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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

Statistical model of meteor streams. IV - A study of radio streams from the synoptic year

The paper gives a detailed description of a search procedure for radio meteor streams and reports the detection of 275 streams in a synoptic-year sample of 19,698 radio meteors observed by the radar system of the Radio Meteor Project at Havana, Illinois. The orbital elements and related parameters of the detected streams are presented, the two parameters of the D-distribution of the streams are determined, and their mean radiants are plotted. Possible associations between the streams and possible parent objects are identified; it is found that streams may be associated with seven periodic comets, five other comets, at least nine asteroids (especially Adonis), and several fireballs. The mean space density in the streams is found to be much lower than the sporadic density, although the central density may be considerably greater than the sporadic density. It is shown that the derived absolute stream-density values are in agreement with the order of magnitude of the space densities estimated from cometary production rates for solid material of comparable particle size.

Sekanina, Z.↗
NASA NTRS1975

A statistical model for telecommunication link design

An evaluation is conducted of the current telecommunication link design technique and a description is presented of an alternative method, called the probability distribution method (PDM), which is free of the disadvantages of the current technique while retaining its advantages. The PDM preserves the simplicity of the design control table (DCT) format. The use of the DCT as a management design control tool is continued. The telecommunication link margin probability density function used presents the probability of achieving any particular value of link performance. It is, therefore, possible to assess the performance risk and other tradeoffs.

Yuen, J. H.↗
NASA NTRS1979

A statistical model for the initial stellar mass function

The nonlinear stages of the fragmentation of a collapsing molecular cloud are modeled by coagulation theory. Several distinct physical processes are discussed, including protostellar fragment coalescence, gas accretion, and binary formation. This work confirms and extends the earlier result of Nakano (1966) that an approximately self-similar limiting form of the mass spectrum develops after several mean initial collision times. An approximate solution to the velocity-averaged coagulation equation is given for an arbitrary power-law dependence of the coalescence rate on mass, with dimensional dependence proportional to m to the power lambda; i.e., the asymptotic mass spectrum varies as m to the -3 lambda/2 power at small masses and cuts off exponentially at large masses, the characteristic mass depending on the number of collision times elapsed. Simple physical arguments suggest that lambda may increase with increasing mass, but is restricted to the range from 2/3 to 4/3. A large fraction of collisions could result in binary formation.

Silk, J.↗
NASA NTRS1983

A non-Gaussian statistical model for surface elevation of nonlinear random wave fields

Probability density function of the surface elevation of a nonlinear random wave field is obtained. The wave model is based on the Stokes expansion carried to the third order for both deep water waves and waves in finite depth. The amplitude and phase of the first-order component of the Stokes wave are assumed to be Rayleigh and uniformly distributed and slowly varying, respectively. The probability density function for the deep water case was found to depend on two parameters: the root-mean-square surface elevation and the significant slope. For water of finite depth, an additional parameter, the nondimensional depth, is also required. An important difference between the present result and the Gram-Charlier representation is that the present probability density functions are always nonnegative. It is also found that the 'constant' term in the Stokes expansion, usually neglected in deterministic studies, plays an important role in determining the details of the density function. The results compare well with laboratory and field experiment data.

Huang, N. E.↗
NASA NTRS1993

Improved Statistical Model Of 10.7-cm Solar Radiation

Improved mathematical model simulates short-term fluctuations of flux of 10.7-cm-wavelength solar radiation during 91-day averaging period. Called "F10.7 flux", important as measure of solar activity and because it is highly correlated with ultraviolet radiation causing fluctuations in heating and density of upper atmosphere. F10.7 flux easily measureable at surface of Earth.

Vedder, John D.↗
NASA NTRS1993

Statistical modeling of capture, association, and exit-channel dynamics in the CH3(+)/CH3CN system

The ion-molecule CH3(+) + CH3CN reaction is presently modeled by means of a master equation treatment incorporating weak collisions. The parameter required for the Rice-Ramsberger-Kassel-Markus (RRKM) treatment is derived from an ab initio investigation of the energy surface in question, and a means of including the capture rate coefficients in the RRKM approach is developed in which only the hindered dipole rotation is coupled in the reaction coordinate at large separations. Unimolecular rate coefficients for the (CH3CNCH3+)-activated complex are calculated for all product channels in the 300-600 K temperature range.

Smith, S. C.↗
NASA NTRS1997

Statistical Models of Areal Distribution of Fragmented Land Cover Types

Imagery of coarse resolution, such weather satellite imagery with 1 square kilometer pixels, is increasingly used to monitor dynamic and fragmented types of land surface types, such as scars from recent fires and ponds in wetlands. Accurate estimates of these land cover types at regional to global scales are required to assess the roles of fires and wetlands in global warming, yet difficult to compute when much of the area is accounted for by fragments about the same size as the pixels. In previous research, we found that size distribution of the fragments in several example scenes fit simple two-parameter models and related effects of coarse resolution to errors in area estimates based on pixel counts. We summarize our model based approach to improved area estimations and report on progress to develop accurate areas estimates based on modeling the size distribution of the fragments, including analysis of size distributions on an expanded set of maps developed from digital imagery.

Hlavka, C.↗