Intermittency of Fast MHD Modes and Regions of Anomalous Gradient Orientation in Low-beta Plasmas

TL;DR

This paper investigates how intermittent fast MHD modes in low-beta plasmas affect magnetic field tracing, introducing a new gradient amplitude technique to improve accuracy by suppressing fast mode interference.

Contribution

It reveals the intermittent nature of fast modes in low-beta plasmas and proposes a new gradient amplitude method to mitigate their impact on magnetic field measurements.

Findings

Fast modes are highly intermittent with small filling factors.

Gradient amplitude techniques reduce fast mode interference.

Synchrotron intensity gradients are less affected by fast modes.

Abstract

The strong alignment of small-scale turbulent Alfv\'enic motions with the direction of the magnetic field that percolates the small-scale eddies and imprints the direction of the magnetic field is a property that follows from the MHD theory and the theory of turbulent reconnection. The Alfv\'enic eddies mix magnetic fields perpendicular to the direction of the local magnetic field, and this type of motion is used to trace magnetic fields with the velocity gradient technique (VGT). The other type of turbulent motion, fast modes, induces anisotropies orthogonal to Alfv\'enic eddies and interferes with the tracing of the magnetic field with the VGT. We report a new effect, i.e., in a magnetically dominated low-\beta subsonic medium, fast modes are very intermittent, and in a volume, with a small filling factor the fast modes dominate other turbulent motions. We identify these localized regions as the cause of the occasional change of direction of gradients in our synthetic observations. We show that the new technique of measuring the gradients of gradient amplitudes suppresses the contribution from the fast-mode-dominated regions, improving the magnetic field tracing. In addition, we show that the distortion of the gradient measurements by fast modes is also applicable to the synchrotron intensity gradients, but the effect is reduced compared to the VGT.