Statistics – Computation
Scientific paper
Dec 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009agufm.p21a1199g&link_type=abstract
American Geophysical Union, Fall Meeting 2009, abstract #P21A-1199
Statistics
Computation
[0545] Computational Geophysics / Modeling, [1510] Geomagnetism And Paleomagnetism / Dynamo: Theories And Simulations, [1595] Geomagnetism And Paleomagnetism / Planetary Magnetism: All Frequencies And Wavelengths, [2756] Magnetospheric Physics / Planetary Magnetospheres
Scientific paper
It has been proposed that the small magnetosphere of Mercury may influence magnetic field generation within the planet, on the grounds that sufficiently large magnetospheric currents can induce magnetic fields at the top of the core that are comparable to the internal field. We have simulated numerically the effects of feedback from a time-varying axial external field on a three-dimensional core dynamo. The external, magnetosphere-induced field has been modeled as having constant magnitude and a direction that remains opposite to that of the internal axial dipole, even in the case of a dipole reversal. The effect of such an external field for a dipole-dominated dynamo is not important unless the external field magnitude is at least half that of the internally generated field (a situation not likely for a magnetosphere-induced field). However, if the external field is applied to a dynamo operating in a fully convective core in which the dipolar component has been weakened relative to higher-order mutlipoles, such a weak-dipole state may be stable for external fields as small as 15% or less of the dynamo-generated field at the core-mantle boundary (CMB). These weak-dipole dynamos exhibit a time-variable multipolar geometry with few dipole-dominated episodes, the lengths of which increase with decreasing external field magnitude. The flow fields of weak-dipole dynamos exhibit distinctive length scales and speeds when compared with undisturbed cases; such differences in behavior are consequences of the differences in magnetic boundary conditions. We speculate that Mercury may have originally been pushed to a weak-dipole state by interaction of its magnetosphere with an early Sun that was much more active than today. Magnetospheric feedback could have acted thereafter to stabilize the weak-dipole dynamo, an inference that is consistent with spacecraft measurements of the internal field and estimates of magnetospheric currents. Magnetospheric feedback is not important when external fields are as small as 0.1% of the internally generated field at the CMB. In such a case, the dynamo returns to its strongly dipolar state. This situation may account for why Earth is stable in a dipole-dominated state, because the external field is too low (about 0.05% of the CMB field) to have stabilized a weak dipole.
Gomez Perez Natalia
Solomon Stanley C.
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