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Hall Cascade with Fractional Magnetic Helicity in Neutron Star Crusts
Stockholms universitet, Nordiska institutet för teoretisk fysik (Nordita).ORCID-id: 0000-0002-7304-021X
Rekke forfattare: 12020 (engelsk)Inngår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 901, nr 1, artikkel-id 18Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

The ohmic decay of magnetic fields in the crusts of neutron stars is generally believed to be governed by Hall drift, which leads to what is known as a Hall cascade. Here we show that helical and fractionally helical magnetic fields undergo strong inverse cascading like in magnetohydrodynamics (MHD), but the magnetic energy decays more slowly with time t: proportional to t(-2/5) instead of proportional to t(-2/3) in MHD. Even for a nonhelical magnetic field there is a certain degree of inverse cascading for sufficiently strong magnetic fields. The inertial range scaling with wavenumber k is compatible with earlier findings for the forced Hall cascade, i.e., proportional to k(-7/3), but in the decaying cases, the subinertial range spectrum steepens to a novel k(5) slope instead of the k(4) slope in MHD. The energy of the large-scale magnetic field can increase quadratically in time through inverse cascading. For helical fields, the energy dissipation is found to be inversely proportional to the large-scale magnetic field and proportional to the fifth power of the rms magnetic field. For neutron star conditions with an rms magnetic field of a few times 10(14) G, the large-scale magnetic field might only be 10(11) G, while still producing magnetic dissipation of 10(33) erg s(-1) for thousands of years, which could manifest itself through X-ray emission. Finally, it is shown that the conclusions from local unstratified models agree rather well with those from stratified models with boundaries.

sted, utgiver, år, opplag, sider
2020. Vol. 901, nr 1, artikkel-id 18
Emneord [en]
Magnetohydrodynamical simulations, Magnetars
HSV kategori
Identifikatorer
URN: urn:nbn:se:su:diva-187846DOI: 10.3847/1538-4357/abad92ISI: 000573452500001OAI: oai:DiVA.org:su-187846DiVA, id: diva2:1510536
Tilgjengelig fra: 2020-12-16 Laget: 2020-12-16 Sist oppdatert: 2022-02-25bibliografisk kontrollert

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