The signature of evolving turbulence in quiet solar wind as seen by ULYSSES

Abstract

Solar wind fluctuations, such as magnetic field or velocity, show power law power spectra suggestive both of an inertial range of intermittent turbulence (with $\sim -5/3$ exponent) and at lower frequencies, of fluctuations of coronal origin (with $\sim -1$ exponent). The ULYSSES spacecraft spent many months in the quiet fast solar wind above the Sun's polar coronal holes in a highly ordered magnetic field. We use statistical analysis methods such as the generalized structure function (GSF) and extended self-similarity (ESS) to quantify the scaling of the moments of the probability density function of fluctuations in the magnetic field. The GSFs give power law scaling in the ``$f^{-1}$'' range of the form $<| y(t+τ)-y(t)|^{m}>\simτ^{ζ(m)}$, but ESS is required to reveal scaling in the inertial range, which is of the form $<| y(t+τ)-y(t)|^{m}>\sim [g(τ)]^{ζ(m)}$. We find that $g(τ)$ is independent of spacecraft position and $g(τ)\simτ^{-log_{10}(\tildeλτ)}$. The ``$f^{-1}$'' scaling fluctuates with radial spacecraft position. This confirms that, whereas the ``$f^{-1}$'' fluctuations are directly influenced by the corona, the inertial range fluctuations are consistent with locally evolving turbulence, but with an ``envelope'' $g(τ)$, which captures the formation of the quiet fast solar wind.