22,000 research outputs found
The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?
In the majority of the literature on plasma shock waves, electrons play the
role of "ghost particles," since their contribution to mass and momentum flows
is negligible, and they have been treated as only taking care of the electric
plasma neutrality. In some more recent papers, however, electrons play a new
important role in the shock dynamics and thermodynamics, especially at the
solar-wind termination shock. They react on the shock electric field in a very
specific way, leading to suprathermal nonequilibrium distributions of the
downstream electrons, which can be represented by a kappa distribution
function. In this paper, we discuss why this anticipated hot electron
population has not been seen by the plasma detectors of the Voyager spacecraft
downstream of the solar-wind termination shock. We show that hot nonequilibrium
electrons induce a strong negative electric charge-up of any spacecraft
cruising through this downstream plasma environment. This charge reduces
electron fluxes at the spacecraft detectors to nondetectable intensities.
Furthermore, we show that the Debye length
grows to values of about compared to the classical value in this
hot-electron environment. This unusual condition allows for the propagation of
a certain type of electrostatic plasma waves that, at very large wavelengths,
allow us to determine the effective temperature of the suprathermal electrons
directly by means of the phase velocity of these waves. At moderate
wavelengths, the electron-acoustic dispersion relation leads to nonpropagating
oscillations with the ion-plasma frequency , instead of
the traditional electron plasma frequency.Comment: 6 pages, 2 figure
Distribution functions in percolation problems
Percolation clusters are random fractals whose geometrical and transport
properties can be characterized with the help of probability distribution
functions. Using renormalized field theory, we determine the asymptotic form of
various of such distribution functions in the limits where certain scaling
variables become small or large. Our study includes the pair-connection
probability, the distributions of the fractal masses of the backbone, the red
bonds and the shortest, the longest and the average self-avoiding walk between
any two points on a cluster, as well as the distribution of the total
resistance in the random resistor network. Our analysis draws solely on
general, structural features of the underlying diagrammatic perturbation
theory, and hence our main results are valid to arbitrary loop order.Comment: 15 pages, 1 figur
Broad Histogram Method for Continuous Systems: the XY-Model
We propose a way of implementing the Broad Histogram Monte Carlo method to
systems with continuous degrees of freedom, and we apply these ideas to
investigate the three-dimensional XY-model with periodic boundary conditions.
We have found an excellent agreement between our method and traditional
Metropolis results for the energy, the magnetization, the specific heat and the
magnetic susceptibility on a very large temperature range. For the calculation
of these quantities in the temperature range 0.7<T<4.7 our method took less CPU
time than the Metropolis simulations for 16 temperature points in that
temperature range. Furthermore, it calculates the whole temperature range
1.2<T<4.7 using only 2.2 times more computer effort than the Histogram Monte
Carlo method for the range 2.1<T<2.2. Our way of treatment is general, it can
also be applied to other systems with continuous degrees of freedom.Comment: 23 pages, 10 Postscript figures, to be published in Int. J. Mod.
Phys.
Gauss-Bonnet lagrangian G ln G and cosmological exact solutions
For the lagrangian L = G ln G where G is the Gauss-Bonnet curvature scalar we
deduce the field equation and solve it in closed form for 3-flat Friedman
models using a statefinder parametrization. Further we show, that among all
lagrangians F(G) this L is the only one not having the form G^r with a real
constant r but possessing a scale-invariant field equation. This turns out to
be one of its analogies to f(R)-theories in 2-dimensional space-time. In the
appendix, we systematically list several formulas for the decomposition of the
Riemann tensor in arbitrary dimensions n, which are applied in the main
deduction for n=4.Comment: 18 pages, amended version, accepted by Phys. Rev.
On the relation between 2+1 Einstein gravity and Chern Simons theory
A simple example is given to show that the gauge equivalence classes of
physical states in Chern Simons theory are not in one-to-one correspondence
with those of Einstein gravity in three spacetime dimensions. The two theories
are therefore not equivalent. It is shown that including singular metrics into
general relativity has more, and in fact a quite counter-intuitive, impact on
the theory than one naively expects.Comment: 14 pages, LaTeX2e, 3 eps figure
Numerical modeling of the wind flow over a transverse dune
Transverse dunes, which form under unidirectional winds and have fixed
profile in the direction perpendicular to the wind, occur on all celestial
objects of our solar system where dunes have been detected. Here we perform a
numerical study of the average turbulent wind flow over a transverse dune by
means of computational fluid dynamics simulations. We find that the length of
the zone of recirculating flow at the dune lee --- the {\em{separation bubble}}
--- displays a surprisingly strong dependence on the wind shear velocity,
: it is nearly independent of for shear velocities within
the range between ms and $0.8\,$ms but increases linearly with
for larger shear velocities. Our calculations show that transport in
the direction opposite to dune migration within the separation bubble can be
sustained if is larger than approximately ms, whereas a
larger value of $u_{\ast}$ (about $0.49\,$ms) is required to initiate this
reverse transport.Comment: 11 pages, 8 figure
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