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mean free path

the force balance equation of the conducting fluid. In historical terms, this was in some ways an improvement on “kinematic dynamo” models which only solve for the magnetic field and not for the forces within the fluid, although the kinematic models did not have the axisymmetric simplification. The advent of high performance computing has allowed fully three-dimensional simulation of the dynamo process, which alleviates the need for mean field parameterization.

mean free path In normal spatial diffusion the average distance a particle travels between two subsequent collisions with particles from its surroundings. The specified type of interaction may be some particular types, such as collision, scattering, capture, or ionization. The total mean free path is the definition that specifies for all types of interaction. If we consider scattering processes where the direction of motion is changed only slowly by successive interactions, such as in pitch-angle scattering, the mean free path can also be interpreted as the average distance traveled by a particle before its direction of motion is reversed. In one-dimensional diffusion, the relation between the diffusion coefficient D and the mean free path λ is D = vλ/2, in three-dimensional diffusion it is D = vλ/3, (where v is the mean particle speed). In photonmatter interactions, the mean free path equals the inverse of the beam attenuation coefficient [m].

mean motion For an object in motion, n = 2π/P , where P is the period of the orbit, an average of the angular velocity of the orbit.

mean motion (short-period) resonances

The relationship between motions of two bodies orbiting a common central object where the mean motions n, that is, the average angular speeds of these bodies are in the ratio of whole numbers. For instance, the Neptune-Pluto 3:2 mean motion resonance means nN : nP = 3 : 2 = 1/167 yr : 1/250 yr. Thus, while Pluto makes two revolutions around the sun, Neptune makes three revolutions and the two planets pass one another once, which results in a perturbative force with a short period of the order of the synodic period 1/(nN − nP ) = 500 years. The

(i + N) : i mean motion resonances are called those of order N.

mean sea level The average level of sea by averaging height of the sea surface for all stages of the tide. It is obtained by using long period tidal height observational data. The longer the tidal data is, the more reliable and stable the value of the mean sea level will be. It is a widely used standard datum level in geodesy.

mean solar day The average length of the day over one year. Because the Earth’s orbit is an ellipse with the sun at one focus, noon-to- noon time varies over the year and is shortest in Northern hemisphere winter. This variation is described by the equation of time.

mean solar time Time based conceptually on the diurnal motion of a fictitious mean sun, under the assumption that the Earth’s rotational rate is constant, as opposed to apparent solar time, to which it is related by the equation of time.

mean water level The average water level at a site, with the averaging period taken to be long enough to remove wind waves. It may, however, include the influence of wave setup on water level. As reported in tide tables, the mean water level typically represents a 19-year average.

mechanics The study of forces and the resulting motions that arise from contact between bodies, or from interaction with conservative fields (e.g., gravitational or electromagnetic fields).

mechanics, Newtonian The fundamental laws on which classical dynamics is based. (1) A physical body will maintain a state of rest or uniform motion in a straight line unless forces act upon it. (2) The rate of change of momentum is proportional to the applied force, and it is in the same direction as the applied force. (3) For every force of one body acting on a second, there is an equal force of the second body acting on the first of the same magnitude and in opposite direction. Named after Isaac Newton (1642– 1727).

© 2001 by CRC Press LLC

merging, magnetic

median diameter, median grain size

The

median particle size in a soil sample. Generally reported in millimeters or φ-units.

Meiyu front A persistent surface front that separates cold air to the north and warm, humid air to the south over East Asia and northwestern Pacific in early summer. It slants slightly northeastward along which precipitating low pressure systems develop and travel eastward. It first forms in May in South China, progresses northward and covers the Yangtze (Changjiang) River, Korea and Japan in mid-June. In late July, it starts to move further northward and weakens substantially in intensity. Called Changma in Korea and Baiu in Japan, the Meiyu (Plum Rain in Chinese) is the single most important climate feature that controls the floods and droughts in East Asia.

membrane paradigm A viewpoint in black hole physics which regards the event horizon of a black hole to be a two-dimensional membrane made of a viscous charged electrically conducting fluid. The physics of a black hole are then governed by standard equations of fluid mechanics, electrodynamics, and thermodynamics. Although the membrane paradigm is not equivalent to the full mathematical theory of black holes, it is useful for intuitive understanding of complex astrophysical processes involving black holes.

Mercalli intensity scale An arbitrary scale, devised by Italian geologist Giuseppi Mercalli in 1902, to quantify the intensity of earthquake shaking at a given location, regardless of the magnitude of the earthquake. The scale ranges from I (detectable only by sensitive instruments) to XII (almost total destruction). It has been modified by different countries according to local conditions and has been largely replaced by the Richter scale.

Mercury Named after the Roman messen-

ger god, Mercury is the planet closest to the sun, having a mass of M = 3.3022 × 1026

g, and a radius of R = 2444 km, giving it a mean density of 5.43 g cm−3 and a surface gravity of 0.38 that of Earth. Its rotational period of 58.65 days is exactly 2/3 of its orbital pe-

riod around the sun, 87.97 days. The rotation axis has an obliquity of 0◦. The slow rotation means that the planet’s oblateness is very nearly 0. Mercury’s orbit around the sun is characterized by a mean distance of 0.39 AU, an eccentricity of e = 0.206, and an orbital inclination of i = 7.0◦. Its synodic period is 116 days. An average albedo of 0.11 gives it an average surface temperature of 620 K on the day side, and around 100 K on the night side. It does not have any significant atmosphere. Mercury is probably composed of a large Fe-rich core surrounded by a silicate mantle, but because its moment of inertia is not known, the relative sizes of the core and mantle can only be estimated from the mean density of the planet. Mercury has no known satellites. Less than half the surface of Mercury was mapped during the Mariner 10 flybys in the 1970s so relatively little is known about this planet.

merger The remnant of a collision between two galaxies, in which the stars of the two galaxies have been mixed together to form a single galaxy. In the process of merging, two galaxies behave like highly inelastic bodies that interpenetrate, and transfer part of their orbital energy to their stars. Collisions among galaxies of similar mass with orbital velocity comparable to their internal velocity dispersion (typically a few hundred km s−1) can produce mergers. If at least one of the merging galaxies is gasrich, the compression of the gas in the disk leads to extensive star formation which, in turn, temporarily enhances the galactic luminosity. The morphology of a late-stage merger is expected to be nearly indistinguishable from that of an elliptical galaxy. It is, however, unclear which fraction of elliptical galaxies has been formed by merging of disk galaxies. A prototypical merger is the galaxy NGC 7252, also known as the “Atoms for Peace” galaxy. NGC 7252 appears still highly disturbed in a deep image taken with a long exposure time, where tidal tails are visible, but resembles a regular elliptical galaxy in its central parts, with radial surface brightness profile following a de Vaucouleurs’ law, in an image taken with a shorter exposure. See de Vaucouleurs’ law, starburst galaxy.

merging, magnetic

See reconnection.

© 2001 by CRC Press LLC

Merian’s equation

Merian’s equation An equation for determination of the natural periods of a basin of water, such as a lake or harbor. The natural period is also referred to as the seiching period.

meridian A great circle passing through the poles of a sphere; in astronomy the meridian of an observer is the meridian passing through the zenith (i.e., directly overhead).

meridional In the direction of the meridians, the lines of constant longitude (i.e., northsouth). See also zonal.

meromixis Refers to a natural water body, which does not experience deep convection (such as due to cooling) on a regular basis (e.g., annual). Meromixis is mostly linked to permanent salinity gradients at greater depth than occurs in isolated ocean basins (such as in the Canada Basin) as well as in many deep lakes (such as Baikal, Tanganyika, Malawi, etc.), as well as in eutrophic lakes.

Merope Magnitude 4.2 type B5 star at RA 03h46m, dec +23◦56 ; one of the “seven sisters” of the Pleiades.

mesogranulation Intermediate scale of convection on the sun. Mesogranules are typically 5,000 to 10,000 km in size with speeds (vertical or horizontal) of 60 m s−1.

meson An unstable bosonic massive particle which undergoes strong nuclear interactions.

mesoscale A motion scale in the atmosphere smaller than a synoptic scale like the scale of a weather system, but larger than a microscale like the scale of an individual fair weather cumulus clouds. Mesoscale phenomena include the weather fronts, tornadoes, and storm and maintain waves. In the ocean, the scale of the Gulf Stream eddies or rings is considered a mesoscale.

mesosphere In atmospheric physics, the atmospheric layer which lies between the stratosphere and the thermosphere. It is bounded on the bottom by the stratopause and at the top by the mesopause. On Earth, the mesosphere lies

between 50 and 85 km altitude. The mesosphere is characterized by a decrease in temperature with altitude. The transport of radiation within the terrestrial mesosphere is primarily due to the absorption and emission of heat by carbon dioxide. The mesosphere and the stratosphere combined are often referred to as the middle atmosphere.

In solid earth geophysics, the lower portion of the upper mantle to the lower mantle underlying the asthenosphere. The Earth’s crust and mantle can be divided into three different portions, namely, lithosphere, asthenosphere, and mesosphere, in terms of their depth range and dynamic properties. The mesosphere has difficulty flowing compared to the asthenosphere. Since motion of the mesosphere is considered to be much slower than plate motions near the Earth’s surface, absolute plate motion is understood as relative motion to the mesosphere.

mesotrophic water Water with moderate concentrations of phytoplankton biomass; chlorophyll a concentration ranges from between approximately 0.5 to 10 mg m−3.

Messier The Messier catalog is a list of nebulous objects in the sky compiled by Charles Joseph Messier (1730–1817). Messier was a comet hunter and kept a list of fuzzy objects in the sky which were not comets, starting with the Crab Nebula. There are 103 objects in the catalog and each is indicated by the letter “M” followed by its number in the list. The catalog includes galaxies (e.g., M31, the Andromeda Galaxy), planetary nebulae (e.g., M42, the Great Nebula in Orion), and star clusters (e.g., M13, the Great Cluster in Hercules).

metallic hydrogen When hydrogen molecules are placed under sufficiently high pressure, the molecules dissociate and the resultant atomic phase has electrons that are free to move about the lattice, giving the solid metallic properties. The exact pressure of the transition depends on the temperature, but shock experiments indicate a transition pressure of 140 GPa at 3000 K.

metallicity The abundance, in stars or galaxies, of all elements heavier than hydrogen and

© 2001 by CRC Press LLC

meteor shower

helium (for which the early tracers were metals like Ca, Fe, and Na). The solar value is about 1.7%, or Z = 0.017. Observed values range from 10−4 of solar metallicity (for the oldest stars in the halos of our own and other galaxies) to perhaps 2 to 3 times solar (for stars in the centers of large elliptical galaxies and the ionized gas near the centers of quasars). Frequently metallicity is given as the log10 of the ratio of iron/hydrogen of the star divided by iron/hydrogen in the sun. This is abbreviated [Fe/H]. See elliptical galaxies, halo, quasar.

metamorphism If sedimentary rocks are buried in a sedimentary basin, the temperature can become sufficiently high so that chemical reactions can occur. These reactions can transform a sedimentary rock into a metamorphic rock.

meteor Any phenomenon in the air (a cloud is a humid meteor), but especially the glowing track of a meteoroid as it travels through the air.

Such meteors also are often called shooting stars or falling stars. Most of the debris creating meteors is extremely small (dust to sand-sized) and burns up during the atmospheric passage. If the material survives the passage through the atmosphere and lands on the surface, it is called a meteorite. A meteor shower occurs when large numbers of meteors are seen radiating from the same area of the sky.

meteorite Meteoroids that survive to impact onto a body’s surface. Meteorites are classified as stones, meteorites that are composed primarily of silicates and are similar to terrestrial rocks; stony-iron meteorites which have approximately equal amounts of iron and silicates; iron meteorites, composed almost entirely of nickel-iron; and further subdivided according to compositional details. Stony-irons are the least common. Most meteorites show thermal processing, and are believed to be fragments of larger parent bodies, probably asteroids, although Martian meteorites and lunar meteorites have also been identified. Meteorites are also often characterized as falls (seen to fall and recovered shortly thereafter) and finds, which are found long after they have landed. Stony meteorites are the most common types of falls and are thus believed to rep-

resent the majority of meteoritic material. Irons are the most common types of finds simply because they do not erode as quickly, nor are they easily confused with terrestrial rocks. See meteorite parent body, SNC meteorites, and under the different classes of meteorites.

meteorite parent body Different classes of meteorites have been differentiated to various degrees, and have been subjected to elevated temperatures and pressures. These processes presumably occurred while the meteorite was still part of a parent body. The breakup of such parent bodies resulted in the various meteortite classes observed. Particular asteroid classes have been identified with parent bodies of various meteorite types based on spectral similarities.

meteoroid A metallic or rocky small body orbiting the sun in space; small particles that may derive from comets; larger (1 cm or larger) particles that probably derive from the asteroid belt. Meteoroids seen passing through an atmosphere are called meteors, and if they survive to land on the surface, they are called meteorites. Small meteoroids are also often called interplanetary dust particles (IDPs). Sunlight reflecting off this material gives rise to the zodiacal light and the gegenschein.

meteor shower A large number of meteors radiating from the same area of the sky. Meteor showers are caused when the Earth passes through an area of space with an enhanced amount of small debris. Most meteor showers are associated with cometary debris left behind in the comet’s orbit. Meteors in a meteor shower appear to radiate from the same area of the sky since the material causing the shower is located in a relatively small region of space. The constellation encompassing the area of the sky from which the meteors appear to radiate provides the name of the meteor shower (e.g., the Perseids appear to radiate from the constellation Perseus, the Leonids come from the constellation Leo, etc.). Debris large enough to survive passage through the atmosphere and land on the surface as a meteorite is very rare during meteor showers.

© 2001 by CRC Press LLC

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