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| Titel |
Predicting the evolution of the SAA by characterisation and modelling of reversed flux patches |
| VerfasserIn |
Maurits Metman, Phil Livermore, Jon Mound |
| 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 |
250134327
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| Publikation (Nr.) |
 EGU/EGU2016-15036.pdf |
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| Zusammenfassung |
| The South Atlantic Anomaly (SAA) is the region at the Earth’s surface where the intensity of
the magnetic field is very low, typically 30,000 nT and lower. Satellites operating in this
region are relatively prone to upsets due to solar winds and cosmic rays. The SAA is
related to regions at the core-mantle boundary (CMB) where the sign of the radial
magnetic field is opposite to that of the dipole state, also known as reversed flux
patches (RFPs). Reversed flux patches are time-dependent properties and have
changed significantly in number, location and total area over the past four centuries. In
general, such temporal variation in the magnetic field B is governed by the induction
equation:
∂B- = ∇ ×(u × B)+ η∇2B
∂t
(1)
with u and η the core fluid flow velocity and magnetic diffusivity respectively. The second
term on the right hand side corresponds to magnetic diffusion and is thought to be small
compared to the first, advective term. It is therefore neglected in most studies. However, it has
been suggested that RFPs can be explained by the diffusion of a radially expelled
toroidal field (Bloxham, 1986). Thus, when explaining RFP evolution, it is well
possible that magnetic diffusion plays a sigfinicant role and can therefore not be
neglected. Additionally, diffusion could have a stronger control on secular variation than
previously thought, as the outer part of the core might be stratified (Pozzo et al.,
2012).
In this presentation, I will first describe my method of defining the magnetic equator and
RFPs. The RFP evolution for the past four centuries, obtained with those methods and the
gufm1 field model (Jackson, 2000) as well as its implications for the SAA will then be
shown. My results indicate that the ratio of RFP area to the total CMB area has increased
greatly and that this growth has predominantly taken place at the Southern hemisphere. More
importantly, I will show that the largest part of the dipole moment decay is due to RFP
growth, indicating that RFPs are dominant in controlling dipole decay. Then, I will present
results of numerical diffusion models to demonstrate to what extent diffusion is responsible
for these changes. Lastly, a comparison between advection and diffusion will be given,
by showing a spatial distribution of how well either mechanism explains secular
variation.
References
- Bloxham, J. (1986). The expulsion of magnetic flux from the Earth’s core. Geophys. J. Int.
87, pp. 669-687.
- Jackson, A., Jonkers, A. R. T. and Walker, M. R. (2000). Four centuries of geomagnetic
secular variation from historic records. Phil. Trans. R. Soc. Lond. A, 358, pp. 957-990.
- Pozzo, M. et al. (2012) Thermal and electrical conductivity of iron at Earth’s core
conditions. Nature, 485, pp. 355-358. |
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