of the Dorman's widely accepted coupling function. The two sets are discussed in detail in order to deduce the possible suitability of Quantum Field Theory tools to cosmic ray events.

Evolution of Coronal Mass Ejections and the Corresponding Forbush Decreases: Modeling vs. Multi-Spacecraft Observations

One of the very common in situ signatures of interplanetary coronal mass ejections (ICMEs), as well as other interplanetary transients, are Forbush decreases (FDs), i.e. short-term reductions in the galactic cosmic ray (GCR) flux. A two-step FD is often regarded as a textbook example, which presumably owes its specific morphology to the fact that the measuring instrument passed through the ICME head-on, encountering first the shock front (if developed), then the sheath and finally the CME magnetic structure. The interaction of GCRs and the shock/sheath region, as well as the CME magnetic structure, occurs all the way from Sun to Earth, therefore, FDs are expected to reflect the evolutionary properties of CMEs and their sheaths. We apply modelling to different ICME regions in order to obtain a generic two-step FD profile, which qualitatively agrees with our current observation-based understanding of FDs. We next adapt the models for energy dependence to enable comparison with different GCR measurement instruments (as they measure in different particle energy ranges). We test these modelling efforts against a set of multi-spacecraft observations of the same event, using the Forbush decrease model for the expanding flux rope (ForbMod). We find a reasonable agreement of the ForbMod model for the GCR depression in the CME magnetic structure with multi-spacecraft measurements, indicating that modelled FDs reflect well the CME evolution.

Cosmic Rays during Forbush Effects in March 1989 and March 1991: Spectra of Variation, Anisotropy, and Variations of Geomagnetic Cutoff Rigidity

Two Forbush effects that occurred during geomagnetic storms in March 1989 and March 1991 are analyzed based on ground-based measurements of cosmic rays at a global network of stations via spectrographic global survey. The rigidity spectra and variation spectra are presented, as well as the pitch-angle anisotropy of cosmic rays at different phases of the development of Forbush effects and variations in the planetary system of geomagnetic cutoff rigidities. It is shown that, when the variation spectra are approximated by a power-law function of particle rigidity in the range of 10–50 GV, the spectrum index in the phase of maximum modulation is softer than in the decline and recovery phases of cosmic-ray intensity. The distance to the subsolar point and the radius of the ring current, as well as the contribution of the ring current to variations in the geomagnetic cutoff rigidity and the Dst index, are determined within an axisymmetric model of the Earth’s limited magnetosphere that takes into account the magnetopause currents and ring current.

Interplanetary Coronal Mass Ejection Associated Forbush Decreases in Neutron Monitors

Abstract Interplanetary coronal mass ejections (ICMEs) are eruptions of plasma that propagate outward through the heliosphere. ICMEs, and the shocks they drive, cause a sudden decrease in the cosmic-ray flux in their local area of the heliosphere, called a Forbush decrease (FD). A method of defining FDs is established, and an automated process for identifying FDs in neutron monitor (NM) data is created. The correlation between ICME properties and FD magnitude in 12 different NMs is examined for 91 ICME-associated FD events occurring from 2001 through 2019 August. A number of ICME properties show positive correlation with FD magnitude, with decreasing correlation strength as NM cutoff rigidity increases.

Evolution of planar magnetic structure within the stream interaction region and its connection with a recurrent Forbush decrease

ABSTRACT In general, stream interaction region (SIR)-induced Forbush decreases are recurrent and low magnitude in nature. The diffusion–convection associated with the SIR plays an important role in their modulation. Here, we study the evolution of planar magnetic structure (PMS) within the SIR and its contribution to cosmic ray modulation. Interestingly, we found the presence of PMS structures within the SIR from the leading part of the SIR to the minimum of the cosmic ray intensity in two events. The PMS may have originated due to the high compression caused by the fast solar wind, which amplifies and aligns the pre-existing discontinuities in the ambient slow solar wind. The study also suggests that the existence of PMS, enhanced initial mass function (IMF) strength, and associated turbulent regions decreases the perpendicular diffusion coefficient and causes a decrease in the cosmic ray intensity observed on Earth. Moreover, a slow decrease in IMF magnitude concurs with the recovery phase of cosmic ray intensity.

Ring of Stations Method in Cosmic Rays Variations Research

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.

Modulation of Galactic Cosmic Rays by Plasma Disturbances Propagating Through the Local Interstellar Medium in the Outer Heliosheath

Abstract The modulation of cosmic rays by a propagating plasma disturbance, a global merged interaction region (GMIR), in the heliosheath is simulated using a Vlasov–Fokker–Planck equation for the transport of energetic particles with significant anisotropy. The prescribed plasma structure of the GMIR contains a shock front and plasma rarefaction region behind the shock, which propagate through a simplified paramagnetic shielding model of the heliosheath. When a GMIR goes through the heliospheric magnetic field in the inner heliosheath, its modulation effects on cosmic rays are consistent with typical Forbush decreases. When a GMIR goes through the interstellar magnetic field in the outer heliosheath, only cosmic rays with large pitch angles with respect to the magnetic field vector (cosine values close to zero) are modulated by it. The difference is due to the very weak scattering of particles by the interstellar turbulence. Particles trapped in the rarefied magnetic field inside a GMIR suffer a significant amount of adiabatic cooling, which results in a considerable intensity decrease and a bidirectional anisotropy. The simulation result can be used to explain what Voyager 1 observed in the very local interstellar medium. Depending on the strength of plasma compression inside a GMIR, some cosmic rays may be accelerated, but the GMIR effect on the cosmic-ray intensity is much weaker than that due to adiabatic cooling because particles have only a brief interaction with a GMIR without trapping.

Exploring the common origins of the Forbush decrease phenomenon caused by the interplanetary counterpart of coronal mass ejections or corotating interaction regions

The interplanetary counterpart of coronal mass ejections (ICMEs) and the interaction regions of slow-fast solar wind (CIRs) have both been known as potential drivers of Forbush decrease (FD). However, reported studies often take an independent approach for investigating FD caused by ICMEs and CIRs, since both the structures show different signature in in-situ observations. In this paper, we explore the common origin of the FD profile caused by these two large scale structures, within the framework of a diffusion-convection model. As a case study, we present one event of each type, in both of which, the solar wind is the most prominent driver. Possible extensions of this model could incorporate other parameters such as magnetic field strength, turbulence, etc which influence the observed FD features. This attempt could help to resolve the complex problem of the diversity in observed FD profiles.

Geomagnetic Storms and Forbush-Decreases Related to Single and Complex Transient Structures of Solar Wind

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.