13,623 research outputs found
Convection in the Earth's core driven by lateral variations in the core-mantle boundary heat flux
Moving core fluid maintains an isothermal core-mantle boundary (CMB), so lateral variations in the CMB heat flow result from mantle convection. Such variations will
drive thermal winds, even if the top of the core is stably stratified. These flows may contribute to the magnetic secular variation and are investigated here using a simple,
non-magnetic numerical model of the core. The results depend on the equatorial symmetry of the boundary heat flux variation. Large-scale equatorially symmetric
(ES) heat flux variations at the outer surface of a rapidly rotating spherical shell drive
deeply penetrating flows that are strongly suppressed in stratified fluid. Smaller-scale
ES heat flux variations drive flows less dominated by rotation and so less inhibited
by stratification. Equatorially anti-symmetric flux variations drive flows an order of
magnitude less energetic than those driven by ES patterns but, due to the nature of the Coriolis force, are less suppressed by stratification. The response of the rotating core fluid to a general CMB heat flow pattern will then depend strongly on the subadiabatic temperature profile. Imposing a lateral heat flux variation linearly related to a model of seismic tomography in the lowermost mantle drives flow in a density stratified fluid that
reproduces some features found in flows inverted from geomagnetic data
The DSI small satellite launcher
A new launcher has been developed by DSI, that is compatible with the GAS canisters. It has the proven capability to deploy a satellite from an orbiting Shuttle that is 18 inches in diameter, 31 inches long, and weighing 190 pounds. These DSI Launchers were used aboard the Discovery (STS-39) in May 1991 as part of the Infrared Background Signature Survey (IBSS) to deploy three small satellites known as Chemical Release Observation (CRO) satellites A, B, and C. Because the satellites contained hazardous liquids (MMH, UDMH, and MON-10) and were launched from GAS Cylinders without motorized doors, the launchers were required to pass NASA Shuttle Payload safety and verification requirements. Some of the more interesting components of the design were the V-band retention and separation mechanism, the separation springs, and the launcher electronics which provided a properly inhibited release sequence operated through the Small Payload Accommodations Switch Panel (SPASP) on board the Orbiter. The original plan for this launcher was to use a motorized door. The launcher electronics, therefore has the capability to be modified to accommodate the door, if desired
Critical Pebbling Numbers of Graphs
We define three new pebbling parameters of a connected graph , the -,
-, and -critical pebbling numbers. Together with the pebbling number, the
optimal pebbling number, the number of vertices and the diameter of the
graph, this yields 7 graph parameters. We determine the relationships between
these parameters. We investigate properties of the -critical pebbling
number, and distinguish between greedy graphs, thrifty graphs, and graphs for
which the -critical pebbling number is .Comment: 26 page
Moduli, Scalar Charges, and the First Law of Black Hole Thermodynamics
We show that under variation of moduli fields the first law of black
hole thermodynamics becomes , where are the scalar charges. We also show
that the ADM mass is extremized at fixed , , when the moduli
fields take the fixed value which depend only on electric
and magnetic charges. It follows that the least mass of any black hole with
fixed conserved electric and magnetic charges is given by the mass of the
double-extreme black hole with these charges. Our work allows us to interpret
the previously established result that for all extreme black holes the moduli
fields at the horizon take a value depending only
on the electric and magnetic conserved charges: is such
that the scalar charges .Comment: 3 pages, no figures, more detailed versio
The Action of Instantons with Nut Charge
We examine the effect of a non-trivial nut charge on the action of
non-compact four-dimensional instantons with a U(1) isometry. If the instanton
action is calculated by dimensionally reducing along the isometry, then the nut
charge is found to make an explicit non-zero contribution. For metrics
satisfying AF, ALF or ALE boundary conditions, the action can be expressed
entirely in terms of quantities (including the nut charge) defined on the fixed
point set of the isometry. A source (or sink) of nut charge also implies the
presence of a Misner string coordinate singularity, which will have an
important effect on the Hamiltonian of the instanton.Comment: 25 page
Vacuum decay via Lorentzian wormholes
We speculate about the spacetime description due to the presence of
Lorentzian wormholes (handles in spacetime joining two distant regions or other
universes) in quantum gravity. The semiclassical rate of production of these
Lorentzian wormholes in Reissner-Nordstr\"om spacetimes is calculated as a
result of the spontaneous decay of vacuum due to a real tunneling
configuration. In the magnetic case it only depends on the field theoretical
fine structure constant. We predict that the quantum probability corresponding
to the nucleation of such geodesically complete spacetimes should be actually
negligible in our physical Universe
Nucleating Black Holes via Non-Orientable Instantons
We extend the analysis of black hole pair creation to include non- orientable
instantons. We classify these instantons in terms of their fundamental
symmetries and orientations. Many of these instantons admit the pin structure
which corresponds to the fermions actually observed in nature, and so the
natural objection that these manifolds do not admit spin structure may not be
relevant. Furthermore, we analyse the thermodynamical properties of
non-orientable black holes and find that in the non-extreme case, there are
interesting modifications of the usual formulae for temperature and entropy.Comment: 27 pages LaTeX, minor typos are correcte
Charged Dilaton Black Holes with a Cosmological Constant
The properties of static spherically symmetric black holes, which are either
electrically or magnetically charged, and which are coupled to the dilaton in
the presence of a cosmological constant, are considered. It is shown that such
solutions do not exist if the cosmological constant is positive (in arbitrary
spacetime dimension >= 4). However, asymptotically anti-de Sitter black hole
solutions with a single horizon do exist if the cosmological constant is
negative. These solutions are studied numerically in four dimensions and the
thermodynamic properties of the solutions are derived. The extreme solutions
are found to have zero entropy and infinite temperature for all non-zero values
of the dilaton coupling constant.Comment: 12 pages, epsf, phyzzx, 4 in-text figures incl. (minor typos fixed, 1
reference added
Two loop and all loop finite 4-metrics
In pure Einstein theory, Ricci flat Lorentzian 4-metrics of Petrov types III
or N have vanishing counter terms up to and including two loops. Moreover for
pp-waves and type-N spacetimes of Kundt's class which admit a non-twisting, non
expanding, null congruence all possible invariants formed from the Weyl tensor
and its covariant derivatives vanish. Thus these Lorentzian metrics suffer no
quantum corrections to all loop orders. By contrast for complete non-singular
Riemannian metrics the two loop counter term vanishes only if the metric is
flat.Comment: 4 pages Latex file, no figure
Bohm and Einstein-Sasaki Metrics, Black Holes and Cosmological Event Horizons
We study physical applications of the Bohm metrics, which are infinite
sequences of inhomogeneous Einstein metrics on spheres and products of spheres
of dimension 5 <= d <= 9. We prove that all the Bohm metrics on S^3 x S^2 and
S^3 x S^3 have negative eigenvalue modes of the Lichnerowicz operator and by
numerical methods we establish that Bohm metrics on S^5 have negative
eigenvalues too. We argue that all the Bohm metrics will have negative modes.
These results imply that higher-dimensional black-hole spacetimes where the
Bohm metric replaces the usual round sphere metric are classically unstable. We
also show that the stability criterion for Freund-Rubin solutions is the same
as for black-hole stability, and hence such solutions using Bohm metrics will
also be unstable. We consider possible endpoints of the instabilities, and show
that all Einstein-Sasaki manifolds give stable solutions. We show how Wick
rotation of Bohm metrics gives spacetimes that provide counterexamples to a
strict form of the Cosmic Baldness conjecture, but they are still consistent
with the intuition behind the cosmic No-Hair conjectures. We show how the
Lorentzian metrics may be created ``from nothing'' in a no-boundary setting. We
argue that Lorentzian Bohm metrics are unstable to decay to de Sitter
spacetime. We also argue that noncompact versions of the Bohm metrics have
infinitely many negative Lichernowicz modes, and we conjecture a general
relation between Lichnerowicz eigenvalues and non-uniqueness of the Dirichlet
problem for Einstein's equations.Comment: 53 pages, 11 figure
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