Powerful parametric instability of Alfven waves in astrophysical pair plasma

TL;DR

This paper reveals that nonlinear Alfven waves in highly magnetized pair plasmas undergo strong modulational instability, leading to large density fluctuations, with implications for astrophysical phenomena like Fast Radio Bursts.

Contribution

It develops an analytic framework and uses PIC simulations to demonstrate the powerful modulational instability of Alfven waves in relativistic pair plasmas.

Findings

Large density fluctuations develop near the critical wave-number $k_0$.

Charge separation effects are temporary; fluctuations become charge neutral over time.

High magnetization leads to generation of high-frequency modes.

Abstract

We demonstrate that in highly magnetized pair plasmas, nonlinear Alfven waves with wave-number $k \leq k_0 = \omega_p^2 /(\delta \omega_B)$ ($\delta =( \delta B)/B_0$ are relative fluctuations of the magnetic field) experience powerful modulational instability. In the two-fluid approximation, we develop an analytic set-up for circularly polarized (CP) Alfven mode in its frame (where the initial configuration is stationary; it is moving with relativistic, amplitude-dependent Alfven velocity $v_A (\sigma, \delta ) $, while both charges experience different, amplitude-dependent, synchrotron gyration). PIC simulations using EPOCH code demonstrate that for Alfven waves with $k$ near $k_0$, large, parametrically-driven density fluctuations develop, and lead to fast modulational instability. Charge separation effects, for a CP wave in magnetized pair plasma, might be temporarily important; on longer time-scales the density fluctuations are charge neutral and in symmetric pair plasma quickly grow to large amplitudes. In highly magnetized plasma, $\sigma \gg 1$, high frequency modes $k / k_0 \sim (2-3 ) \times \sigma \gg 1 $ are quickly generated; for smaller plasma magnetization, the dominant mode is at the Bragg's condition $k = 2 k_0$. Long term behavior of CP and LP modes is similar. We discuss application of the results to the physics of Fast Radio Bursts generated/propagating in the magnetospheres of magnetars.