Magnetic turbulent spectra in the magnetosheath: new insights, Ann

Abstract. The spectrum of the magnetic fluctuations mea-sured by the Cluster satellites in the inner magnetosheath is investigated using the k-filtering technique. On a case study, it is shown first that the wave vectors calculated from the Flux Gate Magnetometer (FGM) data fit well with those de-termined from the Spatio-Temporal Analysis of Field Fluc-tuations (STAFF) data for their common range of frequency, which allows one to confirm that the high pass filter applied to STAFF data does not alter the spatial characteristics of its spectra. Both analyses confirm the dominance of the mirror mode for frequencies up to 1.4 Hz. Furthermore, by com-paring the experimental charateristics of the identified mir-ror mode to the prediction of the linear theory, it is shown that the predicted maximum growth rate is observed in the frequency range 0–0.15 Hz, i.e. the FGM range. All the rest of the mirror mode, identified for higher frequencies is more likely to be a non linear extension of the most instable one. This cascade on the spatial scales is, in turn, observed in the satellite frame as a temporal spread due to Doppler shift. Fur-ther implications on the real nature of the observed spectrum are discussed. Key words. Magnetospheric physics (magnetosheath, plasma waves and enstabilities). Space plasma phusics (turbulence)