1,648 research outputs found
The GEOS-3 orbit determination investigation
The nature and improvement in satellite orbit determination when precise altimetric height data are used in combination with conventional tracking data was determined. A digital orbit determination program was developed that could singly or jointly use laser ranging, C-band ranging, Doppler range difference, and altimetric height data. Two intervals were selected and used in a preliminary evaluation of the altimeter data. With the data available, it was possible to determine the semimajor axis and eccentricity to within several kilometers, in addition to determining an altimeter height bias. When used jointly with a limited amount of either C-band or laser range data, it was shown that altimeter data can improve the orbit solution
Comparing various multi-component global heliosphere models
Modeling of the global heliosphere seeks to investigate the interaction of
the solar wind with the partially ionized local interstellar medium. Models
that treat neutral hydrogen self-consistently and in great detail, together
with the plasma, but that neglect magnetic fields, constitute a sub-category
within global heliospheric models. There are several different modeling
strategies used for this sub-category in the literature. Differences and
commonalities in the modeling results from different strategies are pointed
out. Plasma-only models and fully self-consistent models from four research
groups, for which the neutral species is modeled with either one, three, or
four fluids, or else kinetically, are run with the same boundary parameters and
equations. They are compared to each other with respect to the locations of key
heliospheric boundary locations and with respect to the neutral hydrogen
content throughout the heliosphere. In many respects, the models' predictions
are similar. In particular, the locations of the termination shock agree to
within 7% in the nose direction and to within 14% in the downwind direction.
The nose locations of the heliopause agree to within 5%. The filtration of
neutral hydrogen from the interstellar medium into the inner heliosphere,
however, is model dependent, as are other neutral results including the
hydrogen wall. These differences are closely linked to the strength of the
interstellar bow shock. The comparison also underlines that it is critical to
include neutral hydrogen into global heliospheric models.Comment: 10 pages, 4 figures, submitted to a special section at A&A of an ISSI
team "Determination of the physical Hydrogen parameters of the LIC from
within the Heliosphere
GEOS-3 ocean geoid investigation
A determination of the fine scale sea surface topography in the GEOS-3 calibration area using the radar altimeter data is presented. Estimates of the north-south and east-west components of the deflections of the vertical as well as values of the geoidal heights were made. Three major stages of processing were used in obtaining the final results. The first two use pass processors; in the final stage, the processor combines all the pass results to compute the final results. The results obtained compare favorably with gravimetrically determined geoids for this calibration area
Localization of Charged Quantum Particles in a Static Random Magnetic Field
We consider a charged quantum particle in a random magnetic field with
Gaussian, delta-correlated statistics. We show that although the single
particle properties are peculiar, two particle quantities such as the diffusion
constant can be calculated in perturbation theory. We map the problem onto a
non-linear sigma-model for Q-matrices of unitary symmetry with renormalized
diffusion coefficient for which all states are known to be localized in
dimensions. Our results compare well with recent numerical data.Comment: REVTEX, 12 pages, 1 figure attached as a postscript file. To appear
in Phys.Rev.
Wavefunction and level statistics of random two dimensional gauge fields
Level and wavefunction statistics have been studied for two dimensional
clusters of the square lattice in the presence of random magnetic fluxes.
Fluxes traversing lattice plaquettes are distributed uniformly between - (1/2)
Phi_0 and (1/2) Phi_0 with Phi_0 the flux quantum. All considered statistics
start close to the corresponding Wigner-Dyson distribution for small system
sizes and monotonically move towards Poisson statistics as the cluster size
increases. Scaling is quite rapid for states close to the band edges but really
difficult to observe for states well within the band. Localization properties
are discussed considering two different scenarios. Experimental measurement of
one of the considered statistics --wavefunction statistics seems the most
promising one-- could discern between both possibilities. A real version of the
previous model, i.e., a system that is invariant under time reversal, has been
studied concurrently to get coincidences and differences with the Hermitian
model.Comment: 12 twocolumnn pages in revtex style, 17 postscript figures, to be
published in PRB, send comments to [email protected]
Diversity, genetic structure and evidence of outcrossing in British populations of the rock fern Adiantum capillus-veneris using microsatellites
Microsatellites were isolated and a marker system was developed in the fern Adiantum capillus-veneris. Polymorphic markers were then used to study the genetic diversity and structure of populations within the UK and Ireland where this species grows at the northern edge of its range, requiring a specific rock habitat and limited to a few scattered populations. Three dinucleotide loci detected a high level of diversity (23 alleles and 28 multilocus genotypes) across the UK and Ireland, with nearly all variation partitioned among rather than within populations. Of 17 populations represented by multiple samples, all except four were monomorphic. Heterozygosity was detected in three populations, all within Glamorgan, Wales (UK), showing evidence of outcrossing. We make inferences on the factors determining the observed levels and patterns of genetic variation and the possible evolutionary history of the populations
The Tidal Tails of NGC 5466
The study of substructure in the stellar halo of the Milky Way has made a lot
of progress in recent years, especially with the advent of surveys like the
Sloan Digital Sky Survey. Here, we study the newly discovered tidal tails of
the Galactic globular cluster NGC 5466. By means of numerical simulations, we
reproduce the shape, direction and surface density of the tidal tails, as well
as the structural and kinematical properties of the present-day NGC 5466.
Although its tails are very extended in SDSS data (> 45 degrees), NGC 5466 is
only losing mass slowly at the present epoch and so can survive for probably a
further Hubble time. The effects of tides at perigalacticon and disc crossing
are the dominant causes of the slow dissolution of NGC 5466, accounting for
about 60 % of the mass loss over the course of its evolution. The morphology of
the tails provides a constraint on the proper motion -- the observationally
determined proper motion has to be refined (within the stated error margins) to
match the location of the tidal tails.Comment: MNRAS, in pres
Magnetic field dependence of the exciton energy in a quantum disk
The groundstate energy and binding energy of an exciton, confined in a^M
quantum disk, are calculated as a function of an external magnetic field. The
confinement potential is a hard wall of finite height. The diamagnetic shift is
investigated for magnetic fields up to 40. Our results are applied to
self-assembled quantum dots and very good
agreement with experiments is obtained. Furthermore, we investigated the
influence of the dot size on the diamagnetic shift by changing the disk radius.
The exciton excited states are found as a function of the magnetic field. The
relative angular momentum is not a quantum number and changes with the magnetic
field strength.Comment: 10 pages, 17 figure
Mass segregation of different populations inside the cluster NGC6101
We have used ESO telescopes at La Silla and the Hubble Space Telescope (HST)
in order to obtain accurate B,V,I CCD photometry for the stars located within
200" (~= 2 half-mass radii, r_h = 1.71') from the center of the cluster NGC
6101. Color-Magnitude Diagrams extending from the red-giant tip to about 5
magnitudes below the main-sequence turnoff MSTO (V = 20.05 +- 0.05) have been
constructed.
The following results have been obtained from the analysis of the CMDs: a)
The overall morphology of the main branches confirms previous results from the
literature, in particular the existence of a sizeable population of 73 "blue
stragglers", which had been already partly detected (27).They are considerably
more concentrated than either the subgiant branch or the main sequence stars,
and have the same spatial distribution as the horizontal branch stars (84%
prob. from K-S test). An hypothesis on the possible BSS progeny is also
presented. b) The HB is narrow and the bulk of stars is blue, as expected for a
typical metal-poor globular cluster. c) The derived magnitudes for the HB and
the MSTO, $V(ZAHB) = 16.59+-0.10, V(TO) = 20.05+-0.05, coupled with the values
E(B-V) = 0.1, [Fe/H] = -1.80, Y = 0.23 yield a distance modulus (m-M)_V = 16.23
and an age similar to other ``old'' metal-poor globular clusters. In
particular, from the comparison with theoretical isochrones, we derive for this
cluster an age of 13 Gyrs. d) By using the large statistical sample of Red
Giant Branch (RGB) stars, we detected with high accuracy the position of the
bump in the RGB luminosity function. This observational feature has been
compared with theoretical prescriptions, yielding a good agreement within the
current theoretical and observational uncertainties.Comment: 13 pages, 17 figures, uses documentclass 'aa' v 5.01 with package
'graphicx'. Accepted for publication in Astronomy & Astrophysic
- …