OBSERVATION OF UNIVERSALITY IN THE GENERALIZED SIMILARITY OF EVOLVING SOLAR WIND TURBULENCE AS SEEN BY ULYSSES

We perform statistical analysis of the fluctuating magnetic field observed in-situ by the Ulysses spacecraft, from the perspective of quantitative characterization of the evolving magnetohydrodynamic (MHD) turbulence. We focus on two successive polar passes around solar minimum which provide extended intervals of quiet, fast solar wind at a range of radial distances and latitudes: the south polar pass of 1994 and the north polar pass of 1995. Fully developed inertial range turbulence has a characteristic statistical similarity property of quantities that characterize the flow, such as the magnetic field components Bk (t), so that the pth moment of fluctuations has power-law dependence on scale τ such that < |Bk (t + τ ) − Bk (t)|p >∼ τ ζ (p) . We instead find a generalized similarity < |Bk (t + τ ) − Bk (t)|p >∼ g(τ/τ0 )ζ (p) consistent with extended self-similarity; and in particular all of these Ulysses observations, from both polar passes, share the same single function g(τ/τ0 ). If these observations are indeed characteristic of MHD turbulence evolving in-situ, then this quantifies for the first time a key aspect of the universal nature of evolving MHD turbulence in a system of finite size, with implications both for theoretical development, and for our understanding of the evolving solar wind.