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Lerche, I.

Publications and source records attributed to Lerche, I..

At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1986

Diffusive electron acceleration at SNR shock fronts and the observed SNR radio spectral indices

The radio synchrotron emission from relativistic electrons in shell supernova remnants (SNRs) provides a unique opportunity to probe the energy distribution of energetic electrons at their acceleration site (SNR shock fronts). This information provides insight into the acceleration mechanism(s). The implications of these observations for the diffusive (first-order Fermi) acceleration of electrons at the SNR shock fronts are discussed.

Bogdan, T. J.↗
NASA NTRS1985

Evidence for a dynamical halo around the edge-on galaxy NGC 4631

Radio continuum observations at five frequencies between 327 MHz and 10.7 GHz of the edge on galaxy NGC 4631 confirm the prediction concerning the frequency dependence of the halo extent and the spatial variations of the radio spectral indices in the dynamical halo model made by Lerche and Schlickeiser. The measurements are presented, and a detailed comparison with theoretical predictions is made.

Lerche, I.↗
NASA NTRS1985

Why do leaky-box models work so fine?

By introducing the concept of the age distribution of cosmic rays it is possible to decouple spatial from momentum transport, and simple leaky-box type equations result. The influence of spatial inhomogeneities, geometries, and source distributions enters the spatially homogeneous, infinite (i.e., leaky box) problem through appropriate mean lifetimes. A precise prescription of how to obtain these mean lifetimes, i.c., for comparison with data measured in the vicinity of the solar system they have to be calculated from the age distribution at the spatial position of the observer.

Schlickeiser, R.↗
NASA NTRS1985

Influence of the source distribution on the age distribution of galactic cosmic rays

The age distribution of galactic cosmic rays in the diffusion approximation is calculated. The influence of the scale height of the spatial source distribution on the mean age of particles arriving at the solar system is discussed. The broader the source distribution with respect to the galactic plane, the longer the mean age. This result provides a natural explanation for the shorter mean age of secondary cosmic rays compared to primary cosmic rays necessary for the understanding of the observed secondary/primary ratio.

Lerche, I.↗
NASA NTRS1985

Explanation of the secondary to primary ratio within the continuous Fermi accelerator model

The secondary to primary ratio in galactic cosmic radiation at relativistic momenta is calculated in a model, where the primaries are continuously accelerated from the thermal galactic background medium by 1st and 2nd order Fermi acceleration. It is shown that the measured decrease with momentum does not exclude that cosmic rays are accelerated in the interstellar medium as a whole. Once a momentum dependence of the mean lifetime and the different spatial source distributions are adequately taken into account, the measured decreasing ratio can be explained.

Lerche, I.↗
NASA NTRS1985

Momentum-dependent diffusive particle acceleration in modified shock fronts

In the presently derived analytic solutions of the steady transport equation for diffusive particle acceleration in a modified, planar shock front having free escape boundaries, the fluid velocity profile through the shock transition decreases monotonically between the upstream and downstream boundaries. The spatial diffusion coefficient's spatial dependence is linked to that of the fluid velocity profile. Attention is given to the solution corresponding to monoenergetic particle injection at the shock front, with free particle escape at finite distances both upstream and downstream of the shock. The accelerated particle spectrum is dominated at high energies by an exponential cutoff, due to the competition between acceleration by the first-order Fermi mechanism and particle loss through the free escape boundaries.

Webb, G. M.↗
NASA NTRS1981

On the transport and propagation of relativistic electrons in galaxies

Analytic solutions to the time-dependent equation describing the transport of relativistic electrons that are being convected and diffused, and may be either gaining or losing energy, are described. The calculations yield a body of solutions from which specific ones can be chosen for the modelling of synchrotron radiation in the Galaxy and in other galaxies. The solutions allow for (1) spatially varying convection velocity; (2) an energy and spatially dependent diffusion coefficient; (3) adiabatic acceleration effects; (4) a term describing spatially dependent synchrotron loss and/or Fermi acceleration; (5) an injection source for the electrons that may be impulsive, steady-state, or variable-injection; (6) a spatial injection or spatially more diffuse source; (7) arbitrary energy dependence of the injection source; and (8) spatial and energy boundaries where the spatial or energy form of the convection, diffusion, and energy supply terms in the basic equation can change character.

Lerche, I.↗
NASA NTRS1981

Pulsar statistics and their interpretations

It is shown that a lack of knowledge concerning interstellar electron density, the true spatial distribution of pulsars, the radio luminosity source distribution of pulsars, the real ages and real aging rates of pulsars, the beaming factor (and other unknown factors causing the known sample of about 350 pulsars to be incomplete to an unknown degree) is sufficient to cause a minimum uncertainty of a factor of 20 in any attempt to determine pulsar birth or death rates in the Galaxy. It is suggested that this uncertainty must impact on suggestions that the pulsar rates can be used to constrain possible scenarios for neutron star formation and stellar evolution in general.

Arnett, W. D.↗
NASA NTRS1981

A variational approach to the question of temporal stability of equilibrium models of solar prominences. I - The formal theory

A Lagrangian approach to the equations describing small amplitude departures from equilibrium of solar prominences is presented. Seven quantities are derived which, by analogy to the concepts of energy, momentum, and angular momentum, are conserved under conditions corresponding to ignorable coordinates of classical mechanics. It is expected that these conservation laws will be useful as criteria of accuracy in obtaining eigenfrequencies for the perturbation equations when numerical techniques are used. The method is capable of handling complicated spatial dependences and has internal criteria for assessing the accuracy of any given numerical convergence scheme.

Lerche, I.↗
NASA NTRS1981

On the birthrate of galactic supernovae

The birthrate of galactic supernovae is estimated in three different ways. The three ranges overlap, and it is suggested that 70 + or - 35 yr represents a more realistic estimate of the rate than some that have previously been made. The galactic supernova remnants and their observed systematic brightness gradients perpendicular to the galactic plane imply a scale height of about 200 pc for the remnant progenitors and a galactic magnetic field scale height of about 300 pc. Long standing anomalies associated with the young remnant AD 1006, the galactic loops, and faint remnants, are accounted for by the brightness gradient effect. This provides firm, independent evidence for the validity of the remnant method of deducing the birthrate of galactic supernovae.

Lerche, I.↗
NASA NTRS1981

Mathematical theory of cylindrical isothermal blast waves in a magnetic field

An analysis is conducted regarding the properties of cylindrically symmetric self-similar blast waves propagating away from a line source into a medium whose density and magnetic field (with components in both the phi and z directions) both vary as r to the -(omega) power (with omega less than 1) ahead of the blast wave. The main results of the analysis can be divided into two classes, related to a zero azimuthal field and a zero longitudinal field. In the case of the zero longitudinal field it is found that there are no physically acceptable solutions with continuous postshock variations of flow speed and gas density.

Lerche, I.↗
NASA NTRS1980

On the equilibrium of heated self-gravitating masses - Cooling by conduction

An investigation is given of the equilibrium states available to a self-gravitating mass of gas, cooling by conduction, and being heated at a rate proportional to the local gas density. The plane geometry situation is shown to be reducible to quadratures for the pressure, density, temperature, and gravitational potential. For a constant thermal conductivity it is shown that the gas density has either a central maximum or a central minimum, depending on the ratio of the thermal conductivity to a parameter taken to be a measure of the rate of heating. For a thermal conductivity which is a positive power of the temperature, it is shown that the gas density always has a central minimum and a maximum at the outer boundary of the configuration. For cylindrical and spherical geometrical configurations the same general properties are obtained. The physical origin of this behavior is discussed, and it is suggested that these exploratory calculations provide an effect which may not only aid in understanding thin filamentary structure observed in supernova remnants, but also help to assuage the difficulties of producing maser activity in the interior regions of 'cocoon' protostars.

Lerche, I.↗
NASA NTRS1980

On equilibrium states of heated self-gravitating gas clouds cooling by conduction in an external gravitational field

Exact analytic solutions are presented for equilibrium states of a self-gravitating one-dimensional cloud of gas, embedded in an external gravitational field due to a plane of 'stars' being heated at a rate proportional to the local gas density, and cooling by thermal conduction. It is found that the general topology of the solutions is such that the gas density has a minimum on the plane of 'stars', rising to an infinite but integrable peak away from the plane so that the total mass of gas in the cloud is finite. The results may be of interest in investigations of interstellar molecular clouds and of filamentary structures in supernova remnants as well as in the modeling of gas distributions around 'cocoon' protostars.

Lerche, I.↗
NASA NTRS1980

The heating of a thermally conducting stratified medium. II - A simple plane model of an atmosphere

Exact solutions of the following theoretical problem are presented: A plane atmosphere is in hydrostatic equilibrium with a uniform gravity. The ideal gas law is assumed. Heat is generated everywhere at a rate proportional to the local density. The atmosphere is maintained in a steady state through cooling by thermal conduction and radiation. This problem is reducible to quadratures for a thermal conductivity which is an arbitrary, but prescribed, function of the temperature, and for a radiative loss which is expressible as the product of the density and an arbitrary, but prescribed, function of the pressure. The analysis is carried out for the case of power law thermal conductivity, and a radiative loss proportional to the square of the density and to the first power of the temperature. The radiative cooling function adopted here has the basic mathematical form for an optically thin medium. The solutions reproduce the macroscopic ordering of a hot 'corona' separated from a 'photosphere' by a layer of temperature minimum. The analytic solutions allow direct illustration of the interplay between steady energy transport and the requirements of hydrostatic equilibrium.

Lerche, I.↗
NASA NTRS1980

On the equilibrium of a cylindrical plasma supported horizontally by magnetic fields in uniform gravity

The mechanical equilibrium of a cylinder of plasma suspended horizontally by magnetic fields in uniform gravity is examined. A set of exact solutions describing the equilibrium is derived assuming the plasma distribution to be cylindrically symmetric to obtain an exact force balance between plasma pressure, the Lorentz pressure, and gravity in space. The set of solutions considers a case of uniform temperature and cases where the temperature rises from zero at the center of the plasma cylinder to reach a constant asymptotic value outside the cylinder.

Lerche, I.↗
NASA NTRS1980

On the equilibrium structures of self-gravitating masses of gas containing axisymmetric magnetic fields

The general equations describing the equilibrium shapes of self-gravitating gas clouds containing axisymmetric magnetic fields are presented. The general equations admit of a large class of solutions. It is shown that if one additional (ad hoc) asumption is made that the mass be spherically symmetrically distributed, then the gas pressure and the boundary conditions are sufficiently constraining that the general topological structure of the solution is effectively determined. The further assumption of isothermal conditions for this case demands that all solutions possess force-free axisymmetric magnetic fields. It is also shown how the construction of aspherical (but axisymmetric) configurations can be achieved in some special cases, and it is demonstrated that the detailed form of the possible equilibrium shapes depends upon the arbitrary choice of the functional form of the variation of the gas pressure along the field lines.

Lerche, I.↗