ULF wave identification in the magnetosheath: The k-filtering technique applied to Cluster II data

The magnetic fluctuations in the magnetosheath are studied, thanks to Cluster II data. The k-filtering technique is applied to explore ULF magnetic fluctuations using STAFF (Spatio-Temporal Analysis of a Field Fluctuations) data. Based on multipoint measurements, the k-filtering technique allows, for the first time, to estimate the Magnetic Field Energy Distribution (MFED) in both the angular frequency and wave vector space. We show how the localisation of the magnetic energy in the (ω, k) domain can be used to identify the linear modes that can propagate in the magnetosheath. A comparison between k-filtering results and prediction of the linear theory is performed. For the frequencies examined the magnetic energy seems to be distributed over the low frequency modes: mirror, Alfvén, and slow modes. Estimation of Doppler shift shows that each frequency observed is the superposition of different frequencies in the plasma frame. This "mixture of modes" at a given observed frequency explains why the fluctuations are generally not observed to be polarized, as shown in previous studies. Some other implications on a weak turbulence approach of the magnetic fluctuations in the magnetosheath are discussed.