Merged Interaction Regions Research Articles

The simulation of step decreases in the cosmic ray intensity with a cosmic-ray-hydrodynamic model

A self-consistent time-dependent cosmic ray-hydrodynamic model is used in spherical symmetry to solve the standard cosmic ray transport equation for combined anomalous and galactic proton transport, and the one-fluid equations for the solar wind/pickup ion fluid including the dynamic effects of cosmic rays. The model calculates the evolution of a single global merged interaction region including its interaction with the heliospheric termination shock, and the consequent effect on cosmic ray modulation. A series of such regions is also simulated to study their combined contribution on the cosmic-ray modulation cycle.

Magnetic field strength distributions and spectra in the heliosphere and their significance for cosmic ray modulation: Voyager 1, 1980–1994

Annual variations of the hourly averages of the heliospheric magnetic field strengthBobserved by Voyager 1 (V1) have been analyzed for the 15‐year interval 1980–1994 inclusive. During this period, V1 moved from a radial distance of 6.9–58.1 AU and from a heliolatitude of −5.4 to 32.7°. The annual distribution function ofBis nearly lognormal. The width of the distribution, w, does not change systematically with time, distance, latitude or solar activity: 〈w〉 = 0.61 ± 0.02. The standard deviation ofB, SD(B), normalized by the average,Bav, is also invariant: 〈SD(B)/Bav〉 = 0.67 ± 0.03. There is a decadal frequency range for which the power spectral density (PSD) ofB(t) is well described by a power law with an exponent s that ranges from −2.0 near solar maximum (1980, 1990) when GMIRs were present to −1.5 near solar minimum (1987–1988). A Global Merged Interaction Region (GMIR) is a shell‐like region that encircles the Sun, propagates away from the Sun, has a radial thickness of the order of a few AU and contains intense, disturbed magnetic fields. The lower limit of the scaling range corresponds to a period of ≈ 5 days and is invariant in the 15 years studied. The relative amplitude of the PSD at the lower limit of this range is also invariant. However, the relative amplitude of the fluctuations at the solar rotation period of 26 days decreases with increasing distance from the sun asR−(0–9 ± 0.2). These derived observational results in time and frequency domains require theoretical models of the solar wind expansion into the distant heliosphere that include both physical and statistical features. Such models could be based on (1) energy and/or entropy principles or (2) conventional MHD along with temporally complex but statistically simple input functions. Our results suggest an anticorrelation between the annual magnetic field strength and cosmic ray intensity, as is observed during the 15‐year period considered here. Statistical models of the magnetic field might provide a basis for new models of cosmic ray modulation.

Self‐consistent stochastic preacceleration of interstellar pickup ions in the solar wind including the effects of wave coupling and damping

A self‐consistent model was developed to study the stochastic acceleration (second‐order Fermi) of interstellar pickup ions in the quiet solar wind (SW) and inside corotating merged interaction regions (CMIRs) in the presence of the effects of wave coupling and the damping of magnetohydrodynamic waves by pickup ions. Our simulations of the solar wind wave spectrum show that (1) Pickup H+ is mainly responsible for wave damping at large wave numbers with pickup He+ contributing very little and pickup O+ contributing nothing. (2) Strong wave‐wave interactions wash out wave‐damping effects for radial distances r ≤ 10 AU from the Sun in the quiet solar wind and for r ≤ 23 AU inside corotating merged interaction regions. This is to be contrasted with previous work which calculated strong wave damping inside 10 AU from the Sun in the absence of wave coupling. For the simulated pickup ion spectra the main results are as follows: (1) The pickup He+ and O+ spectra are unaffected by the wave damping. (2) In contrast, the accelerated pickup H+ spectra have concave features for kinetic energies 1–5 KeV which become more pronounced with increasing radial distance. This suppression of the pickup H+ acceleration is attributed to the wave damping caused by the pickup H+. (3) Inside corotating merged interaction regions the preacceleration of pickup ions is stronger because of the enhanced heliospheric magnetic field. The suppression of pickup H+ acceleration is smaller and only starts to show for r > 23 AU. (4) Under the assumption that the seed population of anomalous cosmic rays should have speeds v > 400 km s−1 the H+/He+ density ratio in this population can be as low as ∼2.6 in the quiet solar wind and ∼13 inside corotating merged interaction regions. Compared with the observed ratio of ∼6 for anomalous cosmic rays (ACRs), our conclusion is that if CMIR and quiet SW regions are considered together, then the observed under abundance in ACR H+ could in principle be produced on average.

Perpendicular Diffusion and Drift at Intermediate Cosmic‐Ray Energies

We present a new derivation of perpendicular diffusion and drift coefficients, based on the Taylor-Green-Kubo equation and physical arguments as to the form of the particle velocity correlation function. In contrast to previous derivations, which are restricted to particle rigidities greater than 4 GV for typical conditions at 1 AU, our derivation is valid for intermediate-energy, as well as high-energy, cosmic rays. Our results provide a natural explanation for several hitherto puzzling observations, including (1) the form of the perpendicular diffusion coefficient derived from measurements of anomalous cosmic rays, (2) the fact that drift models often describe observations better when drifts are reduced, and (3) the CR-B relation, which associates cosmic-ray modulation with enhanced magnetic fields in merged interaction regions.

Radial evolution of corotating merged interaction regions and flows between ≈14 AU and ≈43 AU

During 1993 and 1994 the solar coronal holes and the heliospheric current sheet were relatively stationary, and recurrent streams and interaction regions with periods of the order of the solar rotation period were present within 5 AU. One expects that during 1994 Voyager 2 (V2) (located at ≈43 AU and at ≈12°S latitude, in the sector zone) would have observed some evolutionary form of corotating streams and interaction regions. The “sector zone” is the latitude band in which a spacecraft observes both positive and negative sectors [Burlaga and Ness, 1996]. We present the observations of the magnetic field strength (B), the speed (V), density (N), and proton temperature (T) made by V2‐94, and for reference we also discuss the observations made by V2 at ≈14 AU during 1983 (V2‐83) a solar cycle earlier. Correlated, quasiperiodic variations in B and N with a period of ≈26 days (corotating merged interaction regions) were observed at ≈14 AU but not at ≈43 AU. The speed and temperature profiles were irregular at ≈14 AU but quasiperiodic at ≈43 AU. An ƒ−2 spectrum of the magnetic field strength B (indicating the dominance of shocks) was observed at ≈14 AU, but an ƒ−5/3 spectrum (indicating the dominance of Kolmogorov turbulence) was observed at ≈43 AU in the range (2.7 × 10−6 to 2.3 × 10−5)Hz. An ƒ−2.5 spectrum of the speed fluctuations was observed at ≈14 AU, but an ƒ−2 spectrum was observed at ≈43 AU in the range (8.8 × 10−7 to 2.3 × 10−5)Hz. We suggest the hypothesis that the qualitative differences between the observations at ≈14 AU and ≈43 AU represent a change in the state of the solar wind as it moves between these two positions. This change involves a transition from a quasiperiodic (ordered) state in B and N at ≈14 AU to a disordered state at ≈43 AU and from an aperiodic state in V and T at ≈14 AU to a quasiperiodic state at ≈43 AU. The standard MHD models for the radial evolution of corotating streams and interaction regions have not predicted such a transition. Our results suggest that it would be fruitful to develop a new MHD model of such flows, which should include (1) three‐dimensional effects, (2) the intermediate‐scale fluctuations, and (3) the interstellar pickup ion pressure.

Helioradius Dependence of Interplanetary Carbon and Oxygen Abundances during 1991 Solar Activity

A comparison of the heliocentric dependence of the relative abundance of low energy (~1 MeV nucleon-1) carbon and oxygen on the Ulysses (~3-4 AU) and Voyager 1 and 2 (~35 and ~47 AU) spacecraft shows that the C/O abundance ratio is significantly smaller at the larger radial distances. This finding is attributed to a more abundant seed population for anomalous cosmic-ray oxygen than for carbon in the outer solar system, where the seed population can be accelerated by shocks in global merged interaction regions. The energy spectra of carbon and oxygen at the different locations are approximately the same.

Long‐term enhancements in solar wind speed

Long‐term enhancements in solar wind speed over timescales of the order of a year appear to be a common feature throughout the heliosphere over heliocentric distances that range from less than 0.72 AU to greater than 60 AU. The origin of these events remains to be determined, but they are almost certainly associated with long‐term variations at the solar wind source, in contrast with smaller‐scale structures such as corotating interaction regions (CIRs), merged interaction regions (MIRs), and global merged interaction regions (GMIRs), which are dynamical in origin. We present a survey of the long‐term speed enhancements observed at the Pioneer Venus Orbiter (PVO), IMP 8, Voyager 2, and Pioneer 10 between 1974 and 1994 and compare this with published reports of smaller‐scale events such as MIRs. We examine several of these long‐term speed enhancements in detail to identify and characterize aspects of their structure, and then describe how that structure evolves with heliocentric distance. Finally, we discuss some of the implications of these events.

Locations of the termination shock and the heliopause

The locations of the termination shock and the heliopause are restudied by taking into account the effects of pickup protons. The study uses available plasma and magnetic field data from Voyagers over a 14‐year period (1978–1991) and Voyager observations of the 1992–1993 radio emission event, outside 30 AU, pickup protons have a significant influence on dynamical structures of the outer heliosphere. The solar wind is treated as a mixture of electrons, solar wind protons, and interstellar pickup protons. If the magnitude of the interstellar magnetic field Bint is given, one can quantitatively study the motion and location of the termination shock. We find that the location is anticorrelated with the sunspot number. The absolute mean of the shock speed is 19 km/s, and the quadratic mean of the shock speed is 24 km/s. Because Bint is poorly known, additional information is needed in studying the termination shock. Cummings et al. (1994) have used observations of anomalous cosmic rays to estimate the location of the shock. The observations of the 1991 global merged interaction region (GMIR) and GMIR shock and the 1992–1993 radio emission event provide another handle for the study of the termination shock and the heliopause. After its penetration through the termination shock, the GMIR shock continued to propagate in the subsonic region of the solar wind and eventually interacted with the heliopause. This interaction produced a transmitted shock propagating outward in the interstellar medium and a reflected shock propagating inward toward the Sun in the subsonic solar wind. The plasma frequencies behind the reflected and the transmitted shocks can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. We assume that the impingement of the GMIR shock at the heliopause occurred at the time when Voyagers started receiving the radio emissions. Taking into account the effects of pickup protons, we find that the locations of the termination shock and the heliopause in 1991–1992 are at approximately 66 AU and 150 AU, respectively.

The time-dependent transport of cosmic rays in the heliosphere

The modulation of cosmic rays (CRs) in the heliosphere is a dynamic and therefore a highly time-dependent process. Numerical models with only a time-dependent neutral sheet prove to be successful when moderate to low solar activity occurs but fail to describe large and discrete steps in modulated CRs when solar activity is high. To explain this feature of heliospheric modulation, the concept of global merged interaction regions (GMIRs) is required. The combination of gradient, curvature and neutral sheet drifts with these GMIRs has so far been the most successful approach in explaining the 11-year and 22-year cycles in the long-term modulation of CRs.

Magnetic fields and cosmic rays in the distant heliosphere at solar maximum: Voyager 2 observations near 32 AU during 1990

The intensity of >70 MeV cosmic ray protons observed during solar cycle 22 by Voyager 2 (V2) in the distant heliosphere reached a minimum in 1990, corresponding to the maximum of solar activity in mid‐1989 (allowing for the propagation time of the solar wind from the sun to Voyager 2 near 32 AU). A step decrease in the cosmic ray intensity observed within the first ≈ 125 days of 1990 was caused by a global merged interaction region (GMIR), consisting of a cluster of large‐amplitude magnetic field strength fluctuations, in which the field was predominantly stronger than average. The step decrease consists of four events in each of which the decrease in cosmic ray intensity is relatively large, and the recovery is either brief or absent. The relation between the cosmic ray intensity and the magnetic field strength observed near 32 AU during this period at the maximum of solar activity in 1989/1990 is similar to that which Voyager 2 observed near 11 AU in 1982/1983 following the maximum of solar activity in the previous solar cycle in 1980. The local changes in the cosmic ray intensity profile are related to the magnetic field strength. Near solar maximum the emission rate of ejecta, which contain magnetic fields that deviate significantly from the spiral direction, is a maximum. Nevertheless, the distribution of elevation angles of the magnetic field observed by Voyager 2 during 1990 is essentially the same as that observed during earlier parts of the solar cycle from 1986 through 1989, including solar minimum in 1986 and 1987. During 1990, the width of the distribution of elevation angles at ≈ 2 AU was the same as that observed at 1 AU. Sectors were observed, but no sector structure was present during 1990, just as in 1988, 1989, and 1991. The distribution of daily averages of the magnetic field strength is lognormal for 0.05 nT < B < 0.25 nT. However, there are more strong magnetic fields than the lognormal distribution predicts for 0.25 nT < B < 0.4 nT, owing to the large MIRs. The large‐scale fluctuations of the magnetic field strength have a multifractal structure in the low‐frequency range from 2.7 days to at least 21.3 days that influences the cosmic ray intensity profile.

The simulation of complete 11 and 12 year modulation cycles for cosmic rays in the heliosphere using a drift model with global merged interaction regions

Two-dimensional, time-dependent drift models have done exceptionally well in explaining major modulation features, especially during the A less than 0 magnetic polarity cycle of the heliospheric magnetic field when positively charge particles are drifting in along the heliospheric neutral sheet (HNS). These models were found to do well when the heliospheric 'tilt angles' alpha less than approximately 30 deg (le Roux & Potgeiter). However, they seem to do less well when alpha greater than approximately 30 deg during A less than 0 cycles seem to fail when this happens in A greater than 0 cycles. Progress was made in understanding these phases of the modulation cycle when merged interaction regions (MIRs) were incorporated in time-dependent drift models (Potgieter et al.). It was also explicitly shown that in obtaining large step decreases in cosmic rays, the MIRs had to be global, i.e., having a latitudinal extent of more than approximately 60 deg. Other classes of MIRs, such as local MIRs and co-rotating MIRs were found to be of secondary importance for establishing long-term modulation. In a previous paper we studied the effects of two consecutive, identical global MIRs, together with a changing wavy HNS, on long-term modulation (Potegieter & le Roux). This approach gave a very natural and convincing explanation for the observed step decreases in cosmic-ray modulation. Emphasis was placed in the declining and recovery phases of the 11 yr modulation cycle. In this paper, four consecutive, nonidentical global MIRs, in combination with a varying wavy HNS, were included in our time-dependent drift model in order to do simulations closer to what was observed between 1977 and 1987. By doing this we could model, for the first time, complete 11 and 22 yr cycles in the heliospheric modulation of galactic cosmic-rays, including the solar magnetic polarity reversals.

Cosmic ray modulation during solar cycle 22: Solar maximum loops at earth and global transients in the heliosphere

Neutron monitor data from two pairs of cosmic ray stations, Kiel/Tsumeb and Climax/Huancayo, are used to study the rigidity dependence of solar modulation during the solar activity cycle 22. The long‐term decrease of cosmic ray intensity during the ascending phase of cycle 22 is characterized by the same rigidity dependence as for the long‐term recovery during the descending phase of cycle 21. This is equivalent to a lack of any significant spectral hysteresis in cosmic ray modulation for both half cycles surrounding the 1987 cosmic ray maximum. The above is true at least for 27‐day averages of cosmic ray intensity and at neutron monitor rigidities. In December 1988 the first large Forbush decrease of cycle 22 marked the beginning of increased solar activity which resulted in several prominent interplanetary shocks in 1989‐1991. These strong heliospheric disturbances produced global merged interaction regions (GMIRs) which were responsible for large step decreases in cosmic ray intensity seen at all spacecraft throughout the heliosphere, including Voyager 2, Pioneer 10 and 11, as well as Voyager 1. It was found that in the neutron monitor rigidity range, such decreases were followed by rigidity‐dependent recoveries. The rigidity‐dependent recovery manifests itself by formation of hysteresis loops on correlation plots for low‐ versus high‐rigidity cosmic ray intensity changes. The loops, if completed, close approximately at the modulation level from which the corresponding step decrease started. On the other hand, the current long‐term cosmic ray recovery (starting from Bartels solar rotation 2162) follows the "normal" rigidity dependence characteristic of both the A>0 recoveries and A<0 decreases/recoveries (where A<0 and A>0 are two different phases of the solar magnetic cycle). All this together confirms the bimodal character of cosmic ray modulation for 1985‐1992, the same as found previously for cycles 19‐21; the transient modulation (T ≅ 0.5 to ∼1 year) related to GMIRs is accompanied on recovery by the phenomenon of hysteresis, while the long‐term modulation (T>1 year) is hysteresis free. The two explanations of the phenomenon of spectral hysteresis proposed in the literature, i.e., a time‐dependent modulation via convection, diffusion, and drift effects and an acceleration of a normal cosmic ray spectrum in multiple shocks in the outer heliosphere (as proposed by Stoker and Moraal (1986)), are discussed in the context of the presented cosmic ray data.

The long-term modulation of galactic cosmic rays in the heliosphere

The present declining phase of the solar modulation cycle is important to cosmic ray modulation because it should show to what extent gradient and curvature drifts of charged particles through the polar regions of the heliosphere determine the shape of the intensity-time (IT) profiles of long-term modulation. It is well known that standard drift models predict a clear 22-year cycle with alternating peaked-flattened 11-year profiles at all relevant energies. It has also become evident that the IT profile for the 1954 minimum modulation period was not nearly as flat as the 1976 profile. The 1965 and 1987 profiles, however, were perfectly peaked in direct correspondence to what drift models predict. The forthcoming solar minimum modulation period should therefore be most interesting: if not as flat as in 1976, it may indicate that drifts do not occur to the same extent as in 1972–1978 and that other modulation features e.g., merged interaction regions are important in determining the shape of the 22-year cycle. On the other hand, it may imply that predictions by current models are too idealistic because they are based on oversimplified assumptions e.g., a Parker magnetic field in the polar regions of the heliosphere. In this paper it will be discussed what modulation effects are important for the declining phase of the solar cycle. In this review some recent general observations of the long-term modulation of galactic cosmic rays in the heliosphere and the progress made in the modeling thereof will be summarized.

On the correlation between daily GCR intensity values and LDE-type flare index (1987, 1988, 1990 and 1992)

The correlation analysis between daily values of Calgary Galactic Cosmic Ray intensity (CNI) and LDE-type flare index (FI) reveals the following results: in all years under consideration, the CNI-FI anticorrelation is small, but statistically significant. The yearly CNI-FI trend depends on yearly heliospherical conditions and on their 22-year cyclical changes. At the onset of the 22-nd cycle (in 1987) the CNI-FI relation had broad minimum, similar to the result for non-flare SXR background corona in 1987. In 1988 and 1992 the typical short-term CNI-FI lag (from − 1 to − 3 days) was found, indicating the presence of local heliospherical CNI modulation sources. In 1990, the CNI-FI correlation has two comparable, large minima at − 22 as well as − 9 day CNI lags. Such double peak form of the 1990's CNI-FI minimum is consistent with the existence of two, amplitudinally balanced CNI modulation sources in the 1990's heliosphere. The first source seems to be a substantial Global Merged Interaction Region (GMIR) located in the outer heliosphere (producing the − 22 day CNI lag). The second source was a typical local heliospherical structure, indicated by short-term CNI lags.

A computer simulation of large cosmic ray decreases and their effects on the declining modulation cycle

In 1991, large decreases occurred in the long-term modulation of galactic cosmic rays in the heliosphere. These decreases were of such an extent that a global merged interaction region - GMIR - could have been responsible because they were observed at Earth with neutron monitors and IMP 8, at the Ulysses spacecraft and with some delay by the Voyager and Pioneer spacecraft. For this presentation we studied the recovery of the current modulation cycle using a time-dependent model that included drifts and MIRs. It is shown what effects a GMIR, with and without drifts and with several different modulating parameters, has on the recovery of the modulation cycle at Earth, in the outer heliosphere, and also along the Ulysses trajectory.

Daily values of the solar SXR background and modulation of GCRs (1987, 1988, 1990 and 1992)

We present here the results of the intercorrelation analysis between the daily Calgary Galactic Cosmic Ray intensity values (CNI) and daily solar SXR background (XBG - unresolved full-sun SXR GOES flux). The anticorrelation between CNI and XBG is statistically significant for 1987, 1988 and 1992 sequences. In 1990, no anticorrelation between CNI and XBG was found. The latter fact gives evidence for the Global Merged Interaction Region (GMIR) being the most dominant GCR modulation structure of the 1990's outer heliospherical system, while local heliospherical structures seemed to play the secondary role.

Pickup protons and pressure‐balanced structures: Voyager 2 observations in merged interaction regions near 35 AU

Five pressure‐balanced structures, each with a scale of the order of a few hundredths of an astronomical unit (AU), were identified in two merged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma and magnetic field data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasma and magnetic field parameters, and complex uncorrelated variations among the parameters. The changes in the magnetic pressure in these events are balanced by changes in the pressure of interstellar pickup protons. Thus the pickup protons probably play a major role in the dynamics of the MIRs. The solar wind proton and electron pressures are relatively unimportant in the MIRs at 35 AU and beyond. The region near 35 AU is a transition region: the Sun is the source of the magnetic field, but the interstellar medium is the source of pickup protons. Relative to the solar wind proton gyroradius, the thicknesses of the discontinuities and simple magnetic holes observed near 35 AU are at least an order of magnitude greater than those observed at 1 AU. However, the thicknesses of the tangential discontinuity and simple magnetic holes observed near 35 AU (in units of the pickup proton Larmor radius) are comparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons controls the thickness of current sheets near 35 AU. We determine the interstellar pickup proton pressure in the PBSs. Using a model for the pickup proton temperature, we estimate that the average interstellar pickup proton pressure, temperature, and density in the MIRs at 35 AU are (0.53 ± 0.14) × 10−12 erg/cm³, (5.8 ± 0.4) × 106 °K and (7 ± 2) × 10−4 cm−3, respectively.

Interaction of global merged interaction region shock with the heliopause and its relation to the 2‐ and 3‐kHz radio emissions

We use Voyager 2 plasma and magnetic field data together with a one‐fluid magnetohydrodynamics model to study the interactions of the 1991 global merged interaction region (GMIR) shock with the termination shock and the heliopause. The 1991 GMIR is an extraordinarily large global solar wind structure in radial, longitudinal, and latitudinal extents. It has a strong shock at the leading edge. After its penetration through the termination shock, the GMIR shock first propagates through the subsonic solar wind, then interacts with the heliopause. The interaction produces a transmitted shock propagating outward in the interstellar medium, and a reflected shock propagating backward in the subsonic solar wind. We identify the reflected shock and the transmitted shock as the possible source of the radio noise detected at Voyagers (Gurnett, et al., 1993). The plasma frequency behind the reflected and the transmitted shock can be responsible for the 2‐ and 3‐kHz radio emissions, respectively. The two bands of radio noise are emitted from sources on both sides of the heliopause starting at about the same time. If the emissions picked up by Voyager are due to fp radiation, then the heliopause is located at R∼130 AU. If the emissions are due to 2fp radiation, then R∼150 AU. Because the relative speed of the interstellar plasma with respect to the sun appears to be sub‐Alfvenic, it is very unlikely there is a fast‐mode bow shock of the heliosphere.

Merged interaction regions and large‐scale magnetic field fluctuations during 1991: Voyager 2 observations

This paper analyzes Voyager 2 observations of the magnetic field between 33.6 AU and 36.2 AU during 1991 when extraordinary events were observed on the Sun and in the heliosphere. The magnetic field strength signal B(t) has the unusual form of two large transient merged interaction regions (MIRs) on a fluctuating background. The two MIRs moved past the spacecraft in 32 days and 18 days, respectively. The mean field strength in each transient MIR was ≈2.6 times the mean field during the remaining part of the year (0.11 nT). Each of the MIRs is related to a fast stream. The magnetic field is strong throughout each stream, suggesting that the strong fields are carried by the streams as well as produced by shock and stream compression. The fluctuations in B(t) during 1991 are not multifractal, and the MIRs cannot be approximated as multifractal clusters of intense magnetic fields. The distribution of the hour‐averaged magnetic field strengths is approximately lognormal over 90% of its intermediate range, and it has an exponential tail for B greater than the average magnetic field strength. The elevation angles of B have a normal distribution with a standard deviation of 16° ± 4°. The distributions of the azimuthal angles of B in the ranges 0°‐180° and 180°‐360° are approximately normal over a more limited range, and non‐Gaussian tails associated with nearly radial magnetic fields; the standard deviations are ≈40°. Individual sectors are present throughout most of the interval, even in the MIRs, but there is no recurrent sector pattern. A model of the large‐scale fluctuations in 1991 will have to include both deterministic and statistical factors.

Cosmic-ray modulation and long-duration solar flare events

The correlation between the long-term intensity variations of cosmic rays at neutron monitor energies and the LDE index measure of solar flares with long-lasting soft X-ray emissions is reported. Three subsequent solar cycles, 20–22, are taken into account and half-monthly data are analyzed. Possible explanation of this correlation is discussed in terms of the recent concepts of cosmic-ray modulation, in particular with merged interaction regions affecting the cosmic-ray intensity.