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| Titel |
Decadal-period external magnetic field variations resolved with
eigenanalysis |
| VerfasserIn |
Robert Shore, Kathryn Whaler, Susan Macmillan, Ciaran Beggan, Jakub Velimsky, Nils Olsen |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250141266
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| Publikation (Nr.) |
 EGU/EGU2017-4756.pdf |
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| Zusammenfassung |
| Variations in the magnetic field at and above the Earth’s surface permeate the interior of
our planet, and can be used to determine the electrical conductivity of the mantle.
Presently, the annual and semi-annual period fields induced by magnetospheric and
ionospheric currents, suitable to estimate mantle conductivity in the approximate depth
range 1,200–2,000 km, are subject to large uncertainty since they overlap with the
periods on which the core field also changes significantly. It is timely to obtain an
improved determination of the spatial geometry of the external, inducing, fields
in order to better separate their internal, induced, part from that generated in the
core.
We apply the method of Empirical Orthogonal Functions (EOFs) to a dataset of
ground-based magnetic observatory hourly means in order to decompose the external
magnetic field during quiet times over a full 11-year solar cycle into its modes of maximum
variance. This allows us to assess the spatial structures and magnitude changes of its
dominant spatio-temporal patterns. Specifically, our focus is on isolating the long period
external inducing fields as they penetrate to the depths of the mantle where the conductivity is
least constrained.
We expand ground-based measurements in the inertial local-time frame to produce
spherical harmonic models of the dominant long period signals isolated by the EOF method.
Whilst the ring current dominates the decomposition, we show that an annual and a
semi-annual oscillation are important in describing the full inducing field. Each of these
terms exhibits a modulation from the 11-year solar cycle. In summary, the most
important harmonic in the description of the inducing fields is P10, followed by P21.
There are lesser but still significant contributions from the P11 and P20 harmonics. |
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