A statistical study of the solar wind turbulence at ion kinetic scales using the k-filtering technique and cluster data

Plasma turbulence at ion kinetic scales in the solar wind is investigated using the multi-point magnetometer data from the Cluster spacecraft. By applying the k-filtering method, we are able to estimate the full three-dimensional power spectral density P(ω_sc, k) at a certain spacecraft frequency ω_sc in wavevector (k) space. By using the wavevector at the maximum power in P(ω_sc, k) at each sampling frequency ω_sc and the Doppler shifted frequency ω_pla in the solar wind frame, the dispersion plot ω_pla = ω_pla (k) is found. Previous studies have been limited to very few intervals and have been hampered by large errors, which motivates a statistical study of 52 intervals of solar wind. We find that the turbulence is predominantly highly oblique to the magnetic field k_i 蠑 k_ii, and propagates slowly in the plasma frame with most points having frequencies smaller than the proton gyrofrequency ω_pla < Ω_p . Weak agreement is found that turbulence at the ion kinetic scales consists of kinetic Alfvén waves and coherent structures advected with plasma bulk velocity plus some minor more compressible components. The results suggest that anti-sunward and sunward propagating magnetic fluctuations are of similar nature in both the fast and slow solar wind at ion kinetic scales. The fast wind has significantly more anti-sunward flux than sunward flux and the slow wind appears to be more balanced.