Magnetar field dynamics driven by chiral anomalies without magnetic helicity

TL;DR

This study demonstrates that the chiral magnetic effect can generate strong magnetar magnetic fields regardless of initial helicity, highlighting the importance of initial conditions in neutron star magnetic evolution.

Contribution

It shows that CME-driven magnetic field amplification occurs independently of initial helicity, expanding understanding of magnetar formation mechanisms.

Findings

CME can generate magnetar-strength dipoles within decades.

The instability depends on the maximum chiral chemical potential.

Magnetic dipoles can be stable or decay depending on initial helicity.

Abstract

The chiral magnetic effect (CME), arising from the chiral anomaly and enabling a mutual conversion between magnetic topology and fermionic chirality, is a key mechanism in magnetar field evolution. Previous work by Dehman & Pons (2025) demonstrated that the CME can efficiently generate dipolar fields ($B_{\rm dip} \gtrsim 10^{14}~\mathrm{G}$), consistent with magnetar timing measurements, provided that the initial magnetic field carries net helicity. However, whether neutron stars are born with magnetic helicity remains uncertain. In this work, we investigate the CME across a range of initial helicity configurations, including non-helical initial conditions. We find that the CME efficiently generates magnetar-strength dipoles on timescales of decades, independently of the initial helicity content. The instability is driven by localized helical structures that induce a residual chiral asymmetry and is primarily governed by the maximum chiral chemical potential, requiring $\mu_5^{\rm max} \gtrsim \mathrm{few}\times10^{-11}~\mathrm{MeV}$ for onset in the magnetar regime. Our results further show that these dipoles may either remain stable and subsequently evolve through standard Ohmic decay, or become unstable if they acquire sufficient helicity, in which case they decay through the chiral anomaly, transferring energy to less helical modes. This outcome depends sensitively on the initial helicity distribution. These findings extend the applicability of the CME to more realistic magnetic-field configurations and underscore the importance of the helicity distribution at birth, a quantity that remains poorly constrained in neutron star formation, yet is crucial for determining neutron star magnetic evolution and magnetar formation.