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| Titel |
Alfvén-dynamo balance and magnetic excess in MHD turbulence |
| VerfasserIn |
Roland Grappin, Wolf Christian Müller, Andrea Verdini |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250128932
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| Publikation (Nr.) |
 EGU/EGU2016-8978.pdf |
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| Zusammenfassung |
| ——–
3D Magnetohydrodynamic (MHD) turbulent flows with initially magnetic and kinetic
energies at equipartition spontaneously develop a magnetic excess (or residual energy), as
well in numerical simulations and in the solar wind. Closure equations obtained in 1983
describe the residual spectrum as being produced by a dynamo source proportional to the
total energy spectrum, balanced by a linear Alfvén damping term. A good agreement was
found in 2005 with incompressible simulations; however, recent solar wind measurements
disagree with these results.
The previous dynamo-Alfvén theory is generalized to a family of models, leading to
simple relations between residual and total energy spectra. We want to assess these models in
detail against MHD simulations and solar wind data.
The family of models is tested against compressible decaying MHD simulations with low
Mach number, zero cross-helicity, zero mean magnetic field, without or with expansion terms
(EBM or expanding box model).
A single dynamo-Alfvén model is found to describe correctly both solar wind scalings
and compressible simulations without or with expansion. It is equivalent to the 1983-2005
closure equation but with critical balance of nonlinear turnover and linear Alfvén times,
while the dynamo source term remains unchanged. The discrepancy with previous
incompressible simulations is elucidated. The model predicts a linear relation between the
spectral slopes of total and residual energies mR = −1∕2 + 3∕2mT. Examining the solar
wind data as in [?], our relation is found to be valid whatever the cross-helicity, even better so
at high cross-helicity, with the total energy slope varying from 1.7 to 1.55. ——- |
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