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| Titel |
Reconciling westward drift and inner-core super-rotation in very low viscosity models of the geodynamo |
| VerfasserIn |
P. Livermore, R. Hollerbach |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250063504
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| Zusammenfassung |
| In this presentation we reconcile two well known ideas relating to the dynamics of the deep
Earth. The first is the observed westward drift of the non-dipole geomagnetic field, dating
originally back to Halley in the 17th Century. This basic property of the geomagnetic field,
part of a complex picture of the secular variation, has persisted for at least the last 400 years
or so, although archeomagnetic studies show that the main drift was predominantly eastward
in the early 2nd millennium AD.
Independent of geomagnetic observations, mounting evidence from seismological
studies point to a present-day super-rotation of the inner core. One plausible idea
that matches most observational theories is that the rotation rate of the inner core
fluctuates in time — at the present time just we happen to observe a relative eastward
motion.
Numerical models of the geodynamo typically run far from the regions of parameter
space relevant to the Earth’s core. However, by looking only for steady solutions that are
consistent with an imposed Earth-like magnetic field, we are able to run 3D super-computer
models at an unprecedentedly low viscosity. We find that at very low-viscosities of E = 10-7
– 10-9 (where E is the Ekman number), the negative cylindrically-averaged axial
electromagnetic torque drives a strong westward flow in the outer core whilst imparting a
positive (eastward-directed) torque on the inner core, reproducing the apparent drift
direction and the direction of the relative inner-core rotation of the present day. The
signs of these torques are opposite and are the leading order contributors to the net
torque, required to be zero due to the assumed electrically insulating overlying
mantle.
This model linking the directions of outer-core drift and inner-core rotation suggests an
intriguing possibility for the dynamics of the Earth. As we show, should the electromagnetic
torque reverse in sign due to a perturbation of the internal geomagnetic field, the predominant
drift direction in the outer core would be eastward, with a concomitant westward torque on
the inner core. This is precisely the opposite of what is observed today, and suggests that the
eastward drift of 1000 years ago may have been associated with a westward relative
inner-core rotation. |
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