期刊
ASTROPHYSICAL JOURNAL
卷 697, 期 1, 页码 923-933出版社
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/697/1/923
关键词
MHD; turbulence
资金
- Leverhulme Trust
- UKMHD
- QMUL HPC facilities purchased under the SRIF initiative
- National Supercomputer Centre in Linkoping in Sweden
- Science and Technology Facilities Council [PP/C50209X/1] Funding Source: researchfish
- STFC [PP/C50209X/1] Funding Source: UKRI
We use three-dimensional direct numerical simulations of the helically forced magnetohydrodynamic equations in spherical shell segments in order to study the effects of changes in the geometrical shape and size of the domain on the growth and saturation of large-scale magnetic fields. We inject kinetic energy along with kinetic helicity in spherical domains via helical forcing using Chandrasekhar-Kendall functions. We take perfect conductor boundary conditions for the magnetic field to ensure that no magnetic helicity escapes the domain boundaries. We find dynamo action giving rise to magnetic fields at scales larger than the characteristic scale of the forcing. The magnetic energy exceeds the kinetic energy over dissipative timescales, similar to that seen earlier in Cartesian simulations in periodic boxes. As we increase the size of the domain in the azimuthal direction, we find that the nonlinearly saturated magnetic field organizes itself in long-lived cellular structures with aspect ratios close to unity. These structures tile the domain along the azimuthal direction, thus resulting in very small longitudinally averaged magnetic fields for large domain sizes. The scales of these structures are determined by the smallest scales of the domain, which in our simulations is usually the radial scale. We also find that increasing the meridional extent of the domains produces little qualitative change, except a marginal increase in the large-scale field. We obtain qualitatively similar results in Cartesian domains with similar aspect ratios.
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