Measurement of energy reduction by inertial Alfv\'en waves propagating through parallel gradients in the Alfv\'en speed

TL;DR

This study investigates how inertial Alfvén waves propagate through parallel Alfvén speed gradients, revealing reduced transmission with sharper gradients and suggesting an unknown damping process affecting energy transfer.

Contribution

The paper demonstrates that wave transmission depends on the ratio of wavelength to gradient scale and introduces evidence for an unidentified damping mechanism in plasma.

Findings

Transmission decreases with sharper gradients

Reflected waves are below detection limits

A new damping process may be involved

Abstract

We have studied the propagation of inertial Alfv\'en waves through parallel gradients in the Alfv\'en speed using the Large Plasma Device at the University of California, Los Angeles. The reflection and transmission of Alfv\'en waves through inhomogeneities in the background plasma is important for understanding wave propagation, turbulence, and heating in space, laboratory, and astrophysical plasmas. Here we \rev{present inertial Alfv\'en waves, under conditions relevant to solar flares and the solar corona. We find} that the transmission of the inertial Alfv\'en waves is reduced as the sharpness of the gradient is increased. Any reflected waves were below the detection limit of our experiment and reflection cannot account for all of the energy not transmitted through the gradient. Our findings indicate that, for both kinetic and inertial Alfv\'en waves, the controlling parameter for the transmission of the waves through an Alfv\'en speed gradient is the ratio of the Alfv\'en wavelength along the gradient divided by the scale length of the gradient. Furthermore, our results suggest that an as-yet-unidentified damping process occurs in the gradient.