Magnetar field dynamics driven by chiral anomalies without magnetic helicity

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 $\left(B_{\rm dip} \gtrsim 10^{14}\,\mathrm{G}\right)$, 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 chiral 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 newborn neutron stars, yet is crucial for determining their magnetic evolution and the emergence of magnetars.