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.


Monday, February 2, 2015

Cosmic Rays 1

The Earth constantly is bombarded with a large amount of radiation from the cosmos. Much of this radiation is electromagnetic, coming in the form of radio waves, microwaves, infrared rays, visible light, ultraviolet rays, X-rays, and gamma rays. All of these types, however, are composed of photons, massless particles which travel at the speed of light. Other subatomic particles also compose incoming radiation, falling into the umbrella of "cosmic rays".

We cannot take detailed images of the cosmos with cosmic rays like we can with electromagnetic radiation. This is because the particles composing this radiation are charged, and thus change direction and speed when influenced by the magnetic fields of the Sun and Earth. As a result, we cannot directly ascertain the direction from which these particles come. Nevertheless, cosmic rays help us understand the composition of objects in the cosmos. In addition, since the particles making up cosmic rays have mass, they often require huge amounts of acceleration to reach us. The fact that the particles reached Earth at all reveals clues about the nature of the objects from which they came.

Cosmic rays have two main characteristics: composition and energy. We explore each in turn.

Cosmic rays are generally composed of the nuclei of atoms, though other particles appear as well, such as electrons. The extreme energy and speed of these former atoms stripped them of their electrons, leaving positively charged nuclei and negatively charged electrons, each of which are influenced by magnetic fields, as described above. Of these types of particles, hydrogen and helium nuclei (protons and alpha particles), the lightest two elements, are by far the most abundant in cosmic rays. Further, the proportions of hydrogen and helium nuclei in cosmic rays, ~90% and ~9%, respectively, are very consistent with the proportion of atoms in the Universe which are hydrogen and helium.

The above diagram illustrates the relative abundance of the chemical elements (in increasing order) in the Universe, as estimated through indirect means such as spectroscopy, an observational method which uses facts about how different elements absorb and reflect light to discern the composition of celestial objects. Note the relative rarity of lithium, beryllium, and boron, despite the fact that these are very light elements (atomic numbers 3, 4, and 5). The reason for this lack is that these elements are not heavily produced during stellar fusion, occurring only as intermediates in nuclear reactions that either produce helium or heavier elements like carbon. However, the nuclei of these three elements compose 0.25% of cosmic rays, more than a million times greater than what we would expect from the Universe's composition!

It is believed that these elements are disproportionately represented in cosmic rays due to collisions between "original" cosmic rays containing protons and atoms in the interstellar medium, such as carbon or oxygen. Such interactions may produce energetic lithium, beryllium, or boron nuclei as a byproduct, and these particles subsequently reach Earth. The fact that the products of these collisions are energetic and thus more likely to reach Earth explains why these elements are disproportionally represented in cosmic rays. The nuclei of elements of higher atomic numbers also appear more frequently in cosmic rays than we would "expect" from the abundance graph above. This shows that cosmic rays tend to come from sources rich in heavy elements, such as supernovae.

Antiparticles also make infrequent appearances in cosmic rays. An estimated 0.01% of cosmic rays are composed of antimatter. In fact, cosmic rays led to the original discovery of antimatter. In 1928, Paul Dirac, using the mathematics of quantum theory, including Schrödinger's Wave Equation, predicted the existence of antimatter. Antimatter was at the time an undiscovered class of particles each of which would be an "opposite" counterpart to an ordinary particle (opposite in some properties, such as charge, but not in others, such as mass, which must always be positive or zero). One example is the antielectron, also known as the positron, which would have the same mass as the electron, but a positive, rather than negative, charge. This charge, through opposite, is of the same magnitude as the electron's. In 1932, Carl D. Anderson observed a cosmic ray composed of a positron, using a device known as a cloud chamber.

Antimatter, however, does not exist in abundance in the Universe. Matter and antimatter particles annihilate on contact, producing energy, so due to a slight matter-antimatter imbalance following the Big Bang (the origins of which are still unknown), the known Universe is dominated by matter. However, just as matter and antimatter particles can annihilate and form energy, energy may also spontaneously (during certain quantum interactions) be converted into a particle-antiparticle pair through a phenomenon known as pair production.

This diagram illustrates the formation of an electron (e-) and its antiparticle the positron (e+) from a gamma ray photon (γ). This instance of pair production often happens when a high-energy photon impacts the nucleus of an atom. The number of cosmic rays composed of antimatter which impact the Earth over a given period helps us to estimate the frequency of these reactions in space. Cosmic rays also often cause chain reactions in the Earth's atmosphere. In fact, this is how these rays are often detected.

The above image illustrates the cascade of particles that can occur when a cosmic ray (especially a powerful one) hits Earth's atmopshere. The interaction proceeds from top to bottom and begins when the cosmic ray impacts an atom (all atoms involved are denoted by circles). This produces several short-lived particles known as pions (with the symbols π+, π-, and π0 depending on the charge). The left branch indicates the decay of the neutral pion into gamma rays, each of which forms an electron-positron pair through pair production, (presumably through more interaction with atmospheric matter) releasing other gamma rays which do the same. The center shows the main decay mode of the pion into a muon (denoted μ) and a muon antineutrino (not shown) or the corresponding antiparticles (if the initial pion charge is reversed). Muons too decay after around 2.2 microseconds, leaving their characterisitc signiture of particles. Finally, the right branch indicates how incoming cosmic rays might impact and destablize atomuc nuclei. These nuclei then emit protons, neutrons, or tiny particles called neutrinos. Sometimes the emitted neutrons and protons hit other atoms, and the chain reaction continues.

The length of such a chain reaction depends on the energy of the original cosmic ray. Sufficiently energetic cosmic rays can cause huge chain reactions which reach the Earth's surface, allowing us to observe them. To see how energy affects the behavior of cosmic rays and can reveal more about their origins, see the next post.