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storm track

mensionless number that is analogous to speciﬁc yield and is the product of the speciﬁc storage Ss and the aquifer thickness b, or S = bSs.

Störmer orbits Orbits (trajectories) of energetic particles in the terrestrial magnetic ﬁeld. These orbits ﬁrst were calculated for auroral particles by C. Störmer by integrating the equation of motion along the particle orbit. Such calculations are of particular importance for cosmic ray particles because their gyro-radii are comparable to the size of the magnetosphere, thus concepts such as the adiabatic invariants cannot be applied. The particle orbit depends on the location and direction of incidence and can appear to be very irregular with large excursions in longitude and latitude before the particle either hits the denser Earth’s atmosphere and is lost due to interactions or before it escapes back into interplanetary space.

The most important result of Störmer’s calculations is the deﬁnition of allowed and forbidden regions on the ground that can be reached by a charged particle traveling towards the Earth. For instance, to hit the Earth at a certain geomagnetic latitude Qc, the particle’s rigidity must be at least the cut-off rigidity Pc = 14.9 GV·cos4 Qc. The extent of the forbidden region, that is a sphere around the center of the Earth that cannot be penetrated by particles with a rigidity P , is given by the Störmer unit CSt = √ME/P with ME being the magnetic moment of the Earth.

Störmer theory A theory of the motion of energetic ions in the ﬁeld of a magnetic dipole, derived around 1907 by Carl Störmer. Though the theory was intended to explain the aurora, its main application was to cosmic rays. It showed that if all cosmic rays came from great distances and the Earth’s magnetic ﬁeld could be approximated by a dipole, particles of low energy were prevented from reaching parts of the Earth. Each point on Earth has (for protons) a cut-off energy below which protons from inﬁnity cannot reach it, and at energies slightly above that protons coming from the west can arrive but those from the east are cut off. See east-west effect.

Störmer unit A unit of distance rst used in Störmer theory; when distances are expressed in it, the equations become dimensionless. For

particles of mass m, charge Ze (e the electron charge) and momentum p, rst 2 = Zem/p.

storm sudden commencement (SSC) Geomagnetic storms may commence abruptly, the commencement of the storm being observed at nearly the same time at magnetic observatories around the globe. The sharp onset of storm conditions is called a storm sudden commencements, or SSC. The storm sudden commencement can usually be identiﬁed with a shock, propagating in the solar wind, reaching the Earth’s magnetosphere. Since the shock and the solar wind particles responsible for the longerlived geomagnetic storm effects propagate independently, associating a particular shock with a particular storm can be ambiguous. Nevertheless, historically, sudden commencement storms were associated with ﬂares on the sun (although the processes are now recognized to be more complex), and are most dramatic geophysical events, although they may only last for a few days. See geomagnetic storm, gradual commencement storm.

storm surge Superelevation of the mean water level due to the effects of the reduced atmospheric pressure that accompanies a major storm, and the strong wind shear stress that results. Storm surge due to hurricanes or typhoons sometimes exceeds 6 m.

storm track An east-westward oriented zone where subtropical storms prefer to develop and travel. It is often deﬁned as a region of large temporal variance of geopotential height in the upper troposphere. The formation of the storm tracks is closely related to the distribution of the westerly wind velocity. In the Southern Hemisphere, both the westerly wind speed and storm track are nearly uniform in the longitudinal direction. In the Northern Hemisphere, in contrast, winter storminess displays large variations in the longitudinal direction, reaching maximum over the North Paciﬁc and North Atlantic. The North Paciﬁc (North Atlantic) storm track is located downstream of the westerly wind speed maximum over Japan (eastern United States). These longitudinally localized jet streams and storm tracks are caused by largescale orographic features such as the Tibetan

strain

Plateau and the Rockies, and by thermal forcing such as land-sea heat contrast and tropical convection.

strain The deformation of a body that results from an applied stress.

strain hardening A solid yields plastically at a certain state of stress and strain, called its elastic limit. For some solids, subsequent plastic deformation requires greater loading beyond the current elastic limit. This phenomenon is called strain hardening because the solid becomes stronger with increasing plastic strain. It is due to a variety of processes such as dislocation tangles that lead to situations where greater differential stress is required to maintain a given strain rate. If the solid becomes weaker with increasing plastic strain, the phenomenon is called strain softening.

strain partitioning The heterogeneous nature of rocks and their deformation means that on all scales from macroto micro-, strain is partitioned between areas of higher strain and lower strain. Estimates for bulk strain obtained (on whatever scale) represent the average of these partitioned strains.

strain softening Softening, in mechanical terms, can be expressed as a reduction in differential stress to maintain constant strain rate or an increase in strain rate can occur by a number of processes including: change in deformation mechanism, geometric softening, continual recrystallization, reaction softening and chemical softening. See strain hardening.

stratosphere The part of the Earth’s atmosphere extending from the top of the troposphere (typically 10 to 15 km above the surface) to about 50 km. The stratopause, at about 50 km, separates it from the higher mesosphere. Within the stratosphere, the temperature slowly increases with altitude, due primarily to the absorption of ultraviolet radiation by the ozone layer, and reaches 270 to 290 K at stratopause. This temperature variation inhibits vertical air movements and makes the stratosphere stable. Thus, the stratosphere has clear horizontal structure. The heating arises from the absorption of

high-energy solar ultraviolet radiation by ozone molecules. The moisture content in the stratosphere is very small, and in general, there are no complex weather phenomena in the stratosphere. The stratosphere and mesosphere combined are often called the middle atmosphere.

stratovolcano See composite volcanos.

stratus See cloud classiﬁcation.

streakline The line that results if dye is injected at a ﬁxed point within a moving ﬂuid. Used in the study of ﬂuids.

streamer belt Region around the solar equator consisting of closed loops and helmet streamers. The streamer belt is associated with the active regions on the sun, thus its latitudinal extent as well as its inclination with respect to the solar equator varies with the solar cycle. The solar wind originating in the streamer belt can be slow or fast, depending on the topology. Therefore, in interplanetary space, fast and slow solar wind streams can be observed which at larger radial distances eventually form corotating interaction regions. At higher latitudes, that is above the streamer belt, the solar wind always is fast.

stream function wave theory A description of surface water waves that involves a numerical solution for stream function. Typically solved via a numerical technique that determines a set of coefﬁcients that yields a best ﬁt to a particular set of boundary conditions.

streamline A line drawn such that it is everywhere tangent to a velocity vector within a ﬂowﬁeld. Streamlines are used to visualize a ﬂowﬁeld at a particular instant of interest.

stress Force per unit area applied to a body. Tensile stress tends to stretch and compressional stress to compress the body in the direction of the applied force; shear stress tends to distort the body.

stress drop The amount of decrease in average shear stress along the rupture surface of an earthquake as a result of the earthquake. Can also be used to indicate any stress decrease.

structure toe

stress energy tensor A 4-dimensional symmetric tensor Tµν combining the 3-dimensional stress tensor of a substance with its energy density and energy ﬂux:

Too = energy density

Toi = momentum density = energy ﬂux

Tij = 3-dimensional stress tensor.

This tensor is the source of the Einstein equation for the graviational ﬁeld:

Gµv = 8πTµv

where Gµv is the Einstein tensor. (Here µ , ν = 0, 1, 2, 3; i , j = 1, 2, 3.)

striations, auroral A banded structure of auroral arc, believed to be the overlap of moving sections that overtake each other.

strike-slip fault A fault where the movement is horizontal and parallel to the strike (i.e., the angle between true north and the direction of any horizontal planar features) of the fault plane. Strike-slip faults can be right-lateral or left-lateral, depending on the direction of displacement. As one faces a strike-slip fault, if the rocks across the fault move toward the right, the fault is then a right-lateral strike-slip fault. Similarly if the rocks across the fault move toward the left. Transform boundaries at plate margins are usually composed of strike-slip faults.

string model See cosmic string, elastic string model.

string network See scaling solution (cosmic string).

stromatolite The oldest known fossils, dating to 3.5 billion years; the oldest are found in Australia. Colonial structures formed by photosynthesizing cyanobacteria (blue-green algae) and other microbes. These prokaryotes (lacking a cellular nucleus) thrived in warm shallow environments and built layered reefs of calcium carbonate and sediment precipitating from the water, which were trapped within the colony. The colony then grew upward through the sediment to form a new layer. In some cases, bacteria was also fossilized within the mats. This

process still occurs today particularly in Shark Bay in western Australia.

strong anthropic principle The concept that the universe must be such as to admit intelligent observers or, even stronger, that the universe must produce such observers. See anthropic principle.

strong scattering limit In magnetospheric plasma physics, a type of MHD for a guiding center plasma which assumes plasma particles are always isotropic, i.e., they ﬂow with equal intensity in all directions. That is equivalent to assuming that some process keeps scattering the particles, thoroughly mixing their directions of motion. The speciﬁc volume V is an important variable in this theory, and Euler potentials are its natural coordinates.

Strouhal number (Sr) A dimensionless quantity used in ﬂuid mechanics, deﬁned by Sr = lf/v, where l is length, f is frequency, and v is velocity.

structure coefﬁcients		The set of functions
Ca	appearing in the equation
bc

dσ a = 1 Ca σ b σ c .

2 bc

A set of basis 1-forms {σ a} deﬁnes the covariant vector basis. 1-forms are acted on by an exterior derivative to give a 2-form (an antisymmetric rank-2 covariant tensor). Here is the antisymmetric tensor product, i.e.,

σ b σ c = 1 σ b σ c − σ c σ b .

If {ea} is a vector basis dual to the {σ b}, then the structure coefﬁcients also give the commutation rules for the vectors:

[ea , eb] = Cabd ed .

The set of antisymmetric tensor products of the basis 1-forms is itself a basis for the 2-forms. See 1-form, 2-form, tensor.

structure toe The place where the bottom of a structure meets the seaﬂoor. Often a location of severe scour.

St. Venant Equations

St. Venant Equations The equations of mass conservation and momentum for twodimensional ﬂow in an open channel. They assume negligible vertical acceleration, and therefore hydrostatic pressure distribution.

subcritical ﬂow Flow in an open channel with a Froude Number less than unity.√The Froude Number is deﬁned as Fr = V/ gD, where V is the ﬂow speed, g is acceleration of gravity, and D is the hydraulic depth.

subduction The process at ocean trenches where the oceanic lithosphere bends and descends into the interior of the Earth. See convergent boundary.

subduction zone A zone where a cold, dense oceanic plate is subducting beneath a less dense continental plate, located at the convergent boundary of the two plates. Two kinds of systems are formed at a subduction zone: island arc-trench systems and arc of continental margin-trench systems. An example of the former is the Japanese islands where the Paciﬁc plate is subducting, accompanying marginal sea, while an example of the latter is the Andes where the Nazca plate is subducting. Circum-Paciﬁc subduction zones are the most active seismic regions on the Earth, and trench-type great earthquakes take place frequently there. Deep earthquakes occur along inclined planes that deepen from a trench toward the continental side. Volcanic chains are formed in the continental plate, being almost parallel to the trench axis.

subdwarf In astronomy, a star that is smaller, less luminous and of lower metallicity than a main sequence (dwarf) star of the same spectral type.

subgiant A star in which sufﬁcient helium has accumulated in its core to inhibit hydrogen burning. The core begins to contract and hydrogen shell burning begins. The star is then a subdwarf. It then moves off the main sequence and becomes larger, brighter and cooler as the core collapses and shell burning increases, and tracks across the HR diagram along the subgiant branch toward the red giant branch.

subgrain size Sub-grains develop in deformed crystals. Slightly misoriented regions (by a few degrees relative to the parent grain) of a deformed grain, separated by dislocation walls, are deﬁned as subgrains. These subgrains form at the beginning of the deformation and their size (subgrain size) remains constant once they are formed. The subgrain size does not practically depend on temperature but only on the applied stress with the empirical law best written under as:

µb

d = K

where d is the average size of the subgrains, µ is the average shear modulus, b is the Burgers vector of the active dislocation, K is a nondimensional constant of proportionality, and σ is the stress.

submarine canyon A steep valley (the location of a former river valley during a time of lower sea level) found in nearshore waters. Often located near the mouth of modern rivers.

subsidence The sinking of the Earth’s surface via a variety of processes. When subsidence occurs, the surface is often covered with sediments to form a sedimentary basin.

subsolar point The point on the Earth’s magnetopause closest to the sun, also known informally as the “nose” of the magnetosphere. The distance from the center of the Earth to the subsolar point is known as the subsolar distance; a typical value is 10.5 RE, but cases are known of it ranging from 6 to 14 RE. The subsolar points of the Earth’s bow shock, and of magnetopauses and bow shocks of other planets, are similarly deﬁned.

subsonic string model	See elastic string
model.

substorm injection A sudden increase in the ﬂux of energetic ions and electrons at a point in space, associated with the arrival of substormaccelerated particles. For ions in particular, whether the increase at lower energies starts after the one at higher energies (“velocity dispersion”) or is simultaneous (“dispersionless injection”) is believed to indicate whether the ions