Anisotropies of the Magnetic Field Fluctuations at Kinetic Scales in the Solar Wind: Cluster Observations
- LESIA-Observatoire de Paris, PSL Research University, CNRS, UPMC Université Paris 06, Université Paris-Diderot, 5 place Jules Janssen, F-92190 Meudon (France)
We present the first statistical study of the anisotropy of the magnetic field turbulence in the solar wind between 1 and 200 Hz, i.e., from proton to sub-electron scales. We consider 93 ten-minute intervals of the Cluster/STAFF measurements. We find that the fluctuations δB{sub ⊥}{sup 2} are not gyrotropic at a given frequency f, a property already observed at larger scales (∥/⊥ means parallel/perpendicular to the average magnetic B{sub 0}). This non-gyrotropy gives indications of the angular distribution of the wave vectors k: at f< 10 Hz, we find that k{sub ⊥}≫k{sub ∥}, mainly in the fast wind; at f > 10 Hz, fluctuations with a non-negligible k {sub ∥} are also present. We then consider the anisotropy ratio δB{sub ∥}{sup 2}/δB{sub ⊥}{sup 2}, which is a measure of the magnetic compressibility of the fluctuations. This ratio, always smaller than 1, increases with f. It reaches a value showing that the fluctuations are more or less isotropic at electron scales, for f⩾50 Hz. From 1 to 15–20 Hz, there is a strong correlation between the observed compressibility and the one expected for the kinetic Alfvén waves (KAWs), which only depends on the total plasma β. For f>15--20 Hz, the observed compressibility is larger than expected for KAWs, and it is stronger in the slow wind: this could be an indication of the presence of a slow-ion acoustic mode of fluctuations, which is more compressive and is favored by the larger values of the electron to proton temperature ratio generally observed in the slow wind.
- OSTI ID:
- 22875702
- Journal Information:
- Astrophysical Journal, Vol. 848, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
Similar Records
Compressive coherent structures at ion scales in the slow solar wind
Kinetic Cascade in Solar-wind Turbulence: 3D3V Hybrid-kinetic Simulations with Electron Inertia