Local computation of the geostrophic pressure at the top of the core

Details Local computation of the geostrophic pressure at the top of the core Local computation of the geostrophic pressure at the top of the core

Jan 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000pepi..117..309c&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 117, Issue 1-4, p. 309-328.

Physics

13

Scientific paper

According to the frozen-flux hypothesis, the observed secular variation (SV) of the geomagnetic field is generated by the flow at the top of the liquid core. Recovering some information about this flow with the help of magnetic data would therefore be straightforward, were it not for the fact that different flows can share the same magnetic signature. Fortunately, the flow is likely to be tangentially geostrophic, a property which formally ensures that the flow be uniquely determined in a large fraction of the core surface named ``visible belt'' where, in addition, the pressure field associated to the flow is also uniquely determined (to within a global constant). The formally undetermined fraction of the flow is then restricted to so-called regional ``ambiguous patches''. So far, this local property has not been fully exploited. Flows computed up to now under the geostrophic assumption have always been computed with the help of a spectral method consisting in expanding the flow into spherical harmonics and assuming it to be large-scale to force a unique solution. In the present paper, we propose a new method for computing directly and locally the pressure associated with the flow in the visible belt, without any additional assumption concerning its scale. Taking into account possible failures of the frozen-flux and geostrophic assumptions, we show that this method can be implemented to compute the pressure field within a ``geostrophic belt'' included in the visible belt. Despite this ``local method'' being sensitive to the uncertainties in the data, it leads to pressure fields displaying large-scale structures consistent with the large-scale flows obtained with the spectral method. We believe that this result strengthens the previous large-scale assumption. This new approach could also give access to smaller-scale features of the pressure and flow when better main field and SV data will be made available.

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