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arXiv · 2509.09949

The universal growth of magnetic energy during the nonlinear phase of subsonic and supersonic small-scale dynamos

Neco Kriel·James R. Beattie·Mark R. Krumholz·Jennifer Schober·Patrick J. Armstrong
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Abstract

Small-scale dynamos (SSDs) amplify magnetic fields in turbulent plasmas. Theory predicts nonlinear magnetic energy growth $E_\mathrm{mag} \propto t^{p_\mathrm{nl}}$, but this scaling has not been tested across flow regimes. Using a large ensemble of SSD simulations spanning subsonic to supersonic turbulence, we measure linear growth ($p_\mathrm{nl} = 1$) in subsonic flows and quadratic growth ($p_\mathrm{nl} = 2$) in supersonic flows. In all cases, the nonlinear dynamo converts a nearly constant fraction $\sim 1/100$ of the turbulent kinetic energy flux into magnetic energy, and the nonlinear phase has a characteristic duration $\Delta t \approx 20\,t_0$, where $t_0$ is the outer-scale turnover time. By isolating the onset of magnetic backreaction in SSDs, our statistical ensemble approach identifies a robust efficiency and duration for the nonlinear SSD that can be used to interpret more complex astrophysical and laboratory plasmas.

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Neco Kriel, James R. Beattie, Mark R. Krumholz, Jennifer Schober, Patrick J. Armstrong. 2025-09-12. The universal growth of magnetic energy during the nonlinear phase of subsonic and supersonic small-scale dynamos. https://arxiv.org/abs/2509.09949

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