Large Scale Modulation: View From the Earth

The current knowledge and ideas, obtained from groundlevel observations and concerning the solar modulation of cosmic rays, are reviewed. The following topics are discussed: observations of the cosmic ray modulation at the Earth and main characteristics of the accumulated experimental data; manifestations of the solar magnetic cycle in cosmic rays; the effect of hysteresis and its relation to the size of the heliosphere; the rigidity spectrum of long-term cosmic ray variations; the influence of the sporadic effects on long-term modulation; long-term variations of cosmic ray anisotropy and gradients; the place of groundlevel observations in current studies of cosmic ray modulation and their future prospects. Particular consideration is given to the correlation of long-term cosmic ray variations with different solar-heliospheric parameters, and to empirical models of cosmic ray modulation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Cosmic Rays Through the Solar Hale Cycle

The Ulysses spacecraft had been the first to orbit the Sun over its poles and to explore the heliosphere at these high heliolatitudes. It has now completed three fast latitude scans, two at solar minimum and one at solar maximum. Since its launch in October 1990, this mission has led to several surprising discoveries concerning energetic particles, cosmic rays, Jovian electrons, the solar wind, the heliospheric magnetic field and the global features of the heliosphere. This review addresses the propagation and modulation of cosmic rays and other charged particles from an observational point of view with emphasis on what has been learned from exploring the inner heliosphere to high heliolatitudes.     

Solar and Heliospheric Modulation of Galactic Cosmic Rays

The significance of external influences on the environment of Earth and its atmosphere has become evident during recent years. Especially, on time scales of several hundred years, the cosmogenic isotope concentration during the Wolf-, Spoerer-, Maunder- and Dalton-Minimum indicates an increased cosmic ray flux. Because these grand minima of solar activity coincide with cold periods, a correlation of the Earth climate with the cosmic ray intensities is plausible. Any quantitative study of the effects of energetic particles on the atmosphere and environment of the Earth must address their transport to Earth and their interactions with the Earth’s atmosphere including their filtering by the terrestrial magnetosphere. The first problem is one of the fundamental problems in modern cosmic ray astrophysics, and corresponding studies began in the 1960s based on Parker’s cosmic ray modulation theory taking into account diffusion, convection, adiabatic deceleration, and (later) the drift of energetic particles in the global heliospheric magnetic field. It is well established that all of these processes determining the modulation of cosmic rays are depending on parameters that are varying with the solar magnetic cycle. Therefore, the galactic cosmic ray intensities close to Earth is the result of a complex modulation of the interstellar galactic spectrum within the heliosphere. The modern view of this cosmic ray modulation is summarized in our contribution.     

Cosmic-Ray Energy Spectra and Time Variations in the Local Interstellar Medium: Constraints and Uncertainties

The spectra of galactic cosmic rays that are observed inside the heliosphere result from the interaction of the spectra present in the local interstellar medium with the structured but turbulent magnetic field carried by the solar wind. Observational tests of solar modulation theory depend on comparisons between spectra inside and outside the heliosphere. Our knowledge of the local interstellar spectra are indirect, using extrapolations of interplanetary spectra measured at high energies where solar modulation effects are minimal and modeling of the physical processes that occur during particle acceleration and transport in the interstellar medium. The resulting estimates of the interstellar spectra can also be checked against observations of the effects that cosmic rays have on the chemistry of the interstellar medium and on the production of the diffuse galactic gamma-ray background. I review the present understanding of the local galactic cosmic-ray spectra, emphasizing the constraints set by observations and the uncertainties that remain.     

Cosmic Rays at High Heliolatitudes

The Ulysses spacecraft has been the first to orbit the Sun over its poles and to explore the heliosphere at these high heliolatitudes. It has now completed two fast latitude scans, one at solar minimum and one at solar maximum. Since its launch in October 1990, this mission has led to several surprising discoveries concerning energetic particles, cosmic rays, Jovian electrons, the solar wind, the heliospheric magnetic field and the global features of the heliosphere. This review addresses mainly the propagation and modulation of cosmic rays and other charged particles, from both an observational and theoretical point of view, with emphasis on what has been learned from exploring the inner heliosphere to high heliolatitudes. This is done for solar minimum and maximum conditions. The review is concluded with a summary of the main scientific discoveries and insights gained so far from the Ulysses mission.     

Modulation of Galactic Cosmic Rays at Solar Minimum

Cosmic ray particles respond to the heliospheric magnetic field in the expanding solar wind and its turbulence and therefore provide a unique probe for conditions in the changing heliosphere. During the last four years, concentrated around the solar minimum period of solar cycle 22, the exploration of the solar polar regions by the joint ESA/NASA mission Ulysses revealed the three-dimensional behavior of cosmic rays in the inner and middle heliosphere. Also during the last decades, the Pioneer and Voyager missions have greatly expanded our understanding of the structure and extent of the outer heliosphere. Simultaneously, numerical models describing the propagation of galactic cosmic rays are becoming sophisticated tools for interpreting and understanding these observations. We give an introduction to the subject of the modulation of galactic cosmic rays in the heliosphere during solar minimum. The modulation effects on cosmic rays of corotating interaction regions and their successors in the outer heliosphere are discussed in more detail by Gazis, McDonald et al. (1999) and McKibben, Jokipii et al. (1999) in this volume. Cosmic-ray observations from the Ulysses spacecraft at high heliographic latitudes are also described extensively in this volume by Kunow, Lee et al. (1999). This revised version was published online in August 2006 with corrections to the Cover Date.     

Cosmic Rays in the Heliosphere: Requirements for Future Observations

Since the publication of Cosmic Rays in the Heliosphere in 1998 there has been great progress in understanding how and why cosmic rays vary in space and time. This paper discusses measurements that are needed to continue advances in relating cosmic ray variations to changes in solar and interplanetary activity and variations in the local interstellar environment. Cosmic ray acceleration and transport is an important discipline in space physics and astrophysics, but it also plays a critical role in defining the radiation environment for humans and hardware in space, and is critical to efforts to unravel the history of solar activity. Cosmic rays are measured directly by balloon-borne and space instruments, and indirectly by ground-based neutron, muon and neutrino detectors, and by measurements of cosmogenic isotopes in ice cores, tree-rings, sediments, and meteorites. The topics covered here include: what we can learn from the deep 2008–2009 solar minimum, when cosmic rays reached the highest intensities of the space era; the implications of ¹⁰Be and ¹⁴C isotope archives for past and future solar activity; the effects of variations in the size of the heliosphere; opportunities provided by the Voyagers for discovering the origin of anomalous cosmic rays and measuring cosmic-ray spectra in interstellar space; and future space missions that can continue the exciting exploration of the heliosphere that has occurred over the past 50 years.     

Muon Observations

Muon observations are complementary to neutron monitor observations but there are some important differences in the two techniques. Unlike neutron monitors, muon telescope systems use coincidence techniques to obtain directional information about the arriving particle. Neutron monitor observations require simple corrections for pressure variations to compensate for the varying mass of atmospheric absorber over a site. In contrast, muon observations require additional corrections for the positive and negative temperature effects. Muon observations commenced many years before neutron monitors were constructed. Thus, muon data over a larger number of solar cycles is available to study solar modulation on anisotropies and other cosmic ray variations. The solar diurnal and semi-diurnal variations have been studied for many years. Using the techniques of Bieber and Chen it has been possible to derive the radial gradient, parallel mean-free path and symmetric latitude gradient of cosmic rays for rigidities <200 GV. The radial gradient varies with the 11-year solar activity cycle whereas the parallel mean-free path appears to vary with the 22-year solar magnetic cycle. The symmetric latitudinal gradient reverses at each solar polarity reversal. These results are in general agreement with predictions from modulation models. In undertaking these analyses the ratio of the parallel to perpendicular mean-free path must be assumed. There is strong contention in the literature about the correct value to employ but the results are sufficiently robust for this to be, at most, a minor problem. An asymmetric latitude gradient of highly variable nature has been found. These observations do not support current modulation models. Our view of the sidereal variation has undergone a revolution in recent times. Nagashima, Fujimoto and Jacklyn proposed a narrow Tail-In source anisotropy and separate Loss-Cone anisotropy as being responsible for the observed variations. A new analysis technique, more amenable to such structures, was developed by Japanese and Australian researchers. They confirmed the existence of the two anisotropies. However, they found that the Tail-In anisotropy is asymmetric and that both anisotropies had different positions from the prediction. Most 27-day modulations are observed at neutron monitor rigidities but not so readily at higher rigidities. An exception to this is the Isotropic Intensity Wave modulation observed in the early 1980s and again in 1991. This modulation is very strongly related to the heliospheric sector structure and implies a significantly different cosmic ray density on either side of the neutral sheet. The interpretation of most cosmic ray modulation phenomena requires good latitude coverage in both hemispheres. The closure of many muon observatories is a matter of concern. In the northern hemisphere a few new instruments are being constructed and spatial coverage is barely adequate. In the southern hemisphere the situation is far worse with the possibility that within a decade only the Mawson observatory in Antarctica will still be in operation. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Inner Heliosphere - Outer Heliosphere Comparison for Cosmic Ray Modulation

This paper summarizes cosmic ray data on both galactic and anomalous particles in the inner and outer heliosphere near the sunspot minimum in 1995 and 1996 at the end of solar cycle 22. These data come from the IMP spacecraft in the inner heliosphere and the Voyager and Pioneer spacecraft in the outer heliosphere. In the inner heliosphere, the cosmic ray intensities at all energies in 1996 have recovered to almost the same maximum values they had at the last sunspot minimum in 1987 and the intensities are an even closer match to those observed two 11-year cycles earlier in 1976. In the outer heliosphere beyond 40 AU the intensity recovery is very slow and the intensities at all energies and for all species are almost constant in 1995-96 indicating that little further recovery can be expected in this cycle. The intensity of galactic cosmic rays in 1996 is only 0.3-0.5 of that observed at the same radius of 42 AU in 1987 and for anomalous cosmic rays this ratio is only 0.1-0.2. This suggests a dramatically different entry of particles into the heliosphere in the two cycles for both types of particles as well as significantly different particle flow characteristics in the outer heliosphere. The net result of these different characteristics is that near the Earth only a relatively small intensity difference is observed between successive 11-year solar cycles whereas in the outer heliosphere the differences between cycles become very large and may even dominate the overall modulation.     

Cosmic Rays in the Inner Heliosphere: Insights from Observations, Theory and Models

The global modulation of galactic cosmic rays in the inner heliosphere is determined by four major mechanisms: convection, diffusion, particle drifts (gradient, curvature and current sheet drifts), and adiabatic energy losses. When these processes combine to produce modulation, the complexity increases significantly especially when one wants to describe how they evolve spatially in all three dimensions throughout the heliosphere, and with time, as a function of solar activity over at least 22 years. In this context also the global structure and features of the solar wind, the heliospheric magnetic field, the wavy current sheet, and of the heliosphere and its interface with the interstellar medium, play important roles. Space missions have contributed significantly to our knowledge during the past decade. In the inner heliosphere, Ulysses and several other missions have contributed to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated theories and numerical models to explain these observations, and to understand the underlying physics that determines galactic cosmic ray modulation at Earth. An overview is given of some of the observational and modeling highlights over the past decade.     

10Be Production in the Atmosphere by Galactic Cosmic Rays

Galactic cosmic ray nuclei and energetic protons produced in solar flares and accelerated by coronal mass ejections are the main sources of high-energy particles of extraterrestrial origin in near-Earth space and inside the Earth’s atmosphere. The intensity of galactic cosmic rays inside the heliosphere is strongly influenced by the modulation of the interstellar source particles on their way through interplanetary space. Among others, this modulation depends on the activity of the Sun, and the resulting intensity of the energetic particles in the atmosphere is an indicator of the solar activity. Therefore, rare isotopes found in historical archives and produced by spallation reactions of primary and secondary hadrons of cosmic origin in the atmosphere, so-called cosmogenic nuclides, can be used to reconstruct the solar activity in the past. The production rate of ¹⁰Be, one of the cosmogenic nuclides most adequate to study the solar activity, is presented showing its variations with geographic latitude and altitude and the dependence on different production cross-sections present in literature. In addition, estimates for altitude integrated production rates of ¹⁰Be at different locations since the early nineteen sixties are shown.     

What do Cosmogenic Isotopes Tell us About Past Solar Forcing of Climate?

In paleoclimate studies, cosmogenic isotopes are frequently used as proxy indicators of past variations in solar irradiance on centennial and millennial timescales. These isotopes are spallation products of galactic cosmic rays (GCRs) impacting Earth's atmosphere, which are deposited and stored in terrestrial reservoirs such as ice sheets, ocean sediments and tree trunks. On timescales shorter than the variations in the geomagnetic field, they are modulated by the heliosphere and thus they are, strictly speaking, an index of heliospheric variability rather than one of solar variability. Strong evidence of climate variations associated with the production (as opposed to the deposition) of these isotopes is emerging. This raises a vital question: do cosmic rays have a direct influence on climate or are they a good proxy indicator for another factor that does (such as the total or spectral solar irradiance)? The former possibility raises further questions about the possible growth of air ions generated by cosmic rays into cloud condensation nuclei and/or the modulation of the global thunderstorm electric circuit. The latter possibility requires new understanding about the required relationship between the heliospheric magnetic fields that scatter cosmic rays and the photospheric magnetic fields which modulate solar irradiance.     

Three-dimensional cosmic-ray simulations: Heliographic latitude and current-sheet tilt

We present the results from a study of the variations of the cosmic-ray intensity with time, heliographic latitude, and longitude, and for varying interplanetary conditions, using our three-dimensional, time-dependent computer code for cosmic-ray transport in the heliosphere. Our code also produces a solar-wind and interplanetary magnetic field (IMF) configuration which is compared with observations. Because of the fully threedimensional nature of the model calculations, we are able to model time variations which would be expected to be observed along Ulysses's trajectory as it moves to high latitudes. In particular we can model the approximately 13-and 26-day solar-rotation induced variations in cosmic rays, solar wind and IMF, as a function of increasing heliographic latitude, as one moves poleward of the interplanetary current sheet. Our preliminary model results seem to be in general form quite similar to published data, but depend on the physical parameters used such as cosmic-ray diffusion coefficients, boundary conditions, and the nature of the solar wind and IMF and current sheet.     

Voyager 2 Solar Wind Observations in the Outer Heliosphere

We discuss the solar wind parameters measured in the distant heliosphere from the Voyager 2 spacecraft. Periodic variations in the speed of the wind observed at roughly the solar rotation period may correspond to interaction regions between slower and faster streams of wind. Since the interplanetary magnetic field is enhanced in such regions, they are important for the study of modulation of cosmic rays. Unfortunately, direct observation of the enhanced magnetic field from Voyager 2 has been made difficult by spacecraft-associated noise since 1989.     

The modulation of cosmic rays in the high latitude heliosphere: A computer simulation

Modulation models based on the numerical solution of Parker's transport equation for galactic cosmic rays in the heliosphere make clear predictions about modulation in the high latitude heliosphere. However, for these predictions certain assumptions have to be made, for example, what the heliospheric magnetic field (HMF) looks like above the solar poles and what the spatial dependence of the diffusion coefficients are. For this presentation the general predictions of a standard drift model for the modulation of cosmic rays in the high latitude heliosphere, in particular predictions for the Ulysses trajectory, are discussed and critically reviewed. Preliminary results from Ulysses show a significant increase in the solar wind speed towards higher latitudes. The effects of this strong latitudinal dependence together with different modifications of the HMF at these high latitudes on the apparently too large diffusion and drifts predicted by current models are also shown.     

The Modulation of Galactic Cosmic Rays in the Heliosphere: Theory and Models

Almost all theoretical and numerical models for the modulation of cosmic ray in the heliosphere are based on Parker's transport equation which contains all the important basic physical processes. The relative importance of the various mechanisms is however not established and may vary significantly over 22 years. The simultaneous measurements of solar wind parameters, heliospheric magnetic field properties and cosmic rays over a wide range of energies and positions in the heliosphere have brought the realization that modulation is much more complicated than what the original drift models predicted. In the process the sophistication of models based on solving Parker's equation has increased by orders of magnitude. A short review of the global modulation of cosmic rays is given from a theoretical and modelling point of view.     

Cosmic-Ray Transport and Interaction with Shocks

This paper reviews selected topics in cosmic-ray transport in the heliosphere, as well as recent insights on the interaction of cosmic rays with shocks. Topics include: (a) recent observations suggesting very long inferred scattering mean-free paths of cosmic rays, (b) recent insights into the diffusion of cosmic rays normal to a magnetic field, (c) the physics of super-diffusion and sub-diffusion, and (e) the interaction of cosmic rays with shocks moving through large-scale irregular magnetic fields.     

Corotating Particle Events

The corotating particle events give us a unique opportunity to probe the three-dimensional structures of the heliosphere. This is especially true if we have observations over a period of extreme stability of the CIRs, such as existed over the recent solar minimum. We discuss how the observations fit into the context of current heliospheric magnetic field models. The energetic particle signatures of CIRs throughout the regions of the heliosphere covered by the deep-space missions are reviewed. The CIRs accelerate these particles and at the same time modulate both the high energy galactic cosmic rays and the anomalous cosmic rays.     

Transient Effects and Disturbed Conditions

In the present phase of the solar cycle no big transients leading to strong modulation had been observed after 1991. Apart from a few minor disturbances cosmic rays were still recovering to a new intensity maximum. It was suggested, therefore, that existing literature from previous cycles should be critically reviewed. The scene was set by the introductory papers on— phenomenology of cosmic ray modulation in successive solar cycles throughout the heliosphere— the present state of models for long term modulation and their shortcomings— the relation between cosmic ray variations and the magnitude of the interplanetary magnetic field (the CR-B-relation)— charge dependent effects.In the discussions, the study of propagating diffusive disturbances and the CR-B-relation played a central role. The difference was stressed between isolated transient disturbances in the inner solar system (Forbush decreases), and the long lasting, step-like decreases caused by merged interaction regions in the outer heliosphere. The recovery rates following the step-like decreases vary with the phase in the 22-year solar cycle. In some cases this requires a modification of existing drift models. In the outer heliosphere, the CR-B-relation leads to the result 1/ between the diffusion coefficient and the field magnitude . This simple result is a challenge for theoreticians to derive the perpendicular diffusion coefficient fromfirst principles. The three articles in this report essentially follow the list of open points and arguments just presented.The article "Observations and Simple Models" is organised around the model of a propagating diffusive barrier, its application to Forbush effects in the inner heliosphere and to decreases caused by merged interaction regions in the outer heliosphere. Acomparison of observed Forbush decreases with model predictions requires a careful separation of the two steps related to the turbulent region behind the shock front and the closed magnetic field regions of the ejecta (the interplanetary counterparts of coronal mass ejections). It is shown that models for propagating disturbances can be used to derive values of the diffusion coefficients phenomenologically, not only during the disturbance, but also in the ambient medium.The "Modeling of Merged Interaction Regions" summarizes the dynamic and time-dependent process of cosmic ray modulation in the heliosphere. 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 cosmic rays when solar activity is high. To explain this feature of heliospheric modulation, the concept of global merged interaction regions is required. The com-bination of gradient, curvature and neutral sheet drifts with these global merged interaction regions has so far been the most successful approach in explaining the 11-year and 22-year cycles in the long-term modulation of cosmic rays.The "Remarks on the Diffusion Tensor in the Heliosphere" describe available theories of perpen-dicular diffusion and drift, and discuss their relevance to cosmic rays in the heliosphere. In addition, the information about diffusion coefficients and spatial gradients obtained from the analysis of steady state anisotropies at neutron monitor energies is summarized. These topics are intimately related to the other two articles. They are also part of the general discussion about the "Diffusion Tensor throughout the Heliosphere" which played an important role in all working groups.