3,686 research outputs found
Magneto-Acoustic Waves of Small Amplitude in Optically Thin Quasi-Isentropic Plasmas
The evolution of quasi-isentropic magnetohydrodynamic waves of small but
finite amplitude in an optically thin plasma is analyzed. The plasma is assumed
to be initially homogeneous, in thermal equilibrium and with a straight and
homogeneous magnetic field frozen in. Depending on the particular form of the
heating/cooling function, the plasma may act as a dissipative or active medium
for magnetoacoustic waves, while Alfven waves are not directly affected. An
evolutionary equation for fast and slow magnetoacoustic waves in the single
wave limit, has been derived and solved, allowing us to analyse the wave
modification by competition of weakly nonlinear and quasi-isentropic effects.
It was shown that the sign of the quasi-isentropic term determines the scenario
of the evolution, either dissipative or active. In the dissipative case, when
the plasma is first order isentropically stable the magnetoacoustic waves are
damped and the time for shock wave formation is delayed. However, in the active
case when the plasma is isentropically overstable, the wave amplitude grows,
the strength of the shock increases and the breaking time decreases. The
magnitude of the above effects depends upon the angle between the wave vector
and the magnetic field. For hot (T > 10^4 K) atomic plasmas with solar
abundances either in the interstellar medium or in the solar atmosphere, as
well as for the cold (T < 10^3 K) ISM molecular gas, the range of temperature
where the plasma is isentropically unstable and the corresponding time and
length-scale for wave breaking have been found.Comment: 14 pages, 10 figures. To appear in ApJ January 200
Zirconium stable isotope analysis of zircon by MC-ICP-MS: Methods and application to evaluating intra-crystalline zonation in a zircon megacryst
Zirconium (Zr) plays a key role in the development of phases like zircon (ZrSiOâ) and baddeleyite (ZrOâ) in magmatic systems. These minerals are crucial for the study of geologic time and crustal evolution, and their high resistivity to weathering and erosion results in their preservation on timescales of billions of years. Although zircon and baddeleyite may also preserve a robust record of Zr isotope behavior in high-temperature terrestrial environments, little is known about the factors that control Zr isotope partitioning in magmatic systems, the petrogenetic significance of fractionated compositions, or how these variations are recorded in Zr-rich accessory phases. Here, we describe a new analytical protocol for accurately determining the Zr stable isotope composition of zircon by multicollector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), using the double-spike method to correct for procedural and instrumental mass bias. We apply this technique to test whether zircon crystallization in carbonatite magmatic systems is a driver of Zr isotope fractionation by interrogating the internal zonation of a zircon megacryst from the Mud Tank carbonatite (MTUR1). We find the MTUR1 megacryst to lack internal zoning within analytical uncertainties with a mean ÎŒâčâŽ/âčâ°Zr_(NIST) = â55 ± 28 ppm (2 SD, n = 151), which suggests that zircon crystallization is not a driver of Zr isotope fractionation in carbonatite magmas. This observation is in stark contrast with those made in silicate magmatic systems, raising the possibility that the bonding environment of ZrâŽâș ions may be fundamentally different in carbonatite vs. silicate melts. Because of its remarkable homogeneity, the MTUR1 megacryst is an ideal natural reference material for Zr isotopic analysis of zircon using both solution and spatially resolved methods. The reproducibility of a pure Zr solution and our chemically purified zircon fractions indicate that the external reproducibility of our method is on the order of ±28 ppm for ÎŒâčâŽ/âčâ°Zr, or ±7 ppm per amu, at 95% confidence
Urban freight logistics in the European Union
The paper recalls the main challenges of the European urban freight policy: the environment, the need
for sustainable growth and quality of urban life. These are then interpreted by the member states. Some
states have a top down prescriptive approach some others do not. The BESTUFS project, promoted by the
EC, collects and disseminates best practice across the EU. It has identified a deep weakness at a local
level, whilst also a wide range of pilots and successful exceptions
Overview of Grand Unified Models and Their Predictions for Neutrino Oscillations
A brief overview of Grand Unified Models is presented with some attention
paid to their predictions for neutrino oscillations. Given the well-known
features of the two non-unified standard models, SM and MSSM, a listing of the
features of classes of unified models is given, where a GUT flavor symmetry
and/or family symmetry are introduced to reduce the number of model parameters.
Some general remarks are then made concerning the type of predictions that
follow for the neutrino masses and mixings.Comment: 6 pages, contribution to the Proceedings of the NuFACT'00 Workshop
held in Monterey, CA May 22-26, 200
Minimal Family Unification
Absract It is proposed that there exist, within a new , a gauged
discrete group (the order 12 double dihedral group) acting as a family
symmetry. This nonabelian finite group can explain hierarchical features of
families, using an assignment for quarks and leptons dictated by the
requirements of anomaly cancellation and of no additional quarks.Comment: 10 pages, IFP-701-UNC;VAND-TH-94-
Twist Deformations of the Supersymmetric Quantum Mechanics
The N-extended Supersymmetric Quantum Mechanics is deformed via an abelian
twist which preserves the super-Hopf algebra structure of its Universal
Enveloping Superalgebra. Two constructions are possible. For even N one can
identify the 1D N-extended superalgebra with the fermionic Heisenberg algebra.
Alternatively, supersymmetry generators can be realized as operators belonging
to the Universal Enveloping Superalgebra of one bosonic and several fermionic
oscillators. The deformed system is described in terms of twisted operators
satisfying twist-deformed (anti)commutators. The main differences between an
abelian twist defined in terms of fermionic operators and an abelian twist
defined in terms of bosonic operators are discussed.Comment: 18 pages; two references adde
Complete set of Feynman rules for the MSSM -- ERRATUM
This erratum contains the full corrected version of the paper {\em Complete
set of Feynman rules for the Minimal Supersymmetric Standard Model}, published
in Phys. Rev. D41 (3464) 1990. The complete set of Feynman rules for the
R-parity conserving MSSM is listed, including the most general form of flavour
mixing. Propagators and vertices are computed in t'Hooft-Feynman gauge,
convenient for perturbative calculations beyond the tree level.Comment: 46 pages, uses axodraw.sty. This is the "integrated" version of the
erratum, i.e. full text of the paper with errors correcte
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