The k‐filtering technique applied to wave electric and magnetic field measurements from the Cluster satellites

The k‐filtering technique is a method to characterize stationary fluctuations in space plasmas in terms of the wave energy distribution in the frequency and wave vector space. It has the ability to distinguish between wave modes of the same frequency in the spacecraft frame of reference, but with different wave vectors. This method is based on simultaneous multi‐point measurements of the wave field components, where a filter bank is used to enhance the spatial resolution. We have for the first time combined electric field data from the Electric Field and Wave (EFW) instrument and magnetic field data from the Spatio‐Temporal Analysis of Field Fluctuation (STAFF) instrument on the four Cluster spacecraft in order to determine the wave energy distribution. The k‐filtering technique has previously been performed with only the magnetic field measurements. The reason to include the electric field measurements is that it is important to include as much data as possible in order to get the best possible estimation of the wave energy distribution. Another reason is that it also enables comparisons between the electric and the magnetic part of the wave energy distribution to make it possible to differentiate the observed waves according to their polarization. The k‐filtering method has been extended in order to allow for two measured components of the electric field, and also for the possibility that the electric field measurements from one or more satellites cannot be used for k‐filtering. The technique has been applied on satellite data from the magnetosheath and the foreshock, and from these examples it is clear that k‐filtering using both electric and magnetic field measurements is a good tool for characterizing the waves that are observed in space.