2,083 research outputs found
Cosmic ray diffusion: Report of the Workshop in Cosmic Ray Diffusion Theory
A workshop in cosmic ray diffusion theory was held at Goddard Space Flight Center on May 16-17, 1974. Topics discussed and summarized are: (1) cosmic ray measurements as related to diffusion theory; (2) quasi-linear theory, nonlinear theory, and computer simulation of cosmic ray pitch-angle diffusion; and (3) magnetic field fluctuation measurements as related to diffusion theory
Quasi-linear theory via the cumulant expansion approach
The cumulant expansion technique of Kubo was used to derive an intergro-differential equation for f , the average one particle distribution function for particles being accelerated by electric and magnetic fluctuations of a general nature. For a very restricted class of fluctuations, the f equation degenerates exactly to a differential equation of Fokker-Planck type. Quasi-linear theory, including the adiabatic assumption, is an exact theory for this limited class of fluctuations. For more physically realistic fluctuations, however, quasi-linear theory is at best approximate
The partially averaged field approach to cosmic ray diffusion
The kinetic equation for particles interacting with turbulent fluctuations is derived by a new nonlinear technique which successfully corrects the difficulties associated with quasilinear theory. In this new method the effects of the fluctuations are evaluated along particle orbits which themselves include the effects of a statistically averaged subset of the possible configurations of the turbulence. The new method is illustrated by calculating the pitch angle diffusion coefficient D sub Mu Mu for particles interacting with slab model magnetic turbulence, i.e., magnetic fluctuations linearly polarized transverse to a mean magnetic field. Results are compared with those of quasilinear theory and also with those of Monte Carlo calculations. The major effect of the nonlinear treatment in this illustration is the determination of D sub Mu Mu in the vicinity of 90 deg pitch angles where quasilinear theory breaks down. The spatial diffusion coefficient parallel to a mean magnetic field is evaluated using D sub Mu Mu as calculated by this technique. It is argued that the partially averaged field method is not limited to small amplitude fluctuating fields and is hence not a perturbation theory
A new approach to cosmic ray diffusion theory
An approach is presented for deriving a diffusion equation for charged particles in a static, random magnetic field. The approach differs from the usual, quasi-linear one, in that particle orbits in the average field are replaced by particle orbits in a partially averaged field. In this way the fluctuating component of the field significantly modifies the particle orbits in those cases where the orbits in the average field are unrealistic. The method permits the calculation of a finite value for the pitch angle diffusion coefficient for particles with a pitch angle of 90 rather than the divergent or ambiguous results obtained by quasi-linear theories. Results of the approach are compared with results of computer simulations using Monte Carlo techniques
Applications of numerical codes to space plasma problems
Solar wind, earth's bowshock, and magnetospheric convection and substorms were investigated. Topics discussed include computational physics, multifluid codes, ionospheric irregularities, and modeling laser plasmas
Comment on 4D Lorentz invariance violations in the brane-world
The brane-world scenario offers the possibility for signals to travel outside
our visible universe and reenter it. We find the condition for a signal emitted
from the brane to return to the brane. We study the propagation of such signals
and show that, as seen by a 4D observer, these signals arrive earlier than
light traveling along the brane. We also study the horizon problem and find
that, while the bulk signals can travel far enough to homogenize the visible
universe, it is unlikely that they have a significant effect since they are
redshifted in the gravitational field of the bulk black hole.Comment: 21 pages, 6 figures, REVTEX, New section adde
A Comprehensive Trainable Error Model for Sung Music Queries
We propose a model for errors in sung queries, a variant of the hidden Markov
model (HMM). This is a solution to the problem of identifying the degree of
similarity between a (typically error-laden) sung query and a potential target
in a database of musical works, an important problem in the field of music
information retrieval. Similarity metrics are a critical component of
query-by-humming (QBH) applications which search audio and multimedia databases
for strong matches to oral queries. Our model comprehensively expresses the
types of error or variation between target and query: cumulative and
non-cumulative local errors, transposition, tempo and tempo changes,
insertions, deletions and modulation. The model is not only expressive, but
automatically trainable, or able to learn and generalize from query examples.
We present results of simulations, designed to assess the discriminatory
potential of the model, and tests with real sung queries, to demonstrate
relevance to real-world applications
Superconductors with Topological Order
We propose a mechanism of superconductivity in which the order of the ground
state does not arise from the usual Landau mechanism of spontaneous symmetry
breaking but is rather of topological origin. The low-energy effective theory
is formulated in terms of emerging gauge fields rather than a local order
parameter and the ground state is degenerate on topologically non-trivial
manifolds. The simplest example of this mechanism of superconductivty is
concretely realized as global superconductivty in Josephson junction arrays.Comment: 4 pages, no figure
Topological Modes in Dual Lattice Models
Lattice gauge theory with gauge group is reconsidered in four
dimensions on a simplicial complex . One finds that the dual theory,
formulated on the dual block complex , contains topological modes
which are in correspondence with the cohomology group ,
in addition to the usual dynamical link variables. This is a general phenomenon
in all models with single plaquette based actions; the action of the dual
theory becomes twisted with a field representing the above cohomology class. A
similar observation is made about the dual version of the three dimensional
Ising model. The importance of distinct topological sectors is confirmed
numerically in the two dimensional Ising model where they are parameterized by
.Comment: 10 pages, DIAS 94-3
Gauge symmetry breaking on orbifolds
We discuss a new method for gauge symmetry breaking in theories with one
extra dimension compactified on the orbifold S^1/Z_2. If we assume that fields
and their derivatives can jump at the orbifold fixed points, we can implement a
generalized Scherk-Schwarz mechanism that breaks the gauge symmetry. We show
that our model with discontinuous fields is equivalent to another with
continuous but non periodic fields; in our scheme localized lagrangian terms
for bulk fields appear.Comment: 6 pages, 2 figures. Talk given at the XXXVIIth Rencontres de Moriond,
"Electroweak interactions and unified theories", Les Arcs, France, 9-16 Mar
2002. Minor changes, one reference adde
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