Dissipative magnetic structures and scales in small-scale dynamos

Axel Brandenburg, Igor Rogachevskii, Jennifer Schober

Research output: Contribution to journalArticlepeer-review

5 Scopus citations


Small-scale dynamos play important roles in modern astrophysics, especially on galactic and extragalactic scales. Owing to dynamo action, purely hydrodynamic Kolmogorov turbulence hardly exists and is often replaced by hydromagnetic turbulence. Understanding the size of dissipative magnetic structures is important in estimating the time-scale of galactic scintillation and other observational and theoretical aspects of interstellar and intergalactic small-scale dynamos. Here we show that, during the kinematic phase of the small-scale dynamo, the cutoff wavenumber of the magnetic energy spectra scales as expected for large magnetic Prandtl numbers, but continues in the same way also for moderately small values - contrary to what is expected. For a critical magnetic Prandtl number of about 0.3, the dissipative and resistive cutoffs are found to occur at the same wavenumber. In the non-linearly saturated regime, the critical magnetic Prandtl number becomes unity. The cutoff scale now has a shallower scaling with magnetic Prandtl number below a value of about three, and a steeper one otherwise compared to the kinematic regime.

Original languageEnglish
Pages (from-to)6367-6375
Number of pages9
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
StatePublished - 1 Feb 2023


  • MHD
  • dynamo
  • galaxies: magnetic fields
  • turbulence

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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