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#33.     Cosmic Rays

    The atoms involved in our everyday life are not too energetic. Take the air we breathe: its molecules have energies around 0.03 ev (electron volt--see energetic particles) and move as fast as cannonballs, though still quite a bit slower than a typical satellite. Such molecules bounce off each other like billiard balls, with not enough force to affect each other's structure by, say, tearing off electrons.

    The Sun's plasma is much hotter, and that of the magnetosphere is hotter still. Auroral electrons typically have 1000 to 10,000 ev, as do protons in the magnetotail. Ring current protons have more, around 20,000 to 100,000 ev, while inner belt protons go higher still, typically 10,000,000 to 100,000,000 ev. In a nutshell, the magnetosphere is a high-energy environment, where speeds amounting to 1/10 the speed of light are not uncommon.

    How unusual is such an environment? How does the rest of the universe compare? Are the high-energy ions and electrons of the magnetosphere an exceptional and rare population?

    The unexpected answer is that even higher energies seem quite commonplace in the universe. One piece of evidence is a rain of fast ions constantly bombarding Earth, coming from distant space and much more energetic than any found in the magnetosphere. They are known as cosmic rays or cosmic radiation.

Cosmic Rays and Starlight

    Individually the cosmic ray ions are much faster and more energetic than those trapped in the Earth's field, though their overall density is rather small. The radiation is therefore not intense, giving us about as much energy as starlight. That does not sound like much, until one remembers what the stars are--distant suns, about a hundred billion of them traveling together in our galaxy, and untold billions in more distant galaxies. "As intense as starlight" seems to say that our galaxy gives about as much energy to exotic particles moving close to the speed of light, as it gives to the visible light of its billions of stars.

    Actually, the source of cosmic rays is probably not quite as intense, because cosmic ray particles can stay around the galaxy much longer than starlight. Starlight moves in straight lines, one pass through our galaxy and it is gone, into the great emptiness between galaxies. This may require (say) 5000- 50,000 years, going through a thickness of as many light years. Cosmic ray ions, on the other hand, may be trapped by weak magnetic fields in the galaxy--trapped not forever, because sooner or later they hit an atom of the rarefied gas which fills the void between stars, but for a period of the order of 10 million years.

    If cosmic ray ions stay around (on the average) 1000 times longer than starlight, their source only needs 1/1000 of the energy output of the stars to match the intensity of starlight. But even 1/1000 of the energy of starlight is still an enormous amount! If the Sun had invested 1/1000 of its energy input in cosmic radiation, the radiation level around it would have been sufficient to snuff out any life emerging on Earth.

What are they?

  A collision between a high-energy cosmic ray particle and an atom
  in a photographic emulsion, as viewed through a microscope.

    What sort of particles are these? On the ground one rarely encounters the "primary" cosmic rays, because they generally collide high in the atmosphere and all we get below is a shower of very fast fragments. However, sensitive photographic plates have been lifted by balloons to the top of the atmosphere, and have recorded there the passage of "primary" cosmic ray particles. The plates were developed, the tracks were scanned through microscopes, and by the thickness of those tracks, the particles which had caused them were identified. This method showed cosmic ray particles to be ions of a familiar sort--mostly hydrogen, some helium, diminishing amounts of carbon, oxygen etc. and even a few atoms of iron and of heavier elements, to all intents proportions similar to those found on the Sun. The conclusion seems to be that here is ordinary matter, which had undergone some extraordinary process to gain huge energies.

    Those energies are indeed huge. The atmosphere shields us from cosmic rays about as effectively as a 13-foot layer of concrete, yet a large proportion of cosmic ray particles manages to send fragments all the way through it. Some have much, much higher energies, though as one goes up in energy, the numbers drop drastically. Cosmic ray ions at the top of the energy range produce in the atmosphere showers of many millions of fragments, covering many acres, and their more energetic fragments register even in deep mines, a mile underground. Relatively few of the particles are so energetic--an experiment might register them once a week--but their existence is a real riddle. How can a single atomic nucleus gain such extreme energies?


    To all intents, cosmic rays arrive evenly from all directions in the sky, but this does not necessarily mean their sources are evenly spread around us. More likely, they are constantly deflected and scattered by magnetic fields in the galaxy, until any trace of their original motion is lost. In a similar way, sunlight on a heavily overcast day seems to arrive evenly from the entire sky, and we have no idea where the Sun actually is, because its light is thoroughly diffused by water droplets in the clouds.

    Where direct evidence is lacking, one can only guess, using physics and whatever else is known about the universe. The consensus these days is that cosmic ray ions are energized by shock waves which expand from supernovas.

 Supernova in the Crab nebula
 seen in X-rays by the Chandra spacecraft
    Sooner or later, a star must run out of its "nuclear fuel" of light elements (especially hydrogen). Its "nuclear burning" gradually converts light elements into heavier ones, and the heat produced keeps the star puffed up, resisting the pull of gravity which draws it together. When it can no longer produce nuclear heat, it collapses; gravitational energy can keep it hot for a while, but not for long. If it is of the size of our sun, it may end up as a dim dwarf star.

    However, if the star is much bigger than the Sun (say, 10 times more massive), the collapse can be catastrophic and rapid. It then quickly releases an enormous amount of gravitational energy. Nuclear processes quickly consume part of that energy, but another part is then spent in a grand explosion, blowing the star's outer layers out to space and creating a huge expanding shock front.     By good fortune, such an explosion was observed in 1987 in a nearby galaxy, and its shock wave (inner brightness, picture below) has recently been observed, together with some earlier emissions (large circles) which still puzzle astronomers:

 Remnant of
 Supernova 1987
Note: A much more detailed discussion of the way energy is released in stars and of their final collapse can be found in section (S-7) The Energy of the Sun of "From Stargazers to Starships"; see

Recent evidence from Gamma Rays

Added December 2004

    As noted under Supernovas above, cosmic rays are scattered from their original directions by magnetic fields in space. The magnetic force is weak, but acting over distances on the interstellar scale, it thoroughly mixes the directions of cosmic ray particles and makes them arrive evenly from all directions.

    Presumably, however, cosmic ray ions start from matter denser than the interstellar medium, and may collide with it on their way out. Nuclear collisions produce gamma rays (after some intermediate steps) which, like light, move in straight lines, If only we could detect ultra-energetic gamma rays from the sky, we might pin-point sources of cosmic rays.

    At long last, gamma rays of energies of the order of 800 Gev have been detected and tracked. (For comparison, the rest energy E = mc2 of a proton is a little below 1 GeV). Such gamma rays interact strongly with the atmosphere, to produce pairs of very fast electrons and positrons (positive counterparts of electrons), which quickly produce gamma rays, each of which again produces a pair of lower energy, each of which... and so on, ending with an "air shower" of thousands of electrons, positrons and gamma rays, all still moving at close to the original direction.

    In a vacuum, no particle can move faster than light. The atmosphere however slows down light by a slight amount (related to the refraction of light by air), and therefore in air electrons can outrun light, if they are fast enough. When that happens, they emit a sort of "shock front" of light, like the sonic shock ahead of a wing moving at supersonic speed. This light is known, after its Russian discoverer, as "Cherenkov light" (Ch as in Church); if you have seen the glow coming from a nuclear research reactor operating inside a pool of water--that is one example.

    Flashes of Cherenkov light from air showers has been studied for many years. They clearly indicate the presence of high-energy gamma rays, but for a long time there was no way of telling if those rays originated in distant space or (more likely) in nuclear collisions of cosmic ray ions in our own atmosphere. Recent studies, however, have not just detected flashes, they also used giant telescopes to focus the light and observe an image of its sources. It turns out, gamma rays from distant space give a different signature, and are readily distinguished. The HESS telescope array

    Several such telescopes exist, and more are being added. As early as in 1989, a gamma ray shower was observed to come from the Crab Nebula, the remnant of a recent supernova explosion; this was accomplished by the Whipple telescope at Kitt Peak, Arizona. Now an array of 4 giant mirror-telescopes has joined the search, each measuring 13 meters across (for the scale, note small truck in front of the nearest telescope); the optical quality is not even close to that of astronomical telescopes, and the ability to resolve a location in the sky is only like that of the unaided human eye. The huge size is however needed to capture a sufficient intensity of the weak Cherenkov light emission. (Science, vol 305, p. 1392-3, 3 September 2004; also Physics Today, vol 58, p. 19-21, January 2005)

    This is the HESS telescope array--its name standing for High Energy Spectroscopic System, and also honoring Victor Hess, who discovered cosmic rays in 1912. Rising in a balloon in what is now the Czech Republic (then part of Austria), Hess measured the "background rate" of nuclear radiation, finding it actually increased with height. For that he was awarded the Nobel Prize in 1936.

Gamma rays from supernova     Located in Namibia, in southern Africa, HESS quickly discovered in the southern skies a ring-like source of very high-energy gamma rays, about 1 degree across. It is centered at the remnant of supernova RX J1713.7-3946, believed to be about 1000 years old ("High-energy particle acceleration in the shell of a supernova remnant," [a collaboration by about 100 co-authors], Nature, 432, p. 75-7, 4 November 2004). The picture on the right gives the result: colors indicate gamma-ray intensity, while black lines are contours of previously observed X-ray emission from the same supernova remnant. As Paula Chadwick of the University of Durham said, "If you had gamma-ray eyes and were in the southern hemisphere, you could see a large, brightly glowing ring in the sky every night." (Or at least you could, if you were able to collect all the gamma-ray signal of 26 hours into a single bright display, as HESS did.)

    Though these results were reported recently, the observations date to the middle of 2003. More data should already be available, and other Cherenkov telescopes are operating, too. We should soon know a lot more about supernova origin of cosmic radiation!

Cosmic Rays and the Magnetosphere

    Where does the magnetosphere enter all this? Neither acceleration by collision-free shocks nor other particle acceleration processes observed or proposed in space can be duplicated in the laboratory. We have no way of reproducing the large distances and low densities of space, and the phenomena cannot be scaled down properly to laboratory dimensions.

    In trying to understand the physics of such phenomena, the Earth's space environment is our best laboratory, and satellites are the probes which can provide us with relevant information. For instance, the Earth's bow shock (a relatively mild shock wave) can be studied for varying solar wind speeds and magnetic field angles, and some acceleration processes indeed seem to occur there.

    Shock acceleration can also take place inside the magnetosphere (click here for the story of one such event, in March 1991). Yet other acceleration modes exist too, in substorms and auroral beams, and similar processes may also occur in the distant universe and on the Sun. In the long run, the most important reason for studying the magnetosphere might well be that here is our own "cosmic laboratory," replicating the processes which affect the distant universe.

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Next Stop: #34.  High Energy Particles in the Universe

Last updated 24 January 2005