Solar wind Cluster observations: Turbulent spectrum and role of Hall effect

Turbulent spectrum of magnetic fluctuations in the solar wind nearly follows Kolmogorovʹs law ∼ f - 5 / 3 below the ion cyclotron frequency f ci . Above this frequency, the observed steeper power law is believed to be a ‘dissipative range’ of the solar wind turbulence. In this paper we analyze magnetic field fluctuations measured onboard Cluster, lasting two decades above f ci . Well-defined power law and a strong increase of intermittency with frequency in this range indicates that turbulence cannot be characterized by a ‘dissipative range’. Rather we conjecture that above f ci the turbulence is generated by a different nonlinear cascade. We suggest that the presence of dispersive effects is responsible for a steepening of the spectral energy density, simply because a cascade can be realized in a time that is shorter than the usual eddy-turnover time. In the Hall MHD formulation, the magnetic power follows a law ∼ f - 7 / 3 + 2 α , which depends on the degree of plasma compressibility α .