Dispersion relation analysis of solar wind turbulence

1 Frequency versus wave number diagram of turbulent magnetic fluctuations in the solar wind was determined for the first time in the wide range over three decades using four Cluster spacecraft. Almost all of the identified waves propagate quasi-perpendicular to the mean magnetic field at various phase speeds, accompanied by a transition from the dominance of outward propagation from the Sun at longer wavelengths into mixture of counter-propagation at shorter wavelengths. Frequency-wave number diagram exhibits largely scattered populations with only weak agreement with magnetosonic and whistler waves. Clear identification of a specific normal mode is difficult, suggesting that nonlinear energy cascade is operating even on small-scale fluctuations.