Ask A Physicist

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I worry from your question that you don't really understand what cosmic rays are. Cosmic rays are sub-atomic particles that are moving at a good fraction of the speed of light. If you slow them down to "collect and store" them, they look just like the matter that makes up you, me, and the rest of the Earth. If you want them moving at high velocities, a particle accelerator can generate many more than you could easily capture and store (although cosmic rays can get up to much higher energies than the biggest particle accelerators on Earth can generate).

Dr. Eric Christian

The velocity of cosmic rays can go from a small fraction of the speed of light up to about .999999999999 times the speed of light. Since cosmic rays are matter (typically the bare nuclei of atoms), they CANNOT exceed the speed of light. They also cannot escape from the event horizon of black holes, but it looks as if black holes can generate relativistic jets of material out along their poles. But these particles are accelerated outside the black hole and so they (and any light generated there as well) can escape.

Dr. Eric Christian

This is a pretty extensive topic. A brief description that I wrote a few years ago for the instrument development branch goes like this:

How Do You Weigh a Particle Moving at Half the Speed of Light?

It is nearly impossible to make a detector that will detect only lithium, for example. What cosmic ray scientists do is detect all particles similar to lithium and then determine which ones are lithium and which aren't. Different cosmic rays slow down at different rates and also bend differently in a magnetic field (see Measuring Cosmic Rays). But both of these also depend upon the initial energy of the cosmic ray. So typically you have to measure at least two of the following quantities: energy, rate of energy loss (rate of slowing down), velocity, or rigidity (which is a measure of how much the particle bends in a magnetic field). From these measurements, you can tell which particles are lithium and which are helium or beryllium, and if you make the measurements accurately enough, you can tell which cosmic rays are lithium-6 (3 protons and 3 neutrons) and which are lithium-7 (3 protons and 4 neutrons).

You can frequently tune your cosmic ray instrument so that its resolution is best for lithium, but it will detect some helium and beryllium and other cosmic rays, too.

Dr. Eric Christian

Some people still call high energy photons (x-rays and gamma rays) cosmic rays, and you'll still see that in some textbooks. The more common usage (at least in scientific circles) is to call particles cosmic rays, and to call photons either x-rays or gamma rays.

Dr. Eric Christian

I should first say that, although Millikan's work was extremely important for cosmic ray studies, he was not the first one to study them. Several people (Wulf on the Eiffel tower, and Hess from balloons) were before him.

Millikan and his collegue, I.S. Bowen, flew things called electroscopes on high-altitude balloons starting in 1922. Electroscopes measure the number of electrons knocked off something like gold foil by the cosmic rays passing through it. At one point, Millikan was convinced that the radiation was local, because the measurement of quantity over Europe was four times that over Texas. We now know that it's the Earth's magnetic field that allows more cosmic rays into the atmosphere the closer you are to the magnetic poles. It wasn't until 1926, after a detailed study of electoscopes in two lakes at different altitudes (Muir Lake and Arrowhead Lake), putting the electroscopes at different depths in the two lakes, was Millikan convinced that the radiation was coming down into the atmosphere. He's the one who coined the term "cosmic rays".

Dr. Eric Christian

To a very good extent, the abundance of cosmic rays is the same as the abundance in the universe as a whole. So the 10 most abundant elements (in order from most abundant down) are hydrogen, helium, oxygen, carbon, neon, nitrogen, magnesium, silicon, iron, and sulfur.

Dr. Eric Christian

Counting only the baryonic matter (matter made from protons and neutrons), hydrogen (including free protons which are counted as ionized hydrogen) is about 72 or 73%. About 26% is helium, and the rest is heavier than helium. If there is dark matter (which is presumed to have mass) that drops the fraction of hydrogen, but dark matter isn't proven.

Dr. Eric Christian

Why don't you check out this web page from Dr. Strous's Answer Book. It gives an estimate of the number of atoms in the Universe, which you will see is much smaller than 10 to the 100th power.

Dr. Louis Barbier

In interplanetary space, there is more hydrogen closer to the Sun, because most of it is due to the solar wind which spreads out further from the Sun. At Earth, there are 5-10 hydrogen atoms per cubic centimeter (cc). For other distances, you have to divide by the distance (in Astronomical Units) squared. This value varies with time and you can check on the current value at the ACE spacecraft.

In interstellar space, the average density in the galaxy is about 1 atom / cc, but the solar system is in a low density region with about 0.1 atoms / cc.

Dr. Eric Christian

It depends upon which of the several types of cosmic rays you are talking about. Solar cosmic rays (or Solar Energetic Particles) are typically less than 1 MeV in energy and are usually partially (approximately half) ionized. At about 10 MeV, Anomalous Cosmic Rays (accelerated at the edge of the solar system) dominate and are singly ionized (missing only one electron). At even higher energies are Galactic Cosmic Rays which have probably been accelerated in supernovae remnants. They have typical energies of about 1 GeV, but go all the way up to 10²¹ eV, and are fully ionized (no electrons).

Dr. Eric Christian

This question is not so easy to answer. Cosmic ray intensity changes by more than a factor of 2 with the 11 year solar cycle, and can increase for short times by a much larger factor when there is a solar flare. I tend to think of cosmic ray intensity in terms of an energy spectra, or flux (particles per MeV * meters^2 * steradian * seconds). Since you're asking about sieverts, I assume you are interested in cosmic rays as a radiation source, not as a scientific study. But to first order:

• Moon = 600 - 1500 milliSievert per year
• Mars = 600 - 1500 milliSievert per year (a little more galactic cosmic rays, a little less solar)
• Polar = 40 - 120 milliSievert per year
• Equator = 3 - 8 milliSievert per year
• Japan = 10 - 30 milliSievert per year

There is also an altitude effect that isn't considered here. A lot of this info came from:

Plasma is a gas that is hot enough that some electrons have been stripped from the atoms. Plasma is just ionized gas, in other words, a gas of nuclei and electrons instead of a gas of atoms and molecules. So everything in a plasma is either positively charged (the atomic nuclei with whatever electrons remain with it) or negatively charged (electrons). The fact that it is an electrically charged gas makes it behave very differently from a mostly neutral gas (like air), which is why it's considered a fourth state of matter. It would glow (with the color depending upon what temperature it's at) just like the Sun and stars. It can be made in laboratories and can occur in nature on Earth (lightning causes plasma to temporarily form).

One place I found on the web is at NASA's Space Academy web site.

In what year was plasma defined (discovered)? Who was responsible?

I don't know who coined the word "plasma". It was known that gas could be ionized long before the word plasma came into use.

Most of the gas in interstellar space is ionized (astronomers can tell by the wavelengths of light the gas absorbs and emits), and all of the gas in stars in ionized, that's where the 99% comes from. The 99% ignores any dark matter which might be out there. (See Imagine the Universe! for more on dark matter).

Dr. Eric Christian