Cosmic rays Electrons and the nuclei of atoms—largely hydrogen—that impinge upon Earth from all directions of space with nearly the speed of light. These nuclei with relativistic speeds are often referred to as primary cosmic rays, to distinguish them from the cascade of secondary particles generated by their impact against air nuclei at the top of the terrestrial atmosphere. The secondary particles shower down through the atmosphere and are found all the way to the ground and below.

The primary cosmic rays provide the only direct sample of matter from outside the solar system. Measurement of their composition can aid in understanding which aspects of the matter making up the solar system are typical of the Milky Way Galaxy as a whole and which may be so atypical as to yield specific clues to the origin of the solar system. Cosmic rays are electrically charged; hence they are deflected by the magnetic fields which are thought to exist throughout the Galaxy, and may be used as probes to determine the nature of these fields far from Earth. Outside the solar system the energy contained in the cosmic rays is comparable to that of the magnetic field, so the cosmic rays probably play a major role in determining the structure of the field.

Collisions between the cosmic rays and the nuclei of the atoms in the tenuous gas which permeates the Galaxy change the cosmic-ray composition in a measurable way and produce gamma rays which can be detected at Earth, giving information on the distribution of this gas.
Cosmic-ray detection. All cosmic-ray detectors are sensitive only to moving electrical charges. Neutral cosmic rays (neutrons, gamma rays, and neutrinos) are studied by observing the charged particles produced in the collision of the neutral primary with some type of target. At low energies the ionization of the matter through which they pass is the principal means of detection. A single measurement of the ionization produced by a particle is usually not sufficient both to identify the particle and to determine its energy. However, since the ionization itself represents a significant energy loss to a low-energy particle, it is possible to design systems of detectors which trace the rate at which the particle slows down and thus to obtain unique identification and energy measurement.

At energies above about 500 MeV per nucleon, almost all cosmic rays will suffer a catastrophic nuclear interaction before they slow appreciably. An ionization measurement is commonly combined with measurements of physical effects which vary in a different way with mass, charge, and energy. Cerenkov detectors and the deflection of the particles in the field of large superconducting magnets or the magnetic field of the Earth itself provide the best means of studying energies up to a few hundred GeV per nucleon. Detectors employing the phenomenon of x-ray transition radiation promise to be useful for measuring composition at energies up to a few thousand GeV per nucleon.
Above about 1012 eV, direct detection of individual particles is no longer possible since they are so rare. Such particles are studied by observing the large showers of secondaries they produce in Earth’s atmosphere. These showers are detected either by counting the particles which survive to strike ground-level detectors or by looking at the flashes of light the showers produce in the atmosphere with special telescopes and photomultiplier tubes.

Atmospheric cosmic rays. The primary cosmic-ray particles coming into the top of the terrestrial atmosphere make inelastic collisions with nuclei in the atmosphere. When a high-energy nucleus collides with the nucleus of an air atom, a number of things usually occur. Rapid deceleration of the incoming nucleus leads to production of pions with positive, negative, or neutral charge. A few protons and neutrons (in about equal proportions) may be knocked out with energies up to a few GeV. They are called knock-on protons and neutrons.
All these protons, neutrons, and pions generated by collision of the primary cosmic-ray nuclei with the nuclei of air atoms are the first stage in the development of the secondary cosmic-ray particles observed inside the atmosphere. Since several secondary particles are produced by each collision, the total number of energetic particles of cosmic-ray origin will increase with depth, even while the primary density is decreasing.

The uncharged n0 mesons decay into two gamma rays with a life of about 8 x 10-17 s. The two gamma rays each produce a positron-electron pair. Upon passing sufficiently close to the nucleus of an air atom deeper in the atmosphere, the electrons and positrons convert their energy into bremsstrahlung. The bremsstrahlung in turn create new positron-electron pairs, and so on. This cascade process continues until the energy of the initial n0 has been dispersed into a shower of positrons, electrons, and photons with insufficient individual energies (< 1 MeV) to continue the pair production. The electrons and photons of such showers are referred to as the soft component of the atmospheric (secondary) cosmic rays.

The n ± mesons produced by the primary collisions have a life of about 2.6 x 10-8 s before they decay into muons. Most low-energy n ± decay into muons before they have time to undergo nuclear interactions. Except at very high energy (above 500 GeV), muons interact relatively weakly with nuclei, and are too massive (207 electron masses) to produce bremsstrahlung. They lose energy mainly by the comparatively feeble process of ionizing an occasional air atom as they progress downward through the atmosphere. Because of this ability to penetrate matter, they are called the hard component.

The high-energy nucleons—the knock-on protons and neutrons—produced by the primary-particle collisions and a few pion collisions proceed down into the atmosphere. They produce nuclear interactions of the same kind as the primary nuclei, though of course with diminished energies. This cascade process constitutes the nucleonic component of the secondary cosmic rays.
Solar modulation. The cosmic-ray intensity is lower during the years of high solar activity and sunspot number, which follow an 11-year cycle. This effect has been extensively studied with ground-based and spacecraft instruments.

The primary cause of solar modulation is the solar wind, a highly ionized gas (plasma) which boils off the solar corona and propagates radially from the Sun at a velocity of about 250 mi s (400 km/s). The wind is mostly hydrogen, with typical density of 80 protons per cubic inch (5 protons per cubic centimeter). This density is too low for collisions with cosmic rays to be important. Rather, the high conductivity of the medium traps part of the solar magnetic field and carries it outward.

In addition to the bulk sweeping action, another effect of great importance occurs in the solar wind, adiabatic deceleration. Because the wind is blowing out, only those particles which chance to move upstream fast enough are able to reach Earth. However, because of the expansion of the wind, particles interacting with it lose energy. Thus, particles observed at Earth with energy of 10 MeV per nucleon actually started out with several hundred MeV per nucleon in nearby interstellar space, and those with initial energy of only 100-200 MeV per nucleon probably never reach Earth at all.

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The dimension of the physical universe that orders the sequence of events at a given place; also, a designated instant in this sequence, such as the time of day, technically known as an epoch, or sometimes as an instant.

Measurement.

Time measurement consists of counting the repetitions of any recurring phenomenon and possibly subdividing the interval between repetitions. Two aspects to be considered in the measurement of time are frequency, or the rate at which the recurring phenomena occur, and epoch, or the designation to be applied to each instant.
A determination of time is equivalent to the establishment of an epoch or the correction that should be applied to the reading of a clock at a specified epoch. A time interval may be measured as the duration between two known epochs or by counting from an arbitrary starting point, as is done with a stopwatch. Time units are the intervals between successive recurrences of phenomena, such as the period of rotation of the Earth or a specified number of periods of radiation derived from an atomic energy-level transition. other units are arbitrary multiples and subdivisions of these intervals, such as the hour being 1/24 of a day, and the minute being 1/60 of an hour.

Bases.

Several phenomena are used as bases with which to determine time. The phenomenon traditionally used has been the rotation of the Earth, where the counting is by days. Days are measured by observing the meridian passages of the Sun or stars and are subdivided with the aid of precision clocks. The day, however, is subject to variations in duration because of the variable rotation rate of the Earth. Thus, when a more uniform time scale is required, other bases for time must be used.

Sidereal time.

The angle measured along the celestial equator between the observer’s local meridian and the vernal equinox is the measure of sidereal time. In practice, a conventionally adopted mathematical expression provides this time as a function of civil time. It is reckoned from 0 to 24 hours, each hour being subdivided into 60 sidereal minutes and the minutes into 60 sidereal seconds. Sidereal clocks are used for convenience in many astronomical observatories because a star or other object outside the solar system comes to the same place in the sky at virtually the same sidereal time.

Solar time.

The angle measured along the celestial equator between the observer’s local meridian and the Sun is the apparent solar time. The only true indicator of local apparent solar time is a sundial. Mean solar time has been devised to eliminate the irregularities in apparent solar time that arise from the inclination of the Earth’s orbit to the plane of the Sun’s motion and the varying speed of the Earth in its orbit. In practice it is defined by a conventionally adopted mathematical expression. Intervals of sidereal time can be converted into intervals of mean solar time by dividing by 1.002 737 909 35. Both sidereal and solar time depend on the rotation of the Earth for their time base.

Universal Time (UT).

Historically, the mean solar time determined for the meridian of 0° longitude using astronomical observations was referred to as UT1. Currently UT1 is used only as an angle expressed in time units that depends on the Earth’s rotation with respect to the celestial reference system. It is defined by a conventional mathematical expression and continuing astronomical observations. These are made at a number of observatories around the world. The International Earth Rotation and Reference System Service (IERS) receives these data and provides daily values of the difference between UT1 and civil time.
Because the Earth has a nonuniform rate of rotation and a uniform time scale is required for many timing applications, a different definition of a second was adopted in 1967. The international agreement calls for the second to be defined as 9,192,631,770 periods of the radiation derived from an energy-level transition in the cesium atom. This second is referred to as the international or SI (International System) second and is independent of astronomical observations. International Atomic Time (TAI) is maintained by the International Bureau of Weights and Measures (BIPM) from data contributed by time-keeping laboratories around the world.
Coordinated Universal Time (UTC) uses the SI second as its time base. However, the designation of the epoch may be changed at certain times so that UTC does not differ from UT1 by more than 0.9 s. UTC forms the basis for civil time in most countries and may sometimes be referred to unofficially as Greenwich Mean Time. The adjustments to UTC to bring this time scale into closer accord with UT1 consist of the insertion or deletion of integral seconds. These “leap seconds” may be applied preferably at 23 h 59 m 59 s of June 30 or December 31 of each year according to decisions made by the IERS. UTC differs from TAI by an integral number of atomic seconds.

Dynamical time.

Dynamical time is based on the apparent orbital motion of the Sun, Moon, and planets. It is the time inferred in the ephemerides of the positions of these objects, and from its inception in 1952 until 1984 was referred to as Ephemeris Time. Barycentric Dynamical Time (TDB) refers to ephemerides that have been computed by using the barycenter of the solar system as a reference. Terrestrial Dynamical Time (TDT) is the practical realization of dynamical time and is defined as being equal to TAI + 32.184 seconds. In 1991, the International Astronomical Union recommended that TDT be renamed Terrestrial Time (TT), that Geocentric Coordinate Time (TCG) be the time coordinate for the geocenter, and that Barycentric Coordinate Time (TCB) be the time coordinate for the barycenter of the solar system. These times are related by the appropriate relativistic transformations.

Civil and standard times.

Because rotational time scales are local angular measures, at any instant they vary from place to place on the Earth. When the mean solar time is 12 noon at Greenwich, the mean solar time for all places west of Greenwich is earlier than noon and for all places east of Greenwich later than noon, the difference being 1 hour for each 15° of longitude. Thus, at the same instant at short distances east of the 180th meridian the mean solar time is 12:01 A.M., and at a short distance west of the same meridian it is 11:59 P.M. of the same day. Thus persons traveling westward around the Earth must advance their time 1 day, and those traveling eastward must retard their time 1 day in order to be in agreement with their neighbors when they return home. The International Date Line is the name given to a line where the change of date is made. It follows approximately the 180th meridian but avoids inhabited land. To avoid the inconvenience of the continuous change of mean solar time with longitude, zone time or civil time is generally used. The Earth is divided into 24 time zones, each approximately 15° wide and centered on standard longitudes of 0°,15°,30°, and so on (see illustration). Within each of these zones the time kept is related to the mean solar time of the standard meridian.
Zone time is reckoned from 0 to 24 hours for most official purposes, the time in hours and minutes being expressed by a four-figure group followed by the zone designation. For example, “1009 zone plus five” refers to the zone 75° west of Greenwich, where zone time must be increased by 5 hours to obtain UTC. The various zones are sometimes designated by letters, especially the Greenwich zone which is Z, “1509 Z” meaning 1509 UTC. The zone centered on the 180th meridian is divided into two parts, the one east ofthe date line being designated plus 12 and the other minus 12. The time July 2,2400 is identical with July 3,0000.
In civil life the designations A.M. and PPM. are often used, usually with punctuation between hours and minutes. Thus 1009 may be written as 10:09 A.M. and 1509 as 3:09 P.M. The designations for noon and midnight, however, are often confused, and it is better to write 12:00 noon and July 2-3, 12:00 midnight, in order to avoid ambiguity. In some occupations where time is of special importance, there is a rule against using 12:00 at all, 11:59 or 12:01 being substituted. The time 1 minute after midnight is 12:01 A.M. and 1 minute after noon is 12:01 PPM.
The illustration shows the designations ofthe various time zones, the longitudes ofthe standard meridians, and the letter designations and the times in the various zones when it is noon at Greenwich. In the United States the boundaries of the time zones are fixed by the Department of Transportation. Frequently the actual boundaries depart considerably from the meridians exactly midway between the standard meridians. Ships at sea and transoceanic planes usually use UTC for navigation and communication, but for regulating daily activities onboard they use any convenient approximation to zone time, avoiding frequent changes during daylight hours.
Many countries, including the United States, advance their time 1 hour, particularly during the summer months, into “daylight saving time.” For example, 6 A.M. is redesignated as 7 A.M. Such a practice effectively transfers an hour of little-used early morning light to the evening.
Time scales are coordinated internationally by the BIPM. Most countries maintain local time standards to provide accurate time within their borders by radio, telephone, and TV services. These national time scales are often intercompared by using the Global Positioning System (GPS) or time signals transferred by artificial Earth satellites.

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