19,534 research outputs found
Planetary astronomy program
Observations and analyses of asteroids, Trojans and cometary nuclei are presented. Spectrophotometry was used to observe the cometary nuclei. The spectra are plotted as a function of semimajor axis and eccentricity. Trojans and other asteroids at great solar distances show a variety of spectra, many of them quite red despite the low measured albedoes for many of these asteroids. The asteroid spectra are grouped according to diameter and taxonomic class
Pseudo-nonstationarity in the scaling exponents of finite-interval time series
The accurate estimation of scaling exponents is central in the observational study of scale-invariant phenomena. Natural systems unavoidably provide observations over restricted intervals; consequently, a stationary stochastic process (time series) can yield anomalous time variation in the scaling exponents, suggestive of nonstationarity. The variance in the estimates of scaling exponents computed from an interval of N observations is known for finite variance processes to vary as ~1/N as N for certain statistical estimators; however, the convergence to this behavior will depend on the details of the process, and may be slow. We study the variation in the scaling of second-order moments of the time-series increments with N for a variety of synthetic and “real world” time series, and we find that in particular for heavy tailed processes, for realizable N, one is far from this ~1/N limiting behavior. We propose a semiempirical estimate for the minimum N needed to make a meaningful estimate of the scaling exponents for model stochastic processes and compare these with some “real world” time series
Comparing the performance of baseball players : a multiple output approach
This article extends ideas from the economics literature on multiple output production and efficiency to develop methods for comparing baseball players that take into account the many dimensions to batting performance. A key part of this approach is the output aggregator. The weights in this output aggregator can be selected a priori (as is done with batting or slugging averages) or can be estimated statistically based on the performance of the best players in baseball. Once the output aggregator is obtained, an individual player can then be measured relative to the best, and a number between 0 and 1 characterizes his performance as a fraction of the best. The methods are applied to hitters using data from 1995-1999 on all regular players in baseball's major leagues
Multi-Spacecraft Measurement of Turbulence within a Magnetic Reconnection Jet
The relationship between magnetic reconnection and plasma turbulence is
investigated using multipoint in-situ measurements from the Cluster spacecraft
within a high-speed reconnection jet in the terrestrial magnetotail. We show
explicitly that work done by electromagnetic fields on the particles,
, has a non-Gaussian distribution and is
concentrated in regions of high electric current density. Hence, magnetic
energy is converted to kinetic energy in an intermittent manner. Furthermore,
we find the higher-order statistics of magnetic field fluctuations generated by
reconnection are characterized by multifractal scaling on magnetofluid scales
and non-Gaussian global scale invariance on kinetic scales. These observations
suggest within the reconnection jet has an analogue
in fluid-like turbulence theory in that it proceeds via coherent structures
generated by an intermittent cascade. This supports the hypothesis that
turbulent dissipation is highly nonuniform, and thus these results could have
far reaching implications for space and astrophysical plasmas.Comment: 5 pages, 3 figures, submitted to Physical Review Letter
The impact of space and space-related activities on a local economy. a case study of boulder, colorado. part i- the input-output analysis
Impact of space and space-related activities on industry and general economy of Boulder, Colorad
Examples of mathematical modeling tales from the crypt
Mathematical modeling is being increasingly recognized within the biomedical sciences as an important tool that can aid the understanding of biological systems. The heavily regulated cell renewal cycle in the colonic crypt provides a good example of how modeling can be used to find out key features of the system kinetics, and help to explain both the breakdown of homeostasis and the initiation of tumorigenesis. We use the cell population model by Johnston et al. (2007) Proc. Natl. Acad. Sci. USA 104, 4008-4013, to illustrate the power of mathematical modeling by considering two key questions about the cell population dynamics in the colonic crypt. We ask: how can a model describe both homeostasis and unregulated growth in tumorigenesis; and to which parameters in the system is the model most sensitive? In order to address these questions, we discuss what type of modeling approach is most appropriate in the crypt. We use the model to argue why tumorigenesis is observed to occur in stages with long lag phases between periods of rapid growth, and we identify the key parameters
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