#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) as fast as a cannonball, though still quite a bit slower than the typical satellite. Such molecules bounce off each other like billiard balls, lacking the 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 still hotter. Auroral electrons typically have 1000 to 10,000 ev, as do protons in the magnetotail. Ring current protons reach higher, 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 this? 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 number 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?

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 a microscope, 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 and had gained 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 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.

Remnant of Supernova 1987

A supernova is a star which has run out of the "nuclear fuel" of light elements (especially hydrogen), needed to keep it shining. Its "nuclear burning" gradually converts light elements into heavier ones, and the heat it produces keeps the star puffed up, resisting the pull of gravity which would like to todraw it together. When the star can no longer produce nuclear heat, it suddenly collapses to a small volume, releasing in the process an enormous amount of gravitational energy. Much of that energy is 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 above) has recently been observed, together with some earlier emissions (large circles) which still puzzle astronomers:

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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Last updated March 13, 1999