Generation of the magnetic helicity in a neutron star driven by the electroweak electron-nucleon interaction

TL;DR

This paper proposes a novel mechanism driven by electroweak electron-nucleon interactions that can amplify seed magnetic fields in neutron stars by five orders of magnitude, potentially explaining magnetar magnetic fields.

Contribution

It introduces a new instability mechanism involving magnetic helicity growth due to electroweak interactions in neutron star cores, explaining magnetic field amplification.

Findings

Magnetic fields can be amplified from 10^{12} G to 10^{17} G within 10^3-10^5 years.

The mechanism relies on the growth of magnetic helicity driven by anomalous electric currents.

Initial magnetic helicity can be arbitrary, with evolution tending toward maximal helicity.

Abstract

We study the instability of magnetic fields in a neutron star core driven by the parity violating part of the electron-nucleon interaction in the Standard Model. Assuming a seed field of the order $10^{12}\thinspace\text{G}$, that is a common value for pulsars, one obtains its amplification due to such a novel mechanism by about five orders of magnitude, up to $10^{17}\thinspace\text{G}$, at time scales $\sim (10^3 - 10^5)\thinspace\text{yr}$. This effect is suggested to be a possible explanation of the origin of the strongest magnetic fields observed in magnetars. The growth of a seed magnetic field energy density is stipulated by the corresponding growth of the magnetic helicity density due to the presence of the anomalous electric current in the Maxwell equation. Such an anomaly is the sum of the two competitive effects: (i) the chiral magnetic effect driven by the difference of chemical potentials for the right and left handed massless electrons and (ii) constant chiral electroweak electron-nucleon interaction term, which has the polarization origin and depends on the constant neutron density in a neutron star core. The remarkable issue for the decisive role of the magnetic helicity evolution in the suggested mechanism is the arbitrariness of an initial magnetic helicity including the case of non-helical fields from the beginning. The tendency of the magnetic helicity density to the maximal helicity case at large evolution times provides the growth of a seed magnetic field to the strongest magnetic fields in astrophysics.