Alfv\'{e}n wave mode conversion in pulsar magnetospheres

TL;DR

This study investigates how Alfvén waves in pulsar magnetospheres convert into fast magnetosonic waves, affecting radio emission anomalies, using force-free simulations to understand energy loss mechanisms and wave propagation.

Contribution

It provides the first detailed simulation-based analysis of Alfvén to fast mode conversion in pulsar magnetospheres, highlighting the conditions and efficiency of this process.

Findings

Mode conversion can be efficient during initial wave passage.

Dephasing reduces subsequent conversion efficiency.

Significant Alfvén power remains on closed field lines.

Abstract

The radio emission anomaly coincident with the 2016 glitch of the Vela pulsar may be caused by a star quake that launches Alfv\'{e}n waves into the magnetosphere, disturbing the original radio emitting region. To quantify the lifetime of the Alfv\'{e}n waves, we investigate a possible energy loss mechanism, the conversion of Alfv\'{e}nwaves into fast magnetosonic waves. Using axisymmetric force-free simulations, we follow the propagation of Alfv\'{e}n waves launched from the stellar surface with small amplitude into the closed zone of a force-free dipolar pulsar magnetosphere. We observe mode conversion happening in the ideal force-free regime. The conversion efficiency during the first passage of the Alfv\'{e}n wave through the equator can be large, for waves that reach large amplitudes as they travel away from the star, or propagate on the field lines passing close to the Y-point. However, the conversion efficiency is reduced due to dephasing on subsequent passages and considerable Alfv\'{e}n power on the closed field lines remains. Thus while some leakage into the fast mode happens, we need detailed understanding of the original quenching in order to say whether mode conversion alone can lead to reactivation of the pulsar on a short timescale.