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planetary circulation

sub-layer, tens of meters thick; and the Ekman layer which can reach to about 1 to 1.5 km altitude.

planetary circulation The circulation systems and circulation cells with 8,000 to 10,000 km horizontal scale. Planetary circulation includes the planetary wave, high and low pressure centers, mean meridional circulations, and mean zonal circulations as well as planetary scale monsoon circulations. The variation in planetary circulations is very slow with more than 1 to 2 week time scales. Thus the planetary circulations are the basic factors that determine mediumand long-term weather evolutions. They are also the background circulation fields for short-term weather evolutions. In general, planetary circulations are generated by forcing effects from large-scale orography, thermal effects of heat sources and sinks.

planetary magnetic fields Magnetic fields believed to be generated within the deep interior (inner or outer core) by the rotation of the planet creating currents in molten material that conducts electricity. These electrical currents cause a magnetic field to form and the boundaries of that field extend far beyond the surface of the planet. A simple model of the field can be represented by a dipole (having North and South magnetic poles like a bar magnet) and is represented by field lines that originate at the North magnetic pole and terminate at the South magnetic pole. Planetary magnetic fields vary in size and shape because of the different properties within the planets generating the fields, and because of external influences like the sun’s magnetic field and the solar wind. Earth’s field is generally dipole-like until about 70,000 km from the planet. There the solar wind causes the Earth’s field to be swept away from the sun, creating a teardrop shaped cavity where the influence of Earth’s field dominates on the inside of the teardrop and the sun dominates on the outside. The long tail of the teardrop results from Earth’s magnetic field lines being pushed downwind of the sun similar to the wake behind a rock in a stream.

planetary magnetosphere The region of space surrounding a planet where the influence

of the planetary magnetic field dominates over any external fields. The magnetic field originates within the body of the planet and the field extends outward until the magnetopause boundary. This boundary is the point where the intensity of the planetary magnetic field equals that of some other external field (usually the sun). The upper boundary can also be explained as the point where the most distant field lines are closed and are connected back to the planet. The lower boundary is the point where the neutral atmosphere dominates over the ionized (charged) region in space. For Earth, the magnetosphere in the direction toward the sun begins at a distance of about 50,000 km above the surface of the planet and ends at approximately 70,000 km. For the midnight side (pointing away from the sun), the magnetosphere can stretch millions of kilometers from the Earth due to the long magnetotail formed by the solar wind.

planetary nebula The ejected outer envelope of a 0.8 to about 8 solar mass star that has completed hydrogen and helium burning, leaving a carbon-oxygen core. The envelope is ejected over a period of about 104 years, while the star is near the tip of the asymptotic giant branch. Planetary nebulae often show spectroscopic evidence of the helium, carbon, nitrogen, and other elements produced by nuclear reactions in the parent stars. They expand at a few ×10 km/sec. A few tens of thousands of years after the gas begins to glow, a “fast wind” from the remaining material is blown out at high velocity (300 km/s or more), and snowplows through the slower moving gas. It is the interaction of the “slow” and “fast” moving material that contributes to the extravagant shapes of planetary nebulae. The nebulae then fade as the gaseous envelope becomes diffuse and the central star cools and no longer provides much ionizing radiation. About one planetary nebula is born in the Milky Way per year, leading to an inventory of 104 nebulae, of which about 1000 have been cataloged. Their complex shapes are due to some combination of the effects of companions, rotation, and the collisions of the primary wind with material ejected earlier and later. The nuclei of planetary nebulae fade to white dwarfs. The term “planetary nebula” is a misnomer that originated with the discovery of

© 2001 by CRC Press LLC

plasma mantle

these faint, fuzzy objects. At first, astronomers thought they were planets like Uranus.

planetary radio astronomy The study of radio wavelength emissions (long wavelength, low energy) from the planets within our solar system. The radio astronomy spectrum consists of the wavelength range from approximately 1 mm to 300 km corresponding to a frequency range of 300 GHz to about 1 kHz. Planetary radio emissions result from plasma interactions within the planetary magnetosphere and can be driven by the solar wind or some other particle source such as a satellite of a planet (e.g., Io in Jupiter’s system). Due to the absorptive properties of Earth’s ionosphere and the relative strengths of the radio emissions from the planets, only certain frequencies can be monitored from ground-based radio antennas (Jupiter is the only planet with intense enough radio emission so that it is observable from ground-based antennas). The other frequency bands must be monitored via spacecraft.

planetary rotation periods The rotation periods of planets can be determined by using several different techniques: optical observations, RADAR measurements, and radio observations. Optical observations are appropriate for terrestrial planets when the surface can be seen (Mercury, Earth, Mars), but cloud penetrating radar is needed for Venus. Rotation period determinations are more complex for the gas giant planets since they do not have a “true” surface that is observable. Rotation period measurements can be made from cloudtop observations, but those were quickly shown to be variable. Observations of the radio emissions have proven to be the most reliable since the radio emission source regions are directly linked to the planetary magnetic field. The magnetic field is believed to be generated in the planetary interior; therefore, it rotates at the same rate as the interior (inner or outer core) of the planet. Radio rotation periods are the most accurate measurements of the rotation rate for Jupiter, Saturn, Uranus, and Neptune. Pluto’s rotation period has been determined from changes in albedo markings on the surface and also from its tiny moon Charon which is locked in a synchronous orbit. The current rotation periods of the planets are:

Mercury — 58.646 days

Venus — 243.019 days (retrograde) Earth — 23h 56.11m

Mars — 24h 37.44m

Jupiter — 9h 55.5m

Saturn — 10h 39.4m

Uranus — 17h 14.4m (retrograde) Neptune — 16h 6.5m

Pluto — 6.3872 days.

planetesimal A small planetary solar system object of size 1 to 100 km.

plankton Passively drifting or weakly swimming organisms.

plankton bloom An unusually high concentration of phytoplankton, usually producing a discoloration of the water body.

plasma An ionized gas containing ions and electrons whose behavior is controlled by electromagnetic forces among the constituent ions and electrons.

plasma frequency Characteristic frequency of electron plasma oscillations, fluctuations associated with deviations from charge neutrality. The plasma frequency in cgs units is ωp = √ 4πNee2/me , where Ne is the electron number density, e is the electron charge, and me is the electron mass. An analogous frequency for ions is sometimes defined. Transverse electromagnetic waves cannot propagate through a plasma at frequencies lower than the local plasma frequency; this fact can be used to infer an upper limit on plasma density. See critical frequency.

plasma mantle The boundary region separating the high latitude lobes of the Earth’s magnetic tail from the magnetosheath region outside it. It is believed to contain plasma which has passed through the reconnection region or the polar cusp. Many details of the plasma mantle are uncertain, mainly because of the scarcity of observational data, but it is widely believed that it widens gradually with increasing distance down the tail, until it fills the entire tail, contributing the flowing plasma observed in the tail 100 RE or more past Earth.

© 2001 by CRC Press LLC

plasma sheet boundary layer (PSBL)

plasma sheet boundary layer (PSBL) In the Earth’s magnetotail, the region between the lobe and the central plasma sheet. According to some theories, this region is linked to the “distant neutral line” at which lobe field lines reconnect and begin to return sunward, which might explain rapidly flowing streams of electrons and ions observed in it.

plasmasphere The region from about 250 km, the F-layer peak of the ionosphere or the upper atmospheric thermosphere, to about 1500 km is known as “topside” ionosphere. Above the topside ionosphere and roughly below about 4 to 6 earth radii is the region known as the plasmasphere. It is populated by thermal ions and electrons of energy around 1 ev. The plasmasphere like the ring current overlaps parts of the inner and outer radiation belts. The outer surface of the plasmasphere is called the plasmapause, where the electron density drops by a factor of 10 to 100 a few electrons within a distance of fraction of the Earth radius. Magnetic tubes inside the plasmasphere are significantly depleted during geomagnetic disturbances and are refilled during recovery periods. The plasmasphere does not corotate with the Earth, and the plasmapause moves closer to the Earth during geomagnetic disturbances. Beyond the plasmapause, the magnetic field lines are less dipole like and solar wind – magnetospheric interaction dominates the particles and the fields.

plasma stress tensor In plasma kinetic theory, the second moment P of the velocity distribution,

P = m (v − V) (v − V) f d3v ,

where f (v, x, t) is the velocity distribution, m is the mass of the particles whose statistical properties are described by the kinetic theory, v and x are the velocity and spatial coordinates, and t is the time. Also, V is the mean velocity. P is also referred to as the pressure tensor; in the literature the term “stress tensor” is sometimes used for −P. In the momentum equation for each charge species, the tensor divergence ·P replaces the pressure gradient of ordinary fluid mechanics.

For spatial and temporal variations of hydromagnetic scale in a collisionless plasma, the velocity distributions are gyrotropic, and the stress tensor is of the form

P = P 1 + P − P BB/B2 ,

where 1 is the unit tensor, and the two scalars P and P are the pressures transverse and parallel to the magnetic field B. The pressures may also be written in terms of the transverse and parallel kinetic temperatures T and T , defined by P = NkT and P = NkT , where N is the particle number density and k is Boltzmann’s constant. Note that in general the electrons and the various ion species that comprise a plasma have differing number densities and kinetic temperatures. See Vlasov equation.

plastic anisotropy A plastic property for a given crystal; when a crystal is subject to a plastic deformation its plastic strength (or plasticity) is predominantly controlled by crystallographic orientation, and changes substantially with respect to the orientation from which the crystal is deformed, this property is called plastic anisotropy.

plastic (permanent) deformation Deformation of a body that is not recovered upon unloading, as opposed to elastic (temporary) deformation. During plastic deformation, work done by the loading force is dissipated into heat; while during elastic deformation, the work is converted into strain energy.

plateau material A thin mantling of material over the southern lunar highland plains, the surface of which is often etched, and degraded by runoff channels. It contains a morphologically distinct crater type, which lacks a raised rim, evidence for an ejecta blanket, and in which the floors are leveled by the plateau material. The origin of the plateau material is uncertain. It may be volcanic or produced by erosional stripping, for example, by transient ice-rich deposits.

plate tectonics The theory and study of how the segments of the Earth’s lithosphere form, move, interact, and are destroyed. The concept was introduced by A. L. Wegener in 1912, and has now been validated by matching fossil

© 2001 by CRC Press LLC

pocket beach

types across (now separated) continental coastlines, by similarities of geology in such situations, and by direct measurement via laser satellite geodesy, very long baseline interferometry, and the Global Positioning System. The theory states that the Earth’s lithosphere is broken up into segments, or plates, which move atop the more fluid asthenosphere. Currently the Earth’s surface is composed of six major plates and nine smaller plates. The interaction of the plates along their boundaries produces most of the volcanic and tectonic activity seen on Earth. New crust is created and plates move away from each other at divergent boundaries (mid-ocean ridges). Plates collide and are destroyed or severely deformed at convergent boundaries, where one plate is subducted under the other (deep sea trenches) or the two plates are uplifted to form a mountain range. Thus, the Atlantic Ocean is growing, and the Pacific Ocean is shrinking. Plates slide past one another at transform boundaries. The driving mechanism for plate tectonics is believed to be convection cells operating in the Earth’s asthenosphere. It is understood that a giant hot plume ascends from the core-mantle boundary, whereas cold slabs drop through the mantle as a cold plume, producing descending flow. The surface expression of hot plumes are the volcanism of Hawaii, Iceland, and other hot spots not directly associated with plate tectonics.

The plates move with velocities of between 1 and 8 cm/yr. Earth appears to be the only body in the solar system where plate tectonics currently operates; remnant magnetism suggests plate tectonics may have been effective in the past on Mars.

platonic year

See year.

Pleiades Young star cluster, Messier number M45, which is a bright star forming region, and has numerous young hot (B-type) stars, of which seven are prominent; the Pleiades are the seven sisters, daughters of Atlas.

Pleione Variable type B8 star at RA 03h49m, dec +24◦07’; “Mother” of the “seven sisters” of the Pleiades.

Plimsoll’s mark After Samuel Plimsoll; the name given to the load marks painted on the sides of merchant ships indicating the legal limit of submergence. The British began the system around 1899, and the U.S. adopted a similar system in 1930. Load lines include FW (fresh water), S (summer), W (winter) WNA (winter in the North Atlantic), and IS (Indian Summer) for the relatively calm period October to April in the Indian Ocean.

Pluto The ninth planet from the sun. Named

after the Roman god of the underworld, Pluto has a mass of M = 1.29 × 1025 g, and a radius

of R = 1150 km, giving it a mean density of 2.03 g cm−3 and a surface gravity of 0.05 that of Earth. Its rotational period of 6.3867 days, and its rotation axis has an obliquity of 122.5◦. The slow rotation means that the planet’s oblateness should be very nearly 0, but its small size may allow significant deviations from sphericity. Pluto’s orbit around the sun is characterized by a mean distance of 39.44 AU, an eccentricity of e = 0.250, and an orbital inclination of i = 17.2◦. Its large eccentricity means that for part of its orbit it is closer to the sun than Neptune, most recently during the 1980s and 1990s. Its sidereal period is 247.69 years, and its synodic period is 367 days. An average albedo of 0.54 gives it an average surface temperature of about 50 K. Its atmosphere is very thin, and mostly CH4. Pluto is probably composed of a mixture of ices, organic material, and silicates. Because of its small size and icy composition, Pluto is more properly an inner member of the Kuiper belt, rather than a full fledged planet. Pluto has one satellite, Charon, which is nearly as massive as it is.

pluton A magma body (intrusion) that has solidified at depth in the Earth’s crust.

plutonic Indicative of igneous rock consisting of large crystals. This suggests slow crystalization, as would happen at depths beneath the Earth.

pocket beach A beach comprised of sediments that are essentially trapped in the longshore direction by headlands which block longshore sediment transport.

© 2001 by CRC Press LLC

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