Journal of Atmospheric and Terrestrial Physics最新文献

Collective Light Scattering (CLS) from gas density irregularities is analysed using electromagnetic theory and fluid mechanics. The scattered signal is shown to provide three decisive pieces of information.: (1) the signal frequency spectrum gives the Doppler shifted velocity probability distribution; (2) new processing of the modulus and phase spectra of the complex signal provides the velocity fluctuations frequency spectrum; (3) appropriate filtering of the signal phase time derivative provides the real time instantaneous convection velocity. This information is illustrated and extracted from CLS signals obtained in a turbulent mixing layer experiment.

We present a detailed investigation of asymmetric ion features of Thomson scattering spectra observed in the compression phase of a gas-liner pinch discharge. The asymmetries can be explained by drift velocities between electrons and ions, and an appropriate form factor taking into account drift velocities and different atomic species in the plasma can be fitted to the detected spectra. By varying the scattering volume in the midplane of the discharge the magnitude and direction of the drift are measured. These results and pinhole pictures of the whole plasma column show that the drifts are produced by Rayleigh-Taylor instabilities resulting in high radial current density components of the discharge current.

The alternating code technique, primarily designed for incoherent scatter radars, is found to be suitable for HF magnetospheric sounding, providing simultaneously good range and spectral resolutions. Problems related to its implementation at the ‘Sura’ facility are considered. The data processing algorithm has been modified, taking into account data oversampling. The influence of ground clutter at HF due to reciprocal oblique scattering is also investigated, as is the autocorrelation function zero lag problem for relative measurements.

The principles of coherent backscatter from irregularities created by plasma instabilities in the ionospheric E-region are described in this paper. It is intended as an introduction for readers who are not familiar with this subject and not as a review article. The general principles of radar backscatter are outlined as well as the nature of plasma instabilities that cause the irregularities. In subsequent sections the occurrence of the irregularities at different geographical latitudes is addressed and the spectrum of the backscatter is discussed. Finally, some applications, particularly of auroral radars, are briefly mentioned.

The problem of measuring horizontal winds in the lower atmosphere with a VHF/UHF bistatic radar is reconsidered because of recent progress in the analysis of troposcatter systems with joint diversity. The measurement procedures for spatial diversity of small antennas and angular diversity of large antennas are substantiated. It has been established that for the joint measurements of crosspath mean wind velocity and large-scale air vorticity the size of the scattering volume along the path also has to be estimated. Radar wind velocity estimates over a 495 km sea trans-horizon path are compared with radiosonde data over the height range up to 4 km.

We propose a new configuration for laser-based optical measurements of density fluctuations in plasmas. The system is a combination of the Doppler and time-of-flight methods. The characteristics of the system are compared with those of previous approaches. We show that the proposed configuration provides better spatial resolution than the Doppler method.

This paper deals with a brief overview of new experimental results that can be obtained with the collective light scattering (CLS) method in supersonic flows. Wake and mixing layer configurations are used in order to illustrate the possibilities of the method. The collective light scattering bench can be considered as a directional densitometer (with spectral analysis of the density fluctuations), a non-particle anemometer, a Mach-meter (or thermometer) and a directional microphone.

Non-thermal phenomena are a common occurrence in the aurora] ionosphere. The incoherent scatter technique is now frequently used to study several of the features associated with the departure of the plasma from a thermal equilibrium configuration. In this review we limit our discussion to E and F region phenomena. The first observation that we discuss is that of a large increase in the auroral E region electron temperature during periods of strong convection electric fields. We illustrate these observations with a few examples and summarize the current state of the theoretical debate on electron heating by turbulent ion-acoustic waves. Strongly asymmetric incoherent scatter spectra offer a second example of the effect of ion-acoustic turbulence on the wave spectra. We describe the observations and review three different possible physical mechanisms which have been proposed in the literature. The final part of this review deals with the signature of non-Maxwellian ion velocity distributions on incoherent scatter spectra. These non-Maxwellian distributions appear in the presence of a strong electric field. The first theories dealing with the creation of these distributions and with its effect on the incoherent scatter spectrum were developed in the mid-1970s and evolved in the 1980s. The most important problem, currently, is with the interpretation in terms of ionospheric parameters of the incoherent scatter spectra during the disturbed periods that lead to the non-Maxwellian signatures. We emphasize recent advances in the data analysis and discuss the remaining limitations while evaluating the direction of future work on this subject.

A general solution of Equation (1) in a three-dimensional inhomogeneous medium is found. Use is made of the available ray-optical approximate solution of (1) and the Riemann geometry of wave rays. This allows an exact solution in series form, where the first term coincides with the approximate solution. The second term of the exact solution contains the major part of the scattered wave and its computation gives the right value of the coherent scattering. The second term and its transformation leading to easier computation are analysed. The main result of this work is that the mathematical approach used is convenient for a description of the coherent scattering in inhomogeneous media when solving direct and inverse problems of wave propagation. The description is of a very general character and contains Bragg scattering as a particular case. The application of this approach to the development of the computer codes can provide a powerful tool for studying inhomogeneous media. Developing these codes would be time consuming but provide a fast solution of many difficult problems.

A new compact formulation is given of the equation of transfer for a scattering system for a magnetized plasma. The formulation is based on the low temperature kinetic model and accounts for scattering from fluctuations in density, magnetic field, electric field and current. It is demonstrated that scattering from several types of fluctuations can be significant and that the relative phases of the fields scattered by these fluctuations must then be taken into account by including cross-correlations between fluctuations. It is shown that significant enhancement or cancellation of scattered power can result from cross-correlations, and that this is of practical consequence to existing microwave scattering experiments on fusion plasmas.