Wavelet determination of scaling exponents and intermittency seen by Solar Orbiter

The solar wind provides a natural laboratory for plasma turbulence at high Reynolds number. We use Solar Orbiter (SO) observations from the Magnetometer (MAG) and the Solar Wind Analyser Suite (SWA) to study extended intervals of homogeneous turbulence. Intervals which exhibit a clear scaling range of magnetohydrodynamic (MHD) turbulence, and transitions to both the ‘1/f’ range at low frequencies, and the kinetic range at frequencies where MHD is no longer valid, are selected. We ensure that all intervals are of steady solar wind flow and do not contain isolated structures such as shocks, pressure pulses and discontinuities.

Solar wind turbulence is anisotropic due to the presence of a background magnetic field. We first rotate the magnetic field into orthogonal coordinate systems with one coordinate parallel to the average direction of the magnetic field B₀, a second coordinate perpendicular to both B₀ and average solar wind flow direction U, and the third in the plane of both B₀ and U. We then perform a Haar wavelet decomposition to obtain the timeseries of magnetic field fluctuations on multiple temporal scales. The Haar wavelet decomposition is by linearly spaced intervals in logarithmic frequency space and hence provides a precise determination of the power spectral exponents, discriminating between 5/3, 3/2 and other relevant values. It also directly estimates the Local Intermittency Measure, which characterizes localized coherent turbulent structures, and the structure functions, which quantify higher order scaling exponents.

We apply these methods to SO intervals in order to test for systematic dependencies on the properties of the turbulence with different plasma conditions and at different distances from the sun.