Physics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp11b0088s&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #GP11B-0088
Physics
1507 Core Processes (1213, 8115), 1510 Dynamo: Theories And Simulations, 1517 Magnetic Anomalies: Modeling And Interpretation, 1595 Planetary Magnetism: All Frequencies And Wavelengths
Scientific paper
The geomagnetic field is (according to current knowledge) generated by Geodynamo (a hydromagnetic dynamo) action in the outer Earth's core. For simplicity, it can be divided into its poloidal and toroidal components. Toroidal component of geomagnetic field is confined to the Earth's core and we cannot observe it. The outer Earth's core is surrounded by the electrically very slightly conductive region - Earth's mantle, in consequence of it the toroidal field is shielded. Thus, an information about the toroidal field is available only indirectly - it is necessary to solve an inverse problem. The so-called invisible dynamo problem belongs to the group of the inverse problems. The invisible dynamo problem gives possibility to tell us whether any hydromagnetic dynamo exists which produces invisible magnetic field (a magnetic field confined in the area of its generation which does not penetrate the surroundings and remains invisible for any observer in the surrounding area). The investigations of the invisible dynamos in the cylinder showed that an invisible magnetic field exists ("an invisible solution") but this magnetic field decays - no dynamo action (no growing field has been found). Consequently, this problem is still open. A working invisible dynamo was found only in the sphere as a mode of α2 mean field dynamo. The inverse dynamo problem asks for a velocity field given a magnetic field observed in the vacuum region surrounding the conducting fluid, such that the magnetic field is maintained by a dynamo process. Solutions to the inverse dynamo are presumably not unique, but no example seems to be known of two velocity fields producing the exact same magnetic field in the vacuum region. Looking for the such flows which produce the same external magnetic field, is the main goal of our study. The problem is solved in a cylinder. The preliminary results show that a solution exists, i.e. we have found such flows (modes of a kinematic dynamo) in a cylinder, which lead to the identical external magnetic fields (outside of the cylinder). Looking for a general theorem for such flows (modes of a kinematic dynamo which is close to Ponomarenko dynamo) constitutes the main goal of our recent investigations.
Šimkanin Ján
Tilgner Andreas
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