On the role of Alfv\'en waves as precursors of quasi-static acceleration processes in the Earth auroral zone

TL;DR

This paper investigates how counter-propagating Alfvén waves in the Earth's auroral zone can generate non-propagating electric fields, leading to particle acceleration and plasma density perturbations, through simulation studies.

Contribution

It demonstrates that Alfvén wave interactions can produce static electric fields and plasma density fluctuations, advancing understanding of auroral acceleration processes.

Findings

Non-propagating electric fields are generated at wave crossing points.

Electric fields align with the magnetic field and cause plasma acceleration.

Wave interactions induce plasma density fluctuations on Alfvén wavelength scales.

Abstract

In the Earth auroral zone, the electron acceleration by Alfv\'en waves is sometimes a precursor of the non-propagating acceleration structures. In order to investigate how Alfv\'en waves could generate non-propagating electric fields, a series of simulations of counter-propagating waves in a uniform plasma is presented. The waves (initially not configured to accelerate particles) propagate along the ambient magnetic field direction. It is shown that non propagating electric fields are generated at the locus of the Alfv\'en waves crossing. These electric fields have a component orientated along the direction of the ambient magnetic field, and they generate acceleration and a significant perturbation of the plasma density. The non-linear interaction of down and up-going Alfv\'en waves might be a cause of plasma density fluctuations (with gradients along the magnetic field) on a scale comparable to those of the Alfv\'en wavelengths.