3,168 research outputs found
Schutzvorrichtungen gegen Strahlenschäden auf Schiffen mit Kernenergieantrieb. EUR 255. = Protection against radiation on ships with nuclear-powered engines. EUR 255.
F-region drift velocities from incoherent-scatter measurements at Millstone Hill
F-region drift velocities measured at Millstone Hill from 1968 to 1974 are presented in tabular form. A brief description of the measurement procedures is also given
Calculation of conductivities and currents in the ionosphere
Formulas and procedures to calculate ionospheric conductivities are summarized. Ionospheric currents are calculated using a semidiurnal E-region neutral wind model and electric fields from measurements at Millstone Hill. The results agree well with ground based magnetogram records for magnetic quiet days
Spanning Trees on Hypercubic Lattices and Non-orientable Surfaces
We consider the problem of enumerating spanning trees on lattices.
Closed-form expressions are obtained for the spanning tree generating function
for a hypercubic lattice of size N_1 x N_2 x...x N_d in d dimensions under
free, periodic, and a combination of free and periodic boundary conditions.
Results are also obtained for a simple quartic net embedded on two
non-orientable surfaces, a Moebius strip and the Klein bottle. Our results are
based on the use of a formula expressing the spanning tree generating function
in terms of the eigenvalues of an associated tree matrix. An elementary
derivation of this formula is given.Comment: latex, 9 pages, no figures, to appear in Lett. Appl. Mat
Electric fields in the ionosphere
F-region drift velocities, measured by incoherent-scatter radar were analyzed in terms of diurnal, seasonal, magnetic activity, and solar cycle effects. A comprehensive electric field model was developed that includes the effects of the E and F-region dynamos, magnetospheric sources, and ionospheric conductivities, for both the local and conjugate regions. The E-region dynamo dominates during the day but at night the F-region and convection are more important. This model provides much better agreement with observations of the F-region drifts than previous models. Results indicate that larger magnitudes occur at night, and that daily variation is dominated by the diurnal mode. Seasonal variations in conductivities and thermospheric winds indicate a reversal in direction in the early morning during winter from south to northward. On magnetic perturbed days and the drifts deviate rather strongly from the quiet days average, especially around 13 L.T. for the northward and 18 L.T. for the westward component
Vacancy localization in the square dimer model
We study the classical dimer model on a square lattice with a single vacancy
by developing a graph-theoretic classification of the set of all configurations
which extends the spanning tree formulation of close-packed dimers. With this
formalism, we can address the question of the possible motion of the vacancy
induced by dimer slidings. We find a probability 57/4-10Sqrt[2] for the vacancy
to be strictly jammed in an infinite system. More generally, the size
distribution of the domain accessible to the vacancy is characterized by a
power law decay with exponent 9/8. On a finite system, the probability that a
vacancy in the bulk can reach the boundary falls off as a power law of the
system size with exponent 1/4. The resultant weak localization of vacancies
still allows for unbounded diffusion, characterized by a diffusion exponent
that we relate to that of diffusion on spanning trees. We also implement
numerical simulations of the model with both free and periodic boundary
conditions.Comment: 35 pages, 24 figures. Improved version with one added figure (figure
9), a shift s->s+1 in the definition of the tree size, and minor correction
Equatorial ozone characteristics as measured at Natal (5.9 deg S, 35.2 deg W)
Ozone density profiles obtained through electrochemical concentration cell (ECC) sonde measurements at Natal were analyzed. Time variations, as expected, are small. Outstanding features of the data are tropospheric densities substantially higher than those measured at other stations, and also a total ozone content that is higher than the averages given by satellite measurements
Theory of resistor networks: The two-point resistance
The resistance between arbitrary two nodes in a resistor network is obtained
in terms of the eigenvalues and eigenfunctions of the Laplacian matrix
associated with the network. Explicit formulas for two-point resistances are
deduced for regular lattices in one, two, and three dimensions under various
boundary conditions including that of a Moebius strip and a Klein bottle. The
emphasis is on lattices of finite sizes. We also deduce summation and product
identities which can be used to analyze large-size expansions of two-and-higher
dimensional lattices.Comment: 30 pages, 5 figures now included; typos in Example 1 correcte
Network representations of non-equilibrium steady states: Cycle decompositions, symmetries and dominant paths
Non-equilibrium steady states (NESS) of Markov processes give rise to
non-trivial cyclic probability fluxes. Cycle decompositions of the steady state
offer an effective description of such fluxes. Here, we present an iterative
cycle decomposition exhibiting a natural dynamics on the space of cycles that
satisfies detailed balance. Expectation values of observables can be expressed
as cycle "averages", resembling the cycle representation of expectation values
in dynamical systems. We illustrate our approach in terms of an analogy to a
simple model of mass transit dynamics. Symmetries are reflected in our approach
by a reduction of the minimal number of cycles needed in the decomposition.
These features are demonstrated by discussing a variant of an asymmetric
exclusion process (TASEP). Intriguingly, a continuous change of dominant flow
paths in the network results in a change of the structure of cycles as well as
in discontinuous jumps in cycle weights.Comment: 3 figures, 4 table
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