COSMIC RAYS

The idea that there was some type of unknown energetic radiation falling on the Earth from space arose from studies of radioactivity that began in the 1890s. As instruments and understanding of radioactivity improved, the presence of a residual background of radiation that could not be accounted for became more troubling and significant. As early as 1900, C. T. R. Wilson suggested that there could be some form of "cosmic" radiation. However, it was not until 1912 that Victor Hess, in a classic and daring series of manned balloon flights, proved that indeed some form of energetic radiation was penetrating the atmosphere from space.

Initially this cosmic radiation was assumed, by analogy, to be similar to the most penetrating form of radiation produced by radioactivity, gamma radiation. In the 1930s, it was found that the cosmic radiation was influenced by the Earth's magnetic field and hence had to be made up of charged particles, presumed to be electrons. When further studies showed that these particles were positively charged, the assumption was that these particles were positively charged electrons, positrons, examples of which had recently been discovered in the cosmic rays. By 1940 it was becoming apparent that the majority of the incident particles were not electronic but nuclear, presumably protons. It is now known that although most of the particles are indeed protons, there are also energetic nuclei of all the elements in the periodic table present with roughly the same abundance as those found in the solar system. These particles originate from outside the solar system, although, in addition, the sun sometimes produces bursts of copious but relatively low energy particles, solar energetic particles (SEP). The energies of the cosmic ray particles cover an enormous range, from values typically found in radioactive decays, 10⁶ electron volts (eV), to greater than 10²¹ eV, energies that are greatly above any that can be produced in the largest accelerators made by humans.

Few, if any, of the primary particles that enter the top of the earth's atmosphere reach the ground unaffected. Instead they typically undergo nuclear interactions with the nuclei in the atmosphere and produce showers of secondary particles. If the initial energy is high enough, this process may continue for many generations of interactions, resulting in a large burst of secondary particles reaching sea level. While the majority of these secondary particles are electrons, there are also present many of the unstable elementary particles that are produced in high-energy nuclear interactions. The lower-energy particles will lose their energy by ionization in the atmosphere even if they do not interact. As a result, the number of particles in the atmosphere reaches a maximum at an altitude of about 20,000 meters and declines nearer the surface.

Until about 1954, primary and secondary cosmic ray particles were the only source of available high-energy particles that could be used to study the physics of nuclear interactions at energies greater than those typical of radioactive decays. Many of the fundamental processes of the production and existence of elementary particles were first studied by looking at the secondary cosmic ray particles.

Pair Production and the Positron

The first new particle discovered in the secondary cosmic rays was the positively charged electron, the positron, the first example of antimatter proved to exist. In 1932 Carl Anderson used a cloud chamber to look at the tracks produced by cosmic ray particles passing through the chamber. A magnetic field was applied to the chamber so that the sign of the charge of each particle could be determined from the curvature in the field. A lead plate was placed in the chamber so that the direction of motion could be determined from the loss of energy in the lead. Anderson found an example of a particle with the mass of an electron but with a positive charge. This discovery was rapidly confirmed by pictures in cloud chambers of the tracks of pairs of electrons with opposite curvature, examples of the pair production of electrons and positrons by gamma rays. The discovery of these positrons confirmed the theoretical predictions of Paul Dirac (1928) of negative energy states of the electron.

Light Mesons

The presence of unstable particles in the secondary cosmic rays with masses intermediate between that of the proton mass of 1836 electron masses, m_e, and that of the electron, was established as the result of a wide range of experiments. In 1937 particles were observed which had both positive and negative charges, did not lose energy as fast as electrons, but were not as massive as a proton. These particles, originally called "mesotrons," then "mesons," appeared at first to resemble those predicted in 1935 by Hideki Yukawa to explain the forces within the nucleus. The observed mesons were found to have a mass some 200 times that of the electron and to be unstable. However, they had a half-life of 2.1 × 10⁶ second, which was some twenty times longer than predicted, and did not interact strongly with matter, as the Yukawa particle should.

After World War II rapid advances in experimental techniques and in particular the development of sensitive nuclear photographic emulsions by Cecil Powell and his group solved the problem of the discrepancies between the predicted and observed properties of these mesons. Emulsions exposed on mountaintops and on high-altitude balloons showed that there were two types of light meson. Pi-mesons, with a mass of about 273 m_e, were observed coming to rest in the emulsion and decaying into a lighter mu-meson with a mass of about 207 m_e. This mu-meson then also came to rest and decayed into an electron. Now we know that charged pi-mesons decay to mu-mesons and a neutrino. The mu meson then decays into an electron and two more neutrinos. The lifetime of the pi meson is 2.5 × 10^-8 second, much less than that of the mu meson. In addition, negatively charged pi mesons, which are not repelled by the positive nuclei in atoms, tend to be captured and interact with nuclei before they can decay, proving that they are strongly interacting particles of the sort predicted by Yukawa. Mu mesons, on the other hand, do not interact but decay even though they have a much longer lifetime. These pi mesons are now known to exist with positive, negative, and neutral charges, with the neutral pi mesons having very short lifetimes (8.4 × 10^-17 second) and generally decaying into two energetic gamma rays.

Heavier Mesons and Hyperons

Although a light meson was predicted theoretically, the existence of additional mesons heavier than the pi meson was not anticipated. However, as early as 1944 there was a cosmic ray report of the observation of a particle with a mass about 1,000 m_e. Further isolated examples were found over the next few years. Examples were seen of particles that came to rest in emulsion and decayed into three pi-mesons. Other events showed decays, both in flight and at rest, of heavy mesons into various lighter particles. It became clear from observations in cloud chambers and emulsions that there were a number of different modes of decay of so called "K mesons" with masses around 1,000 m_e being produced in high-energy nuclear interactions. By 1954 there had been observations of at least six different modes of decay of charged K mesons and several modes of decay of neutral K mesons.

At the same time there were also observations of unstable particles with masses between that of the proton and deuteron. These "hyperons" were initially regarded as being excited states of the nucleon. They could be charged or neutral, decaying into a proton and a charged or neutral pi-meson.

In every case only a few examples of each type of particle were observed. In many cases there was limited information on the properties of the incident particles and the decay products. Only when artificial accelerators were built with sufficient energy to create these particles and study them under controlled conditions was it possible to begin to understand the underlying physics of nuclei and their forces.

Current Status

The studies using cosmic ray particles only just touched on the problem of these unstable particles. The full complexity of the possible unstable forms of matter that could be created in high-energy nuclear interactions was not unraveled until extensive experiments using high-energy particles from artificial accelerators led to the development of the quark theory of matter. In this theory strongly interacting particles known as hadrons consisted either of baryons (nucleons and hyperons) composed of three quarks or mesons composed of two quarks. Hence, unlike the earlier assumption in the cosmic ray studies, mesons could include particles with masses greater than that of a nucleon. Hyperons also were found to be more than just excited nucleons.

See also:Astrophysics

Bibliography

Dirac, P. A. M. "The Quantum Theory of the Electron." Proceedings of the Royal Society of LondonA117 , 610–624 (1928).

Friedlander, M. W. Cosmic Rays (Harvard University Press, Cambridge, MA, 1989).

Millikan, R. A. Cosmic Rays (Cambridge University Press, Cambridge, UK, 1939).

Montgomery, D. J. X. Cosmic Ray Physics (Princeton University Press, Princeton, NJ 1949).

Sekido, Y., and Elliot, H., eds. Early History of Cosmic Ray Studies (D. Reidel Publishing Company, Dordrecht, Netherlands, 1983).

Wilson, J. G. Cosmic Rays (Springer-Verlag, New York, 1976).

C. Jake Waddington

Cosmic Rays

Introduction

Cosmic rays are naturally occurring high-energy particles— protons, helium nuclei, and electrons—that travel near the speed of light. Some scientists have argued that cosmic rays may cause cloud droplets to form in Earth's atmosphere. If so, the cloudiness of Earth's atmosphere might increase when the sun is emitting more cosmic rays or when the solar system is passing through a part of the galaxy where cosmic rays are more abundant. Increased clouds could affect climate.

As of 2007, most climate scientists did not agree that cosmic rays are a significant influence on Earth's climate, but a small number of scientists disagreed. An experimental device under construction at the European Organization for Nuclear Research (CERN) particle accelerator laboratory in Switzerland, due to be completed in 2010, could decide the question.

Historical Background and Scientific Foundations

Scientists have long known that slight changes in the energy output of the sun might affect the climate of Earth. American astronomer Jack Eddy pointed out in the 1970s that a cold period in Earth's climate during the seventeenth and eighteenth centuries, the Little Ice Age, coincided with a historic low point in the number of sunspot numbers known as the Maunder minimum. In 1997, Danish scientists Henrick Svensmark and Eigil Friis-Christensen noted that Earth's global cloudiness had decreased by 3% during the period 1987–1990, at the same time that cosmic rays had decreased by 3.5% due to the sun's regular cycle of activity.

They proposed that cosmic rays might increase cloud cover by the following mechanism: Because cosmic rays have high energy, they can strip electrons from many numbers of atoms when they strike Earth's

atmosphere. These atoms, now bearing positive electric charges, might cause water to start condensing out of humid air into cloud droplets. This, in turn, might increase the number of clouds, their density, or both.

The cosmic-ray theory was controversial because most scientists agree that the global warming occurring rapidly today is caused by an increase in the amount of carbon dioxide in Earth's atmosphere, not by changing levels of solar activity. In 2003, two other scientists, Nir J. Shaviv and Ján Veizer, took the dispute to a new level by arguing that over the last 545 million years, about two thirds of Earth's changes in climate could be attributed to rises and falls in the number of cosmic rays striking Earth from outside the solar system. Independently of changes in cosmic ray flow from our own star, rises and falls in cosmic ray abundance also occur as the solar system's orbital path around the center of the galaxy takes it into and out of our galaxy's spiral arms over millions of years.

Impacts and Issues

There is evidence from several independent sources that changing energy output from the sun has affected Earth's climate in the past. Moreover, the solar hypothesis and the carbon dioxide hypothesis do not necessarily exclude each other: Earth's climate might be influenced by multiple factors. As of 2008, however, most scientists had not been convinced that galactic and solar cosmic rays ever had a significant influence on climate.

In 2004, German scientist Stefan Rahmstorf and a group of 10 other scientists from the United States, Switzerland, and elsewhere published a paper replying to the 2003 paper by Shaviv and Veizer arguing that cosmic rays dominate Earth's climate. The new paper was widely reported in the scientific press as a refutation of the cosmic-ray idea. In 2007, British scientists Mike Lockwood and Claus Fröhlich published a study finding that “the observed rapid rise in global mean temperatures seen after 1985 cannot be ascribed to solar variability,” no matter what mechanism for solar influence on Earthly climate is invoked or how that influence might be amplified, as for example by cosmic rays. Present-day climate change, the authors concluded, is due to human activity, not solar influence.

WORDS TO KNOW

COSMICS LEAVING OUTDOOR DROPLETS (CLOUD): Experiment at the CERN particle physics laboratory in Geneva, Switzerland, designed to investigate whether cosmic rays (high-energy particles from outer space) can influence Earthly weather and climate by triggering the formation of cloud particles. Began collecting data in 2006.

SUNSPOTS: Comparatively dark, cool patches that appear on the sun's surface in synchrony with increased solar activity every 11 years. By interacting with stratospheric ozone, sunspot activity affects Earth's climate, mostly at high altitudes but subtly at the surface (perhaps a few tenths of a degree of warming in the Northern Hemisphere).

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Nevertheless, the possibility of a cosmic-ray connection to climate during other periods, whether slight or great, remains open. At the CERN particle accelerator laboratory in Geneva, Switzerland, an experiment called CLOUD (Cosmics Leaving Outdoor Droplets) is under construction. Due to be completed in 2010, CLOUD will enable scientists to observe the effects of artificial cosmic rays on air and water vapor and may settle the question of whether cosmic rays can create clouds. Even if they can, some scientists assert, it is unlikely that solar cosmic rays could explain the recent warming of Earth.

See Also Atmospheric Chemistry.

BIBLIOGRAPHY

Periodicals

Kanipe, Jeff. “A Cosmic Connection.” Nature 443 (2006): 141–143.

Lockwood, Mike, and Claus Fröhlich. “Recent Opposite Directed Trends in Solar Climate Forcings and the Global Mean Surface Air Temperature.” Proceedings of the Royal Society A, May 2007. Also available online athttp://www.pubs.royalsoc.ac.uk/media/proceedings_a/rspa20071880.pdf(accessed August 8, 2007).

Rahmstorf, Stefan, et al. “Cosmic Rays, Carbon Dioxide, and Climate.” Eos 85 (2004): 38–40.

Schiermeier, Quirin. “No Solar Hiding Place for Greenhouse Skeptics.” Nature 448 (2007): 8–9.

Shaviv, Nir J., and Ján Veizer. “Celestial Driver of Phanerozoic Climate?” GSA [Geological Society of America] Today (July, 2003): 4–10.

Web Sites

“Cosmic Rays Are Not the Cause of Climate Change, Scientists Say.” American Geophysical Union, January 21, 2004. < http://www.agu.org/sci_soc/prrl/prrl0405.html> (accessed August 8, 2007).

Larry Gilman

Cosmic Rays

Cosmic rays are, in fact, not rays, but high energy subatomic particles of cosmic origin that continually bombard Earth. The measurements scientists make of them, both on the ground and from probes in space, are the only direct measurements that are made of matter originating outside the solar system.

Among the cosmic rays are electrons, protons, and the complete nuclei of all the elements. Their energies range from below the rest mass of an electron, easily attainable in terrestrial accelerators, up to energies 10¹¹ times the rest mass of a proton. Matter with such energies is moving at speeds so close to the speed of light that there is an enormous relativistic time dilation , so that in its proper frame only 10^-11 of the time has elapsed that an observer on Earth would have measured. An early verification of German-born American physicist Albert Einstein's special theory of relativity came from explaining how unstable mesons produced by cosmic rays impinging on the upper atmosphere (whose lifetime was less than the time it took for them to reach the detectors on Earth) managed to survive without decaying. According to special relativity, these high energy mesons would not have had enough time in their own reference frame to decay.

Although cosmic rays have been known for more than a century, neither their precise origin nor their source of energy is known. Austrian-born American physicist Viktor Hess demonstrated their cosmic origin in 1912, using balloon flights to show that the penetrating, ubiquitous, ionizing radiation increases in intensity with altitude. It was not until the 1930s, with increased understanding of nuclear physics, that the "radiation" was recognized to be charged particles.

The low energy particles measured—below 10¹⁰ eV —are dominated by the effects of our environment in the solar system and the unpredictability of space weather. Incoming galactic cosmic rays are scattered on magnetic irregularities in the solar wind, resulting in "solar modulation" of the galactic cosmic ray spectrum. At low energies, many of the particles themselves originate in solar flares, or are accelerated by shocks in the solar wind.

At mid-energies, 10¹⁰ to 10¹⁵ eV, the particles measured are galactic, show a smooth power law energy spectrum , and show a composition of nuclei roughly consistent with supernovae ejecta , modified by their subsequent diffusion through the galaxy. Bulk acceleration in the supernova blast wave, and diffusive acceleration in shocks in the remnant can probably account for particles up to 10¹⁴ eV. They diffuse throughout the interstellar medium , but remain trapped within the galaxy for several million years by the magnetic field and scattering by magnetohydrodynamic waves .

Particles have been detected with energies up to about 10²¹ eV. There is no generally accepted mechanism for accelerating them above about 10¹⁵eV. One speculation is that collapsing superstrings could produce particles with the grand unified theory (GUT) energy of 10²⁵ eV; the particles then decay and lose energy. Above 10¹⁹ eV neither the spectrum nor the composition are well-known because the events are rare and the detection methods indirect.

In 1938 French physicist Pierre Auger discovered extensive air showers. When a single high energy particle impinges on the atmosphere, it generates a cascade that can contain 10⁹ particles. Information about the primary nucleus can be deduced from the lateral distribution of the muons and electrons that reach the ground, and from the pulse of Čerenkov light emitted as the shower descends through the atmosphere. If the spectrum, composition, and anisotropy above 5x10¹⁹ eV, where there should be a cutoff in the spectrum because of interactions on the 2.7°K cosmic microwave background photons, can be measured, and these are consistent, these cosmic rays will identify sites where some of the most exotic and energetic events in the universe occur.

Cosmic rays represent a significant component in the energy balance of our galaxy. The energy density in cosmic rays in the galactic disk is comparable to that found in starlight and in the galactic magnetic field, and therefore must play an important, if so far poorly understood, role in the cycle of star formation. By maintaining a residual ionization in the cores of dense molecular clouds, star formation is inhibited because the magnetic field cannot diffuse out. On the other hand, cosmic rays streaming along the magnetic field in the diffuse interstellar medium could provoke cloud condensation through MHD instabilities.

see also Galaxies (volume 2); Solar Particle Radiation (volume 2); Solar Wind (volume 2); Space Environment, Nature of the (volume 2); Stars (volume 2); Sun (volume 2); Supernova (volume 2); Weather, Space (volume 2).

Susan Ames

Bibliography

Friedlander, Michael W. A Thin Cosmic Rain: Particles from Outer Space. Cambridge, MA: Harvard University Press, 2000.

Gaisser, Thomas K. Cosmic Rays and Particle Physics. Cambridge, UK: Cambridge University Press, 1990.

Sokolsky, Pierre. Introduction to Ultrahigh Energy Cosmic Ray Physics. Boston, MA: Addison-Wesley Publishing Company, Inc. 1989.