Anisotropy of Solar Wind Turbulence between Ion and Electron Scales

Abstract

The anisotropy of turbulence in the fast solar wind, between the ion and electron gyroscales, is directly observed using a multispacecraft analysis technique. Second order structure functions are calculated at different angles to the local magnetic field, for magnetic fluctuations both perpendicular and parallel to the mean field. In both components, the structure function value at large angles to the field S{⊥} is greater than at small angles S{∥}: in the perpendicular component S{⊥}/S{∥}=5±1 and in the parallel component S{⊥}/S{∥}>3, implying spatially anisotropic fluctuations, k{⊥}>k{∥}. The spectral index of the perpendicular component is -2.6 at large angles and -3 at small angles, in broad agreement with critically balanced whistler and kinetic Alfvén wave predictions. For the parallel component, however, it is shallower than -1.9, which is considerably less steep than predicted for a kinetic Alfvén wave cascade.

Turbulent spectra and spectral kinks in the transition range from MHD to kinetic Alfvén turbulence

“…The 2-k pattern described above can be noticed in many high-resolution high-frequency Cluster measurements (as reported by Kiyani et al, 2009;Sahraoui et al, 2010;Chen et al, 2010) and can also be noticed in some previous measurements where the frequency range extended to 1 Hz or a little higher (see e.g. ACE data reported by Smith et al, 2006, Fig.…”

“…This last range may extend to the dissipative wavenumber k ⊥de at the electron length scale (Sahraoui et al, 2010). The turbulence spectrum measured by Chen et al (2010) can be explained by the combination of above spectra, as is shown in Fig. 1.…”

“…The transition wavenumber for the γ GS k = γ NL k↑↑ transition is practically the same, k ⊥1 ρ p 0.6. However, the above estimations did not take into account the weakening of the MHD nonlinear interactions by the dynamic alignment between velocity and magnetic perturbations (Boldyrev, 2005) and/or by the nonlocal decorrelation mechanism proposed by Gogoberidze (2007). In general, the interaction rate can be written as a reduced GS rate (RGS)…”

“…Turbulent Alfvénic spectrum extending over three ranges, with the steepest slope in the weakly/mildly dispersive KAW range (interpretation of the spectrum shown in Fig. 1 by Chen et al, 2010). general, the "non-dissipative" scenario is as follows: the turbulence, driven at a large MHD scale…”

Density Fluctuations Upstream and Downstream of Interplanetary Shocks

“…At magnetohydrodynamic (MHD) scales, solar wind turbulence has been studied for decades using data from different spacecraft, but some properties (e.g., scaling laws, anisotropy) at kinetic scales are only now beginning to be investigated. To understand the processes of energy cascading and dissipation, it is important to estimate experimentally where key processes such as energy dissipation and particle heating occur (Kiyani et al 2009(Kiyani et al , 2013Sahraoui et al 2009Sahraoui et al , 2010Schekochihin et al 2009;Chen et al 2010Chen et al , 2013.…”