Evidence of the ion cyclotron resonance at proton kinetic scales in the solar wind

The k-filtering and wave polarization techniques are applied to Cluster magnetic field data at four intervals of fast free (not connected to Earth's foreshock) solar wind. In addition to the commonly observed population of magnetic field fluctuations propagating at quasi-perpendicular angles to the global mean field B₀ , a population of fluctuations propagating at quasi-parallel angles are also observed when no local plasma instability can be identified. At low wavenumbers (kv_A /Ω_p ≤ 0.6) both components are present, and have powers of similar strength, while at higher wavenumbers (kv_A Ω_p > 0.6) only the perpendicular component can be identified. Here v_A and Ω_p are the Alfvén speed and the proton (angular) gyration frequency, k denotes the wavenumber. The dispersion curve obtained (and the ratio of the magnitudes of left and right hand polarized fluctuations) shows a sharp transition (and decrease) at (kv_A /Ω_p ) ∼ 0.6, which corresponds to a spacecraft frequency f _sc = 0.32 Hz. At higher wavenumbers the measurements agree with previous studies indicating the presence of kinetic Alfvén wave (KAW) turbulence or a mixture of KAW turbulence and convected structures. The parallel component displays a scaling of where the spectral index δ ranges between about 5/3 and 2 although the number of data points does not warrant an accurate determination. The origins of the observed parallel component are unclear. The presence of a parallel component is consistent with ion cyclotron waves, and we interpret the sharp drop of this component at higher wavenumbers as evidence that the ion cyclotron resonance is occurring in the solar wind intervals studied.