# Effective description of non-equilibrium currents in cold magnetized plasma

### Submission summary

 As Contributors: Nabil Iqbal Arxiv Link: https://arxiv.org/abs/1909.12609v1 (pdf) Date submitted: 2021-03-09 12:44 Submitted by: Iqbal, Nabil Submitted to: SciPost Physics Academic field: Physics Specialties: High-Energy Physics - Theory Approach: Theoretical

### Abstract

The dynamics of cold strongly magnetized plasma -- traditionally the domain of force-free electrodynamics -- has recently been reformulated in terms of symmetries and effective field theory, where the degrees of freedom are the momentum and magnetic flux carried by a fluid of cold strings. In physical applications where the electron mass can be neglected one might expect the presence of extra light charged modes -- electrons in the lowest Landau level -- propagating parallel to the magnetic field lines. We construct an effective description of such electric charges, describing their interaction with plasma degrees of freedom in terms of a new collective mode that can be thought of as a bosonization of the electric charge density along each field line. In this framework QED phenomena such as charged pair production and the axial anomaly are described at the classical level. Formally, our construction corresponds to gauging a particular part of the higher form symmetry associated with magnetic flux conservation. We study some simple applications of our effective theory, showing that the scattering of magnetosonic modes generically creates particles and that the rotating Michel monopole is now surrounded by a cloud of electric charge.

###### Current status:
Editor-in-charge assigned

### Submission & Refereeing History

Submission 1909.12609v1 on 9 March 2021

## Reports on this Submission

### Strengths

1- Makes progress in an important astrophysical question (What is the non-linear dynamics of magnetic field lines in a plasma in the presence of out-of-equilibrium charges?) using a clean theoretical approach.
2- The formal development of the effective field theory is accompanied by several applications.

### Weaknesses

1- The problem of extending the framework to massive charge carriers is left unresolved.
2- While the paper contains quite a bit of background, the discussion of the EFT of force free electrodynamics is not self-contained (as the author also points out).

### Report

This paper determines the dynamics of non-equilibrium currents in cold magnetized plasma using an effective field theory (EFT) description. It builds on prior work of the author that developed an EFT description for force free electrodynamics, and solves the (astro)physically important problem of how to couple this EFT to non-equilibrium currents. Both the original EFT and its coupling to non-equilibrium currents is governed by unconventional and intricate symmetries that the author presents very clearly. The paper would deserve publication even as purely theoretical exercise, and its value is further enhanced by the promise of applying its formalism to astrophysical problems and its preliminary discussion of some applications.

### Requested changes

1- Present the EFT of force free electrodynamics in a more self-contained manner, especially the meaning of the EFT degrees of freedom (perhaps through a figure).
2- Explain explicitly (around eqs. (18)-(22)) what the EFT is an expansion in, i.e. what physical scale makes the derivative expansion dimensionless. (I understand that this issue is discussed later in the paper.)
3- Explain why in Sec IV. one higher derivative term is kept. Why isn't it enough to analyze the coupled theory of FFE and $\Theta$?

• validity: high
• significance: good
• originality: good
• clarity: high
• formatting: perfect
• grammar: perfect