Large Pm small-scale kinematic dynamo in protoneutron stars
Abstract
Magnetars are young, isolated neutron stars that possess an exceptionally strong magnetic field, with surface dipolar strengths on the order of 1015 G. One of the plausible scenarios for generating such a strong field is an exponential amplification by a turbulent convective dynamo during the protoneutron star phase. However, the short expected duration of the convection (∼ 10 s) imposes a stringent constraint on the dynamo growth rate. We perform an extensive set of 82 three-dimensional convective dynamo simulations in the anelastic approximation and investigate the kinematic phase to quantify the dynamo growth rate γ. We find that γ increases with both the magnetic Prandtl number Pm and the Rayleigh number Ra, with the most unstable mode becoming highly non-axisymmetric and multipolar. We further observe a gradual transition from large-scale to small-scale dynamo as the magnetic Reynolds number Rm increases, resulting in a magnetic field that is predominantly concentrated at small scales. The trend remains unchanged when the outer magnetic boundary condition is varied. Since resolving the increasingly small scales becomes numerically impractical, we employ the theoretical small-scale Kazantsev dynamo model to explore the large Pm regime characteristic of protoneutron stars. The model qualitatively captures the growth rate behaviour observed in simulations and, upon extrapolation to the large Pm limit, indicates that γ is only weakly dependent on the resistivity in a PNS. Under conditions relevant to the PNS, this model predicts a magnetic energy growth rate of the order of ∼ 1 ms−1.
Versions
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BibTeX
@misc{verma2026large,
title = {{Large $\rm Pm$ small-scale kinematic dynamo in protoneutron stars}},
author = {Shipra Verma and Kannabiran Seshasayanan and Raphaël Raynaud and Jérôme Guilet},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.17365},
eprint = {2609.17365},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.17365},
note = {CC BY-NC-SA 4.0},
}
RIS
TY - UNPB
TI - Large Pm small-scale kinematic dynamo in protoneutron stars
AU - Shipra Verma
AU - Kannabiran Seshasayanan
AU - Raphaël Raynaud
AU - Jérôme Guilet
PY - 2026
DA - 2026/09/15
PB - arXiv
DO - 10.48550/arxiv.2609.17365
UR - https://arxiv.org/abs/2609.17365
AB - Magnetars are young, isolated neutron stars that possess an exceptionally strong magnetic field, with surface dipolar strengths on the order of 10^15 G. One of the plausible scenarios for generating such a strong field is an exponential amplification by a turbulent convective dynamo during the protoneutron star phase. However, the short expected duration of the convection (∼ 10 s) imposes a stringent constraint on the dynamo growth rate. We perform an extensive set of 82 three-dimensional convective dynamo simulations in the anelastic approximation and investigate the kinematic phase to quantify the dynamo growth rate γ. We find that γ increases with both the magnetic Prandtl number Pm and the Rayleigh number Ra, with the most unstable mode becoming highly non-axisymmetric and multipolar. We further observe a gradual transition from large-scale to small-scale dynamo as the magnetic Reynolds number Rm increases, resulting in a magnetic field that is predominantly concentrated at small scales. The trend remains unchanged when the outer magnetic boundary condition is varied. Since resolving the increasingly small scales becomes numerically impractical, we employ the theoretical small-scale Kazantsev dynamo model to explore the large Pm regime characteristic of protoneutron stars. The model qualitatively captures the growth rate behaviour observed in simulations and, upon extrapolation to the large Pm limit, indicates that γ is only weakly dependent on the resistivity in a PNS. Under conditions relevant to the PNS, this model predicts a magnetic energy growth rate of the order of ∼ 1 ms^−1.
ER -