The Wave Telescope Technique

Unambiguous determination of wavevectors (wavelengths and propagation directions) is one of the major challenges of space plasma observations, since conventional wave analysis methods for the single spacecraft data are limited to the time series data analysis and the wave properties are determined as a function of the frequencies in the spacecraft frame. The wave telescope technique overcomes the problem of wavevector determination by making extensive use of the multi-spacecraft interferometric data. The wave telescope techniques do not assume any wave mode or plasma mode, and is suited to testing for the wave and turbulence theories against the multi-spacecraft data. This article presents an updated report on the wave telescope technique on the basis of adaptive filtering (Capon's minimum variance projection), the construction of an estimator for the multi-point magnetic field data, the performance and limits in the wavevector analysis (aliasing and low-wavenumber behavior), the applications to the Cluster fluxgate magnetometer data (dispersion relations, phase velocity diagrams, propagation pattern, energy spectra, helicities, and higher-order statistics), and possible extensions of the technique. Plain Language Summary The wave telescope technique is a wave analysis method based on the adaptive filter theory, and provides an algorithm to determine the wavelengths and propagation directions from the multi-spacecraft data without any assumption of wave modes or plasma model a priori. The technique has successfully been applied to the fluxgate magnetometer data from four spacecraft of the Cluster mission. The theoretical background, performance limits, and scientific applications (such as observational dispersion relations and spatial pattern of wave propagations) and further possible extensions are reported in the article.