The Earth is subjected to cosmic bombardment … No, this is not a quote from a fantastic action movie, but the reality – our planet is constantly “fired” by the streams of charged particles from deep space.
In collisions with atoms of gases in the air, cosmic particles trigger branching chains of nuclear reactions that produce a variety of secondary products. The proton, which has entered the atmosphere with energy of tens and hundreds TeV, gives rise to other high-energy particles that are scattered on surrounding atoms and cause the next generations of particles to live. As a result, cascade production of particles occurs in the air basin, many of which are unstable and decay rapidly. This is how many-particle atmospheric showers occur, which Dmitrii Skobeltsyn observed for the first time in the late 1920s.
From a drop to showers
The area of rainfall and the total number of its “drops” increase sharply as the energy of the primary particle increases. A proton with an energy of the order of 1015 eV gives rise to about a million secondary particles, 1016 eV to ten million, and 1020 eV to several billion. Cascade processes of this magnitude, called wide atmospheric showers, were first observed in 1938 by the French physicist Pierre Auger. His name has been operating since 2005, a large international observatory of cosmic rays, located in the west of Argentina.
Registration of heavy rains is not an easy task. On the square kilometer of the upper boundary of the atmosphere, on average one particle with an energy of 1019 eV falls annually, while a particle with an energy of 1020 eV crosses the same area much less frequently than once in a century. Therefore, for the detection of showers generated by such particles, installations of giant sizes are constructed. Thus, the main complex of the Pierre Auger Observatory consists of 1600 cisterns with ultrapure water and Cerenkov radiation sensors scattered over an area of 3000 km².
For the formation of the shower, there are two types of processes: hadronic and electromagnetic processes. The primary proton collides with the atomic nucleus and breaks it into fragments. If its energy does not exceed several hundred MeV, this all ends, but protons with energies of tens and hundreds of GeV cause far more serious consequences. After the first collision, such a proton continues to move with less energy (about 30% of the original). The entrance of this meeting, as a rule, is charged with charged and neutral pions, but more massive particles may also appear. A charged pion either collides with the nucleus of another atom and gives rise to new nuclear processes, or does not have time to do so and breaks up into a muon of the same sign and the muon neutrino (there is another decay channel, but its probability is very small). The muon, whose lifetime is measured by a pair of microseconds, moves almost at the speed of light and interacts very weakly with atomic nuclei, slightly losing energy only when passing through their electronic shells. Therefore, it has excellent chances to reach the earth’s surface and even penetrate deep into the earth.
In the end, muons also decay, and almost always an electron or positron (depending on their sign) and a pair of neutrinos, muon and electron. Neutral peony, which lives about a hundred million times less than the charged one, is likely to not collide with anything and will turn into an atmosphere of a couple of gamma-ray photons. They are scattered by atoms and produce electron-positron pairs, and positrons are rapidly annihilated, giving rise to new gamma quanta. Thus, an electromagnetic storm cascade is triggered, leading to the birth of a soft component of cosmic radiation. At the same time, the primary proton, even if it gave part of the energy, as well as pions and other unstable particles that do not have time to decay, continue to collide with atomic nuclei, giving rise to all the new interacting particles of the hadronic cascade. In the course of all these transformations, not only pions appear, but also other hadrons, such as kaons and hyperons.
The atmosphere under fire
The cosmic rays quite really affect the earth’s atmosphere. If the protons simply break the nuclei that they have found, then their more massive partners may themselves fracture (for example, the core of magnesium that has flown from space can split into six alpha particles). Two such reactions deserve special mention. Among the secondary products cosmic rays produce neutrons, some of them are so slowed down by collisions with air atoms that merge with the nuclei of atmospheric nitrogen. In this way, nuclei of unstable carbon isotope 14C with a half-life of 5730 years arise at a 15-km altitude. Combining with oxygen, it forms a radioactive carbon dioxide 14CO2, which, along with ordinary carbon dioxide, is absorbed by plants and participates in the processes of photosynthesis. This circumstance underlies the method of radiocarbon dating, which is widely used in paleontology and archeology. With the help of carbon-14 and the much more long-lived radioactive isotope of beryllium, 10Be of cosmic origin, one can even reconstruct the history of the oscillations of the intensity of cosmic rays themselves to a depth of up to 200,000 years (this trend is called experimental paleoastronomy).
The composition of “Rain”
According to data published by NASA in 2010, the flux of space charged particles is 98% composed of baryons and only 2% of stable leptons (electrons and positrons). The baryon component, in turn, contains protons (87%), alpha particles (12%), and nuclei of elements heavier than helium, which astronomers call metals (1%). Among them, the first place is occupied by carbon, nitrogen and oxygen, followed by lithium, beryllium and boron. This six is responsible for about 90% of space “metals”, so that all others remain very few. Approximately four-fifths of the remaining particles are represented by elements with atomic numbers from 9 to 25, lying in the periodic table between oxygen and iron. Almost all of the residue was captured by iron, to which nickel and cobalt adjoin. The total proportion of elements heavier than cobalt is measured in hundred-thousandths of a percent. But they still occur – so, in the primary cosmic rays, the nuclei of gold, mercury, platinum, lead, and even uranium have been discovered. On the other hand, there are no radioactive elements with a short lifespan.
Atmospheric showers can trigger and ultrarelativistic electrons coming from space. However, they fall out infrequently, since the density of such electrons is very small. In space, they arise in abundance, but they quickly decelerate, dissipating on photons and emitting electromagnetic waves as they pass through magnetic fields. Therefore, electrons with energies of the order of 1000 GeV come to Earth only from rather close sources, the distance to which does not exceed 3000 light years. Cosmic protons of high energies cover immeasurably large distances.
The energy density of primary cosmic rays in the vicinity of the Sun is approximately equal to 1 eV / cm3. The energy makeup they provide to our planet is very stable and approximately equal to 100 MW. This value is two billion times less than the energy of the sun’s rays, but comparable to the energy of the incident starlight. True, cosmic rays, unlike stars, do not inspire poets – they are invisible.
Mystery of origin
The pedigree of almost all cosmic particles is established quite reliably. In 1934, American astronomers Fritz Zwicky and Walter Baade assumed that their source could be supernova explosions. In the 1950s, this hypothesis was strongly strengthened and since then it has been generally accepted.
Nevertheless, she immediately meets an obvious objection. It is natural to assume that the lion’s share of cosmic rays is born in our Galaxy. However, stars, including supernovae, are concentrated in the equatorial plane of the Milky Way (more precisely, in the spiral arms lying there), while the rays come to Earth from all directions. The fact is that protons and other charged particles move in space is not at all rectilinear. Their paths are repeatedly bent by the galactic magnetic field and by collisions with atoms and molecules scattered in interstellar space. The situation is complicated by the fact that particles of cosmic rays create their own magnetic fields, which are superimposed on the general field of the Galaxy and deform its structure. So the movement of particles from sources to the Earth is very confusing, and for its modeling over the past decades, very complex computer codes have been created.
Cosmic rays have been studied and studied using detectors installed in ground and underground observatories, on airplanes, balloons and space vehicles. Few people know that one such observatory for 10 years operated in the bombproof after the Second World War air-raid shelters at the Moscow metro stations Kropotkinskaya and Park Kultury. As the professor-consultant of the Physics Department of the Moscow State University, Irina Vakaslavovna Rakobolskaya, told us, there were installed 144 multilayer chambers in the late 1960s that registered muons generated by primary nucleons with energy up to 1015-1016 eV. The muons left traces on piles of double-sided X-ray films with a total area of 4,000 square meters, interlaid with lead plates. Moscow physicists received very interesting results, which allowed to correct mistakes made by their American counterparts.
Will the supernova have enough energy to produce cosmic rays? As already mentioned, the density of their energy near the Sun is 1 eV / cm3; the average density throughout the galactic disk may be greater, but most likely it does not exceed 2 eV / cm3. Since the volume of the disk is equal to 1067 cm³, the total maximum energy of cosmic rays is 2х1067 eV, or 6×1055 erg. The average lifetime of the wandering particles of cosmic radiation in our Galaxy is estimated at 15 million years, or 5.4 × 1014 s. The quotient of these values, equal to 6×1040 erg / s, is equal to the average energy, which is spent every second to maintain a stable density of cosmic radiations. On the other hand, supernovas explode in our Galaxy at least every 50 years, or 1.5x109s, and each explosion emits particles with an average total energy of 1050 erg. So every second generation of energy is at least 6×1040 ergs – as much as required. No matter how rough this estimate is, it works on the hypothesis of Zwicky and Baade.
The energy of cosmic protons, which reach the vicinity of our planet, varies from 108 to 1020 eV. It is believed that almost all of them, except for very rare particles at the upper boundary of this interval, are accelerated by shock waves that accompany explosions of intragalactic supernovae. Such an explosion throws out the substance of the outer shell of the dying star into space with speeds of up to ten percent of the speed of light. This is much more than the speed of sound in the interstellar medium, which leads to the appearance of shock waves. In this case, chaotic magnetic fields are generated, which force the protons to jump many times between the shock wave fronts and the substance of the interstellar medium that has not yet been compressed. At each hop the proton increases the kinetic energy due to the energy of the shock wave.
Protons, which undergo the maximum number of transitions, gain the highest energy, but they remain numerically in the minority. As a result, a supernova explosion ejects into a cosmos the nuclei of hydrogen with energy up to 1012 eV, but in much smaller quantities generates particles with high energies. “This mechanism explains the acceleration of protons and compound nuclei to an energy of the order of 1016 eV,” says Angela Olinto, professor of astronomy and astrophysics at the University of Chicago. – It is possible that explosions of the most massive collapsing stars accelerate protons even up to 1018 eV. Possible sources of protons with high energies within the Milky Way have not yet been found, so they almost certainly come from other galaxies. ”
Explosions of supernovae are generated also by superfast electrons with positrons. However, these particles are easily inhibited and scattered in the interstellar medium and, for the most part, do not reach the Earth (and positrons also annihilate). Therefore, their fraction in primary cosmic rays is small, and the energies are not too great.
Now these particles are studied only in three places – the Auger Observatory, the Telescope Array complex in Utah since 2007 and the Russian EAS installation in the Oktemtsi settlement south of Yakutsk (the only one with muon detectors). The origin of these particles is still unknown; there is not even complete certainty that they are all protons, alpha particles or metal nuclei. By the most common version, they are born in active nuclei of galaxies. But there are other explanations that connect them with gamma bursts, accretion processes near strongly magnetized neutron stars, the fusion of black holes and even the decay of hypothetical massive particles of dark matter or the disintegration of even more hypothetical topological space defects inherited from the Big Bang era.
But no matter how protons arise with energies of hundreds of EEV, their sources are not too far from our Galaxy – at least not at cosmological distances. Traveling in space, they interact with quanta of microwave background radiation, whose density is approximately 400 photons per 1 cm³. These collisions lead to the birth of pions, both positively charged and neutral. A charged pion appears together with a neutron, after which the two particles decay – the first very quickly, the second in minutes. A neutral pion, which decays even faster, appears together with a proton, whose energy is noticeably inferior to the energy of the parental particle (the same applies to protons that were born as a result of neutron decay). As a result, at distances over 50 megaparsecs from the source (160 million light-years) there are no protons with energies above 50 EeV. This effect in the mid-1960s was predicted by Professor Cornell University Kenneth Greisen and then-staff members of the FIAN Georgy Zatsepin and Vadim Kuzmin.
Follow the trail
Ultrarelativistic baryons are very weakly deflected by intergalactic magnetic fields, so that their trajectories roughly indicate the direction to the source. Astronomers are trying to get out this way on the sources themselves, however, according to Professor Olinto, without much success. To facilitate the solution of this problem, it is necessary to register more particles of ultrahigh energies. This is aimed at the international project JEM-EUSO (Japanese Experiment Module – Extreme Universe Space Observatory), which involves the installation in 2016 in the Japanese module of the International Space Station unique wide-angle telescope. This apparatus will track ultraviolet photons that arise in atmospheric showers, generated by particles with energies of tens and hundreds of EeV. Since the orbiting telescope will have a wider field of view than ground installations, it will be able to catch a lot more particles.
For several years, Russian scientists have been involved in the preparation of the JEM-EUSO project. “Within the framework of this program we have designed devices for scientific mini-satellites Tatiana-1 and Tatiana-2, and we hope to launch a much heavier Lomonosov satellite next year,” says Mikhail Skobeltsyn Institute of Nuclear Physics at the Moscow State University Panasyuk. – One of the goals of these launches is to develop methods for separating ultraviolet flares from cosmic rays on the general background of ultraviolet glow of the atmosphere. This is a very difficult task, and information from satellites will help solve it. We are also engaged in modeling atmospheric processes that are relevant to the operation of the telescope and its mechanical systems: the telescope will be brought into orbit in a folded state, after which it will be brought into operation. Unfortunately, so far the fate of this experiment is not clear, because in September last year NASA refused to participate in the project. Because of this, Japan has not yet made a final decision to launch the telescope, although this experiment is actively supported and subsidized by the European Space Agency. ”
Towards a new physics
In recent years, cosmic rays have again entered the sphere of fundamental physics. “Particles of low energies that do not exceed 1012 eV are very numerous, they are easily detected by ground, air and space based instruments. This is also done by the PAMELA detector complex installed on the Russian satellite Resurs-DK1, launched in June 2006, explains Sergei Troitsky, a leading researcher at the Institute of Nuclear Physics of the Russian Academy of Sciences, to Popular Mechanics. – Instruments recorded an excess of positrons of certain energies, which is rather difficult to explain. There are suspicions that “superfluous” positrons arise when annihilation of particles of dark matter that have not yet been discovered. If these suspicions are confirmed, there will be chances to extract information about its properties from observations of cosmic radiations.
The second possibility consists in using the most energetic cosmic particles as a kind of complement to the Geneva Large Hadron Collider. The consequences of collisions of these particles with air atoms depend on their energy in the reference system tied to the center of mass of the “atom-particle” pair. It is much smaller than their energies of the order of hundreds of EeV in the laboratory reference frame, but it is still ten times larger than the corresponding energy achievable in experiments at the LHC. If you register in detail the different components of a broad shower, you can get information about the processes immediately following the first collision of the “parent” particle.
There is also a more exotic search line. Some data indicate that 2-3% of particles with energies of the order of 10 EeV arrive from lacertides, powerful sources of electromagnetic radiation in the nuclei of some galaxies. Near the Milky Way, they simply do not exist, they are at least a hundred and fifty megaparsec away from us. However, the point is that none of the known neutral particles can fly such a distance. Protons and atomic nuclei are capable of this, however, they would deviate in intergalactic magnetic fields by far greater angles from the directions to the assumed lacertic sources than the observations show. So the question arises: is not there some new physics here? ”