Cosmic Rays

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 10 ¹⁰ 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 10 ¹⁴ 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.