SPECTRAL EVOLUTION OF ALFVÉNIC TURBULENCE
Résumé
A correct description of solar wind acceleration relies critically on a good understanding of the turbulent cascade in the solar wind. However, no cascade theory is presently able to reproduce the variability of the observed spectral indices in the large scale range of the spectrum (Grappin et al. 1991; Chen et al. 2013). We propose here to test numerically the possibility that expansion is at the origin of some of the still not understood spectral properties, and focus on the scaling of the Elsasser spectra E±, especially with strong Alfvénicity. We use 3D MHD simulations, with moderate ratio B0/brms, with and without expansion. We find that with zero expansion, small-scale pinning of the dominant and subdominant spectra lead to (unobserved) different indices for the two Elsasser spectra, while with expansion, one finds nearly equal spectral exponents, as observed, and a slow spectral steepening with distance, thus leading naturally to the observed variability of spectral indices.
Domaines
Planète et Univers [physics]Origine | Fichiers éditeurs autorisés sur une archive ouverte |
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