MHD-simulation of corotating interaction regions in the heliosphere using different boundary conditions and codes

The paper presents the results of MHD modeling of corotating interaction regions (CIRs) at distances of 0.1 AU from the Sun (inner boundary) to much larger distances (20–30 AU) in two variants in which the magnetic field on the photosphere (1) is determined from a detailed synoptic map and (2) is represented only by the dipole component. The calculations are made for Carrington rotation 2066 (January–February 2008), using two independent software packages of Russian and Chinese groups. The time period under study is characterized by the presence of long-lived coronal holes on the Sun and a stable recurrent variation in heliosphere characteristics, as well as in the intensity of galactic cosmic rays. We discuss the advantages and disadvantages of modeling CIRs by detailed and dipole models of the photospheric magnetic field, as well as with the two mentioned software packages. The mechanisms of formation and evolution of CIRs with distance in the two models are compared and correlated to the conclusions of our previous works.

An analysis of the data of spacecraft that scanned large areas of the heliosphere, as well as the resultsof magnetohydrodynamic calculations, indicates that the corotating interaction regions of solar wind (SW),which are almost always present in the low- and mid-latitude heliosphere, sometimes strongly change thelarge-scale characteristics of the heliosphere that are important for long-term variations in the intensity ofgalactic cosmic rays (GCRs). In particular, for Carrington rotation no. 2066 (January–February 2008), theseregions enhance magnetic fields in the inner (r 3–5 AU) heliosphere and weaken them in the middle andfar heliosphere, as well as significantly changing the polarity distribution of heliospheric magnetic fields. Theassumption is made that in this situation the influence of the corotating interaction regions should lead to anincrease in the GCR intensity in many regions of the heliosphere. This paper discusses the process of changingthe polarity distribution of heliospheric magnetic fields due to the interaction of SW streams for Carringtonrotation no. 2066 of different speeds, the simple model of the heliospheric magnetic field without aninteraction between the SW streams of different speeds, as well as the results of numerical two-dimensionalfinite-difference calculations of longitude-averaged GCR intensity with the use of this model in comparisonwith a three-dimensional Monte Carlo calculation based on three-dimensional magnetohydrodynamic simulationof the heliosphere.

The effects of the 22-year variation of solar magnetic fields in the galactic cosmic ray (GCR) intensity were first observed and interpreted as manifestations of inversion of the high-latitude solar magnetic field in properties of heliospheric magnetic fields by the Lebedev Physical Institute team in 1973. Since then, these effects have been studied already for 50 years.
 The situation with the heliospheric magnetic field is clear for periods of medium and low sunspot activity — the heliosphere consists of two unipolar “hemispheres” separated by a wavy global heliospheric current sheet and characterized by a general polarity A (unit quantity with the sign of the radial component of the heliospheric magnetic field in the northern hemisphere). Yet there is no consensus on what the inversion of the heliospheric magnetic field is and which effects in the GCR intensity are connected with this phenomenon.
 In this article, we briefly formulate general concepts of the 22-year variation in characteristics of the Sun, heliosphere, and GCR intensity and discuss the observed effects in the GCR intensity, which we attribute to the heliospheric magnetic field reversal. Models for this phenomenon and the results of GCR intensity calculations with these models will be discussed in the next article.

The intensity of galactic cosmic rays (GCRs) is modulated by solar activity on various timescales. In this study, we performed comprehensive numerical modeling of the solar rotational recurrent variation in GCRs caused by a corotation interaction region (CIR). A recently developed magnetohydrodynamic numerical model is adapted to simulate the background solar wind plasma with a CIR structure present in the inner heliosphere. As for the outer heliospheric plasma background, from 27 to 80 au, the Parker interplanetary magnetic field model is utilized. The output of these plasma and magnetic field models is incorporated into a comprehensive Parker-type transport model for GCRs. The local interstellar spectrum for galactic protons is transported to 80 au, specifying the outer boundary condition. The obtained solutions of this hybrid model, for studying the CIR effect, are as follows: (1) the onset of the decrease in the GCR intensity inside the CIR coincides with the increase of the solar wind speed with the intensity depression accompanied by a magnetic field and plasma density enhancement. Additionally, the CIR effect weakens with increasing heliocentric radial distance. (2) This decrease in GCR intensity also appears at different heliolatitudes and varies with changing latitude; the amplitude of the GCR depression exhibits a maximum in the low-latitude region. (3) The CIR affects GCR transport at different energy levels as well. Careful analysis has revealed a specific energy dependence of the amplitude of the recurrent GCR variation in the range of 30–2000 MeV.

The effects of the 22-year variation of solar magnetic fields in the galactic cosmic ray (GCR) intensity were first observed and interpreted as manifestations of inversion of the high-latitude solar magnetic field in properties of heliospheric magnetic fields by the Lebedev Physical Institute team in 1973. Since then, these effects have been studied already for 50 years.
 The situation with the heliospheric magnetic field is clear for periods of medium and low sunspot activity — the heliosphere consists of two unipolar “hemispheres” separated by a wavy global heliospheric current sheet and characterized by a general polarity A (unit quantity with the sign of the radial component of the heliospheric magnetic field in the northern hemisphere). Yet there is no consensus on what the inversion of the heliospheric magnetic field is and which effects in the GCR intensity are connected with this phenomenon.
 In this article, we briefly formulate general concepts of the 22-year variation in characteristics of the Sun, heliosphere, and GCR intensity and discuss the observed effects in the GCR intensity, which we attribute to the heliospheric magnetic field reversal. Models for this phenomenon and the results of GCR intensity calculations with these models will be discussed in the next article.

A Corotating Interaction Region (CIR) is formed when the fast solar wind catches up with the slow solar wind. It is known that the intensity of Galactic Cosmic Rays (GCRs) is modulated by CIRs with the GCR intensity suppressed inside the CIR. Previous numerical studies were mainly confined to GCR protons. For this study we utilize a hybrid GCR transport model which incorporates a Magnetohydrodynamic (MHD) simulated solar wind plasma background with a CIR structure. Additionally, adopting appropriate mass, charge and local interstellar spectra, the hybrid transport model is applied to GCR protons and the two helium isotopes. It is found that both proton and total helium studied at 0.3 GV are modulated by the CIR with their fluxes being depressed. However, the modulation for protons and helium is different with helio-longitude, and interestingly the ratio of the proton to helium flux varies with longitude. Similar results are found for the way in which the two helium isotopic fluxes vary.

Quasi-biennial oscillation (QBO) is a well-known quasi-periodical variation with characteristic time 0.5-4 years in different solar, heliospheric and cosmic ray characteristics. Recently it has been shown that there are low correlation between the solar and heliospheric QBOs and rather high anticorrelation between the QBOs in galactic cosmic ray (GCR) intensity near the Earth and in the strength of the heliospheric magnetic field (HMF). Besides, it was suggested that both steplike changes of the GCR intensity and Gnevyshev Gap effect (a temporal damping of the solar modulation around the sunspot maxima) could be viewed as the manifestations of QBO. Some suggestions were also made on the mechanisms of QBO in the GCR intensity. In this paper a hypothesis is checked on the causes of the apparent lack of correlation between solar and heliospheric QBOs, then the possible mechanisms of QBO in the GCR intensity are discussed as well as the idea of the same nature of the step-like changes and Gnevyshev Gap effects in the GCR intensity. Our main conclusions are as follows: 1. In thefirst approximation the hypothesis is justified that the change in the sunspot and QBO cycles in the transition from the Sun to the heliosphere is due to 1) the different magnitude and time behavior of the large-scale and small-scale photospheric solar magnetic fields and 2) the stronger attenuation of the small-scale fields in this transition. 2. As the QBO in the HMF strength influences both the diffusion coefficients and drift velocity, it can give rise to the complex QBO in the GCR intensity with respect to the dominating HMF polarity. The description of drift velocity field for the periods of the HMF inversion is suggested, although it has drawbacks. 3. As the conditions in the heliosphere are quite different around the sunspot maximum and during the periods of low solar activity (both with respect to the HMF polarity distribution and with the presence or absence of the large-scale barriers), the suggestion that both the step-like changes of the GCR intensity and Gnevyshev Gap effect could have the same nature, looks questionable.

Study of the development and mechanism of large amplitude decreases in cosmic ray intensity during geomagnetic disturbances in the magnetosphere

The intensity variation of cosmic rays associated with corotating interaction regions

This paper reviews three important effects on energetic particles of corotating interaction regions (CIRs) in the solar wind that are formed at the leading edges of high-speed solar wind streams originating in coronal holes. A brief overview of CIRs and their important features is followed by a discussion of CIR-associated modulations in the galactic cosmic ray intensity, with an emphasis on observations made by spacecraft particle telescope 'anti-coincidence' guards. Such guards combine high counting rates (hundreds of counts/s) and a lower rigidity response than neutron monitors to provide detailed information on the relationship between cosmic ray modulations and CIR structure. The modulation of Jovian electrons by CIRs is then described. Finally, the acceleration of ions to energies of ∼ 20 MeV/n in the vicinity of CIRs is reviewed. Corotating interaction regions (CIRs) are regions of compressed plasma formed at the leading edges of corotating high-speed solar wind streams originating in coronal holes as they interact with the preceding slow solar wind. They are par- ticularly prominent features of the solar wind during the declining and minimum phases of the 11-year solar cycle, but may also be present at times of higher solar activity, interspersed with slow solar wind and transient flows associated with coronal mass ejections (CMEs) (e.g., Richardson, Cane, and Cliver, 2002). This paper reviews three important effects of CIRs on energetic particles in the heliosphere. The first effect is the tendency for the galactic cosmic ray intensity to be depressed temporarily ('modulated') during the passage of a CIR and high- speed stream. This phenomenon has been studied for many years, typically using ground-based neutron monitors. However, the emphasis in this review is on obser- vations made in the inner heliosphere and near the Earth by the 'anti-coincidence' guards of certain spacecraft particle telescopes. Such guards combine a large de- tector volume and integral energy response (> several tens of MeV) to give high counting rates (hundreds of counts/s) which can provide detailed information on the relationship between cosmic ray modulations and CIR structure. The observa-

Ground neutron monitors (NMs) sometimes observe a sudden reduction in galactic cosmic ray intensity—the so‐called Forbush decrease (FD) event. Such events are mainly associated with interplanetary coronal mass ejections passing around the Earth and corotating interaction regions in the heliosphere. Some FD events are observed globally, either simultaneously or nonsimultaneously, at different NM stations in the case that the simultaneity is determined by the overlapping of the FD main phase, with the period of the cosmic ray intensity decreasing before returning to a steady state. Previous studies have identified two types of FD events with statistically significant differences in the distributions of the main phase onset time. It has been hypothesized that simultaneous FD events occur when a strong magnetic cloud passes by the Earth through the central part of the magnetic barrier, whereas nonsimultaneous events occur if a weaker magnetic cloud passes on the duskside of the magnetosphere. However, the previous statistical analyses were performed using only data from high geomagnetic latitude NM stations in the Northern Hemisphere. To address this shortcoming and to further test the above hypothesis, we repeated the analysis using data from NM stations located at middle latitudes (Jungfraujoch, Irkutsk, and Climax), employing cutoff rigidities 3–6 GV for the last solar maximum period (1998–2002), spanning the same time period as Oh et al. (2008) that employed high‐latitude NM stations. The results of the present statistical analysis support the above hypothesis with high confidence levels.

Interplanetary structures such as shocks, sheaths, interplanetary counterparts of coronal mass ejections (ICMEs), magnetic clouds, and corotating interaction regions (CIRs) are of special interest for the study of the transient modulation of galactic cosmic rays (GCRs). These structures modulate the GCR intensity with varying amplitudes and recovery-time profiles. It is known that ICMEs are mainly responsible for Forbush decreases in the GCR intensity. However, not all of the ICMEs produce such decreases in GCR intensity. We utilize GCR intensity data recorded by neutron monitors and solar-wind plasma/field data during the passage of ICMEs with different features and structures, and we perform a superposed-epoch analysis of the data. We also adopt the best-fit approach with suitable functions to interpret the observed similarities and differences in various parameters. Using the GCR-effectiveness as a measure of the cosmic-ray response to the passage of ICMEs, about half of the ICMEs identified during 1996 – 2009 are found to produce moderate to very large intensity depressions in GCR intensity. The ICMEs associated with halo CMEs, magnetic-cloud (MC) structures, bidirectional superthermal electron (BDE) signatures, and those driving shocks are 1.5 to 4 times more GCR effective than the ICMEs not associated with these structures/features. Further, the characteristic recovery time of GCR intensity due to shock/BDE/MC/halo-CME-associated ICMEs is larger than those due to ICMEs not associated with these structures/features.

We investigate the formation and evolution of corotating interaction regions (CIRs) in the solar wind and their effects on galactic cosmic rays (GCR) during the recent solar cycle 23/24 solar minimum. The output from a three‐dimensional MHD model serves as background for kinetic time‐dependent simulations of GCR transport based on the Parker equation. The results show that the CIR forward/reverse shock pairs or compression/rarefaction regions play important roles in the transport of GCR particles and directly control the observed 27 day periodic intensity variations. We find that stream interfaces (SIs) in CIRs and the heliospheric current sheet (HCS) are both closely associated with the GCR depression onset, in agreement with the observations at 1 AU. The HCS is more important when its tilt angle becomes small during the declining phase of the solar minimum, while the passages of SIs control the onset of GCR depressions for larger HCS tilt angles. The mechanism of GCR intensity variation near 1 AU can be explained through an interplay between the effects of particle drift and diffusion. The simulated plasma background and GCR intensity are compared with the observations from spacecraft and a neutron monitor on the ground, to find good qualitative agreement. Evidently, CIRs had a substantial modulational effect on GCR during the recent solar minimum.

We present a study of the long-term evolution of coronal mass ejections (CMEs) observed by the Large Angle and Spectrometric Coronograph (LASCO) on board SOHO during the ascending, maximum, and part of the descending phases of solar cycle 23 and their relation with the modulation of galactic cosmic-ray (GCR) intensity observed at 1 AU by the Climax neutron monitor and IMP-8 spacecraft. We compare the long-term GCR modulation with the CME occurrence rate at all, low, and high latitudes, as well as the observed CME parameters (width and speed). Twenty-seven day averages of CME occurrence rates and CME properties from 1996 January to 2003 December are presented in the Appendix. The general anticorrelation between GCR intensity and the CME rate is relatively high (~-0.88). However, when we divide the CME rate into low- and high-latitude rates and compare them with the GCR intensity during the ascending phase of solar cycle 23, we find a lower anticorrelation between the low-latitude the CME rate and GCR intensity (~-0.71) and a very high anticorrelation between the high-latitude CME rate and GCR intensity (~-0.94). This suggests that, in general, CMEs could cause the decrease in the GCR flux in the inner heliosphere, as stated by the global merged interaction region (GMIR) theory. In particular, during the ascending phase of cycle 23 (qA > 0), this flux comes mainly from heliospheric polar regions. Thus, high-latitude CMEs may play a central role in the long-term cosmic-ray modulation during this phase of the cycle by blocking the polar entrance of GCRs to the inner heliosphere. This study supports the scenario in which CMEs, among other structures, are the building blocks of GMIRs, although we propose that the spherical shells (GMIRs) are closed separately at polar and equatorial regions by CMEs of different latitudes. Our results suggest that all CME properties show some correlation with the GCR intensity, although there is no specific property (width, speed, or a proxy of energy) that definitely has a higher correlation with GCR intensity.

The Forbush decrease (Fd) of the Galactic cosmic ray (GCR) intensity and disturbances in the Earth’s magnetic field generally take place simultaneously and are caused by the same phenomenon, namely a coronal mass ejection (CME) or a shock wave created after violent processes in the solar atmosphere. The magnetic cut-off rigidity of the Earth’s magnetic field changes because of the disturbances, leading to additional changes in the GCR intensity observed by neutron monitors and muon telescopes. Therefore, one may expect distortion in the temporal changes in the power-law exponent of the rigidity spectrum calculated from neutron monitor data without correcting for the changes in the cut-off rigidity of the Earth’s magnetic field. We compare temporal changes in the rigidity spectrum of Fds calculated from neutron monitor data corrected and uncorrected for the geomagnetic disturbances. We show some differences in the power-law exponent of the rigidity spectrum of Fds, particularly during large disturbances of the cut-off rigidity of the Earth’s magnetic field. However, the general features of the temporal changes in the rigidity spectrum of Fds remain valid as they were found in our previous study. Namely, at the initial phase of the Fd, the rigidity spectrum is relatively soft and it gradually becomes hard up to the time of the minimum level of the GCR intensity. Then during the recovery phase of the Fd, the rigidity spectrum gradually becomes soft. This confirms that the structural changes of the interplanetary magnetic field turbulence in the range of frequencies of 10−6 – 10−5 Hz are generally responsible for the time variations in the rigidity spectrum we found during the Fds.