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.     

Cosmic Rays,Clouds, and Climate

A correlation between a global average of low cloud cover and the flux of cosmic rays incident in the atmosphere has been observed during the last solar cycle. The ionising potential of Earth bound cosmic rays are modulated by the state of the heliosphere, while clouds play an important role in the Earth's radiation budget through trapping outgoing radiation and reflecting incoming radiation. If a physical link between these two features can be established, it would provide a mechanism linking solar activity and Earth's climate. Recent satellite observations have further revealed a correlation between cosmic ray flux and low cloud top temperature. The temperature of a cloud depends on the radiation properties determined by its droplet distribution. Low clouds are warm (>273K) and therefore consist of liquid water droplets. At typical atmospheric supersaturations (1%) a liquid cloud drop will only form in the presence of an aerosol, which acts as a condensation site. The droplet distribution of a cloud will then depend on the number of aerosols activated as cloud condensation nuclei (CCN) and the level of super saturation. Based on observational evidence it is argued that a mechanism to explain the cosmic ray-cloud link might be found through the role of atmospheric ionisation in aerosol production and/or growth. Observations of local aerosol increases in low cloud due to ship exhaust indicate that a small perturbation in atmospheric aerosol can have a major impact on low cloud radiative properties. Thus, a moderate influence on atmospheric aerosol distributions from cosmic ray ionisation would have a strong influence on the Earth's radiation budget. Historical evidence over the past 1000 years indicates that changes in climate have occurred in accord with variability in cosmic ray intensities. Such changes are in agreement with the sign of cloud radiative forcing associated with cosmic ray variability as estimated from satellite observations.     

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.     

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.     

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.     

Cosmic ray Implications for Human Health

There appears to be concern among some people about the possible effects of cosmic radiation on everyday life. The amount of cosmic radiation that reaches the Earth and its environment is a function of solar cycle, altitude and latitude. The possible effect of naturally occurring cosmic radiation on airplane crews and space flight personal is a subject of current study. This paper discusses the variables controlling the cosmic ray flux in the atmosphere and describes models and software that have been developed that provide quantitative information about the cosmic radiation exposure at flight altitudes. The discussion is extended to include the cosmic radiation exposure to manned spacecraft. This revised version was published online in August 2006 with corrections to the Cover Date.     

Hard rock cosmic ray archaeology

Recent examinations of extraterrestrial materials exposed to cosmic rays for different intervals of time during the geological history of the solar system have generated a wealth of new information on the history of cosmic radiation. This information relates to the temporal variations in

1. the flux and energy spectrum of low energy (solar) protons of ? 10 MeV kinetic energy;
2. the flux and energy spectrum of (solar) heavy nuclei of Z > 20 of kinetic energy, 0.5–10 MeV/n;
3. the integrated flux of protons and heavier nuclei of ? 0.5 GeV kinetic energy, and
4. the flux and energy spectrum of nuclei of Z > 20 of medium energy — 100–2000 MeV/n kinetic energy.

The above studies are entirely based on the natural detector method which utilises two principal cosmogenic effects observed in rocks, (i) isotopic changes and (ii) changes in the crystalline structure of rock constituents, due to cosmogenic interactions. The information available to date in the field of hard rock cosmic ray archaeology refers to meteorites and lunar rocks/soil. Additional information based on study of cosmogenic effects in man-made materials exposed to cosmic radiation in space is also discussed. It is shown that the natural detectors inspite of their extreme simplicity have begun to provide cosmic ray information in a very quantitative and precise manner comparable to the most sophisticated electronic particle detectors. The single handicap in using the hard rock detectors is however the uncertainty regarding their manner of exposure, geometry etc. At present, a variety of techniques are being used to study the evolutionary history of extraterrestrial materials and as this field grows, uncertainties in cosmic ray archaeology will correspondingly decrease.     

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.     

Time-Variability in the Interstellar Boundary Conditions of the Heliosphere: Effect of the Solar Journey on the Galactic Cosmic Ray Flux at Earth

During the solar journey through galactic space, variations in the physical properties of the surrounding interstellar medium (ISM) modify the heliosphere and modulate the flux of galactic cosmic rays (GCR) at the surface of the Earth, with consequences for the terrestrial record of cosmogenic radionuclides. One phenomenon that needs studying is the effect on cosmogenic isotope production of changing anomalous cosmic ray fluxes at Earth due to variable interstellar ionizations. The possible range of interstellar ram pressures and ionization levels in the low density solar environment generate dramatically different possible heliosphere configurations, with a wide range of particle fluxes of interstellar neutrals, their secondary products, and GCRs arriving at Earth. Simple models of the distribution and densities of ISM in the downwind direction give cloud transition timescales that can be directly compared with cosmogenic radionuclide geologic records. Both the interstellar data and cosmogenic radionuclide data are consistent with two cloud transitions, within the past 10,000 years and a second one 20,000–30,000 years ago, with large and assumption-dependent uncertainties. The geomagnetic timeline derived from cosmic ray fluxes at Earth may require adjustment to account for the disappearance of anomalous cosmic rays when the Sun is immersed in ionized gas.     

The evolution of the anomalous component of the cosmic rays during the Ulysses mission

We have studied the evolution of the anomalous cosmic ray component at ULYSSES using observations of the quiet time helium fluxes obtained with the COSPIN Low Energy Telescope during the years from launch in October 1990 to the latest data in 1994. Shown are preliminary results of measurements in the energy range from 4.0 to 19 MeV/n. Following the drop in solar activity in late 1991, the low energy cosmic ray helium flux started to increase and continue to rise until the end of 1993, signifying the appearance of the anomalous component at ULYSSES.     

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.     

The Galactic Cosmic Ray Intensity over the Past 106–109 Years as Recorded by Cosmogenic Nuclides in Meteorites and Terrestrial Samples

Concentrations of stable and radioactive nuclides produced by cosmic ray particles in meteorites allow us to track the long term average of the primary flux of galactic cosmic rays (GCR). During the past ～10 Ma, the average GCR flux remained constant over timescales of hundreds of thousands to millions of years, and, if corrected for known variations in solar modulation, also during the past several years to hundreds of years. Because the cosmic ray concentrations in meteorites represent integral signals, it is difficult to assess the limits of uncertainty of this statement, but they are larger than the often quoted analytical and model uncertainties of some 30%. Time series of concentrations of the radionuclide ¹⁰Be in terrestrial samples strengthen the conclusions drawn from meteorite studies, indicating that the GCR intensity on a ～0.5 million year scale has remained constant within some ±10% during the past ～10 million years. The very long-lived radioactive nuclide ⁴⁰K allows to assess the GCR flux over about the past one billion years. The flux over the past few million years has been the same as the longer-term average in the past 0.5–1 billion years within a factor of ～1.5. However, newer data do not confirm a long-held belief that the flux in the past few million years has been higher by some 30–50% than the very long term average. Neither does our analysis confirm a hypothesis that the iron meteorite data indicate a ～150 million year periodicity in the cosmic ray flux, possibly related to variations in the long-term terrestrial climate.     

Energetic Particles and Corotating Interaction Regions in the Solar Wind

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.     

Composition of Anomalous Cosmic Rays

The “classic” anomalous cosmic ray (ACR) component originates as interstellar neutral atoms that drift into the heliosphere, become ionized and picked up by the solar wind, and carried to the outer heliosphere where the pickup ions are accelerated to hundreds of MeV, presumably at the solar wind termination shock. These interstellar ACRs are predominantly singly charged, although higher charge states are present and become dominant above ～350 MeV. Their isotopic composition is like that of the solar system and unlike that of the source of galactic cosmic rays. A comparison of their energy spectra with the estimated flux of pickup ions flowing into the termination shock reveals a mass-dependent acceleration efficiency that favors heavier ions. There is also a heliospheric ACR component as evidenced by “minor” ACR ions, such as Na, Mg, S, and Si that appear to be singly-ionized ions from a source likely in the outer heliosphere.     

Cosmic Rays in Relation to Space Weather

A review of selected experimental results relevant for the use of cosmic ray records in Space Weather research is presented. Interplanetary perturbations, initiated in the solar atmosphere, affect galactic cosmic rays. In some cases their influence on the cosmic ray intensity results in data signatures that can possibly be used to predict geomagnetic storm onsets. Case studies illustrating the complexity of the cosmic ray effects and related geomagnetic activity precursors are discussed. It is shown that some indices for cosmic ray activity are good tools for testing the reliability of cosmic ray characteristics for Space Weather forecasts. A brief summary of the influence of cosmic rays on the ozone layer is also given. The use of cosmic ray data for Space Weather purposes is still in its infant stage, but suggestions for both case and statistical studies are made. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

High energy cosmic ray nuclei results on Ulysses: 1. Mission overview

The cosmic ray flux observed with the Kiel Electron Telescope on board the ULYSSES spaceprobe varies with solar activity as well as with heliospheric position. Determination of the latitudinal gradients requires a careful analysis of the influences of the current sheet tilt angle, the number of major solar flares, interplanetary shocks and interaction regions evolving in the expanding solar wind. In this paper we concentrate on nuclei with rigidity above 1 GV. We discuss the effects of the variable solar activity in the declining phase of the present solar cycle and the variation with radial distance as a basis for separating latitudinal effects. We show that during this phase of the solar cycle modulation of GV nuclei is ordered by temporal evolution, radial distance and negligible latitudinal effects even at latitudes between 30° and 50° South.     

Solar cosmic ray phenomena

This review attempts to present an integrated view of the several types of solar cosmic ray phenomena. The relevant large and small scale properties of the interplanetary medium are first surveyed, and their use in the development of a quantitative understanding of the cosmic ray propagation processes summarised. Solar cosmic ray events, in general, are classified into two phenomenological categories: (a) prompt events, and (b) delayed events. The properties of both classes of events are summarised. The properties considered are the frequency of occurrence, dependence on parent flare position, the time profile, energy spectra, anisotropies, particle species, velocity dispersions, etc. A single model is presented to explain the various species of delayed event. Thus the halo and core events, energetic storm particle events, EDP events and proton recurrent regions are suggested to be essentially of common origin. The association of flare particle events with electromagnetic phenomena, including optical, X-ray and microwave emissions is summarised. The conditions in a sunspot group, and solar flare that are considered to be conducive to cosmic ray acceleration processes are discussed. Considerable discussion is devoted to physical processes occurring near the Sun. Near Sun particle storage, and diffusion, and secondary injection processes that are triggered by a far distant solar flare are reviewed. In order to explain the considerable differences between aspects of the prompt and delayed events, we propose selective diffusion processes that only occur at early times in a solar flare. The type IV radio emissions at metric wave-lengths are suggested to yield direct evidence for the storage processes that are necessary to explain the properties of the delayed events, and also as yielding direct evidence of secondary injection processes. We conclude by briefly summarising the ionospheric effects of the solar cosmic radiation.     

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.     

The multifuid solar wind termination shock and its influence on the threedimensional plasma structure upstream and downstream

The solar wind termination shock is described as a multi-fluid phenomenon taking into account the magnetohydrodynamic self-interaction of a multispecies plasma consisting of solar wind ions, pick-up ions and shock-generated anomalous cosmic ray particles. The spatial diffusion of these high energy particles relative to the resulting, pressure-modified solar wind flow structure is described by a coupled system of differential equations describing mass-, momentum-, and energy-flow continuities for all plasma components. The energy loss due to escape of energetic particles (MeV) from the precursor into the inner heliosphere is taken into account. We determine the integrated properties of the anomalous cosmic ray gas and the low-energy solar wind. Also the variation of the compression ratio of the shock structure is quantitatively determined and is related to the pick-up ion energization efficiency and to the mean energy of the downstream anomalous cosmic ray particles. The variation of the resulting shock structure and of the solar wind sheath plasma extent beyond the shock is discussed with respect to its consequences for the LISM neutral gas filtration and the threedimensional shape of the heliosphere.