Fw Frans Sluijter

A model‐independent algorithm for ionospheric tomography: 1. Theory and tests

This paper presents a model-independent algorithm for tomography of the ionosphere. Prior knowledge consists of the following pieces of information: electron density cannot be negative, the ionosphere is basically smooth and stratified, and electron density is low at high (≳700 km) and low (≲100 km) altitudes. Tests based on simulated measurements show that the method recovers the latitudinal structure well, whereas the vertical structure is recovered with moderate success: the estimated height of the layer of maximum electron density may be as much as 90 km in error. Because of the imposed smoothness the method tends to underestimate the peak in electron density by as much as one third in unfavorable cases.

An algorithm for quadratic optimization with one quadratic constraint and bounds on the variables

This paper presents an efficient algorithm to solve a constrained optimization problem with a quadratic object function, one quadratic constraint and (positivity) bounds on the variables. Against little computational cost, the algorithm allows for the inclusion of positivity of the solution as prior knowledge. This is very useful for the solution of those (linear) inverse problems where negative solutions are unphysical. The algorithm rewrites the solution as a function of the Lagrange multipliers, which is achieved with the help of the generalized eigenvectors, or equivalently, the generalized singular value decomposition. The next step is to find the Lagrange multipliers. The multiplier corresponding to the quadratic constraint, which is known to be active, is easy to find. The Lagrange multipliers corresponding to the positivity constraints are found with an iterative method that can be likened to the active set methods from quadratic programming.

TM-modes in a planar optical waveguide with a graded index of the symmetric Epstein type

The eigenvalue problem for the TM-modes in a planar optical waveguide is rigorously solved by analytical and numerical methods in the case of a symmetric Epstein-type variation of the dielectric permittivity. It is shown that there exists a finite number of guided TM-modes. The TM-modal fields are expressed in terms of Heun functions and the corresponding eigenvalues (or propagation coefficients) are numerically determined from the minimal solution of a three-term recurrence relation. Upper and lower bounds are established for the difference between corresponding eigenvalues of the TE- and TM-modes.

A model‐independent algorithm for ionospheric tomography: 2. Experimental results

This paper presents the results of an ionospheric tomography experiment over mid-latitude Europe. The campaign has yielded 539 reconstructions over a 45 day period in spring 1995. Special attention is paid to the performance of the reconstruction algorithm, specifically by comparison with ionosonde data. Tomography and soundings tend to agree on electron density, but they do not agree on the height of maximum electron density: tomography seems to overestimate Hmax. Whereas tomography finds a daily oscillation in Hmax between 250 km (daytime) and 450 km (nighttime), the soundings indicate an oscillation between 200 and 250 km..

Some generalizations of radial Bremmer series

In this paper we address the problem of finding a solution of the wave equation with a non-constant propagation velocity whereby the attention is focussed on the case of spherical waves. By a subdivision of the medium into spherical shells one particular so-called Bremmer series is derived and it is shown that other Bremmer series are possible too. It turns out that each of these series has a different convergence criterion, the fulfillment of which depends on the physical situation under consideration. Hence in each physical situation there is some freedom in choosing an appropriate Bremmer series.

Wave equations for a relativistic magnetoplasma

The linearized relativistic Vlasov-Maxwell system for a hot inhomogeneous relativistic magnetized electron plasma is studied through particle orbit theory using the techniques of Fourier transform. An analytical integral expression in the (k, omega )-space is obtained for the current density for waves propagating across an externally applied uniform static magnetic field (k is the wavenumber and omega is the wave frequency). After applying inverse Fourier transform, differential equations for the electric field are obtained from the expression for the current density combined with Maxwells equations. These fully relativistic equations are correct up to second order in rc/L. where rc is the electron gyroradius and L is the gradient length of the plasma inhomogeneity.

On the effect of laser and plasma parameters on stimulated Raman scattering and fast-electron generation

The effect of Raman instabilities on the production of fast electrons in laser–plasma interaction has been investigated for laser intensities well above the electron trapping threshold. The results of one-dimensional particle-in-cell simulations show that in this regime the presence of Raman backscattering (RBS) hampers fast-electron production, and that its suppression increases the yield of high-energy electrons (

One‐dimensional relativistic Vlasov–Maxwell equilibria of charge‐separation‐free plasmas

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On the application of the Boussinesq approximation for nonlinear gravity waves in the atmosphere

15 MeV). Such suppression has been realized either through deletion of all backscattered radiation from the simulations or through direct stimulation of Raman forward scattering (RFS). An increased high-energy electron yield has been observed for both methods. In addition, the influence of various laser and plasma parameters on the production of highly energetic electrons has been investigated. Specifically, the effects of plasma density ramps, skews in the temporal envelopes of the laser pulses, and laser frequency chirp (both pulse-length preserving and bandwidth preserving) have been examined. For each parameter, its influence on the yield of high-energy electrons can be explained from the way it affects the balance between RBS and RFS excitation in laser–plasma interaction.

Nonlinear wave equations for low-frequency acoustic gravity waves

The conditions under which charge separation is negligible or even absent are studied for one‐dimensional relativistic Vlasov–Maxwell equilibria. These conditions are then employed to model the boundary between a vacuum magnetic field and a plasma that is free of charge separation for the cases with and without trapped particles.