Solar wind Cluster observations: turbulent spectrum and role of Hall effect O. Alexandrova a , V. Carbone b , P. Veltri b , L. Sorriso-Valvo c a LESIA/CNRS, Observatoire de Paris, 5 pl. J. Janssen, 92195 Meudon, France b Dip. Fisica/CNISM, Ponte P. Bucci, Cubo 31C, 87036 Rende (CS), Italy c LICRYL/CNR, Ponte P. Bucci, Cubo 33B, 87036 Rende (CS), Italy Abstract Turbulent spectrum of magnetic fluctuations in the solar wind nearly follows Kol- mogorov’s 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 on- board 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 can- not 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 -7/3-4h/3 , which depends on the plasma compressibility. Key words: Space Plasma Physics, Turbulence, Interplanetary Physics, Magnetohydrodynamics and Plasmas PACS: 94.05.-a, 94.05.Lk, 96.50.-e, 95.30.Qd 1 Introduction Interplanetary space, which is highly turbulent, represents the unique oppor- tunity we have to experimentally investigate turbulence in natural plasmas [6]. In this paper we analyze solar wind magnetic fluctuations on the scales of the order of the ion cyclotron frequency f ci . Frequencies close to f ci are criti- cal, because at this scale the usual Magnetohydrodynamic (MHD) description breaks down in favour of a more complex description of plasma. Within the standard fluid-like framework, it is generally believed that at this critical scale Preprint submitted to Elsevier 24 January 2007