On the pulsar Y-point

TL;DR

This paper investigates the structure of the pulsar magnetosphere, revealing that the system naturally favors a configuration where the closed-line region extends to 90% of the light cylinder, and discusses the geometry at the Y-point.

Contribution

The study introduces an improved numerical method to solve the pulsar equation, demonstrating a preferred magnetospheric configuration with minimal electromagnetic energy and analyzing the Y-point geometry.

Findings

The magnetosphere's energy is minimized when the closed-line region reaches 90% of the light cylinder.

The Y-point forms a T-shape with the equatorial current sheet, at right angles.

Time-dependent and particle-in-cell simulations show different closed-line region extents.

Abstract

The pulsar magnetosphere is divided into a corotating region of closed field lines surrounded by open field lines that emanate from the two poles of the star, extend to infinity and are separated by an equatorial current sheet. The three regions meet at a magnetospheric Y-point. In steady-state solutions of the ideal force-free magnetosphere, the Y-point may lie at any distance inside the light cylinder. Time-dependent force-free simulations, however, develop closed-line regions that extend all the way to the light cylinder. On the other hand, particle (PIC) solutions consistently develop smaller closed-line regions. In order to understand this effect, we solve the pulsar equation with an improved numerical method. We show that the total electromagnetic energy stored in the ideal force-free magnetosphere manifests a subtle minimum when the closed-line region extends to only 90% of the light cylinder, and thus argue that the system will spontaneously choose this particular configuration. Furthermore, we argue that the intersection of the corotating region with the equatorial current sheet is at right angles, literally leading to a T-point.