5,524 research outputs found
Directional Statistics for Polarization Observations of Individual Pulses from Radio Pulsars
Radio polarimetry is a three-dimensional statistical problem. The
three-dimensional aspect of the problem arises from the Stokes parameters Q, U,
and V, which completely describe the polarization of electromagnetic radiation
and conceptually define the orientation of a polarization vector in the
Poincar'e sphere. The statistical aspect of the problem arises from the random
fluctuations in the source-intrinsic polarization and the instrumental noise. A
simple model for the polarization of pulsar radio emission has been used to
derive the three-dimensional statistics of radio polarimetry. The model is
based upon the proposition that the observed polarization is due to the
incoherent superposition of two, highly polarized, orthogonal modes. The
directional statistics derived from the model follow the Bingham-Mardia and
Fisher family of distributions. The model assumptions are supported by the
qualitative agreement between the statistics derived from it and those measured
with polarization observations of the individual pulses from pulsars. The
orthogonal modes are thought to be the natural modes of radio wave propagation
in the pulsar magnetosphere. The intensities of the modes become statistically
independent when generalized Faraday rotation (GFR) in the magnetosphere causes
the difference in their phases to be large. A stochastic version of GFR occurs
when fluctuations in the phase difference are also large, and may be
responsible for the more complicated polarization patterns observed in pulsar
radio emission.Comment: 9 pages, 1 figure. Conference proceedings of Mathematics and
Astronomy: A Joint Long Journey, Madrid, Spain, 23-27 November 2009. See
http://link.aip.org/link/?APCPCS/1283/175/1. Copyright (2010) American
Institute of Physics. This article may be downloaded for personal use only.
Any other use requires prior permission of the author and the American
Institute of Physic
The LEAP of Pulsars in the Milky Way
The location of objects on the celestial sphere is a fundamental measurement
in astronomy, and the distribution of these objects within the Milky Way is
important for understanding their evolution as well as the large scale
structure of the Galaxy. Here, physical concepts in Galactic astronomy are
illustrated using straightforward mathematics and simplifying assumptions
regarding the geometry of the Galaxy. Specifically, an analytical model for a
smooth distribution of particles in an oblate ellipsoid is used to replicate
the observed distributions of the Galactic coordinates for pulsars and
supernova remnants. The distributions and the Lambert equal area projections
(LEAPs) of the coordinates suggest that the dominant factors determining the
general shape of the distributions are the heavy concentration of objects in
the Galactic plane and the offset of the Galactic center from the coordinate
system origin. The LEAPs and the distributions also show that the dispersion of
pulsars about and along the plane are much larger than that for their
progenitor supernovae. Additionally, the model can be used to derive an
analytical expression for the dispersion measure along any line of sight within
the Galaxy. The expression is used to create a hypothetical dispersion
measure-distance map for pulsars in the Galaxy.Comment: 15 pages, 5 figures. Published in proceedings of Mathematics and
Astronomy: A Joint Long Journey, Madrid, Spain, 23-27 November 2009. See
http://link.aip.org/link/?APCPCS/1283/294/1. Copyright (2010) American
Institute of Physics. This article may be downloaded for personal use only.
Any other use requires prior permission of the author and the American
Institute of Physic
Metaphors in and for the Sociology of Religion : Towards a Theory after Nietzsche
Peer reviewedPreprin
The Religious, the Paranormal and Church Attendance : A Response to Orenstein
Peer reviewedPostprin
Sociological Definitions, Language Games and the "Essence" of Religion
Peer reviewedPostprin
Triton's surface age and impactor population revisited in light of Kuiper Belt fluxes: Evidence for small Kuiper Belt objects and recent geological activity
Neptune's largest satellite, Triton, is one of the most fascinating and
enigmatic bodies in the solar system. Among its numerous interesting traits,
Triton appears to have far fewer craters than would be expected if its surface
was primordial. Here we combine the best available crater count data for Triton
with improved estimates of impact rates by including the Kuiper Belt as a
source of impactors. We find that the population of impactors creating the
smallest observed craters on Triton must be sub-km in scale, and that this
small-impactor population can be best fit by a differential power-law size
index near -3. Such results provide interesting, indirect probes of the unseen
small body population of the Kuiper Belt. Based on the modern, Kuiper Belt and
Oort Cloud impactor flux estimates, we also recalculate estimated ages for
several regions of Triton's surface imaged by Voyager 2, and find that Triton
was probably active on a time scale no greater than 0.1-0.3 Gyr ago (indicating
Triton was still active after some 90% to 98% of the age of the solar system),
and perhaps even more recently. The time-averaged volumetric resurfacing rate
on Triton implied by these results, 0.01 km yr or more, is likely
second only to Io and Europa in the outer solar system, and is within an order
of magnitude of estimates for Venus and for the Earth's intraplate zones. This
finding indicates that Triton likely remains a highly geologically active world
at present, some 4.5 Gyr after its formation. We briefly speculate on how such
a situation might obtain.Comment: 14 pages (TeX), plus 2 postscript figures Stern & McKinnon, 2000, AJ,
in pres
Design and development of a six degree of freedom hand controller
The design objectives of a six degree of freedom manual controller are discussed with emphasis on a space environment. Details covered include problems associated with a zero-g environment, the need to accommodate both 'shirt sleeve' and space suited astronauts, the combination of both manipulator operation and spacecraft flight control in a single device, and to accommodate restraints in space. A variable configuration device designed as a development tool in which rotational axes can be moved relative to one another, is described and its limitations discussed. Two additional devices were developed for concept testing. Each device combines the need for good quality with its ability achieve a wide range of adjustments
Multi-axis manual controllers: A state-of-the-art report
A literature search was carried out to examine the feasibility of a six degree of freedom hand controller. Factors addressed included related areas, approaches to manual control, applications of manual controllers, and selected studies of the human neuromuscular system. Results are presented
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