#33.     Cosmic Rays

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

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?

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?

Supernovas

Cosmic Rays and the Magnetosphere

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."