Cosmic rays are energetic particles originating from outer space Outer space is the void that exists beyond any celestial body including the Earth. It is not completely empty (i.e. a perfect vacuum), but contains a low density of particles, predominantly hydrogen plasma, as well as electromagnetic radiation, magnetic fields, and neutrinos. Theoretically, it also contains dark matter and dark energy that impinge on Earth's atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21%. About 89% of all the incoming cosmic ray particles are simple protons The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom, along with neutrons, but is also stable by itself and has a second identity as the hydrogen ion, H+. It is composed of three fundamental particles: two up quarks and one down quark, with nearly 10% being helium Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except nuclei (alpha particles Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is produced in the process of alpha decay. The alpha particle can be written as He2+, 42He2+ or 42He (as it is possible that the ion gains electrons from the environment. Also, electrons are not important in nuclear chemistry)), and slightly under 1% are heavier elements; electrons The electron is a subatomic particle carrying a negative electric charge. It has no known components or substructure, and therefore is believed to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton. The intrinsic angular momentum of the electron is a half integer value in units of ħ, which means that (beta particles Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay. They are designated by the Greek letter beta . There are two forms of beta) constitute about 1% of galactic cosmic rays.^[1] The term ray is a misnomer, as cosmic particles arrive individually, not in the form of a ray or beam of particles. However, when they were first discovered, cosmic rays were thought to be rays. When their particle In physics, subatomic particles are the small particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles. Particle physics and nuclear physics study these particles and how they interact nature needs to be emphasized, "cosmic ray particle" is written.

The variety of particle energies reflects the wide variety of sources. The origins of these particles range from energetic processes on the Sun The Sun is the star at the center of the Solar System. It has a diameter of about 1,392,000 kilometers , about 109 times that of Earth, and its mass (about 2 × 1030 kilograms, 330,000 times that of Earth) accounts for about 99.86% of the total mass of the Solar System. About three quarters of the Sun's mass consists of hydrogen, while the rest is all the way to as yet unknown events in the farthest reaches of the visible universe The universe is commonly defined as the totality of everything that exists, including all physical matter and energy, the planets, stars, galaxies, and the contents of intergalactic space, although this usage may differ with the context . The term universe may be used in slightly different contextual senses, denoting such concepts as the cosmos,. Cosmic rays can have energies of over 10²⁰ eV In physics, the electron volt is a unit of energy equal to approximately 1.602×10−19 J. By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt. Thus it is 1 volt (1 joule per coulomb) multiplied by the electron charge (1 e, or 1.60217, far higher than the 10¹² to 10¹³ eV that man-made particle accelerators can produce. (See Ultra-high-energy cosmic rays for a description of the detection of a single particle with an energy of about 50 J The joule , named after James Prescott Joule, is the derived unit of energy in the International System of Units. It is the energy expended in applying a force of one Newton through a distance of one metre (1 Newton·metre or N·m). In terms of dimensions:, the same as a well-hit tennis ball at 42 m/s [about 150 km/h].) There has been interest in investigating cosmic rays of even greater energies.^[2]

Composition

Cosmic rays may broadly be divided into two categories, primary and secondary. The cosmic rays that arise in extrasolar astrophysical sources are primary cosmic rays; these primary cosmic rays can interact with interstellar matter In astronomy, the interstellar medium is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar to create secondary cosmic rays. The Sun also emits low energy cosmic rays associated with solar flares A solar flare is a large explosion in the Sun's atmosphere that can release as much as 6 × 1025 joules of energy. The term is also used to refer to similar phenomena in other stars, where the term stellar flare applies. The exact composition of primary cosmic rays, outside the Earth's atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21%, is dependent on which part of the energy spectrum An energy spectrum is a distribution of energy among a large assemblage of particles. It is a statistical representation of the wave energy as a function of the wave frequency, and an empirical estimator of the spectral function. For any given value of energy, it determines how many of the particles have that much energy is observed. However, in general, almost 90% of all the incoming cosmic rays are protons The proton is a subatomic particle with an electric charge of +1 elementary charge. It is found in the nucleus of each atom, along with neutrons, but is also stable by itself and has a second identity as the hydrogen ion, H+. It is composed of three fundamental particles: two up quarks and one down quark, about 9% are helium Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except nuclei (alpha particles Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is produced in the process of alpha decay. The alpha particle can be written as He2+, 42He2+ or 42He (as it is possible that the ion gains electrons from the environment. Also, electrons are not important in nuclear chemistry)) and nearly 1% are electrons The electron is a subatomic particle carrying a negative electric charge. It has no known components or substructure, and therefore is believed to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton. The intrinsic angular momentum of the electron is a half integer value in units of ħ, which means that. The ratio of hydrogen to helium nuclei (28% helium by mass) is about the same as the primordial elemental abundance ratio of these elements (24% by mass He) in the universe.

The remaining fraction is made up of the other heavier nuclei which are abundant end products of stars' nuclear synthesis. Secondary cosmic rays consist of the other nuclei which are not abundant nuclear synthesis end products, or products of the Big Bang The Big Bang theory is the prevailing cosmological theory of the early development of the universe. The theory postulates that the Big Bang event took place at some finite time in the past: according to the best available measurements as of 2009[update], around 13.7 billion years ago. According to the Big Bang model, the universe, originally in an, primarily lithium Lithium is a soft, silver-white metal that belongs to the alkali metal group of chemical elements. It is represented by the symbol Li, and it has the atomic number 3. Under standard conditions it is the lightest metal and the least dense solid element. Like all alkali metals, lithium is highly reactive and flammable. For this reason, lithium metal, beryllium Beryllium is the chemical element with the symbol Be and atomic number 4, and boron Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a trivalent metalloid element which occurs abundantly in the evaporite ores borax and ulexite. These light nuclei appear in cosmic rays in much greater abundance (about 1:100 particles) than in solar atmospheres, where their abundance is about 10⁻⁷ that of helium Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except.

This abundance difference is a result of the way secondary cosmic rays are formed. When the heavy nuclei components of primary cosmic rays, namely the carbon and oxygen nuclei, collide with interstellar matter, they break up into lighter nuclei (in a process termed cosmic ray spallation) - lithium, beryllium and boron. It is found that the energy spectra of Li, Be and B fall off somewhat more steeply than those of carbon or oxygen, indicating that less cosmic ray spallation occurs for the higher energy nuclei presumably due to their escape from the galactic A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The name is from the Greek root galaxias [γαλαξίας], meaning "milky," a reference to the Milky Way galaxy magnetic field Magnetic fields surround magnetic materials and electric currents and are detected by the force they exert on other magnetic materials and moving electric charges. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field. Spallation is also responsible for the abundances of scandium Scandium is a chemical element with symbol Sc and atomic number 21. A silvery-white metallic transition metal, it has historically been sometimes classified as a rare earth element, together with yttrium and the lanthanoids. In 1879, Lars Fredrik Nilson and his team, found a new element with spectral analysis, in the minerals euxenite and, titanium Titanium is a chemical element with the symbol Ti and atomic number 22. Sometimes called the "space age metal", it has a low density and is a strong, lustrous, corrosion-resistant (including sea water, aqua regia and chlorine) transition metal with a silver color, vanadium Vanadium is the chemical element with the symbol V and atomic number 23. It is a soft, silvery gray, ductile transition metal. The formation of an oxide layer stabilizes the metal against oxidation. Andrés Manuel del Río discovered vanadium in 1801 by analyzing the mineral vanadinite, and named it erythronium. Four years later, however, he was, and manganese Manganese is a chemical element, designated by the symbol Mn. It has the atomic number 25. It is found as a free element in nature (often in combination with iron), and in many minerals. As a free element, manganese is a metal with important industrial metal alloy uses, particularly in stainless steels ions An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge in cosmic rays, which are produced by collisions of iron and nickel nuclei with interstellar matter In astronomy, the interstellar medium is the gas and dust that pervade interstellar space: the matter that exists between the star systems within a galaxy. It fills interstellar space and blends smoothly into the surrounding intergalactic space. The energy that occupies the same volume, in the form of electromagnetic radiation, is the interstellar. (See environmental radioactivity#Natural Environmental radioactivity is the study of radioactive materials in the human environment. While some radioisotopes are only found on Earth as a result of human activity, such as Strontium-90 and Technetium-99 (99Tc), and some isotopes like Potassium-40 (40K) are only present due to natural processes, a few isotopes are present as a result of).

In the past, it was believed that the cosmic ray flux One could argue, based on the work of James Clerk Maxwell, that the transport definition precedes the more recent way the term is used in electromagnetism. The specific quote from Maxwell is "In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of this operation is has remained fairly constant over time. Recent research has, however, produced evidence for 1.5 to 2-fold millennium-timescale changes in the cosmic ray flux in the past forty thousand years.^[3]

Modulation

The flux One could argue, based on the work of James Clerk Maxwell, that the transport definition precedes the more recent way the term is used in electromagnetism. The specific quote from Maxwell is "In the case of fluxes, we have to take the integral, over a surface, of the flux through every element of the surface. The result of this operation is (flow rate) of cosmic rays incident on the Earth’s upper atmosphere is modulated (varied) by two processes; the Sun’s solar wind The solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It mostly consists of electrons and protons with energies usually between 10 and 100 eV. The stream of particles varies in temperature and speed over time. These particles can escape the sun's gravity because of their high kinetic energy and the high and the Earth's magnetic field Earth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). The cause of the field can be explained by dynamo theory. Magnetic fields extend infinitely,. The Solar wind The solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It mostly consists of electrons and protons with energies usually between 10 and 100 eV. The stream of particles varies in temperature and speed over time. These particles can escape the sun's gravity because of their high kinetic energy and the high is expanding magnetized plasma In physics and chemistry, plasma is a gas in which a certain portion of the particles are ionized. The presence of a non-negligible number of charge carriers makes the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma, therefore, has properties quite unlike those of solids, liquids, or gases and is generated by the Sun, which has the effect of decelerating the incoming particles, as well as excluding some of the particles with energies below about 1 GeV. The amount of solar wind The solar wind is a stream of charged particles ejected from the upper atmosphere of the sun. It mostly consists of electrons and protons with energies usually between 10 and 100 eV. The stream of particles varies in temperature and speed over time. These particles can escape the sun's gravity because of their high kinetic energy and the high is not constant due to changes in solar activity, for instance over its regular eleven-year cycle. Hence the level of modulation varies in anticorrelation with solar activity. Also the Earth's magnetic field Earth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole (see Magnetic North Pole) and the other magnetic field N pole near the Earth's geographic south pole (see Magnetic South Pole). The cause of the field can be explained by dynamo theory. Magnetic fields extend infinitely, deflects some of the cosmic rays, giving rise to the observation that the intensity of cosmic radiation is dependent on latitude Latitude, usually denoted by the Greek letter phi gives the location of a place on Earth (or other planetary body) north or south of the equator. Lines of Latitude are the imaginary horizontal lines shown running east-to-west (or west to east) on maps (particularly so in the Mercator projection) that run either north or south of the equator, longitude Longitude is the angular distance, in degrees, minutes, and seconds, of a point east or west of the Prime Meridian. Lines of longitude are often referred to as meridians (pronounced /ˈlɒndʒɨtjuːd/ or /ˈlɒŋɡɨtjuːd/), identified by the Greek letter lambda (λ), is the geographic coordinate most commonly used in cartography and global, and azimuth angle An azimuth ( ˈæzɪməθ ) is an angular measurement in a spherical coordinate system. The vector from an observer (origin) to a point of interest is projected perpendicularly onto a reference plane; the angle between the projected vector and the reference vector on the reference plane is called the azimuth. The cosmic flux varies from eastern and western directions due to the polarity Dipoles can be characterized by their dipole moment, a vector quantity. For the simple electric dipole given above, the electric dipole moment points from the negative charge towards the positive charge, and has a magnitude equal to the strength of each charge times the separation between the charges. For the current loop, the magnetic dipole of the Earth's geomagnetic field and the positive charge dominance in primary cosmic rays. (This is called the "east-west effect"). The cosmic ray intensity at the Equator An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass. The capitalized term Equator refers to the Earth's equator is lower than at the poles as the geomagnetic cutoff value is greatest at the equator. This is because charged particles tend to move in the direction of field lines and not across them, so that they are concentrated in the polar regions (where field lines are closest together). This is the reason the auroras Auroras, also known as northern and southern lights or aurorae (singular: aurora), are natural light displays in the sky, usually observed at night, particularly in the polar regions. They typically occur in the ionosphere. They are also referred to as polar auroras. This is a misnomer however, because they are commonly visible between 65 to 72 occur at the poles, since the field lines curve down towards the Earth’s surface there. Finally, the longitude Longitude is the angular distance, in degrees, minutes, and seconds, of a point east or west of the Prime Meridian. Lines of longitude are often referred to as meridians (pronounced /ˈlɒndʒɨtjuːd/ or /ˈlɒŋɡɨtjuːd/), identified by the Greek letter lambda (λ), is the geographic coordinate most commonly used in cartography and global dependence arises from the fact that the geomagnetic dipole Dipoles can be characterized by their dipole moment, a vector quantity. For the simple electric dipole given above, the electric dipole moment points from the negative charge towards the positive charge, and has a magnitude equal to the strength of each charge times the separation between the charges. For the current loop, the magnetic dipole axis is not parallel to the Earth's rotation axis.

This modulation which describes the change in the interstellar intensities of cosmic rays as they propagate in the heliosphere is highly energy and spatial dependent, and it is described by the Parker's Transport Equation in the heliosphere. At large radial distances, far from the Sun (~94 AU), there exists the region where the solar wind undergoes a transition from supersonic to subsonic speeds called the "solar wind termination shock". The region between the termination shock and the heliopause (the boundary marking the end of the heliosphere) is called the heliosheath. This region acts as a barrier to cosmic rays, decreasing their intensity at lower energies by about 90%; thus it is not only the Earth's magnetic field that protects us from cosmic ray bombardment.

From a scientific modeling point of view, there is a challenge in determining the Local Interstellar Spectra (LIS) due to large adiabatic energy changes these particles experience owing to the diverging solar wind in the heliosphere. However, significant progress has been made in the field of cosmic ray studies with the development of an improved state-of-the-art 2D numerical model that includes the simulation of the solar wind termination shock, drifts and the heliosheath coupled with fresh descriptions of the diffusion tensor, see Langner et al. (2004). But challenges also exist because the structure of the solar wind and the turbulent magnetic field in the heliosheath is not well understood indicating the heliosheath as the region unknown beyond. With lack of knowledge of the diffusion coefficient perpendicular to the magnetic field our knowledge of the heliosphere and from the modelling point of view is far from complete. There exist promising theories like ab initio method approaches, but the drawback is that such theories produce poor compatibility with observations (Minnie, 2006) indicating their failure in describing the mechanisms influencing the cosmic rays in the heliosphere.

Detection

The nuclei that make up cosmic rays are able to travel from their distant sources to the Earth because of the low density of matter in space. Nuclei interact strongly with other matter, so when the cosmic rays approach Earth they begin to collide with the nuclei of atmospheric gases. These collisions, in a process known as a shower, result in the production of many pions and kaons, unstable mesons which quickly decay into muons.

Because muons do not interact strongly with the atmosphere, and because of the relativistic effect of time dilation in the Earth's reference frame (alternately, length contraction in the muon's reference frame) many of these muons are able to reach the surface of the Earth and even penetrate for some distance into shallow mines. Muons are ionizing radiation, and may easily be detected by many types of particle detectors such as cloud chambers or bubble chambers or scintillation detectors. If several muons are observed by separated detectors at the same instant it is clear that they must have been produced in the same shower event.

Cosmic rays impacting other (non-Earth) bodies in the Solar System which are made of elements heavier than hydrogen and helium, can be detected indirectly by observing high energy gamma ray emissions from these bodies using a gamma-ray telescope (see image at right). When such gammas are of energy too high to result from radioactive decay processes (> about 10 MeV) they must be secondary to cosmic ray bombardment.

Detection by particle track-etch technique

Cosmic rays can also be detected directly when they pass through particle detectors flown aboard satellites or in high altitude balloons. In a pioneering technique developed by Robert Fleischer, P. Buford Price, and Robert M. Walker,^[5] sheets of clear plastic such as 1/4 mil Lexan polycarbonate can be stacked together and exposed directly to cosmic rays in space or high altitude. When returned to the laboratory, the plastic sheets are "etched" [literally, slowly dissolved] in warm caustic sodium hydroxide solution, which removes the surface material at a slow, known rate. Wherever a bare cosmic ray nucleus passes through the detector, the nuclear charge causes chemical bond breaking in the plastic. The slower the particle, the more extensive is the bond-breaking along the path; and the higher the charge (the higher the Z), the more extensive is the bond-breaking along the path. The caustic sodium hydroxide dissolves at a faster rate along the path of the damage, but thereafter dissolves at the slower base-rate along the surface of the minute hole that was drilled. The net result is a conical shaped pit in the plastic; typically with two pits per sheet (one originating from each side of the plastic). The etch pits can be measured under a high power microscope (typically 1600X oil-immersion), and the etch rate plotted as a function of the depth in the stack of plastic. At the top of the stack, the ionization damage is less due to the higher speed. As the speed decreases due to deceleration in the stack, the ionization damage increases along the path. This generates a unique curve for each atomic nucleus of Z from 1 to 92, allowing identification of both the charge and energy (speed) of the particle that traverses the stack. This technique has been used with great success for detecting not only cosmic rays, but fission product nuclei for neutron detectors.

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