Multi-Spacecraft Methods of Wave Field Characterisation
Résumé
Space plasmas are collisionless and thus waves play a major role in the collective interaction between particles. Also, space plasmas are subject to a variety of instabilities, generating a plethora of different plasma wave modes. Thus, for many physical processes the role of waves and turbulence is likely to be predominant. The simultaneous four-by-fourpoint measurements available from the Cluster mission enable spatiotemporal effects in data sets to be resolved, which is mandatory for the unambiguous identification of waves and plasma turbulence. The two main methodologies that have been used for wave field characterisation in the frame of Cluster are the k-filtering/wave-telescope technique and the phase differencing technique. Important new results obtained through the applications of both techniques to Cluster data have been presented in numerous studies. The purpose of this chapter is not to discuss the scientific results obtained, but to serve as a guide for the interested reader as to what has been learned regarding wave identification methods using multi-spacecraft data and where to find it. The following is a brief outline of the chapter. In Sections 5.2 and 5.3, respectively, the basic principles of k-filtering/wave-telescope and phase differencing techniques are presented. Section 5.4 describes the main problems and limitations encountered by applying the methods to real data. In Section 5.5 the most recent developments of the k-filtering and phase differencing techniques are presented and the future of wave field characterisation through multi-spacecraft methods is briefly discussed.