Magnetic Discontinuities in Magnetohydrodynamic Turbulence and in the Solar Wind

TL;DR

This study investigates whether MHD turbulence can produce magnetic discontinuities similar to those observed in the solar wind, finding that turbulence can indeed generate such features with statistical properties matching observations.

Contribution

The paper demonstrates that MHD turbulence can reproduce solar wind magnetic discontinuities and characterizes their statistical distribution and scaling properties.

Findings

Discontinuities follow an exponential distribution with a characteristic angle.

MHD turbulence reproduces the magnitude and distribution of solar wind discontinuities.

Intermittent structures exhibit anomalous scaling in magnetic field direction.

Abstract

Recent measurements of solar wind turbulence report the presence of intermittent, exponentially distributed angular discontinuities in the magnetic field. In this Letter, we study whether such discontinuities can be produced by magnetohydrodynamic (MHD) turbulence. We detect the discontinuities by measuring the fluctuations of the magnetic field direction, Delta theta, across fixed spatial increments Delta x in direct numerical simulations of MHD turbulence with an imposed uniform guide field B_0. A large region of the probability density function (pdf) for Delta theta is found to follow an exponential decay, proportional to exp(-Delta theta/theta_*), with characteristic angle theta_* ~ (14 deg) (b_rms/B_0)^0.65 for a broad range of guide-field strengths. We find that discontinuities observed in the solar wind can be reproduced by MHD turbulence with reasonable ratios of b_rms/B_0. We also observe an excess of small angular discontinuities when Delta x becomes small, possibly indicating an increasing statistical significance of dissipation-scale structures. The structure of the pdf in this case closely resembles the two-population pdf seen in the solar wind. We thus propose that strong discontinuities are associated with inertial-range MHD turbulence, while weak discontinuities emerge from near-dissipation-range turbulence. In addition, we find that the structure functions of the magnetic field direction exhibit anomalous scaling exponents, which indicates the existence of intermittent structures.