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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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point object

point object Astronomical source that has an angular diameter smaller than the resolving power of the instrument used to observe it.

point spread function (PSF) In an optical system, the apparent radiance due to an unresolved Lambertian (cosine-emitting) point source, as normalized to the source intensity in the direction of maximum emission [m−2]; numerically equal to the beam spread function. The PSF is the intensity distribution of a point source as it appears on the detector. The PSF is the result of any distorting factors induced by the full optical path including that of the instrument and environmental effects (e.g., atmospheric seeing).

Poiseuille’s law The mean velocity (U) for steady laminar ﬂow through a uniform pipe can be described as U = −(dp/dx)(D2/32µ), where D is the diameter of the pipe, µ is the viscosity of the ﬂuid, and dp/dx is the pressure decrease in the direction of ﬂow. Poiseuille’s law is derived from the Bernoulli equation, assuming that elevation along the pipe is constant. We can generalize the equation by allowing the pipe to be inclined at an angle and adding a term related to the elevation difference, giving

U = −	d		p	+ z	D2ρg
	ds		ρg		32µ

where ds is the length of a short section of pipe, ρg is the ﬂuid weight, and z is the elevation difference. Note that the term in brackets is the hydraulic head h, a term used frequently in hydrology.

Poisson’s ratio When an isotropic elastic object is subject to elongation or compressive stress Tz a strain εz results in the direction of the stress. At the same time a strain arises in the transverse directions, εx , εy. If εz is an elongation, then εx = εy are compressive. The ratio

ν = −εx = −εy

εz εz

is called Poisson’s ratio.

polar cap The region surrounding one of the poles of the Earth. In magnetospheric physics,

the region surrounding the magnetic pole, in general the one inside the auroral oval. That region is believed to be connected to “open” ﬁeld lines which extend to great distances from Earth and which to all intents and purposes can be considered to be linked to Earth at one end only. The existence of polar cap precipitation

(infalling charged particles) suggests the other end is linked to the interplanetary magnetic ﬁeld.

polar cap absorption (PCA) At the polar cusps, even solar energetic particles with relatively low energies (in general protons with energies between about 1 MeV and 100 MeV) from a solar ﬂare or coronal mass ejection can follow magnetic ﬁeld lines to penetrate down to the ionosphere and stratosphere into heights between 30 and 90 km. These particles lead to an increased ionization, which in turn leads to the absorption of radio waves that may last up to days. Since this absorption is limited to the polar regions, this phenomenon is called polar cap absorption (PCA). Owing to the propagation time of particles between the sun and the Earth, a polar cap absorption generally starts a few hours after a ﬂare. In contrast, a similar effect on radio waves on the dayside atmosphere, the sudden ionospheric disturbance (see sudden ionospheric disturbance), starts immediately after the ﬂare because it is caused by hard electromagnetic radiation. Polar cap absorptions are limited to a small latitudinal ring around the geomagnetic pole. Part of the particles’ energy is transferred to electromagnetic radiation in the visible range, the polar glow aurora, a diffuse reddish glow of the entire sky. Since the incoming protons also inﬂuence the upper atmospheric chemistry, in particular the NO production, strong polar cap absorptions lead to a decrease in the total ozone column at high latitudes.

polar cap arc (or sunward arc) A type of aurora most often observed by satellite-borne imagers. Polar cap arcs extend into the polar cap, usually starting near midnight and stretching sunward. Polar cap arcs can extend many hundreds of kilometers or even all the way across the polar cap, in which case they are known as the theta aurora because the auroral conﬁguration seen by orbiting imagers — the auroral oval

polar glow

plus the arc stretching across it — resembles the Greek letter theta (L). Polar cap arcs are associated with northward interplanetary magnetic ﬁelds and their origin is not well understood.

polar cap (Mars) Snow ﬁelds covering polar regions. Their main composition is CO2 ice (dry ice). In winter the atmospheric temperature of the polar region is below the freezing point of the CO2 gas which accounts for 95% of the Martian atmosphere. The polar region in winter does not receive sunlight (the inclination of Mars to its orbit is quite similar to that of Earth). Moreover, the Martian polar regions are covered by the polar hood cloud. Because it grows in darkness, observations have not been made of a growing polar cap. In late winter or early spring, the polar cap appears to us. The north polar cap extends from around 60◦N. It recedes slowly during early spring and then quickly until a permanent cap is exposed in late spring. The north permanent cap extends from about 75◦N. The south polar cap extends from 55◦S in early spring and recedes with constant speed. Mars is near aphelion when the southern hemisphere is in winter, so that the south seasonal cap grows larger than the north seasonal cap. In the south, the permanent polar cap extends from about 85◦S in summer. According to observations of the surface temperature by Viking spacecraft, the north permanent cap consists of H2O ice and the south cap consists of CO2 ice. There is no current explanation of the fact that the south permanent cap consists of dry ice. Martian atmospheric pressure varies with waxing and waning of seasonal polar caps. It reaches primary minimum in late winter of the southern hemisphere. The amplitude of the atmospheric pressure suggests 8.5 × 1015 kg for the mass of the south seasonal cap in late winter. See polar hood.

polar cap precipitation The polar cap region is roughly the position of the so-called “inner” auroral zone and is a circular region around the geomagnetic pole of radius of about 10◦. There are three types of low energy electron precipitation in the polar cap ionosphere: polar rain, polar showers, and polar squalls.

The polar rain particles seem to be of magnetosheath origin and ﬁll the entire cap with ther-

mal electrons of about 100 ev and a ﬂux of about 102 ergs/cm2/s.

The polar showers are embedded in the polar rain and consist of enhanced ﬂuxes of precipitating electrons of mean energy around 1 kev. The showers are probably responsible for “sun aligned” arcs. Polar squalls are also localized intense ﬂuxes of electrons of several kev during geomagnetic storms, which occur as a result of ﬁeld aligned accelerations.

polar crown Region around poles of sun at about latitude 70◦ of ﬁlaments oriented nearly parallel to the equator. The polar crown is occupied by prominences and large arcades which can erupt and result in CMEs.

polar cusp See cusp, polar.

polar dunes Dune-like features evident on Mars. The similarity in size and form between dunes on Earth and Mars indicates that surface materials have responded to wind action in the same way on both planets, despite differences in atmospheric density, and wind speeds. However, the global distribution of dunes differs greatly on the two planets. On Earth the most extensive sand dunes are in the midto low-latitude deserts, whereas on Mars most dunes are in high latitudes. In the Martian North polar region, an almost continuous expanse of dunes forms a collar, in places 500 km across, around the layered terrain, while in the south dune ﬁelds form discrete deposits within craters. The dunes imaged by Mars Global Surveyor Orbiter Camera are classic forms known as barchan and transverse dunes. These two varieties form from winds that persistently come from a single direction (in this case, from the southwest).

The source of material involved in the formation of the Martian dunes is unclear. As an alternative to quartz (silicic rocks are thought to be lacking on Mars) garnet has been proposed. (It is sufﬁciently hard to withstand the erosive action of wind.) Alternatively, the sand-sized particles could be produced by electrostatic aggregation, or frost cementation in the polar regions, of smaller particles.

polar glow Reddish proton aurora at heights between 300 and 500 km. The polar glow covers

polar hood (Mars)

large areas and is less structured than the more common electron aurora and, in contrast to the electron aurora, it is observed in the dayside instead of the nightside magnetosphere. The polar glow is formed when solar wind protons penetrate along the cusps into the dayside magnetosphere. In this case, the excitation of the neutral atmosphere is due to charge exchange: the penetrating proton is decelerated and becomes an excited hydrogen atom, emitting either the Lα line in UV or the Hα line in the red.

polar hood (Mars) A steady cloud entirely covering polar regions of Mars from early autumn through early spring. Polar hoods are easily visible in blue, but not in red. The north polar hood appears in late summer and becomes stable in early autumn. It extends from about 40 N in the early morning and recedes to about 60 N in the afternoon. Bright spots of cloud appear frequently near the edge of the north polar hood in autumn. They drift east and disappear in several days. The south polar hood has not been investigated as much as the north polar hood, for observations are difﬁcult owing to the tilt of the polar axis. It appears to be less stable than the north polar hood; its brightness varies from place to place and day by day, and its boundaries ﬂuctuate frequently. Large basins in south midlatitudes are often covered with thick clouds in late autumn to early winter, which seem to be extensions of the polar hood. They recede poleward in late winter. The cloud on Hellas is especially bright and stable.

Polaris A second magnitude F-star, located at RA 02h 31.5m, dec 89◦16 . Polaris is about 200 pc distant. This is the North Pole star because it is within a degree of the pole (at present).

Polaris is a spectroscopic binary with at least two additional 12th magnitude components, and the principal star is a Cepheid variable with a period of 3.97 days and a variability of 0.15 magnitude.

polarization The orientation of the electric vector of an electromagnetic wave along a preferential direction. Polarization can be linear or circular. In the ﬁrst case, the direction of the electric vector is ﬁxed in space; in the plane perpendicular to the direction of propagation of

the wave. In the case of circular polarization, the electric vector rotates in the plane with constant angular frequency equal to the frequency of the light. Light coming from thermal sources such as stars is typically unpolarized, i.e., the electric vector oscillates randomly in all directions in the plane. Scattering by interstellar dust grains and charged particles can polarize previously non-polarized light analogously to nonpolarized sunlight becoming polarized after being reﬂected by the sea. Non-thermal radiation can be intrinsically highly polarized. For example, synchrotron radiation, which is an important astrophysical emission mechanism at radio frequencies can have a high degree of linear polarization ( < 70 %).

polarization brightness In solar physics, a measure of the polarized light emanating from the white light corona of the sun deﬁned as the degree of polarization multiplied by the brightness. The emission is due to Thomson scattering of photospheric light by coronal free electrons and as such can be directly related to the electron density integrated over the line-of-sight.

polarization fading The polarization of signals propagated by the ionosphere depends on the electron density along the propagation path and changes in this will result in changes in the polarization. When two polarization components are present, each responding to electron density changes, then the two modes will interfere resulting in a fading signal. This is polarization fading and lasts for fractions of a second to a few seconds (or the order of 1 Hz or less). See fading.

polarization state The description of a light ﬁeld using four components. See Stokes parameters.

polar motion The motion of the rotation axis of the Earth relative to the geographical reference frame. The motion is described in coordinates ﬁxed in the Earth, with x measured along the Greenwich meridian, and y, 90◦ west. The zero point is the CTRS pole. The motion has three major components: A free oscillation with period about 435 days (Chandler wobble) and an annual oscillation forced by the seasonal

poloidal/toroidal decomposition

displacement of air and water masses cause an approximately circular motion about the mean rotation pole. The mean pole itself has an irregular drift in a direction 80◦ west. Thus, the wobble is centered at about 3.4” west of the CTRS pole as of this writing. Worldwide observations of polar motion of the CTRS pole are reduced and provided to the public by the International Earth Rotation Service.

polar plume Bright ray-like solar structure of out-ﬂowing gas which occurs along magnetic ﬁeld lines in coronal holes. Plumes deviate noticeably from the radial direction and tend to angle towards lower latitudes as expected if they follow the global solar magnetic ﬁeld. Most prominent at times near solar minimum.

polar wander See polar motion.

polar wind In the polar regions of the Earth where the geomagnetic lines of force are open, the continuous expansion of the ionospheric plasma consisting of O+, H+, He+ leads to supersonic ﬂow into the magnetospheric tail. This is known as polar wind and is analogous to the solar wind from the sun. This supersonic ﬂow is reached beyond about 1500 km for lighter ions depending upon the plasma temperature.

pole-on magnetosphere When the solar wind directly hits one pole of a planet’s magnetic ﬁeld, a pole-on magnetosphere results. Such a magnetosphere is cylinder-symmetric with an axial-symmetric neutral and plasma sheet around the axis of the magneto-tail. Neptune’s magnetosphere oscillates between such a pole-on magnetosphere and an earth-like magnetosphere: its magnetic ﬁeld axis is tilted by 47◦ with respect to its axis of rotation, which itself is inclined by 28.8◦ with respect to the plane of ecliptic. Thus during one rotation of the planet (0.67 days), the inclination of the magnetic dipole axis with respect to the plane of the ecliptic, and thus the solar wind ﬂow, varies between 90◦ − 28.8◦ − 47◦ = 14.2◦, which leads to a nearly pole-on magnetosphere, and 90◦ + 28.8◦ − 47◦ = 71.8◦, which gives an earth-like magnetosphere.

poles of Mars Regions with distinctly different physiography from the rest of Mars. First, layered deposits are unique to the polar regions and extend outwards for a little over 10◦. They are arranged in broad swirls such that individual layers can be traced for hundreds of kilometers. The layer deposits are almost entirely devoid of craters, indicating they are among the youngest features of the planet. In the north the deposits lie on plains and are surrounded by a vast array of sand dunes, which form an almost complete collar around the polar region. Dune ﬁelds of comparable magnitude do not occur in the south. Elevations from the Mars Orbiter Laser Altimeter show the northern ice cap has a maximum elevation of 3 km above its surroundings, but lies within a 5-km deep hemispheric depression that adjoins the area into which the outﬂow channels emptied.

The polar caps are at their minimum size at the start of fall, and are at their maximum size in the spring. Clouds form a polar hood over the north polar cap in the winter months. In the south, on the other hand, the Viking Orbiter showed only discrete clouds existed in the polar regions during the fall and winter.

Pole star North pole star, Polaris. There is no bright star near the southern celestial pole.

Pollux 1.14 magnitude star of spectral type K0 at RA07h45m18.9s, dec +28◦01 34 .

poloidal/toroidal decomposition It can be shown that a sufﬁciently differentiable vector ﬁeld v that vanishes at inﬁnity may be written in the form:

v = φ + × (T r) + × × (P r)

where r is the radius vector from the origin of the coordinate system. The three potentials φ, T , and P are, respectively, the scaloidal, toroidal, and poloidal potentials. If v represents a solenoidal vector such as magnetic ﬁeld or incompressible ﬂow, then the term in φ does not contribute ( φ = 0), leaving the other two terms as the poloidal/toroidal decomposition of v. This is a convenient representation in many geophysical systems because of the near spherical symmetry of the Earth. The toroidal portion of v has no radial component, i.e., ×(T r) lies

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