On the escape of low-frequency waves from magnetospheres of neutron stars

TL;DR

This paper investigates how fast magnetosonic waves decay into Alfvén waves in neutron star magnetospheres, impacting models of pulsar emissions and fast radio bursts by revealing energy transfer mechanisms and limitations.

Contribution

It introduces a nonlinear decay mechanism of fast magnetosonic waves into Alfvén waves in relativistic force-free magnetohydrodynamics, with implications for astrophysical radio emissions.

Findings

Decay rate of waves estimated.

Infinite phase volume for Alfvén waves enables complete energy transfer.

Constraints on magnetospheric models of FRBs and pulsar radio emission.

Abstract

We study the nonlinear decay of the fast magnetosonic into the Alfv\'en waves in relativistic force-free magnetohydrodynamics. The work has been motivated by models of pulsar radio emission and fast radio bursts (FRBs), in which the emission is generated in neutron star magnetospheres at conditions when not only the Larmor but also the plasma frequencies significantly exceed the radiation frequency. The decay process places limits on the source luminosity in these models. We estimated the decay rate and showed that the phase volume of Alfv\'en waves available for the decay of an fms wave is infinite. Therefore the energy of fms waves could be completely transferred to the small-scale Alfv\'en waves not via a cascade, as in the Kolmogorov turbulence, but directly. Our results explain the anomalously low radio efficiency of the Crab pulsar and show that FRBs could not be produced well within magnetar magnetospheres.