Streaming instability in neutron star magnetospheres: No indication of soliton-like waves

TL;DR

This study uses multi-dimensional simulations to investigate whether solitons form in neutron star magnetospheres and finds no evidence of soliton formation in 2D, challenging their role in pulsar radio emissions.

Contribution

The paper demonstrates that solitons generated in 1D models do not form in more realistic 2D simulations, questioning their relevance in neutron star radio emission mechanisms.

Findings

No solitons are generated in 2D simulations of streaming instability.

Only superluminal Langmuir waves are produced, with amplitudes decreasing over time.

Solitons are stable in 1D for high beam Lorentz factors but do not form in 2D.

Abstract

Coherent radiation of pulsars, magnetars, and fast radio bursts could, in theory, be interpreted as radiation from solitons and soliton-like waves. The solitons are meant to contain a large number of electric charges confined on long time-scales and may radiate strongly by coherent curvature emission. However, solitons are also known to undergo a wave collapse, which may cast doubts on the correctness of the soliton radio emission models of neutron stars. We investigate the evolution of the caviton type of solitons self-consistently formed by the relativistic streaming instability and compare their apparent stability in 1D calculations with more generic 2D cases, in which the solitons are seen to collapse. Three representative cases of beam Lorentz factors and plasma temperatures are studied to obtain soliton dispersion properties. We utilized 1D electrostatic and 2D electromagnetic relativistic particle-in-cell simulations at kinetic microscales. We found that no solitons are generated by the streaming instability in the 2D simulations. Only superluminal L-mode (relativistic Langmuir) waves are produced during the saturation of the instability, but these waves have smaller amplitudes than the waves in the 1D simulations. The amplitudes tend to decrease after the instability has saturated, and only waves close to the light line, $\omega = c k$, remain. Solitons in the 1D approach are stable for $\gamma_\mathrm{b} \gtrsim 60$, but they disappear for low beam Lorentz factor $\gamma_\mathrm{b} < 6$. Our examples show that the superluminal soliton branch that is formed in 1D simulations will not be generated by the relativistic streaming instability when more dimensional degrees of freedom are present - unless one can show that there are alternative plasma mechanisms for the soliton generation.