Thursday, February 26, 2015

Cosmic Rays 2

This is the second part of a two-part post about cosmic rays. For the first part, see here.

The first post dealt primarily with the composition of cosmic rays and the abundance of different types. The other major characteristic of cosmic rays is their energy.

The figure above illustrates the abundance of cosmic rays of different energies on a logarithmic scale. The x-axis shows the energy of cosmic rays in electron volts. An electron volt (eV) is a unit of energy, defined to be the energy required to move an electron through an electric potential of 1 volt. Protons, for example, the most common type of cosmic ray, have a rest energy of 9.38x108 eV, at the bottom end of the chart above. Even at rest, protons are considered to have energy given by the mass-energy relationship E = mc2. Protons (and other cosmic rays such as atomic nuclei) of larger velocity have even greater energies.

The y-axis of the chart indicates cosmic ray flux. Flux, generally speaking, refers to the quantity of something passing through a given surface. In this case, the flux refers to the number of cosmic rays of a certain energy passing through a given area of the Earth's atmosphere. For example, (as labeled on the graph) "1 m-2 s-1" indicates that one cosmic ray passes through each square meter of the Earth's atmosphere (viewed as a spherical surface) every second.

The blue line indicates the relationship between the energy of cosmic rays and the frequency with which they impact Earth (measured by flux). The fact that the curve is decreasing represents that higher energy cosmic rays are rarer. Uncertainty in the values causes the curve to widen for higher energies.

Finally, the graph is separated into three sections, indicating the typical origins of cosmic rays of different energies. The leftmost zone, the yellow zone, refers to solar cosmic rays, i.e. those originating in the Solar System.

Solar Cosmic Rays

Solar cosmic rays, also known as Solar Energetic Particles (SEP's), have energies up to a few gigaelectron volts (~1010 eV). The Sun emits these particles during coronal mass ejections (CME), or explosions of the Sun's atmosphere.

The above image shows a solar flare, which is similar but distinct from a CME in that most radiation released in electromagnetic (the explosion is bright) rather than cosmic rays. Solar flares do release cosmic rays, but to a lesser extent. Mass ejections, on the other hand, release huge quantities of SEP's, some of which are acclerated to over half the speed of light (these have the greatest energies). These events can produce particles that can interfere with the performance of satellites by penetrating their outer skin. The volume of high-energy radiation produced by the strongest events, such as the CME of August 1972, would be fatal to a human outside of the Earth's magnetic field.

Very rarely do solar (and other) cosmic rays reach the surface of the Earth, and, when they do, there are too few to cause radiation damage. However, by the chart above, there is an average of about 1000 such particles impacting every square meter of the surface of the outer atmosphere (outside the magnetic field) every second. Thus, there is potential for satellite damage, especially when this value increases during CME's.

Galactic Cosmic Rays

Galactic cosmic rays (GCR's) are cosmic rays originating from outside the Solar System, but within the Milky Way galaxy. Though they may have energies similar to those of solar cosmic rays, they also may be more energetic, falling into the range 1010 eV to 1015 eV, as illustrated by the blue zone of the figure.

The primary sources for GCR's are supernova remnants; the magnetic fields of these supernovae may accelerate ejected particles to over 99.9% of the speed of light. Through a process known as diffusive shock acceleration, the shock waves of the magnetic field of an exploding supernova emits impart massive amounts of energy to accelerate charged particles. However, other objects, such as so-called microquasars, also produce very high energy cosmic rays.

Microquasars are compact objects, such as white dwarfs, neutron stars, or black holes (see here) which have an ordinary companion star in a binary system. The gravitational pull of these objects sucks plasma off of their companion stars, adding them to an accretion disk as shown. Though a process not entirely understood (but known to involve the magnetic fields of the compact objects) matter from the accretion disk is shot out of the polar regions into relativistic jets, so named for the extremely high speeds of particles therein. Cosmic rays in these jets may be extremely energetic, on the order of 1-1000 TeV (1012-1015 eV). An example of a microquasar in the Milky Way is Cygnus X-3, named for being the third brightest X-ray source in the constellation Cygnus.

An X-ray image of Cygnus X-3

Though Cygnus X-3 is the third brightest in X-rays from Earth, it is more distant than Cygnus X-1 and Cygnus X-2, at a distance of 37,000 light-years (though still in the Milky Way). Further, it emits some of the highest-energy cosmic rays known to originate in the Milky Way galaxy, up to 1000 TeV. Note that the most energetic artificially accelerated particles produced on Earth by the Large Hadron Collider (LHC) have energies on the order of 10 TeV. Since the Earth receives about one particle with energy 1000 TeV per square meter per year, our planet is constantly bombarded by particles moving faster than anything manmade particle colliders have produced.

The final category of cosmic rays (corresponding to the purple area of the figure above) is extragalactic cosmic rays. They are discussed in the final post of this series, coming March 22.


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