1,547 research outputs found
Inverse problems of symbolic dynamics
This paper reviews some results regarding symbolic dynamics, correspondence
between languages of dynamical systems and combinatorics. Sturmian sequences
provide a pattern for investigation of one-dimensional systems, in particular
interval exchange transformation. Rauzy graphs language can express many
important combinatorial and some dynamical properties. In this case
combinatorial properties are considered as being generated by substitutional
system, and dynamical properties are considered as criteria of superword being
generated by interval exchange transformation. As a consequence, one can get a
morphic word appearing in interval exchange transformation such that
frequencies of letters are algebraic numbers of an arbitrary degree.
Concerning multydimensional systems, our main result is the following. Let
P(n) be a polynomial, having an irrational coefficient of the highest degree. A
word (w=(w_n), n\in \nit) consists of a sequence of first binary numbers
of i.e. . Denote the number of different subwords
of of length by .
\medskip {\bf Theorem.} {\it There exists a polynomial , depending only
on the power of the polynomial , such that for sufficiently
great .
Fast Ray Tracing of Lunar Digital Elevation Models
Ray-tracing (RT) of Lunar Digital Elevation Models (DEM)'s is performed to virtually derive the degree of radiation incident to terrain as a function of time, orbital and ephemeris constraints [I- 4]. This process is an integral modeling process in lunar polar research and exploration due to the present paucity of terrain information at the poles and mission planning activities for the anticipated spring 2009 launch of the Lunar Reconnaissance Orbiter (LRO). As part of the Lunar Exploration Neutron Detector (LEND) and Lunar Crater Observation and Sensing Satellite (LCROSS) preparations RI methods are used to estimate the critical conditions presented by the combined effects of high latitude, terrain and the moons low obliquity [5-7]. These factors yield low incident solar illumination and subsequently extreme thermal, and radiation conditions. The presented research uses RT methods both for radiation transport modeling in space and regolith related research as well as to derive permanently shadowed regions (PSR)'s in high latitude topographic minima, e.g craters. These regions are of scientific and human exploration interest due to the near constant low temperatures in PSRs, inferred to be < 100 K. Hydrogen is thought to have accumulated in PSR's through the combined effects of periodic cometary bombardment and/or solar wind processes, and the extreme cold which minimizes hydrogen sublimation [8-9]. RT methods are also of use in surface position optimization for future illumination dependent on surface resources e.g. power and communications equipment
Comparison of Image Restoration Methods for Lunar Epithermal Neutron Emission Mapping
Orbital measurements of neutrons by the Lunar Exploring Neutron Detector (LEND) onboard the Lunar Reconnaissance Orbiter are being used to quantify the spatial distribution of near surface hydrogen (H). Inferred H concentration maps have low signal-to-noise (SN) and image restoration (IR) techniques are being studied to enhance results. A single-blind. two-phase study is described in which four teams of researchers independently developed image restoration techniques optimized for LEND data. Synthetic lunar epithermal neutron emission maps were derived from LEND simulations. These data were used as ground truth to determine the relative quantitative performance of the IR methods vs. a default denoising (smoothing) technique. We review and used factors influencing orbital remote sensing of neutrons emitted from the lunar surface to develop a database of synthetic "true" maps for performance evaluation. A prior independent training phase was implemented for each technique to assure methods were optimized before the blind trial. Method performance was determined using several regional root-mean-square error metrics specific to epithermal signals of interest. Results indicate unbiased IR methods realize only small signal gains in most of the tested metrics. This suggests other physically based modeling assumptions are required to produce appreciable signal gains in similar low SN IR applications
Are Slope Streaks Indicative of Global-Scale Aqueous Processes on Contemporary Mars?
Acknowledgments We acknowledge the editorial board of Reviews of Geophysics for inviting the submission of this review article. We extend our gratitude to the efforts of the handling editor and the reviewers. We thank NASA, JPLâCaltech, JPL/Goddard, University of Arizona, Malin Space Science systems, Arizona State University, USGS, ESA/DLR/FU Berlin, and Google Earth for providing various satellite images, maps, and JMARS software free of charge. The paper is theoretical, and no new data have been generated during the work. All the used satellite images of Mars can be rendered on JMARS software using the image ID provided in the respective figure captions, and the image sources have also been duly acknowledged in the respective figure captions. The maps in various figures have been created using ArcGIS version 10.4 (http://desktop.arcgis.com/en/arcmap/latest/getâstarted/setup/arcgisâdesktopâquickâstartâguide.htm). Although we have cited all the previous research results used in the paper, we here acknowledge the efforts of all those researchers in providing the essential inputs for our study. A. B. acknowledges the Swedish Research Council for supporting his research in cold arid environments. L. S. acknowledges the German Academic Exchange Service (DAAD) for her PhD scholarship.Peer reviewedPublisher PD
Isolation Effects on the Moon: High Topographic Slope Observations from the LRO and LOLA Instruments
The extremely low temperatures in the Moon's polar permanent shadow regions (PSR) has long been considered a unique factor necessary for entrapping volatile Hydrogen (H). However, recent discoveries indicate some H concentrations lie outside PSR, suggesting other geophysical factors may also influence H distributions. In this study we consider insolation and its resulting thermal effects as a loss/redistribution process influencing the Moon's near-surface < 1m volatile H budget. To isolate regional (5deg latitude band) insolation effects we correlate two data sets collected from the ongoing, 1.5 year long mapping mission of the Lunar Reconnaissance Orbiter (LRO). Epithermal neutron mapping data from the Lunar Exploration Neutron Detector (LEND) is registered and analyzed in the context of slope derivations from Lunar topography maps produced by the Lunar Observing Laser Altimeter (LOLA). Lunar epithermal neutrons are inferred to be direct geochemical evidence for near-surface H due to the correlated suppression of surface leakage fluxes of epithermal neutrons with increased H concentration. Regional suppressions of neutrons seen in LEND maps are considered localized evidence of H concentration increase in the upper 1 m of the Lunar surface. To quantify spatially localized insolation effects, LEND data are averaged from sparsely distributed pixels, classed as a function of the LOLA slope derivations
Gamma Ray Bursts from the Evolved Galactic Nuclei
A new cosmological scenario for the origin of gamma ray bursts (GRBs) is
proposed. In our scenario, a highly evolved central core in the dense galactic
nucleus is formed containing a subsystem of compact stellar remnants (CSRs),
such as neutron stars and black holes. Those subsystems result from the
dynamical evolution of dense central stellar clusters in the galactic nuclei
through merging of stars, thereby forming (as has been realized by many
authors) the short-living massive stars and then CSRs. We estimate the rate of
random CSR collisions in the evolved galactic nuclei by taking into account,
similar to Quinlan & Shapiro (1987), the dissipative encounters of CSRs, mainly
due to radiative losses of gravitational waves, which results in the formation
of intermediate short-living binaries, with further coalescence of the
companions to produce GRBs. We also consider how the possible presence of a
central supermassive black hole, formed in a highly evolved galactic nucleus,
influences the CSR binary formation. This scenario does not postulate ad hoc a
required number of tight binary neutron stars in the galaxies. Instead, it
gives, for the most realistic parameters of the evolved nuclei, the expected
rate of GRBs consistent with the observed one, thereby explaining the GRB
appearance in a natural way of the dynamical evolution of galactic nuclei. In
addition, this scenario provides an opportunity for a cosmological GRB
recurrence, previously considered to be a distinctive feature of GRBs of a
local origin only. We also discuss some other observational tests of the
proposed scenario.Comment: 25 pages, LATEX, uses aasms4.sty, accepted by Ap
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