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Nature and duration of high–magnetic-Prandtl-number nonlinear dynamos

Characterize the nonlinear, secular dynamo behavior in plasmas with magnetic Prandtl number much greater than unity (Pm ≫ 1), including whether a second nonlinear dynamo emerges as magnetic fields traverse sub-viscous scales, and ascertain the duration of dynamo activity in this regime.

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Background

The simulations in the paper focus on Pm = 1, whereas many hot astrophysical plasmas have Pm ≫ 1. Prior theoretical work suggests that in high-Pm plasmas the magnetic field may evolve through sub-viscous scales and potentially exhibit a distinct, second nonlinear secular dynamo phase.

The authors explicitly state that both the qualitative nature of dynamos at high Pm and the timescales over which they operate remain unresolved, highlighting the need for targeted theoretical and numerical investigations of this regime.

References

We note that our study has focused on Pm = 1 plasmas, whilst many hot astrophysical plasmas are Pm ≫ 1 (Pm ∝ T4/n_e, where T is the plasma temperature and n_e is the electron number density). In this regime, there may emerge a second nonlinear, secular dynamo, as the magnetic field transverses the sub-viscous range of scales [Schekochihin2002model]. The nature of dynamos in this regime, and their duration, also remain open questions for future work.

— The growth of magnetic energy during the nonlinear phase of the subsonic and supersonic small-scale dynamo (2509.09949 - Kriel et al., 12 Sep 2025) in Discussion and Conclusions