Nonlinear firehose relaxation and constant-B field fluctuations

TL;DR

This paper investigates the nonlinear evolution of Alfvénic fluctuations in the firehose unstable regime, revealing self-organized states with constant magnetic field magnitude that may explain observations in the solar wind.

Contribution

It introduces a theoretical and numerical analysis of nonlinear firehose relaxation, showing the emergence of constant-B states as a stable attractor.

Findings

States with broadband magnetic and velocity fluctuations form during nonlinear saturation.

These states tend to maintain a constant magnetic field magnitude.

The results suggest a mechanism for the constant-B states observed in the solar wind.

Abstract

The nonlinear evolution of Alfv\'enic fluctuations in the firehose unstable regime is investigated numerically and theoretically for an anisotropic plasma described by the one-fluid double adiabatic equations. We revisit the traditional theory of the instability and examine the nonlinear saturation mechanism, showing that it corresponds to evolution towards states that minimize an appropriate energy functional. We demonstrate that such states correspond to broadband magnetic and velocity field fluctuations with an overall constant magnitude of the magnetic field. These nonlinear states provide a basin of attraction for the long-term nonlinear evolution of the instability, a self-organization process that may play a role in maintaining the constant-$B$ Alfv\'enic states seen in the solar wind in the high-$\beta$ regime.