Solar wind turbulence: analysis at different scales, Waves and Instabilities in Space Plasmas
Résumé
Magnetic turbulence in the solar wind has been studied since the beginning of space exploration by several spacecraft such as Voyager, Helios, Ulysses, Wind and Stereo. More recently, a real breakthrough has been made in understanding this turbulence thanks to the multi-point Cluster measurements. Using appropriate multi-spacecraft techniques, such as the so-called k-filtering technique, it has been possible to compute 3D wave-number spectra of the turbulence both at MHD scales (L>>i, i being the ion Larmor radius) and at sub-proton scales (L<i) [Narita PRL, 2010; Sahraoui et al., PRL 2010]. Moreover, thanks to the use of high time resolution of the Cluster magnetic field data (up to 100Hz in the spacecraft reference frame), it has been possible for the first time to probe the electron scales where a new kinetic cascade and dissipation has been revealed [Sahraoui et al., PRL 2009, Alexandrova PRL., 2009]. Besides these analyses of power spectra (i.e. the 2nd order moment of the PDFs) other analyses involving the phase can be performed. This is mandatory to understand the role of non-linear effects in the different domains and to investigate the possibility of an energy cascade through different scales (ion and electron scales). Two different methods are applied to diagnose the degree of phase coherence - which is related to the degree of nonlinearities - of the turbulent signals (using single spacecraft data). One is a direct statistical analysis of the phase gradients; the other is the comparison with a surrogate signals through computation of structure functions. These methods will be presented with application to simulations and real data in the solar wind.