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 ~-5/3 exponent), and at lower frequencies, of fluctuations of coronal origin (with ~-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 langle|y(t + τ) - y(t)|^mrangle~ τ^{ζ (m)}, but ESS is required to reveal scaling in the inertial range, which is of the form langle|y(t + τ) - y(t|^mrangle ~ [g(τ)]^{ζ (m)}. We find that g(τ) is independent of spacecraft position and g(τ) ∼ τ ^-{log₁₀(\tilde{λ }τ) }. 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.