High Energy Cosmic Rays and the Atmosphere
Cosmic rays are (mostly) protons from outer space. When a high-energy proton hits the earth's atmosphere, it will collide and interact with one of the nuclei of the atmospheric gas molecules.
Showers and muons
In these high-energy collisions many secondary particles are produced, including lots of high-energy particles called pions. Pions decay rapidly but some may first interact and make even more (somewhat lower energy) pions.
A high-energy (charged) pion decay makes a high-energy muon and two (unseen) neutrinos. Muons have two properties that allow them to reach the earth's surface:
- Muons decay relatively slowly compared to pions.
- Muons penetrate large amounts of material without interacting.
Muons, unlike pions, have no strong interaction properties and unlike electrons they are too massive to be significantly deflected by atomic electric fields that they encounter.
The illustration at right shows happens. A proton from outer
space (yellow) hits the upper atmosphere, and produces a
shower of other particles (green). Some of these particles
(mostly pions) decay into muons (red). Only a small fraction
of the muons reaches the earth's surface, because most decay
in flight. Therefore, at higher altitudes there are more muons,
because fewer have decayed. At sea level, one muon goes through
an area the size of your fingernail about every minute!
Muons are Heavy Electrons
What is a muon anyway? For our purposes, it is sufficient to say that a muon is a heavy electron. It has the same charge and spin as an electron and undergoes the same types of interactions; electrons and muons differ only in their masses. A muon is roughly 200 times heavier than an electron. Because it is so heavy, it is unstable and decays into an electron and two neutrinos (more precisely, a muon-type neutrino and an electron-type antineutrino).
All particles have an anti-matter counterpart. For the electron, the counterpart is the positron. For the muon, the counterpart is called the antimuon. In cosmic ray showers, both muons and antimuons are produced about equally. Each antimuon decays into a positron, a muon-type antineutrino and an electron-type neutrino. Physicists use the term muon to refer to either a negatively charged muon or its positively charged antiparticle. We will follow that convention in the rest of this discussion.
Half-life and Relativity
Muons decay with a half-life of about 1.4 microseconds [*Note]. This means that if you have a sample of muons in the laboratory, at rest, on average half of them will decay in the next 1.4 microseconds.
When they are produced in the upper atmosphere, however, they are not produced at rest. In fact, they are usually produced with velocities that are pretty close to the speed of light. Einstein's theory of relativity tells us that from our point of view, time goes slower in a system that moves fast with respect to ourselves. This is also true for our fast-moving muons. While in a laboratory, at rest, muons have a half-life of 1.4 microseconds, they appear to live much longer when they travel at high speed through the atmosphere. It is, in fact, because of relativity that so many muons actually make it down to the earth's surface!
*Note: There is a difference between half-life and mean-life. The mean-life of a muon is about 2 microseconds.
Cosmic rays and us
Cosmic rays are important to life on earth in a number of ways. We will discuss here only how cosmic rays affect the weather, and how they lead to such useful methods as carbon dating.
Cosmic rays and the weather
While low-energy cosmic rays such as the solar wind cause ionization in the upper atmosphere, muons cause most of the ionization in the lower atmosphere. When a muon ionizes a gas molecule, it strips away an electron, making that molecule into a positive ion. The electron is soon captured, either by another gas molecule turning it into a negative ion, or it may find an already ionized positive ion and neutralize it (this is called recombination). There is a balance between ionization and recombination, and so there is a fairly constant density of positive and negative ions in the atmosphere. But there is a difference between the types of molecules that become negative ions and the ones that are positive. On average, the negative ions are more "mobile" than the positive ones, and this results in the fact that there is an electric field in atmosphere. On a normal quiet day, this electric field is about 100 Volts per meter. When a thunder shower forms, there is an as yet not completely understood mechanism that tends to lift the negative ions up while pushing the positive ones down. This changes the electric field strength to tens of thousands of Volts/meter. When the field strength becomes too high, a discharge occurs: lightning. Clearly, without ionization, thunder and lightning would not happen, so cosmic rays have a direct influence on the types of weather we can have on earth.
There is also evidence that there is a correlation between cosmic ray flux and low-altitude cloud formation. Now, correlation does not always imply causation, and it is also known that the sun is slightly brighter if it is more active, which may also affect cloud formation on earth. But it is at least possible that cosmic rays could have something to do with it. There is a possible mechanism for this: elevated levels of ionization seem to facilitate the coagulation of such molecules as sulfuric acid (H2SO4) in the atmosphere into tiny droplets, which then form condensation nuclei for water vapor. The condensed droplets of water then form clouds. For further information, see for example:
Cosmic rays and carbon dating
The collisions of primary cosmic rays with the atmosphere can also cause nucleons (neutrons or protons) to be kicked out of the nuclei of the atmospheric gas molecules. One of the possible things that can happen is that a resulting high energy neutron collides with the nucleus of a nitrogen atom, which has seven protons and seven neutrons. By a process known as "charge exchange", one of the nitrogen's protons may turn into a neutron, while the incoming neutron turns into a proton. What we're left with is a nucleus that has six protons and 8 neutrons, which is a form of carbon, 14C. Normal carbon (12C), however, has only 6 neutrons. It is completely stable, while 14C is radioactive and decays with a half-life of 5730 years. Many organisms on earth (such as plants and trees and animals that eat them) take in atmospheric carbon (through carbon dioxide). When such organisms die, they obviously no longer do so, and the fraction of radioactive 14C in them starts to decay. By measuring the level of 14C in the remains one can determine fairly accurately when it died. This is known as "carbon dating". See for example:
One can also measure the relative amount of 14C in materials of which the age is known by some other means (for example, the year rings in the trunks of trees, and similar features in stalagmites from caves). From this we can deduce the true amount of 14C present in the atmosphere at that time, after correcting for its decay. This in turn gives a historic record of the cosmic ray flux. From other paleontological measurements we know something about the global history of the weather. As it turns out, the cosmic ray flux is clearly correlated to the weather in prehistoric times: when the weather was generally cold, cosmic ray fluxes were high, and the reverse.
