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Inverse cascading for initial magnetohydrodynamic turbulence spectra between Saffman and Batchelor
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Carnegie Mellon University, USA; Ilia State University, Georgia.ORCID iD: 0000-0002-7304-021X
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC).ORCID iD: 0000-0002-2549-6861
Number of Authors: 32023 (English)In: Journal of Plasma Physics, ISSN 0022-3778, E-ISSN 1469-7807, Vol. 89, no 6, article id 905890606Article in journal (Refereed) Published
Abstract [en]

In decaying magnetohydrodynamic (MHD) turbulence with a strong magnetic field, the spectral magnetic energy density is known to increase with time at small wavenumbers k, provided the spectrum at low k is sufficiently steep. This process is called inverse cascading and occurs for an initial Batchelor spectrum, where the magnetic energy per linear wavenumber interval increases like k(4). For an initial Saffman spectrum that is proportional to k(2), however, inverse cascading has not been found in the past. We study here the case of an intermediate k(3) spectrum, which may be relevant for magnetogenesis in the early Universe during the electroweak epoch. This case is not well understood in view of the standard Taylor expansion of the magnetic energy spectrum for small k. Using high resolution MHD simulations, we show that, also in this case, there is inverse cascading with a strength just as expected from the conservation of the Hosking integral, which governs the decay of an initial Batchelor spectrum. Even for shallower k(alpha) spectra with spectral index alpha > 3/2, our simulations suggest a spectral increase at small k with time t proportional to t(4 alpha/9-2/3). The critical spectral index of alpha = 3/2 is related to the slope of the spectral envelope in the Hosking phenomenology. Our simulations with 2048(3) mesh points now suggest inverse cascading even for an initial Saffman spectrum.

Place, publisher, year, edition, pages
2023. Vol. 89, no 6, article id 905890606
Keywords [en]
astrophysical plasmas
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-225064DOI: 10.1017/S0022377823001253ISI: 001113341700001OAI: oai:DiVA.org:su-225064DiVA, id: diva2:1825223
Available from: 2024-01-09 Created: 2024-01-09 Last updated: 2024-01-09Bibliographically approved

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Brandenburg, AxelSharma, Ramkishor

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Nordic Institute for Theoretical Physics (Nordita)Department of AstronomyThe Oskar Klein Centre for Cosmo Particle Physics (OKC)
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