Force-free wave interaction in magnetar magnetospheres: Computational modeling in axisymmetry

TL;DR

This study models relativistic plasma wave interactions in magnetar magnetospheres, revealing efficient conversion of Alfvén waves to fast magnetosonic waves, with implications for understanding magnetar emissions like FRBs.

Contribution

It provides the first detailed computational modeling of wave conversion processes in magnetar magnetospheres using force-free electrodynamics simulations.

Findings

Alfvén waves convert to FMS waves with up to three times higher efficiency during counter-propagation.

FMS waves are generated at twice the frequency of Alfvén waves during initial crossing.

A significant portion of energy is carried away by the decaying FMS wave tail.

Abstract

Crustal quakes of highly magnetized neutron stars can disrupt their magnetospheres, triggering energetic phenomena like X-ray and fast radio bursts (FRBs). Understanding plasma wave dynamics in these extreme environments is vital for predicting energy transport across scales to the radiation length. This study models relativistic plasma wave interaction in magnetar magnetospheres with force-free electrodynamics simulations. For propagation along curved magnetic field lines, we observe the continuous conversion of Alfv\'en waves to fast magnetosonic (FMS) waves. The conversion efficiency can be up to three times higher when counter-propagating Alfv\'en waves interact in the equatorial region. Alfv\'en waves generate FMS waves of twice their frequency during their first crossing of the magnetosphere. After the initial transient burst of FMS waves, Alfv\'en waves convert to FMS waves periodically, generating variations on timescales of the magnetospheric Alfv\'en wave crossing time. This decaying FMS wave tail carries a significant portion (half) of the total energy emitted. Plastic damping of 'bouncing' Alfv\'en waves by the magnetar crust has minimal impact on the FMS efficiency. We discuss the implications of the identified wave phenomena for magnetar observations. Outgoing FMS waves can develop electric zones, potential sources of coherent radiation. Long-wavelength FMS waves could generate FRBs through reconnection beyond the light cylinder.