The recognition and analysis of plasma structures is one of the important tasks of space missions. This is a rather complex topic because of the variety of possible structures. Observations over a long-term period made at sufficiently numerous positions in space and covering a sufficient number of parameters would be necessary for unambiguous identification of these structures. The real possibilities, however, are very modest. Temporal resolution is relatively simple: Nowadays plasma parameters may be observed with a high time resolution over long periods and transmitted to earth. A comparable spatial resolution is practically impossible. Satellite observations are mostly one-point measurements. The Cluster mission provides the first true multi-point measurements, with four satellites performing synchronous measurements in the magnetosphere. The observation of a sufficient number of physical parameters to identify a plasma structure depends on the instruments of the corresponding mission.
Although the data will always be incomplete in comparison with the requirements mentioned above, sophisticated filtering methods enable us to extract well-defined plasma structures.
The figure demonstrates that with only four sensors irregular distributed in space, the wave number spectrum (k-spectrum) of a wave field consisting of 7 different waves may be reconstructed with great accuracy. To achieve the same result, a conventional Fourier analysis would require an array of approximately 64x64x64 sensors in space.
References
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