Freiburg 2008 – wissenschaftliches Programm
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EP: Fachverband Extraterrestrische Physik
EP 8: The Sun and Heliosphere II
EP 8.3: Vortrag
Dienstag, 4. März 2008, 17:15–17:30, KGI-Aula
Universal Lévy laws and monoscaling in solar wind turbulence — •Wolf-Christian Müller1 and Mahdi Momeni2 — 1Max-Planck-Institut für Plasmaphysik, Boltzmannstr. 2, 85748 Garching — 2Faculty of Physics, Tabriz University, Tabriz 51664, Iran
Probability density functions (PDFs) of scale-dependent energy fluctuations, P[δ E(ℓ)], are studied in high-resolution direct numerical simulations of Navier-Stokes and incompressible magnetohydrodynamic (MHD) turbulence. MHD flows with and without a strong mean magnetic field are considered. For all three systems it is found that the PDFs of inertial range energy fluctuations exhibit self-similarity and monoscaling in agreement with recent solar-wind measurements [B. Hnat et al., Geophys. Res. Lett. 29(10), 86-1 (2002)]. Furthermore, the energy PDFs exhibit similarity over all scales of the turbulent system showing no substantial qualitative change of shape as the scale of the fluctuations varies. This is in sharp and surprising contrast to the well-known behavior of PDFs of turbulent field fluctuations e.g. of velocity and magnetic field. In all three cases under consideration the P[δ E(ℓ)] resemble Lévy-type Gamma distributions ∼ Δ−1exp(δ E/Δ)|δ E|−γ in agreement with the solar-wind observations. The observed Gamma distributions exhibit a scale-dependent width Δ(ℓ) and a system-dependent γ. They are apparently a characteristic and universal consequence of turbulent energy transfer even outside of the self-similar inertial range. A simple theory explains the appearance of Gamma-type PDFs as consequence of a cascade process and also accounts for the observed monoscaling exponents.