Recent changes of the Earth’s core derived from satellite observations of magnetic and gravity fields

Abstract

To understand the dynamics of the Earth's fluid, iron-rich outer core, only indirect observations are available. The Earth's magnetic field, originating mainly within the core, and its temporal variations can be used to infer the fluid motion at the top of the core, on a decadal and subdecadal time-scale. Gravity variations resulting from changes in the mass distribution within the Earth may also occur on the same time-scales. Such variations include the signature of the flow inside the core, though they are largely dominated by the water cycle contributions. Our study is based on 8 y of highresolution, high-accuracy magnetic and gravity satellite data, provided by the CHAMP and GRACE missions. From the newly derived geomagnetic models we have computed the core magnetic field, its temporal variations, and the core flow evolution. From the GRACE CNES/GRGS series of time variable geoid models, we have obtained interannual gravity models by using specifically designed postprocessing techniques. A correlation analysis between the magnetic and gravity series has demonstrated that the interannual changes in the second time derivative of the core magnetic field under a region from the Atlantic to Indian Ocean coincide in phase with changes in the gravity field. The order of magnitude of these changes and proposed correlation are plausible, compatible with a core origin; however, a complete theoretical model remains to be built. Our new results and their broad geophysical significance could be considered when planning new Earth observation space missions and devising more sophisticated Earth's interior models. Earth's interior | core dynamicsO ur planet is a very dynamic system, composed of the core and various layers, such as the mantle, lithosphere, oceans and atmosphere, up to near-Earth space. The fluid core (1), undergoing hydromagnetic motions, contributes to both the origin of the geomagnetic field (2, 3) and the spatial distribution of the Earth's mass (4, 5). Consequently, decadal and subdecadal timescale processes occurring in the core produce signatures in the changes of the geomagnetic (6-8) and gravity (3, 4) fields. To date, short time-scale variations of core origin have only been evidenced in the magnetic field (9-11), and the gravity signals including the signature of the flow inside the core are largely dominated by the water cycle contribution (12). The question that now arises is to what extent core flow effects may be identified in other observables (than magnetic), such as gravity measurements; a core origin has been suggested as a possible cause for rapid geoid flattening variations (13,14).When either a surface observatory or a satellite takes a geomagnetic field measurement, this measure is the result of the superposition of many sources (15). The largest contribution generated by the dynamo action within the fluid, iron-rich core of the Earth is known as the core field, with a dominant dipolar component at the Earth's surface. Sizable contributions come from the static lithospheric field, and e...

Core surface sub-centennial magnetic flux patches: characteristics and evolution

“…In terms of energy of field change, an active area under the northern South America that extends toward north up to the east coast of USA and Canada indicates important displacements in time and intensity variations that have taken place under this region. Also active areas are seen under the southern tip of the African continent and under the Indian Ocean, corroborating studies on rapid evolution of the surface geomagnetic field in that area (Whaler 1986;Mandea et al 2007;Lesur et al 2008Lesur et al , 2010Mandea et al 2012;Kotzé and Korte 2016). Our results indicate that the active areas under the southern tip of the African continent and under the Indian Ocean, together with an active area centered now on the southern South America, visible in the inter-centennial constituent of the field, are the main contributors to the observed, more intense activity in the Southern Hemisphere compared to the Northern one.…”

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Parent magnetic field models for the IGRF-12GFZ-candidates

“…Of course, the parent model of our IGRF candidates also used corrected data, but with a slightly less robust correction process than the one presented below. Outside this re-calibration, the processing path used to obtain accurate models of the magnetic field is similar to that of previous models of the magnetic field model series GRIMM (Lesur et al 2008Mandea et al 2012;Lesur et al 2015) We note however that, unlike during the CHAMP epoch, only a very short time span of satellite data was available at the end of September 2014 when the IGRF candidates were submitted. Therefore, observatory data had to be used to obtain robust models.…”