The Implications of Discontinuities for Testing Theories of Turbulence in the Solar Wind

TL;DR

This study investigates how discontinuities in solar wind magnetic field data affect turbulence analysis, revealing that removing these discontinuities makes the turbulence appear more isotropic and aligns with the Iroshnikov-Kraichnan model.

Contribution

The paper introduces a novel method to remove discontinuities without imposing a scale, clarifying the true nature of turbulence in solar wind data.

Findings

Discontinuities significantly influence the observed anisotropy in PSD.

Removing discontinuities results in quasi-isotropic turbulence consistent with Iroshnikov-Kraichnan.

A surrogate analysis estimates the noise-floor caused by discontinuities.

Abstract

In-situ observations of magnetic field fluctuations in the solar wind show a broad continuum in the power spectral density (PSD) with a power-law range of scaling often identified as an inertial range of magnetohydrodynamic turbulence. However, both turbulence and discontinuities are present in the solar wind on these inertial range of scales. We identify and remove these discontinuities using a method which for the first time does not impose a characteristic scale on the resultant time-series. The PSD of vector field fluctuations obtained from at-a point observations is a tensor that can in principle be anisotropic with scaling exponents that depend on background field and flow direction. This provides a key test of theories of turbulence. We find that the removal of discontinuities from the observed time-series can significantly alter the PSD trace anisotropy. It becomes quasi-isotropic, in that the observed exponent does not vary with the background field angle once the discontinuities are removed. This is consistent with the predictions of the Iroshnikov-Kraichnan model of turbulence. As a consistency check we construct a surrogate time-series from the observations that is composed solely of discontinuities. The surrogate provides an estimate of the PSD due solely to discontinuities and this provides the effective noise-floor produced by discontinuities for all scales greater than a few ion-cyclotron scales.