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twistor

object that is carried away at speed v (from an identical object that remains at rest) to a different location and is returned to its original location accumulates less total time interval than does its nonmoving “twin”. In principle, if the motions are at a substantial fraction of the speed of light and the distances are substantial (light years), one could demonstrate this effect with human twins. For instance, if one newborn twin travels 5 light years at a speed of 0.99c(c = speed of light) and then returns at the same speed, the stay-at-home twin would see the return at age 10, while the elapsed time for (the age of) the traveling twin would be

Age = t 1 − c2

= 10 yr. × 0.14

= 1.4 yr .

The paradox is with everyday experience, where these effects are so small as to be unnoticeable. The effect has been veriﬁed in atomic clocks ﬂown around the Earth, in the operation of atomic accelerators, and in the correct operation of the global positioning satellite system. See Lorentz transformation, special relativity.

twistor An element of the four-dimensional spinor representation space of the group SU(2,2). Following work in 1962–1963 with R.P. Kerr, the notion of a twistor was put forward by R. Penrose as a possible mathematical tool for formulating the quantum theory of gravitation. In the theory of relativity, a twistor represents the state of a massless particle.

two-ﬂow equations The two coupled differential equations for irradiances obtained by integrating the radiative transfer equation over the hemispheres of downward and upward directions.

2-form A rank-2 antisymmetric covariant tensor. A basis for 2-forms is the antisymmetric tensor product of pairs of 1-form basis forms. For instance, if {dxβ } is a basis for 1-forms, then {dxβ dxα} is a basis for 2-forms. Here is the antisymmetric tensor product, i.e.,

dxβ dxγ = 1 dxβ dxγ − dxγ dxβ

with the tensor product. See tensor.

two-ribbon ﬂare A large ﬂare that has developed as a pair of bright strands (ribbons) on both sides of the main inversion (“neutral”) line of the magnetic ﬁeld of the active region, usually most prominent in Hα. Two-ribbon ﬂares result in a substantial reconﬁguration of the coronal magnetic ﬁeld.


two-stream approximation		The two-ﬂow
approximation. See two-ﬂow equations.
two-stream instability	A broad term refer-

ring to any of a number of resonant and nonresonant instabilities driven by the streaming of charged particles relative to the main body of a collisionless plasma.

type I radio burst The metric type I burst is continuous radio emission from the sun, basically the normal solar radio noise. It can be enhanced during the late phase of a ﬂare. The radio noise is unstructured, no frequency drift can be identiﬁed.

type II radio burst In the metric type II burst, a relatively slow frequency drift can be observed, indicating a propagation of the electron source at a speed of about 1000 km/s. It is interpreted as evidence for a shock wave propagating through the solar corona, but it should be noted that the shock itself is not generating the radio emission but rather the electrons accelerated at the shock. As these electrons stream away from the shock, they generate small type III like structures, giving the burst the appearance of a herringbone in the frequency time diagram with the type II burst as the backbone and the type III structures as ﬁsh-bones. The type II burst is often split into two parallel frequency bands, interpreted as forward and reverse shocks.

Type II radio bursts can also be observed with frequencies in the kilometer range (kilometric type II radio burst). The drift characteristic is similar to that of the metric counterpart, the frequency indicates that these bursts are excited in interplanetary space. Consequently, the kilometric type II burst is interpreted as the signature of a shock propagating through interplanetary space. It should be noted, however, that

typhoon

only the fastest (speeds exceeding 1000 km/s) and strongest shocks excite kilometric type II bursts; thus, most interplanetary shocks do not excite these bursts.

type III radio burst Short time scale decreasing frequency solar radio features occurring in association with a type II radio burst.

type IV radio burst Metric type IV emission is a continuum, directly following the metric type II burst. It is generated by gyro-synchrotron emission of electrons with energies of about 100 keV. It consists of two components: a non-

drifting part generated by electrons captured in closed magnetic loops, and a propagating type IV burst generated by electrons moving higher in the corona.

type V radio burst Metric type V emission is a continuum, directly following the metric type III burst. It is stationary, showing no frequency drift. Most likely, the metric type V burst is radiation of the plasma itself.

typhoon A tropical cyclone occurring in the (western) Paciﬁc Ocean, equivalent to a hurricane in the Atlantic Ocean.

uniﬁcation of active galactic nuclei

ultrasound “Sound” waves of frequency above 20,000 Hz (hence inaudible to humans).

ultraviolet Referring to that invisible part of the electromagnetic spectrum with radiation of wavelength slightly shorter than violet-colored visible light.

Ultraviolet Coronagraph Spectrometer (UVCS) An ultraviolet spectrometer/coronagraph on board the SOHO spacecraft. The purpose of the UVCS instrument is to locate and characterize the coronal source regions of the solar wind, to identify and understand the dominant physical processes that accelerate the solar wind, to understand how the coronal plasma is heated in solar wind acceleration regions, and to increase the knowledge of coronal phenomena that control the physical properties of the solar wind as determined by in situ measurements.

Ulysses Mission The ﬁrst spacecraft to explore interplanetary space at high solar latitudes, speciﬁcally the high latitude heliosphere away from the plane of the ecliptic where expanding gases from the solar corona dominate the properties of interstellar space. The spacecraft was launched on October 6, 1990 by the shuttle Discovery. Ulysses ﬂew by Jupiter in February 1992, where a gravity-assisted maneuver placed the spacecraft in a solar polar orbit, allowing it to ﬂy over the south pole of the sun in 1994 and over the north pole in 1995. High latitude observations obtained then were during the quiet (minimum) portion of the 11-year solar cycle. The spacecraft’s orbital period is 6 years and, if kept in operation, high latitude observations will also be obtained during the active (maximum) portion of the solar cycle.

The primary results of the mission have been to discover the properties of the solar corona, the solar wind, the heliospheric magnetic ﬁeld, solar energetic particles, galactic cosmic rays, solar radio bursts, and plasma waves. Other

investigations pertained to the study of cosmic dust, gamma ray bursts, and studies of the Jovian magnetosphere acquired during the Jupiter ﬂy-by.

Ulysses is a joint endeavor of the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA).

umbra The central dark part of a sunspot. During an eclipse, the part of the shadow from which no part of the sun is visible.

Umbriel Moon of Uranus, also designated UII. It was discovered by Lassell in 1851. Its orbit has an eccentricity of 0.005, an inclination of 0.36◦, a precession of 3.6◦ yr−1, and a semimajor axis of 2.66 × 105 km. Its radius is 585 km, its mass 1.27 × 1021 kg, and its density 1.44 g cm−3. It has a geometric albedo of 0.18 and orbits Uranus once every 4.144 Earth days. Umbriel is the darkest of the major Uranian satellites.

unconformities Discontinuities in the sedimentary geological record. In general, younger sediments overlie older sediments in sedimentary basins, and the fossils found in the sediments deﬁne the geological record. At an unconformity, the overlying sediments are significantly younger than the underlying sediments, and unconformities are used to deﬁne the geologic epochs; the boundary between the Tertiary and the Cretaceous and the boundary between the Jurassic and Triassic are unconformities. The origin of unconformities is not fully understood, but some are attributed to low stands of sea level. Some major unconformities are associated with extinctions of species, and these in turn are associated with catastrophic meteorite impacts.

undertow The mean offshore current within the surf zone. Breaking waves drive water shoreward in the upper part of the water column, leading to a net offshore ﬂow in the lower part of the water column.

uniﬁcation of active galactic nuclei Theories attempting to explain the variegated phenomenology of active galactic nuclei on the basis of a few key parameters. In uniﬁcation

uniﬁed soil classiﬁcation

schemes, the central engine is considered to be basically the same for all active galactic nuclei (AGN), i.e., a supermassive black hole surrounded by a hot accretion disk, with orientation, black hole mass, accretion rate, and spin of the black hole (or, alternatively, the morphology of the host galaxy) accounting for all differences observed between AGN types. In the uniﬁcation scheme for radio quiet AGN, different orientation of the accretion disk with respect to the line of sight could give rise to a Seyfert-1 galaxy (if the disk is seen at intermediate inclination), or to a Seyfert-2 galaxy because of obscuration: If the disk is seen at large inclination, the line emitting regions should be obscured by a molecular torus. Uniﬁcation of radio-loud AGN relies on the effect of relativistic beaming to explain the different appearance of radio-loud AGN and to establish a link between the beamed and unbeamed (also called the parent) population. For instance, a low luminosity radio-loud AGN is classiﬁed as BL Lac in the optical and as a compact core in the radio if the disk is seen face-on. On the basis of the radio morphology, the same object would be classiﬁed as a low luminosity Fanaroff–Riley type I radio source if the line of sight is oriented at a large angle from the radio jet. The validity of uniﬁcation models and the identiﬁcation of the parent population for some AGN classes is a subject of current debate.

uniﬁed soil classiﬁcation A classiﬁcation of sediment by particle size (diameter) commonly used by engineers. Based on the Casagrande Classiﬁcation of soils. Ranges from cobbles (>76 mm) down through clays (<0.0039 mm).

unipolar magnetic region Large-scale regions of the solar surface which extend over hundreds of thousand of kilometers in both latitude and longitude. They contain ﬂux elements of predominantly one polarity and are long-lived with lifetimes of 1 year or more. Coronal holes can be found above some large unipolar regions.

unit weight Weight per unit volume.

universal gravitation, Newton law of Every particle in the universe attracts every other particle with a force directed along the line between them, proportional to the product of the masses

of the particles and inversely proportional to the square of the distance between the particles.

Universal Time (UT or UT1) Starting in 1972, Greenwich Mean Solar Time was split into Coordinated Universal Time (UTC), used for civil timekeeping, and Universal Time (UT1), a measure of Earth rotation, which is 12h at Greenwich mean solar noon, i.e., UT1 is Greenwich Mean Time (GMT, also called Zulu Time). Universal Time (UT) could refer to any of UT0, UT1, or UTC, but in the absence of other speciﬁers it means UT1. UT0 is latitude and longitude dependent and is of mostly historical interest; for details see the Astronomical Almanac. UT1 is derived, nowadays, by VLBI observations of quasars. Several national agencies contribute their data to the International Earth Rotation Service (IERS), which integrates the data into published values of the difference UT1 – UTC. Both predictive and deﬁnitive data are routinely available. The difference, UT1 – UTC, tends to slowly decrease over the passage of months, due to the cumulative slowing of Earth rotation, but before this difference reaches −0.9 s due to oceanic and atmospheric tidal friction, the IERS inserts a leap second, which brings UT1 – UTC positive again, starting a new cycle of decrease.

universe The totality of all physical reality. In astronomy it is often used to refer to the scale larger than galaxies, which contains galacticscale structures. Also used to refer to a particular (inevitably simpliﬁed) model of the physical cosmos. Dynamical 4-dimensional solutions to Einstein’s equations, which are in principle descriptions of possible gravitational ﬁelds, are often called universes.

unstable equilibrium In mechanics, a conﬁguration in which the system experiences no net force (equilibrium) but slight displacements from this state cause forces moving the system away from the equilibrium state. The paradigm is an inverted pendulum at equilibrium in the vertical position. See neutral equilibrium, stable equilibrium.