Evidence for monochromatic unstable Weibel modes in asymmetric counterstreaming pair plasmas     

R. C. Tautz  J.-I. Sakai  I. Lerche 《Astrophysics and Space Science》2007,310(1-2):159-167

In this article, an asymmetric counterstreaming distribution function is investigated on the basis of three-dimensional relativistic particle-in-cell simulations for wave propagation at an oblique angle with respect to the axis of the counterstream. For such asymmetric distribution functions, any linear Weibel modes must be isolated and therefore restricted to discrete wavenumber values. Using analytical linear Vlasov theory, this result has recently been proven generally, and has been illustrated by the example of an electron beam counterstreaming with a positron beam. By the means of self-consistent particle-in-cell simulations, in this paper a realistic distribution function is investigated that consists of neutral asymmetric Maxwellian counterstreams. For this scenario, the existence of isolated modes can be confirmed, especially when compared to the case of symmetric counterstreams.   相似文献   

A model of regional fluid flow: Sand concentration factors and effective lateral and vertical permeabilities     

W. H. Glezen  I. Lerche 《Mathematical Geology》1985,17(3):297-315

Permeabilities of sand and shale differ by five to seven orders of magnitude. In the depositional pattern characteristic of the Gulf Coast, lenticular sand bodies occur randomly dispersed within a shale matrix, even though they can be grouped into stratigraphic sequences, contributing to net permeability of the sand-shale system and thus facilitating upward flow of pore water. In addition, a substantial lateral flow develops as fluid flow is directed toward sand bodies, thus concentrating fluid from a wide source area into sand bodies of limited lateral and vertical extent. Calculations reported here determine the effect of randomly distributed sands on both vertical and lateral permeabilities of composite sandshale systems. Effective permeabilities depend only on mean sand percent and local variance about the mean, because the effects of sand body shapes and their orientation and dip are not incorporated in the analysis. Results indicate that at a mean sand percentage of 20–50% a large lateral migration of fluid flow develops as sand bodies attract flow from a region two to seven times the radius of the sand body itself. For a mean sand percentage in excess of 50–60%, large vertical migration results and thus hydrocarbon trapping should not be expected for sand percentages in excess of 50–60%. Results of these calculations are in accord with observations that most larger oil accumulations on the Gulf Coast are found in regions having sandiness between about 20 and 50%.  相似文献   

Inverse methods and kinetic models of hydrocarbon generation: III. Case histories for autoclave product analysis     

Jianchang Liu  Ian Lerche 《Mathematical Geology》1991,23(3):325-347

From a determination of the transformation matrix for three pyrolysis product experimental data sets, an examination is given of both the applicability of the laboratory experimental data to the modeling of oil cracking in a sedimentary basin, and of the appropriateness of an inverse model. The results of the laboratory experimental data sets, which were done under different thermodynamic conditions and using different sources, show that the transformation matrix varies over each data set and also with time. Therefore, it is necessary to check the data sets before applying them to a basin for hydrocarbon modeling. The laboratory experimental data taken at lower temperature and over longer times appear more pertinent for the construction of an oil-cracking kinetic model suitable for geologic conditions.  相似文献   

A self-consistent model of a thermally balanced quiescent prominence in magnetostatic equilibrium in a uniform gravitational field     

I. Lerche  B. C. Low 《Solar physics》1977,53(2):385-396

We present a theoretical model of quiescent prominences in the form of an infinite vertical sheet. Self-consistent solutions are obtained by integrating simultaneously the set of nonlinear equations of magnetostatic equilibrium and thermal balance. The basic features of the models are: (1) The prominence matter is confined to a sheet and supported against gravity by a bowed magnetic field. (2) The thermal flux is channelled along magnetic field lines. (3) The thermal flux is everywhere balanced by Low's (1975b) hypothetical heat sink which is proportional to the local density. (4) A constant component of the magnetic field along the length of the prominence shields the cool plasma from the hot surrounding. We assume that the prominence plasma emits more radiation than it absorbs from the radiation fields of the photosphere, chromosphere and corona, and we interpret the above hypothetical heat sink to represent the amount of radiative loss that must be balanced by a nonradiative energy input. Using a central density and temperature of 10¹¹ particles cm^–3 and 5000 K respectively, a magnetic field strength between 2 to 10 gauss and a thermal conductivity that varies linearly with temperature, we discuss the physical properties implied by the model. The analytic treatment can also be carried out for a class of more complex thermal conductivities. These models provide a useful starting point for investigating the combined requirements of magnetostatic equilibrium and thermal balance in the quiescent prominence.  相似文献   

Spontaneous and induced Čerenkov emission in a turbulent medium     

I. Lerche 《Astrophysics and Space Science》1970,8(1):29-49

It is shown that the presence of spatially random fluctuations in refractive index, about a mean exceeding unity, influences the power output of erenkov waves emitted by a charged particle in several ways.(1) The frequency spectrum of the spontaneous emission is altered by the fluctuations. (2) There is an induced erenkov emission, due to the interaction of the spontaneous field with the random refractive index variations. This induced field can, in certain frequency bands, be as large as the spontaneous field. And it also contains a backscattered component which propagates in theopposite direction to the particle. (3) The conditions for emission of the erenkov waves, be they spontaneous or induced, depend critically on both the mean refractive index, the particle velocity, the intensity and correlation length of the fluctuations. And the dependence is sensitive to the precise functional form of the two-point correlation function.Instruments detect the optical erenkov emission produced by cosmic ray particles penetrating the Earth's atmosphere (below about 5 km). Also gas erenkov counters are triggered by the passage of highly energetic particles through the gas. Since both the Earth's lower atmosphere and the gas counters contain turbulent fluctuations, the present calculation is of some interest in connection with particle energy loss mechanisms in turbulent media and the basic structure of such media.  相似文献   

Reply to the comments of K. Elsässer and P. Gräff     

I. Lerche 《Astrophysics and Space Science》1970,8(2):207-207

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On the application of Cramér's theorem to axisymmetric,incompressible turbulence     

I. Lerche 《Astrophysics and Space Science》1972,19(1):189-193

In the theory of homogeneous, stationary, axisymmetric, incompressible velocity turbulence there arise four scalar functions. The incompressibility condition provides two relations between these four functions.We will demonstrate here that application of Cramér's theorem imposes two additional constraints on the four functions. These constraints do not uniquely define the allowed functional form but they do provide very powerful criteria for limiting the class of functions which are permitted. In view of the growing use of velocity turbulence in kinematic dynamo theory and its importance in astrophysical situations (e.g., Earth, Sun, Galaxy) to maintain or regenerate a large scale magnetic field, we believe that the present constraints are of more than academic interest. In particular, application of the constraints to a form of velocity turbulence used by Steenbeck, Krause and Rädler when computing kinematic dynamo action, shows that their assumed turbulence is not physically realizable in nature.  相似文献   

Models of residual kerogen kinetic evolution: A reply to Nielsen and Burnham     

I. Lerche 《Mathematical Geology》1993,25(1):91-97

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On the propagation of magnetic disturbances in the solar wind     

I. Lerche 《Astrophysics and Space Science》1975,34(2):309-319

Under the geometrical optics approximation we discuss the propagation of a polarized magnetic profile, made up of Alfvén waves, in the solar wind. We show that (i) the profile propagates at an angle to the radial direction (the direction of the solar wind flow), (ii) the radial half-width of the profile stays essentially constant, or even diminishes a little, with distance from the Sun, (iii) the half-width in a direction transverse to the radial direction increases without limit as the magnetic profile moves outward from the Sun. Thus the profile stretches out into a ‘ribbon’ which could, of course, be experimentally identified as a discontinuity. We also give equations for the variation of polarization of the profile, and illustrate the behavior of polarization in a simple case. We have done these calculations to show that the production of ‘discontinuities’ in the solar wind can arise from propagation effects on irregularly shaped ‘blobs’ of magnetic field, as well as from other causes.  相似文献   

On the energy spectrum of galactic primary cosmic rays: Results from the transport equation     

I. Lerche  R. Schlickeiser 《Astrophysics and Space Science》1988,145(2):319-354

The spectrum of galactic primary cosmic rays at relativistic monenta is calculated. The primaries are assumed to be accelerated continuously from the thermal galactic background medium by first- and second-order Fermi acceleration. We show that the observed spectrum is readily obtained from the transport equation conventionally invoked to discuss propagation and loss of cosmic rays in our Galaxy from a distribution of sources. We have previously (Lerche and Schlickeiser, 1985) shown that the observed secondary to primary ratio is satisfactorily explained by a similar use of the transport equation, allowing for secondary production from the primaries. Accordingly, when the results of this paper are added to those concerning the secondary/primary ratio behaviour, it would seem that continuous Fermi acceleration accounts, in a quantitative fashion, for the spectral behaviours observed at Earth.  相似文献