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

¹ Institute for Physics and Astronomy, University of Potsdam, 14476 Potsdam, Germany
² Janusz Gil Institute of Astronomy, University of Zielona Góra, ul. Szafrana 2, 65516 Zielona Góra, Poland
³ Max Planck Institute for Solar System Research, 37077 Göttingen, Germany
⁴ Center for Astronomy and Astrophysics, Technical University of Berlin, 10623 Berlin, Germany
⁵ Max Planck Institute for Radio Astronomy, 53121 Bonn, Germany
⁶ Institute of Theoretical Physics and Astrophysics, Masaryk University, 611 37 Brno, The Czech Republic
⁷ Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, D-15738 Zeuthen, Germany

^⋆ Corresponding author; jan.benacek@uni-potsdam.de

Received: 31 May 2024
Accepted: 15 September 2024

Abstract

Context. 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.

Aims. 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°, 45°, and 90°.

Methods. We use two-dimensional particle-in-cell simulations that include quantum-electrodynamic pair cascades in a charge-limited flow from the star surface.

Results. 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.

Conclusions. 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.