Development of Direction Finding System for Plasma Waves

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We have developed a new ground-based directim finding system for magnetospheric VLF/ELF emissions, which will be of great importance in the study of their generation and propagation mechanisms. This new method enables us to estimate the distribution of wave energy density at the ionospheric base as the inversion problem to the observed data of three possible field canponents (two horizontal magnetic and a vertical electric component) obervable on the ground. The previous methods so far proposed, are of no use for the situations with a very wide distribution at the ionospheric base or with a multiple ducts, and our new method is still effective for these cases. The first wave distribution function. (WDF) method) is based on the concept such that the entropy of the distribution function be maximized. The application of this method to the simulated data consisting of a single and double source, has yielded that this method is able to distinguish between the closely spaced sources. Also, i … More ts application to the actually observed data of whistlers at Yamaoka (geomag. lat. 25ﾟN) (three field components), has provided us with useful information on the ionospheric transmission mechanism of low-latitude vyhistlers. This maximum entropy method has a few demerits ; (1) The validity of the final WDF is not objective and (2) the computing time is rather long because of its nonlinear estimation. These problems are solved by the combined use of the regularization and generalized cross validation (GCV). This method is found to yield very objective way of estimating the final optimum EWDF especially for a single peaked source. The WDF at the regularization parameter giving the minimum GCV is the optimum on the basis of the simulation experiments. However, we have to think about scmeting for the doubly peaked source, and a method fortthis is the regularization with smaller smoothness, which we would like to study in future. Furthermore, in order to examine the accuracy of our proposed methods, we carried out the three-stationed direction finding measurements, the data from which will be analyzed extensively in future. Less