Abstract
Abstract. We investigate the spectral shape, the anisotropy of the wave vector distributions and the anisotropy of the amplitudes of the magnetic fluctuations in the Earth's magnetosheath within a broad range of frequencies [10−3, 10] Hz which corresponds to spatial scales from ~10 to 105 km. We present the first observations of a Kolmogorov-like inertial range of Alfvénic fluctuations δB2⊥}~f−5/3 in the magnetosheath flanks, below the ion cyclotron frequency fci. In the vicinity of fci, a spectral break is observed, like in solar wind turbulence. Above the break, the energy of compressive and Alfvénic fluctuations generally follows a power law with a spectral index between −3 and −2. Concerning the anisotropy of the wave vector distribution, we observe a clear change in its nature in the vicinity of ion characteristic scales: if at MHD scales there is no evidence for a dominance of a slab (kł>>k⊥) or 2-D (k⊥>>kł) turbulence, above the spectral break, (f>fci, kc/ωpi>1) the 2-D turbulence dominates. This 2-D turbulence is observed in six selected one-hour intervals among which the average ion β varies from 0.8 to 10. It is observed for both the transverse and compressive magnetic fluctuations, independently on the presence of linearly unstable modes at low frequencies or Alfvén vortices at the spectral break. We then analyse the anisotropy of the magnetic fluctuations in a time dependent reference frame based on the field B and the flow velocity V directions. Within the range of the 2-D turbulence, at scales [1,30]kc/ωpi, and for any β we find that the magnetic fluctuations at a given frequency in the plane perpendicular to B have more energy along the B×V direction. This non-gyrotropy of the fluctuations at a fixed frequency is consistent with gyrotropic fluctuations at a given wave vector, with k⊥>>kł, which suffer a different Doppler shift along and perpendicular to V in the plane perpendicular to B.
Highlights
In the space plasma turbulence, the presence of a mean magnetic field B gives rise to anisotropies with respect to the field direction
Concerning the anisotropy of the wave vector distribution, we observe a clear change in its nature in the vicinity of ion characteristic scales: if at MHD scales there is no evidence for a dominance of a slab (k k⊥) or 2-D (k⊥ k ) turbulence, above the spectral break, (f >fci, kc/ωpi>1) the 2-D turbulence dominates
Within the range of the 2-D turbulence, at scales [1, 30]kc/ωpi, and for any β we find that the magnetic fluctuations at a given frequency in the plane perpendicular to B have more energy along the B×V direction
Summary
In the space plasma turbulence, the presence of a mean magnetic field B gives rise to anisotropies with respect to the field direction ( means parallel, and ⊥ means perpendicular to B). The second test reveals the dependence of the PSD at a fixed frequency on the angle between the plasma flow and the mean field BV (defined between 0 and 90 degrees): For a PSD decreasing with k (like a power-law, for example), in the case of the slab turbulence, the PSD for a given frequency will be more intense for BV =0◦, and the PSD decreases while BV increases; for the 2-D turbulence the PSD will be more intense for BV =90◦, and so it increases with BV Using these tests, Bieber et al (1996) have shown that the subrange [10−3, 10−2] Hz in the inertial range of the slow solar wind is dominated by a 2-D turbulence; a small percentage of a slab component is present. For Alfvenic fluctuations δB⊥ we perform the first test of Bieber et al (1996), i.e., we analyze the gyrotropy of the PSD of the magnetic fluctuations in the plane perpendicular to B, at a given frequency, as a function of BV
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