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Dictionary of Geophysics, Astrophysic, and Astronomy.pdf

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spread F

are called “sporadic” because of their varied appearance and disappearance times, they are a common phenomenon, mid-latitude ionograms rarely being free of some signs of sporadic E. Mid-latitude sporadic E layers are thought to be formed by a wind shear, in the presence of the local magnetic ﬁeld, acting on long-lived metallic ions. At high latitudes, Es is attributed to ionization by incoming eneregetic particles. The resulting layers may vary in spatial extent from several kilometers to 1,000 km and have typical thickness of 0.5 to 2 km. Much statistical information has been collected on sporadic E from measurements made with ionosondes. Three parameters have been collected: (1) h Es (the sporadic E layer height), (2) foEs (the maximum electron density of the layer), and (3) fbEs (the minimum electron density observable above the layer). The last parameters gives a measure of the layer patchiness. Sporadic E is most common in the summer daytime and diurnally there can be a pre-noon peak followed by a second pre-dusk peak in occurrence. There are also marked global differences; for instance, summertime sporadic E in southeast Asia, especially near Japan, are more intense (higher foEs) than at any other location. Mid-latitude sporadic E can support HF propagation of both wanted and unwanted (interference) signals as well as screen the F region, thereby preventing propagation. At low latitudes, equatorial sporadic E reﬂections from intense irregularities in the equatorial electrojet appear on iongrams as highly transparent reﬂections, sometimes up to 10 MHz. Equatorial sporadic E is most common during the daytime, and shows little seasonal dependence. The daytime electrojet irregularities can support propagation at VHF frequencies. At high latitudes, the auroral E region displays several phenomena including Es layers formed by particle precipitation. Combinations of these processes can give rise to rapid fading and multipath on HF circuits. See E region.

spread E Ionization irregularities can form in the E region at any time of the day and night. While not as severe, or as frequent in occurrence as spread F, it can have signiﬁcant effects on HF radio propagation. The irregularities in the E region have been shown to impose phase front distortions on radio waves reﬂected from

higher layers and are also responsible for signiﬁcant distortion of radio waves reﬂected in the E region. Spread E possibly results from small-structured ionization irregularities caused by large electric ﬁelds. See E region.

spread F Irregularities in the F layer ionization can result in multiple reﬂections from the ionosphere forming a complex F-region ionization structure recorded on ionograms. The name spread F derives from the spread appearance of the echo trace on the ionogram. On occasions, the spread F traces can contain many discrete traces, other times there may be no distinct structure, this being suggestive of different scale size ionization irregularities. Spreading may be found only near the F-region critical frequency, generally called frequency spreading, and on other occasions range spreading may form near the base of the F region and extend upward in virtual height. These phenomena are observed during mid-latitude nighttime, more frequently during winter, and during ionospheric storms. Although spread F is observed mainly at night, during ionospheric storms, daytime spread F is often observed and is a clear indicator of major storm activity. Spread F may be related to traveling ionospheric disturbances. The ionization irregularities responsible for spread F are also responsible for high ﬂutter fading rates at HF and VHF and for scintillation on satellite propagation paths. At high latitudes, spread F is common, and related to changes in particle ﬂuxes into the ionosphere. At low latitudes, equatorial spread F is a distinctive form that restructures the overhead ionosphere removing all impression of the F region critical frequency on ionograms. This form shows a strong solar cycle dependence, being most prevalent at high solar activity, when ionization levels are highest. Coherent radars have shown this form is a result of bubbles of ionization moving upwards through the nighttime ionosphere. These irregularities tend to be ﬁeld-aligned, roughly 100 to 2000 km in extent and extend from the bottomside up into the topside ionosphere (an altitude extent of 600 to 1000 km). Radio waves (HF, VHF) can be ducted within these large structures, that are thought to be one of the principal sources of trans-equatorial propagation. The equatorial Spread F affects the trans-ionospheric propaga-

spring

tion and produces ﬂuctuations in signals coming from the outer ionosphere known as Scintillations. Spread F tend to occur in the night. See F region, ionosphere.

spring Season of the year in the northern hemisphere between the vernal equinox, about March 21 and the summer solstice, about June 21; in the southern hemisphere between the autumnal solstice, about September 21, and the winter solstice, about December 21. See also cosmic spring.

spring tide Tide which occurs when the Earth, sun, and moon are nearly co-linear. Under these conditions the gravitational ﬁeld gradients of the sun and moon reinforce each other. The high tide is higher and low tide is lower than the average. Spring tides occur twice a month near the times of both new moon and full moon.

sprites Short-lived luminosities observed at high altitudes above thunderstorms, apparently associated with upward discharges of thunderstorm electricity. They appear as columnar diffuse reddish glows between 30 km and 80 km above ground, branching into the upper atmosphere lasting tens of milliseconds, following large positive cloud-to-ground lightning strokes. It is currently believed that such lightning strokes leave in the cloud a residual charge of 200 Coulombs or more, creating a signiﬁcant voltage difference between the cloud top and the ionosphere. This induces a heating of the middle atmosphere, which produces the sprites when electrons collisionally excite atmospheric nitrogen, that emits red light in ﬂashes with several milliseconds duration.

Studied from the ground, aircraft and from the space shuttle, sprites appear to be most common above large mesoscale convection features, such as the storm systems of the American Midwest. Ground observations indicate that an active system can generate as many as 100 sprites in the course of several hours. Discovered in 1990. See elves, blue jet.

squall A sudden strong wind or brief storm that persists for only a few minutes, often associated with thunderstorm activity and sometimes accompanied by rain or snow.

s (slow) process The capture of free neutrons by nuclei of iron and heavier elements on a time scale slower than the average for beta-decays of unstable nuclei (cf. r process). The s process is the primary source of those isotopes of elements from Z = 30 to 82 with the largest binding energies (the “valley of beta stability”). It occurs in asymptotic giant branch stars and in the corresponding evolutionary stage of more massive stars. The main sources of free neutrons (which require some mixing between zones dominated by different nuclear reactions) are

13C(α, n)16O and

14N(α, γ )18O(α, γ )22Ne(α, n)25Mg

(the latter being more important in massive stars). Among the important products of the s process are barium (because it is dominated by s process nuclides and can be seen in the atmospheres of cool, evolved stars) and technitium (because of its short half life). s-process production of elements does not reach beyond lead and bismuth because of the set of unstable elements before reaching uranium and thorium. A typical s-process chain is

174Y b(n, γ )175Y b(e−ν¯e)175Lu(n, γ )176Lu(n, γ ) 177Lu(e−ν)¯ 177Hf (n, γ )178Hf (n, γ )179Hf 179Hf (n, γ )180Hf (n, γ )181Hf (e−ν¯e)

181T a(n, γ )182T a e−ν¯e 182W(n, γ )184W(n, γ )

and so forth, onward to Re, Os, etc. The time scales of the beta decays range from minutes to months.

The cross-section to capture the next neutron is smallest for nuclides with ﬁlled shells, so that products of the s process with N = 26, 50, 82, and 126 are particularly abundant. See asymptotic giant branch star, beta decay, r process.

S stars Cool (3000 to 3600 K) giant stars which have greatly enhanced s-process elements (such as zirconium, barium, yttrium, and technetium) and have an atmospheric C/O number ratio very close to unity. Their optical spectra are characterized by strong absorption bands of ZrO, as well as TiO, and carbon-rich compounds such as CN and C2. Most are believed to be on the asymptotic giant branch (AGB) of the H-R

stable causality

diagram. Although S stars might be an intermediate stage of evolution between oxygen-rich (M stars) and carbon rich (C stars) AGB stars, it is possible that they are an end stage and will not go through a carbon rich phase. It has been shown that some S stars have undergone mass transfer from a companion and have white dwarf companions.

stability frequency See buoyancy frequency.

stability of the water column The strength of the stratiﬁcation of a water column is expressed in terms of the intrinsic buoyancy frequency N, that a parcel experiences. The stability is given by

N2 = − gρ−1(∂ρ/∂z)

=g(α∂ /∂z − β∂S/∂z

+ additional terms)

where g is gravitational acceleration, ρ density, z vertical coordinate (positive upward), α thermal expansivity, β saline contraction coefﬁcient, potential temperature and S salinity. Additional terms may become relevant, when (1) stability is very low (e.g., gradient of gases, such as CO2 or CH4, or silica in deep lakes), or when (2) inﬂowing river water or bottom boundary water contain high particle concentrations. N2 varies over 10 orders of magnitude in natural waters from 10−11 s−2 (in well-mixed bottom layers or double-diffusive layers) to 1 s−2 in extreme halocline (merging fresh and salt water).

stability ratio The non-dimensional ratio of the stability due to the stabilizing component divided by the destabilizing component of

α∂T/∂z 1		ρ	=	βs∂S/∂z	or Rρ =
the stratiﬁcation: R				α∂T/∂z
the stratiﬁcation: R
βs∂S/∂z −	(see double diffusion). Here T is

the temperature, S is the salinity, and z is the vertical coordinate. The thermal and saline expansion coefﬁcients of sea water, α and β, respectively, are deﬁned such that they are always positive. Hence, the value of Rρ indicates the type of the stability of the water column. The density of sea water is a function of temperature and salinity, and their vertical gradients determine the stability of the water column. In the oceans, temperature generally decreases with depth and

salinity increases with depth (i.e., for z positive upward, ∂T /∂z > 0 and ∂S/∂z < 0). A value of Rρ in the open interval (−∞, 0) indicates a stable stratiﬁcation, while a Rρ > 1 indicates an unstable water column. Vice versa, if ∂T /∂z < 0, then −∞ < Rρ < 0 means the water column is unstable, while Rρ > 1 indicates stable stratiﬁcation.

In a two-layer situation in which two water masses of different salt and temperature composition are stacked vertically, a value of Rρ in the interval between 0 and approximately 3 indicates the possibility of a double diffusive instability. For ∂T /∂z > 0 the double diffusion will be in the form of salt ﬁngers, for ∂T /∂z < 0 in the form of layering.

Rρ	∂T /∂z > 0	∂T /∂z < 0
−∞ < Rρ ≤ 0	Stable	Unstable
Rρ = 1	Statically unstable	Statically unstable
Rρ > 1	Unstable	Stable
0 < Rρ 3	Fingering	Layering

Infrequently Rρ is called density ratio or Turner number.

stable auroral red (SAR) arcs These are very stable, globe encircling arcs of pure red line emission in OI (3P − D)(λλ6300 − 6364 Å) at about 50◦ geomagnetic latitude region and span several hundred kilometers in north-south direction. They are situated in the upper F region with intensity from several hundreds to several thousands of rayleighs. They are observed during highly geomagnetic disturbed periods close to the plasmapause. The excitation mechnanism is thermal excitation by high electron temperature conducted down from the magnetosphere. The energy comes from the decay of the ring current in the recovery phase of the geomagnetic storm, when it is closest to the plasmapause.

stable causality Theorem (Hawking): A space-time is stably causal, if and only if, it admits a foliation with a family of space-like hypersurfaces.

Stable causality holds in a space-time if no arbitrary but small variation of the metric gives closed causal curves. Stable causality implies strong causality.

stable equilibrium

stable equilibrium In mechanics, a conﬁguration in which the system experiences no net force (equilibrium) and slight displacements from this state cause forces returning the system to the equilibrium state. The paradigm is a pendulum at rest at the bottom of its arc. See neutral equilibrium, unstable equilibrium.

standard atmosphere An idealized atmospheric structure in the middle latitudes up to 700 km level. It is deﬁned in terms of temperature at certain ﬁxed heights; between these levels temperatures are considered to vary linearly and other properties such as density, pressure, and speed of sound are derived from the relevant formulas. In it, at the sea level, the standard gravitational acceleration is 9.80665 m/s2, the temperature is 15◦C, the pressure is 1013.25 hPa, density is 1.225 kg/m3; From ground to 11 km is troposphere with 0.65◦C/100 m lapse rate; From 11 to 20 km is the stratosphere with constant temperature; From 20 to 32 km, the lapse rate is −0.1◦C/100 m.

standard candle An astrophysical object with a speciﬁc, known, or calibrated brightness. Such an object can be used to measure distance by comparing apparent to (known) absolute magnitude. Supernovae of type Ia are standard candles for cosmological distance determinations since all type Ia supernovae have roughly equal maximum brightness.

standard pressure Adopted values of pressure used for speciﬁc purposes; the value of a standard pressure of one atmosphere (1013.25 hPa) is deﬁned as the pressure produced by a 76.0 cm height column of pure mercury whose density is 1.35951 × 104 kg/m3 at temperature of 0◦C, under standard gravitational acceleration g = 9.80665 m/s2.

standard project hurricane A theoretical, parameterized storm used for design purposes, “intended to represent the most severe combination of hurricane parameters that is reasonably characteristic of a region excluding extremely rare combinations” (U.S. Army Corps of Engineers, Shore Protection Manual, 1984).

standard temperature and pressure The values of temperature and pressure deﬁned in the standard atmosphere. See standard atmosphere.

standard time The time in one of 24 time zones generally used as the civil time of the nations therein, except when daylight saving adjustments are made.

standing shock Shock building in front of an obstacle in a super-sonic ﬂow. Examples are planetary bow shocks (super-sonic solar wind hits a planet’s magnetosphere).

standing wave A wave pattern produced by oscillation on a ﬁnite domain with speciﬁc (in general mixed: a linear combination of value and ﬁrst derivative) boundary conditions on the boundary of the domain.

In one dimension this can be visualized as a superposition of waves of equal frequency and amplitude, propagating in opposite directions. Oscillations of a piano string, for instance, constitute a transverse standing wave.

stand-off distance Distance of the sub-solar point of a planetary magnetopause, that is the intersection of the magnetopause with the sunplanet line, from the center of the planet; in general measured in units of the planetary radius. The stand-off distance can be used as a crude measure for the size of the magnetosphere.

Stanton number (St) A dimensionless quantity used in ﬂuid mechanics, deﬁned by St = h/ρvCp, where h is coefﬁcient of heat transfer, ρ is density, v is velocity, and Cp is speciﬁc heat capacity at constant pressure.

starburst galaxy A galaxy undergoing a strong episode of star formation. Starburst galaxies can be more quantitatively deﬁned as galaxies whose total star formation rate cannot be sustained over the age of the universe (the Hubble time). This criterion is very general, and includes spiral galaxies whose nuclei show emission lines typical of HII regions, as well as star-forming dwarf galaxies. Starburst nuclei exhibit, along with optical and UV nuclear spectra typical of star forming regions, an excess of mid and far infrared emission with re-

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