Stern--Gerlach Spin Sorting in Relativistic Magnetic Reconnection

TL;DR

This paper introduces a new spin-kinetic control parameter for magnetic reconnection, evaluates its significance across environments, and demonstrates particle sorting by magnetic moment using simulations, highlighting magnetars as key natural laboratories.

Contribution

It proposes the SG spin-kinetic control parameter, assesses its relevance in various astrophysical environments, and shows particle sorting via SG force without affecting reconnection rates.

Findings

SG transport negligible in magnetotail, solar corona, AGN jets, pulsar nebulae

SG force sorts particles by magnetic moment in simulations

Magnetars identified as prime targets for strong-field spin-kinetic reconnection

Abstract

We introduce a Stern--Gerlach (SG) spin-kinetic control parameter for magnetic reconnection. The fully projected branch parameter, $\Xi_0=<Z>/r_L$ compares the SG cross-sheet displacement accumulated during a diffusion-region transit with the relativistic Larmor radius. For an ensemble or partially participating population the relevant effective parameter is $\Xi_{\rm Eff}=P_{eff}\Xi _0$, where $P_{eff}$ represents the surviving branch weight or effective spin/moment projection. Evaluating $\Xi_{\rm Eff}$ across representative space and astrophysical environments reveals a robust hierarchy: SG transport is negligible in the magnetotail, solar corona, active galactic nuclei (AGN)/blazar jets, and pulsar-wind nebulae, but becomes transitional to strong in magnetar current sheets and extreme near magnetar surfaces. We further show, using electron--positron particle-in-cell simulations, that the SG force sorts particles by magnetic-moment projection into opposite sides of a Harris current sheet without measurably changing the global reconnection rate in the tested regime. This identifies magnetars as the clearest natural target for strong-field spin-kinetic reconnection ($\Xi_{\rm eff}\gg 1$) near the surface; transitional in the outer magnetosphere), while SG transport is safely negligible ($\Xi_{\rm eff}\ll 1$) in all heliophysical and jet environments considered, and provides a falsifiable framework for assessing where SG physics is relevant.