Global Processes that Determine Cosmic Ray Modulation

The global processes that determine cosmic ray modulation are reviewed. The essential elements of the theory which describes cosmic ray behavior in the heliosphere are summarized, and a series of discussions is presented which compare the expectations of this theory with observations of the spatial and temporal behavior of both galactic cosmic rays and the anomalous component; the behavior of cosmic ray electrons and ions; and the 26-day variations in cosmic rays as a function of heliographic latitude. The general conclusion is that the current theory is essentially correct. There is clear evidence, in solar minimum conditions, that the cosmic rays and the anomalous component behave as is expected from theory, with strong effects of gradient and curvature drifts. There is strong evidence of considerable latitude transport of the cosmic rays, at all energies, but the mechanism by which this occurs is unclear. Despite the apparent success of the theory, there is no single choice for the parameters which describe cosmic ray behavior, which can account for all of the observed temporal and spatial variations, spectra, and electron vs. ion behavior.     

Solar Variability and Clouds – Discussion Session 3c

Satellite observations have recently revealed a surprising imprint of the 11-year solar cycle on global low cloud cover. The cloud data suggest a correlation with the intensity of galactic cosmic rays. If this apparent connection between cosmic rays and clouds is real, variations of the cosmic ray flux caused by long-term changes in the solar wind could have a significant influence on the global energy radiation budget and the climate. However a direct link between cosmic rays and clouds has not been unambiguously established and, moreover, the microphysical mechanism is poorly understood. New experiments are being planned to find out whether cosmic rays can affect cloud formation, and if so how.     

Observations of Variability in Cosmic Rays

Cosmic rays are the main source of ionization in the atmosphere at altitudes below 55–60km. This circumstance, together with the fact that cosmic ray flux modulation closely mirrors the solar activity time history, makes cosmic rays a good candidate as a possible mediator in the solar variability – climate relationship. The observed cosmic ray flux variations are described with the aim of emphasizing the features which may be useful in the search of correlation between cosmic rays and atmospheric phenomena.     

Cosmic-Ray Modulation in the Heliosphere A Phenomenological Study

The heliospheric cosmic-ray network–Pioneer 10/11, Voyager 1/2, Ulysses and IMP 8 have provided detailed observations of galactic and anomalous cosmic rays over a period of time that now exceeds 25 years and extends to heliocentric distances beyond 65 AU. These data, when compared over consecutive 11 year solar cycles, clearly establishes the existence of a 22-year cosmic ray modulation cycle that is dominated by the 11-year solar activity cycle but is strongly influenced by gradient and curvature drifts in association with the tilt of the heliospheric neutral current sheet as well as the mediation of the enhanced magnetic turbulence above the solar poles. Over successive solar minima these effects manifest themselves in the remarkable differences in the energetic particle time histories, in the magnitude and sign of the radial and latitudinal intensity gradients and in the changes in the energy spectra of anomalous cosmic rays as a function of heliocentric distance.From solar minimum to solar maximum the long term modulation is principally a combination of two solar related phenomena, the cumulative effect of long-lived global merged interaction regions (GMIRs) and gradient and curvature drifts in the interplanetary magnetic field. For the periods when positive ions flow in over the solar poles and out along the heliospheric current sheet, the modulation of ions is dominated by GMIRs. When this flow pattern is reversed it is found that drifts are an important but not dominant factor for cosmic ray modulation with the current sheet related drift effects decreasing with increasing rigidity R, heliolatitude and heliocentric distance. Over a single solar cycle these conclusions are confirmed at 1 AU by comparing the relative modulation of cosmic-ray helium nuclei and electrons.     

Galactic Cosmic Rays in the Outer Heliosphere: Theory and Models

We review recent advances in the field of galactic cosmic ray transport in the distant heliosphere. The advent of global MHD models brought about a better understanding of the three-dimensional structure of the interface between the solar system and the surrounding interstellar space, and of the magnetic field topology in the outer heliosphere. These results stimulated a development of galactic cosmic ray transport models taking the advantage of the available detailed plasma backgrounds and of the new Voyager results from the heliosheath. It emerges that the heliosheath plays a prominent role in the process of modulation and filtration of low-energy galactic ions and electrons. The heliosheath stores particles for a duration of several years thus acting as a large reservoir of galactic cosmic rays. Cosmic-ray trajectories, transit times, and entry locations across the heliopause are discussed. When compared to observations model calculations of low energy electrons show almost no radial gradient up to the termination shock, irrespective of solar activity, but a large gradient in the inner heliosheath. Intensities are however sensitive to heliospheric conditions such as the location of the heliopause and shock. In contrast, high energy proton observations by both the Voyager spacecraft show a clear solar cycle dependence with intensities also increasing with increasing distance. By comparing these observations to model calculations we can establish whether our current understanding of long-term modulation result in computed intensities compatible to observations.     

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.     

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.     

Modulation of Cosmic Rays in the Heliosphere From Solar Minimum to Maximum: a Theoretical Perspective

The modulation of galactic cosmic rays in the heliosphere seems to be dominated by four major mechanisms: convection, diffusion, drifts (gradient, curvature and current sheet), and adiabatic energy losses. In this regard the global structure of the solar wind, the heliospheric magnetic field (HMF), the current sheet (HCS), and that of the heliosphere itself play major roles. Individually, the four mechanisms are well understood, but in combination, the complexity increases significantly especially their evolvement with time - as a function of solar activity. The Ulysses observations contributed significantly during the past solar minimum modulation period to establish the relative importance of these major mechanisms, leading to renewed interest in developing more sophisticated numerical models, and in the underlying physics, e.g., what determines the diffusion tensor. With increased solar activity, the relative contributions of the mentioned mechanisms change, but how they change and what causes these changes over an 11-year solar cycle is not well understood. It can therefore be expected that present and forthcoming observations during solar maximum activity will again produce very important insights into the causes of long-term modulation. In this paper the basic theory of solar modulation is reviewed for galactic cosmic rays. The influence of the Ulysses observations on the development of the basic theory and numerical models are discussed, especially those that have challenged the theory and models. Model-based predictions are shown for what might be encountered during the next solar minimum. Lastly, modulation theory and modelling are discussed for periods of maximum solar activity when a global reorganization of the HMF, and the HCS, occurs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Polar rain entry of galactic electrons into the inner heliosphere?

The understanding of the relative intensity variations in cosmic ray ions and electrons with respect to solar modulation is a grand challenge for cosmic ray modulation theory. Although effects of the heliospheric neutral sheet, gradient-curvature drifts, and merged interaction regions provide qualitative explanations for observed solar cycle variations of high energy protons and ions, these effects do not account for the anomalously high intensities of high energy galactic electrons at 22-year intervals of the solar magnetic solar cycle. From the similar modulation responses of protons and heavy ions it does not appear that cosmic ray pressure effects, dominated by protons, can account for the chargesign asymmetry of cosmic ray modulation. External factors including modulation in the heliosheath and polar linkage to the interstellar magnetic field are examined as potential causes of symmetry breaking for electron modulation with respect to the solar magnetic polarity at solar minimum.     

Solar cosmic rays

The first observations of solar cosmic rays were made simultaneously by many investigators at worldwide cosmic-ray stations in the periods of powerful chromospheric flares on February 28 and March 7, 1942. The discovery of these and the investigation of cosmic-ray solar-daily variations with maximum time near noon led some authors (Richtmyer and Teller, 1948; Alfvén, 1949, 1950) to a model of apparent cosmic-ray solar origin. We present here the results of the properties of solar cosmic rays from ground events (experimental and theoretical investigations). We also discuss important information from solar experimental data relating to these ground events observed in September and October 1989 and May 1990. Some experimental evidence of acceleration processes in associated phenomena with flares and long-term (solar cycle) variation of the average flux of solar cosmic rays is discussed as also cornal and interplanetary propagation, and that in the terrestrial magnetosphere. Note that the energy spectrum of solar cosmic rays varied very strongly from one flare to another. What are the causes of these phenomena? What is the nature of chemical and isotopic contents of solar cosmic rays? How can its changes occur in the energy spectrum and chemical contents of solar cosmic rays in the process of propagation? Is it possible to recalculate these parameters to the source? What makes solar cosmic rays rich in heavy nucleus and³He? The important data about electrons, positrons, gamma-quanta and neutrons from flares will be discussed in a subsequent paper (Dorman and Venkatesan, 1992). The question is: What main acceleration mechanism of solar flare and associated phenomena are reliable? These problems are connected with the more general problem on solar flare origin and its energetics. In Dorman and Venkatesan (1993) we will consider these problems as well as the problem of prediction of radiation hazard from solar cosmic rays (not only in space, but also in the Earth's atmosphere too).     

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 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.     

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.     

Solar Influence on Earth's Climate

An increasing number of studies indicate that variations in solar activity have had a significant influence on Earth's climate. However, the mechanisms responsible for a solar influence are still not known. One possibility is that atmospheric transparency is influenced by changing cloud properties via cosmic ray ionisation (the latter being modulated by solar activity). Support for this idea is found from satellite observations of cloud cover. Such data have revealed a striking correlation between the intensity of galactic cosmic rays (GCR) and low liquid clouds (<3.2 km). GCR are responsible for nearly all ionisation in the atmosphere below 35 km. One mechanism could involve ion-induced formation of aerosol particles (diameter range, 0.001–1.0 μm) that can act as cloud condensation nuclei (CCN). A systematic variation in the properties of CCN will affect the cloud droplet distribution and thereby influence the radiative properties of clouds. If the GCR-Cloud link is confirmed variations in galactic cosmic ray flux, caused by changes in solar activity and the space environment, could influence Earth's radiation budget. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Modulation of Cosmic Rays and Anomalous Components by CIRs

CIRs produce clearly visible recurrent modulation in the intensity of cosmic rays and anomalous components, but are not principally responsible for determining the overall global level of modulation. However, the localized variations imposed by CIRs in the parameters for propagation of energetic particles through the solar wind provide useful diagnostics for testing models of the propagation against observations. A principal result from Ulysses observations of CIR-induced variations is that the variations persist to very high latitudes, well beyond the range where CIRs are observed. This has driven theoretical models to provide for enhanced latitude transport of energetic particles. On the other hand, observations of Jovian electron intensities vs. latitude do not support enhanced latitude transport. This chapter contains a summary of the interaction between observations and models for the effects of CIRs, and its impact on the understanding of the physics of modulation. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

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.     

A cosmochemical view of cosmic rays and solar particles

The composition of cosmic rays and solar particles is reviewed with emphasis on the question of whether they are representative samples of Galactic and solar matter. The composition of solar particles changes with energy and from flare to flare. A strong excess of heavy elements at energies below a few MeV/nuc decreases with energy, and at energies above 15 MeV/nuc the composition of solar particles resembles that of galactic cosmic rays somewhat better than that of the solar atmosphere. The elements Ne through Pb have remarkably similar abundances in cosmic ray sources and in the matter of the solar system. The lighter elements are depleted in cosmic rays, whereas U and Th may be enriched or not, depending on whether the meteoritic or solar abundance of Th is used. Two prototype sources of cosmic rays are considered: gas with solar system composition but enriched in elements with Z > 8 during acceleration and emission (by analogy with solar particle emission), and highly evolved matter enriched in r-process elements such as U, Th and transuranic elements. The energy-dependence of cosmic ray composition suggests that both sources may contribute at different energies.Miller Institute Professor, 1972–73.