Poynting flux transport channels formed in polar cap regions of neutron star magnetospheres

TL;DR

This study uses advanced simulations to reveal that Poynting flux channels form in neutron star polar caps where currents are minimal, enabling coherent radio wave escape without traditional energy conversion processes.

Contribution

It demonstrates that Poynting flux transport channels form along specific magnetic field lines, providing a new understanding of pulsar radio emission mechanisms without requiring particle-to-wave energy conversion.

Findings

Transport channels for high Poynting flux form where currents approach zero.

Coherent radio waves can escape directly along open magnetic field lines.

The pulsar radio beam is not cone-shaped but directly emitted along certain field lines.

Abstract

Pair cascades in polar cap regions of neutron stars are considered to be an essential process in various models of coherent radio emissions of pulsars. The cascades produce pair plasma bunch discharges in quasi-periodic spark events. The cascade properties, and therefore also the coherent radiation, depend strongly on the magnetospheric plasma properties and vary significantly across and along the polar cap. Importantly, where the radio emission emanates from in the polar cap region is still uncertain. We investigate the generation of electromagnetic waves by pair cascades and their propagation in the polar cap for three representative inclination angles of a magnetic dipole, $0^\circ$, $45^\circ$, and $90^\circ$. We use two-dimensional particle-in-cell simulations that include quantum-electrodynamic pair cascades in a charge-limited flow from the star surface. We find that the discharge properties are strongly dependent on the magnetospheric current profile in the polar cap and that transport channels for high intensity Poynting flux are formed along magnetic field lines where the magnetospheric currents approach zero and where the plasma cannot carry the magnetospheric currents. There, the parallel Poynting flux component is efficiently transported away from the star and may eventually escape the magnetosphere as coherent radio waves. The Poynting flux decreases with increasing distance from the star in regions of high magnetospheric currents. Our model shows that no process of energy conversion from particles to waves is necessary for the coherent radio wave emission. Moreover, the pulsar radio beam does not have a cone structure; rather, the radiation generated by the oscillating electric gap fields directly escapes along open magnetic field lines in which no pair creation occurs.