14,892 research outputs found
The aurora as a source of planetary-scale waves in the middle atmosphere
Photographs of global-scale auroral forms taken by scanning radiometers onboard U.S. Air Force weather satellites in 1972 show that auroral bands exhibit well-organized wave motion with typical zonal wave number of 5 or so. The scale size of these waves is in agreement with that of well-organized neutral wind fields measured by the 1967-50B satellite in the 150- to 220-km region during the geomagnetic storm of May 27, 1967. The horizontal scale size revealed by these observations is in agreement with that of high-altitude traveling ionospheric disturbances. It is conjectured that the geomagnetic storm is a source of planetary and synoptic scale neutral atmospheric waves in the middle atmosphere. The possible existence of a source of waves of the proper scale size to trigger instabilities in middle atmospheric circulation systems may be significant in the study of lower atmospheric response to geomagnetic activity
Effects of turbulence on a kinetic auroral arc model
A plasma kinetic model of an inverted-V auroral arc structure which includes the effects of electrostatic turbulence is proposed. In the absence of turbulence, a parallel potential drop is supported by magnetic mirror forces and charge quasi neutrality, with energetic auroral ions penetrating to low altitudes; relative to the electrons, the ions' pitch angle distribution is skewed toward smaller pitch angles. The electrons energized by the potential drop form a current which excites electrostatic turbulence. In equilibrium the plasma is marginally stable. The conventional anomalous resistivity contribution to the potential drop is very small. Anomalous resistivity processes are far too dissipative to be powered by auroral particles. It is concluded that under certain circumstances equilibrium may be impossible and relaxation oscillations set in
Auroral magnetosphere-ionosphere coupling: A brief topical review
Auroral arcs result from the acceleration and precipitation of magnetospheric plasma in narrow regions characterized by strong electric fields both perpendicular and parallel to the earth's magnetic field. The various mechanisms that were proposed for the origin of such strong electric fields are often complementary Such mechanisms include: (1) electrostatic double layers; (2) double reverse shock; (3) anomalous resistivity; (4) magnetic mirroring of hot plasma; and (5) mapping of the magnetospheric-convection electric field through an auroral discontinuity
A generalized Ginsparg-Wilson relation
We show that, under certain general assumptions, any sensible lattice Dirac
operator satisfies a generalized form of the Ginsparg-Wilson relation (GWR).
Those assumptions, on the other hand, are mostly dictated by large momentum
behaviour considerations. We also show that all the desirable properties often
deduced from the standard GWR hold true of the general case as well; hence one
has, in fact, more freedom to modify the form of the lattice Dirac operator,
without spoiling its nice properties. Our construction, a generalized
Ginsparg-Wilson relation (GGWR), is satisfied by some known proposals for the
lattice Dirac operator. We discuss some of these examples, and also present a
derivation of the GGWR in terms of a renormalization group transformation with
a blocking which is not diagonal in momentum space, but nevertheless commutes
with the Dirac operator.Comment: 16 pages, Latex, no figure
Solutions of the Ginsparg-Wilson Relation
We analyze general solutions of the Ginsparg-Wilson relation for lattice
Dirac operators and formulate a necessary condition for such operators to have
non-zero index in the topologically nontrivial background gauge fields.Comment: 6 pages, latex, no figures, set T to 1 in eqs. (10)--(13
Two-dimensional quasineutral description of particles and fields above discrete auroral arcs
Stationary hot and cool particle distributions in the auroral magnetosphere are modelled using adiabatic assumptions of particle motion in the presence of broad-scale electrostatic potential structure. The study has identified geometrical restrictions on the type of broadscale potential structure which can be supported by a multispecies plasma having specified sources and energies. Without energization of cool thermal ionospheric electrons, a substantial parallel potential drop cannot be supported down to altitudes of 2000 km or less. Observed upward-directed field-aligned currents must be closed by return currents along field lines which support little net potential drop. In such regions the plasma density appears significantly enhanced. Model details agree well with recent broad-scale implications of satellite observations
On the structures and mapping of auroral electrostatic potentials
The mapping of magnetospheric and ionospheric electric fields in a kinetic model of magnetospheric-ionospheric electrodynamic coupling proposed for the aurora is examined. One feature is the generalization of the kinetic current-potential relationship to the return current region (identified as a region where the parallel drop from magnetosphere to ionosphere is positive); such a return current always exists unless the ionosphere is electrically charged to grossly unphysical values. A coherent phenomenological picture of both the low energy return current and the high energy precipitation of an inverted-V is given. The mapping between magnetospheric and ionospheric electric fields is phrased in terms of a Green's function which acts as a filter, emphasizing magnetospheric latitudinal spatial scales of order (when mapped to the ionosphere) 50 to 150 km. This same length, when multiplied by electric fields just above the ionosphere, sets the scale for potential drops between the ionosphere and equatorial magnetosphere
A Linux PC cluster for lattice QCD with exact chiral symmetry
A computational system for lattice QCD with exact chiral symmetry is
described. The platform is a home-made Linux PC cluster, built with
off-the-shelf components. At present this system constitutes of 64 nodes, with
each node consisting of one Pentium 4 processor (1.6/2.0/2.5 GHz), one Gbyte of
PC800/PC1066 RDRAM, one 40/80/120 Gbyte hard disk, and a network card. The
computationally intensive parts of our program are written in SSE2 codes. The
speed of this system is estimated to be 70 Gflops, and its price/performance is
better than $1.0/Mflops for 64-bit (double precision) computations in quenched
QCD. We discuss how to optimize its hardware and software for computing quark
propagators via the overlap Dirac operator.Comment: 24 pages, LaTeX, 2 eps figures, v2:a note and references added, the
version published in Int. J. Mod. Phys.
A Measurement of the Absorption of Liquid Argon Scintillation Light by Dissolved Nitrogen at the Part-Per-Million Level
We report on a measurement of the absorption length of scintillation light in
liquid argon due to dissolved nitrogen at the part-per-million (ppm) level. We
inject controlled quantities of nitrogen into a high purity volume of liquid
argon and monitor the light yield from an alpha source. The source is placed at
different distances from a cryogenic photomultiplier tube assembly. By
comparing the light yield from each position we extract the absorption cross
section of nitrogen. We find that nitrogen absorbs argon scintillation light
with strength of ,
corresponding to an absorption cross section of . We obtain the relationship
between absorption length and nitrogen concentration over the 0 to 50 ppm range
and discuss the implications for the design and data analysis of future large
liquid argon time projection chamber (LArTPC) detectors. Our results indicate
that for a current-generation LArTPC, where a concentration of 2 parts per
million of nitrogen is expected, the attenuation length due to nitrogen will be
meters.Comment: v2: Correct mistake in molecular absorption cross section
calculation, and a minor typo in fig
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