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sudden phase anomaly (SPA)

were convected from a distant location or were accelerated locally.

substorm phases Stages in a typical magnetospheric substorm, identified by S.-I, Akasofu, Robert McPherron and others. They begin with the growth phase, in which field lines on the night side are stretched, typically lasting 40 min. That is followed by onset or breakup, when the midnight aurora brightens and rapidly expands poleward (as well as laterally), tail lines snap back to less stretched shape (“dipolarization”), the auroral electrojets and Birkeland currents of the substorm wedge intensify, and large numbers of ions and electrons in the geotail are convected earthward and energized, either by convection or by a local mechanism. This typically lasts 20 min and is followed by a gradual recovery phase in which the auroral oval returns to its initial state.

substorm triggering The event that initiates a magnetospheric substorm, still a subject of controversy. An interval in which the northsouth component Bz of the interplanetary magnetic field is directed southward is necessary to prepare the magnetosphere for a substorm, but in some storms a sudden northward turning is thought to be the final trigger.

substorm wedge An electric current circuit that becomes active in magnetospheric substorms. It seems to be caused by the diversion of some of the cross-tail current flowing across the plasma sheet, causing part of it to flow in a wedge-shaped circuit—Earthward along magnetic field lines, in the ionosphere along the midnight auroral oval, and back out to the tail. The physical mechanism of the diversion of current (“tail current disruption”) is still poorly understood.

subtropical high-pressure belt High-pres- sure belt that lies over both the Pacific and Atlantic near 30◦N and separates the predominently easterly flow in the subtropics from the southwesterly flow over the high latitude oceans in the Earth’s atmosphere. These subtropical high-pressure belts are characterized by light winds and absence of storms.

sudden frequency deviation (SFD) When a solar flare occurs, abrupt changes in HF radiowave frequency on a link, called a sudden frequency deviation (SFD), occur due to rapid changes in the propagation path as ionization changes. SFD are thought to have their origins in the lower F region. See short wave fadeout.

sudden ionospheric disturbance (SID)

Sudden increase in ionization at the dayside ionosphere due to the enhanced hard electromagnetic radiation emitted in a flare. The enhanced X-rays and ultraviolet (UV) radiations during solar flares cause increased ionization at several levels of the ionosphere and may last 1 h or so. A number of ionospheric effects kown as sudden ionospheric disturbances (SID) result. These are:

1.Short wave fadeout (SWF). A sharp drop in high frequency transmission in the 2 to 30 MHz range.

2.Sudden Cosmic Noise Absorption (SCNA). A decrease in the constant galactic radio noise intensity in the 15 to 60 MHz range.

3.Sudden Phase Anolaly (SPA). A change in phase in very low frequency (VLF) transmissions in the 10 to 150 kHz range relative to a frequency standard.

4.Sudden Enhancement of Signals (SES). The enhancement in the VLF transmission intensity in the 10 to 150 kHz range.

5.Sudden Enhancement of Atmospherics (SEA). An increase in the background of VLF noise from distant thunderstorms.

6.Sudden Frequency Deviation (SFD). A short lived increase in the high frequency (HF) signal from a distant transmitter.

7.Solar Flare Effect (SFE) or Geomagnetic Crochet. A sudden variation in the H component of the Earth’s geomagnetic field.

1, 3, 4, 5 are due to increase in the D region ionization from soft X-rays in the 1 to 8 A wavelength range. 2 and 6 are due to increased ionization and collision frequency in the F region. The lowering of the D region also contributes to

3.7 is produced by increased D and E region conductivities and ionospheric currents.

sudden phase anomaly (SPA)

Coincident

with a solar flare, abrupt phase shifts may occur on VLF and LF radio signals propagated by

© 2001 by CRC Press LLC

sudden stratospheric warming

the ionosphere. When the D-region ionization increases, the reflection height is lowered correspondingly and the altered path-length is responsible for the SPA. See short wave fadeout.

sudden stratospheric warming A sudden increase temperature in the stratosphere that occurs in winter and can be driven by upwardpropagating planetary waves. In a major event of this type, the temperature at the 10 mb (about 31 km) level at the North Pole may increase by 40 to 60 K in less than 1 week.

sulfur dioxide SO2, a colorless, toxic gas. Freezing point −72.7◦C; boiling point −10◦C. Volcanic eruptions provide a natural source of sulfur dioxide in the atmosphere. However, sulfur dioxide is a major anthropogenic primary atmospheric pollutant, originating from fossil fuel combustion. In the atmosphere, sulfur dioxide is oxidized to form sulfur trioxide, that is extremely soluble in water. In the presence of water, droplets of sulfuric acid (acid rain) are formed. Sulfuric acid droplets also appear to act as nucleation centers, increasing the amount of rainfall. High airborne concentrations of sulfur dioxide may aggravate existing respiratory and cardiovascular disease, forming sulfuric acid in the bronchia.

summation convention In equations involving vectors or tensors, and in matrix equations,

one often encounters sums of the form AαBα,

α

where {Aα} and {Bα} are collections of perhaps different kinds of objects, but of equal number labeled by and summed over the full set of inte-

gers α [αmin , αmax].

Einstein noted that in a large number of situations, the expressions unambiguously require the sum. In such cases we can omit writing the

. This is the Einstein summation convention.

α

In the unusual circumstance where one wants to examine the symbol AαBα for one fixed value of α only, one appends a “no sum” to the expression. Formulae in this dictionary use the summation convention. Similar notations have been extended to continuous sums (i.e., integration).

summer solstice The point that lies on the ecliptic midway between the vernal and autumnal equinoxes and at which the sun, in its apparent annual motion, is at its greatest angular distance north of the celestial equator. On the day of the summer solstice, that occurs on about June 21, the length of daytime is at its maximum in the northern hemisphere. After summer solstice, the northern length of daytime will decrease until the winter solstice. Because of complicated interactions with atmosphere ocean and land heat reservoirs, northern surface temperatures continue to increase for a period of time after the summer solstice. In the southern hemisphere, the longest day, and the beginning of southern summer, occur on the winter solstice, about December 21.

sun Our star; an incandescent, approximately spherical star around which there exists a system of nine planets (the “solar system”), rotating in elliptical orbits. The sun is a main sequence star of type G2 V with an absolute magnitude of 4.8. The sun has a mass of 1.99 × 1030 kg (roughly 99.9% of all the matter in the solar system) a radius of 696,000 km, a mean distance from the Earth of 150 million kilometers (denoted an Astronomical Unit), a surface gravity of 274 m/s2, a radiation emission of 3.86 × 1026 W, an equatorial rotation rate of 26 days and an effective temperature of 5785 K. The sun has a radiative interior surrounded by a convective zone both of which participate in the transportation of energy from the nuclear burning core to the surface.

The solar atmosphere consists of a number of distinct temperature regimes: the photosphere, chromosphere, transition region, and corona.

sunlit aurora Aurora occurring in the upper atmosphere around the polar cusps of the Earth, observed in the ultraviolet by satellite imagers.

sunspot A generally irregular dark spot, with considerable internal detail on the surface of the sun. A well-developed sunspot consists of a central circular or elliptical umbra, of lower luminosity, surrounded by a brighter penumbra. Sunspots are violent eruptions of gases cooler than the surrounding surface areas. The temperature of sunspots is about 1000 K lower than the typical temperature of the photosphere of

© 2001 by CRC Press LLC

supercell storm

the sun. Sunspots tend to occur in groups, are relatively short-lived, usually less than a day, but some sunspots can live as long as a month, and some special sunspots can live even to half a year. Sunspots occur in cycles of 11 or 12 years. They are restricted to regions (solar active regions) of 5◦N −40◦N and 5◦S −40◦S, moving from higher to lower latitudes during a sunspot cycle. At the beginning of a sunspot cycle, sunspots appear near 40◦N and 40◦S and move to lower latitude. Their number increase and then decrease during the moving processes, and reach a minimum at about 5◦N and 5◦S regions at the end of the sunspot cycle. Sunspots are characterized by strong magnetic fields (up to several thousand tesla), being concentrations of magnetic flux, typically in bipolar clusters or groups and are associated with magnetic storms on the Earth. Sunspots are the most obvious manifestation of solar activity. See sunspot cycle, sunspot number.

sunspot classification A - A small single unipolar sunspot or very small group of spots without penumbra. B - Bipolar sunspot group with no penumbra. C - An elongated bipolar sunspot group. One sunspot must have penumbra. D - An elongated bipolar sunspot group with penumbra on both ends of the group. E - An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 10◦ but not 15◦. F - An elongated bipolar sunspot group with penumbra on both ends. Longitudinal extent of penumbra exceeds 15◦. H - A unipolar sunspot group with penumbra.

sunspot cycle An approximately 11-year quasi-periodic variation in the twelve-month smoothed sunspot number. Other solar phenomena, such as the 10.7-cm solar radio emission, show similar cyclical behavior. The polarities of solar magnetic fields are now known to reverse with each cycle leading to a roughly 22-year cycle. This is sometimes called the Hale cycle.

sunspot number The number of sunspots apparent on the sun at different times. At the beginning of a sunspot cycle, sunspots appear near 40◦N and 40◦S and move to lower latitude. Their numbers increase and then decrease during the moving processes and reach to the min-

imum at about 5◦N and 5◦S regions at the end of the sunspot cycle.

sunspot number (daily) A daily index of sunspot activity. The sunspot number is computed according to the Wolf (1849) sunspot number R = k(10g + s), where g is the number of sunspot groups (regions), s is the total number of individual spots in all the groups, and k is a variable scaling factor (usually <1) that indicates the combined effects of observing conditions, telescope, and bias of the solar observers. k is equal to 1 for the Zurich Observatory and adjusted for all other observatories to obtain approximately the same R number.

sunspot number (smoothed) An average of 13 monthly sunspot numbers, centered on the month of concern. The 1st and 13th months are given a weight of 0.5.

sunward arc See polar cap arc.

 

Sunyaev–Zeldovich (1972) effect

A mod-

ification of the spectral energy distribution of the background microwave radiation at temperature ≈ 2.7◦K, due to inverse Compton scattering of the microwave radiation photons by free electrons in the hot gas filling the intergalactic space in clusters of galaxies (the intra-cluster medium). The background photons gain energy, causing a minimal yet measurable change in the background radiation temperature. If the distribution of electron temperature and electron density of hot matter inside the cluster is known, then the value of the Hubble constant (and in principle of the deceleration parameter q0) can be derived from observations of the Sunyaev– Zeldovich effect. Such measurements of H0 have the advantage of being independent of the distance ladder built on optical distance indicators but require detailed X-ray observations of massive and dense clusters of galaxies. They

have yielded, as of early 1998, values of H0

<

50 km s−

1

Mpc−

1

 

 

 

 

, which are somewhat lower

than other estimates. See inverse Compton effect.

supercell storm The storm that is so organized that the entire storm behaves as a single entity, rather than as a group of cells. These so-

© 2001 by CRC Press LLC

super cloud cluster

called supercell storms account for most tornadoes and damaging hail. Most supercell storms move continuously toward the right of the environmental winds. See multicell storm.

super cloud cluster Deep convection in the Tropics is organized into a hierarchy of spatial structures. A typical convective cloud is 10 km in horizontal scale. Such clouds often gather into a cloud cluster with horizontal dimensions of 1 to a few 100 km. A super cloud cluster is a group of such cloud clusters and has a longitudinal dimension of a few 1000 km. A super cloud cluster forms in the rising branch of a Madden– Julian Oscillation and is exclusively found in the large warm water region from the equatorial Indian to the western Pacific Ocean.

supercluster A very large, high density cluster of rich clusters of galaxies that is flattened or filamentary in shape, with sizes as large as 150 Mpc. Superclusters appear to surround large voids creating a cellular structure, with galaxies concentrated in sheets; in higher density at the intersection of the sheets (edges); and in very high density superclusters at the intersection of edges. Superclusters typically contain several (2 to 15) clusters and are defined by a number density of galaxies taken to exceed some threshold, typically a factor 20 above the average galaxy number density. The mean separation between superclusters is reported to be approximately 100 Mpc.

superconducting string See Witten conduct- ing string.

supercooling The phenomenon in which a pure material may be cooled below the usual transition temperature but not undergo a phase change. For instance, pure water may be cooled below 0◦C without freezing. The addition of a freezing center, or of a small ice crystal will immediately initiate freezing in such a case. One can also speak of supercooling with respect to the condensation temperature.

supercritical flow Flow in an open channel with a Froude number greater than unity. See Froude number.

superflare A stellar flare having 100 to 10 million times the energy of the most energetic Solar flare. Superflares have been observed on normal solar-like stars. These flares have durations of a fraction of 1 h up to a few days, radiate 1033 to 1038 ergs, and emit light from radio waves through X-rays. One theory of their origin is that they arise from the sudden release of stored magnetic energy. See RS Canum Venaticorum stars.

supergeostrophic wind Real wind which is stronger than the geostrophic wind. Along the axes of low level jets, it is often found that the wind speed is larger than the geostrophic wind. Both supergeostrophic wind and subgeostrophic wind are the non-geostrophic wind fluctuations caused by inertia gravitational waves. In general, there will be severe weather, such as heavy storms over the supergeostrophic wind regions.

supergiant The evolved phase of the life of a star of more than about 5 solar masses. The distinction between giants, bright giants, and supergiants is somewhat arbitrary (see HR diagram), but, in general, supergiants will be the biggest (more than 1,000 solar radio), brightest (more than 104 solar luminosities), and the shortest-lived (less than 106 years). Supergiants, at least those whose initial mass was more than about 10 solar masses, will end their lives as supernovae of Type II. They have vigorous stellar winds that can reduce the initial mass of the star by a factor of two or more during their lives.

supergradient wind Real wind which is stronger than the gradient wind. It is similar to supergeostrophic wind. When the pressure gradient force cannot be balanced by the horizontal Coriolis force and the centrifugal force, the supergradient wind will appear. Supergradient winds often can be found in tornado systems. In general, supergradient winds are not easy to create and will evolve to normal gradient winds due to the Coriolis effect. In practice, it is hard to determine the radius of curvature of wind required to determine the gradient wind; thus, it is hard to determine the existence of the supergradient wind.

© 2001 by CRC Press LLC

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