The determination of fluid flow at the core surface from geomagnetic observations

Bloxham, Jeremy

doi:10.1007/978-94-009-2857-2_9

Cited by 39 publications

(45 citation statements)

References 30 publications

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“…Amit and Pais 2013) or large scales may for some reason dominate the flow (e.g. Bloxham 1988;Whaler and Beggan 2015). Nonetheless, the primary flow structures identified here, in particular equatorward flow in both the northern and southern hemispheres around 100 • E and time-dependent non-axisymmetric westward flow at low latitudes, are sufficient to reproduce the observed rapid field changes, within the uncertainties due to the unresolved small-scale field.…”

Section: Fig 10mentioning

confidence: 48%

Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model

et al. 2016

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We use more than 2 years of magnetic data from the Swarm mission, and monthly means from 160 ground observatories as available in March 2016, to update the CHAOS time-dependent geomagnetic field model. The new model, CHAOS-6, provides information on time variations of the core-generated part of the Earth's magnetic field between 1999.0 and 2016.5. We present details of the secular variation (SV) and secular acceleration (SA) from CHAOS-6 at Earth's surface and downward continued to the core surface. At Earth's surface, we find evidence for positive acceleration of the field intensity in 2015 over a broad area around longitude 90°E that is also seen at ground observatories such as Novosibirsk. At the core surface, we are able to map the SV up to at least degree 16. The radial field SA at the core surface in 2015 is found to be largest at low latitudes under the India-South-East Asia region, under the region of northern South America, and at high northern latitudes under Alaska and Siberia. Surprisingly, there is also evidence for significant SA in the central Pacific region, for example near Hawaii where radial field SA is observed on either side of a jerk in 2014. On the other hand, little SV or SA has occurred over the past 17 years in the southern polar region. Inverting for a quasi-geostrophic core flow that accounts for this SV, we obtain a prominent planetary-scale, anticyclonic, gyre centred on the Atlantic hemisphere. We also find oscillations of non-axisymmetric, azimuthal, jets at low latitudes, for example close to 40°W, that may be responsible for localized SA oscillations. In addition to scalar data from Ørsted, CHAMP, SAC-C and Swarm, and vector data from Ørsted, CHAMP and Swarm, CHAOS-6 benefits from the inclusion of along-track differences of scalar and vector field data from both CHAMP and the three Swarm satellites, as well as east-west differences between the lower pair of Swarm satellites, Alpha and Charlie. Moreover, ground observatory SV estimates are fit to a Huber-weighted rms level of 3.1 nT/year for the eastward components and 3.8 and 3.7 nT/year for the vertical and southward components. We also present an update of the CHAOS high-degree lithospheric field, making use of along-track differences of CHAMP scalar and vector field data to produce a new static field model that agrees well with the MF7 field model out to degree 110.

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Section: Fig 10mentioning

confidence: 48%

Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model

et al. 2016

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“…Cm is the a priori model covariance matrix regularising the flow, in this case defined by the Bloxham (1988) 'strong norm'. We investigated weaker regularisations, but the large damping parameters required to obtain converged solutions significantly over-fitted the data, and the essential features of the models were similar to those obtained using the 'strong norm'.…”

Section: Methodsmentioning

confidence: 99%

Decadal variability in core surface flows deduced from geomagnetic observatory monthly means

Whaler¹,

Olsen

Finlay

2016

Geophys. J. Int.

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“…However, we use an L1-norm iterative reweighting scheme to reduce the effect of any large outliers when fitting the model back to the data. We also truncate the flow model at degree 14 and damp using the Bloxham 'strong' norm (Bloxham 1988) which effectively reduces the influence of the model SV above degree 8. We ignore the 'subgrid' processes which we assume to be small for the purposes of this flow modelling (Pais and Jault 2008).…”

Section: Derivation Of Igrf Candidate Modelsmentioning

confidence: 99%

The BGS magnetic field candidate models for the 12th generation IGRF

et al. 2015

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We describe the candidate models submitted by the British Geological Survey for the 12th generation International Geomagnetic Reference Field. These models are extracted from a spherical harmonic 'parent model' derived from vector and scalar magnetic field data from satellite and observatory sources. These data cover the period 2009.0 to 2014.7 and include measurements from the recently launched European Space Agency (ESA) Swarm satellite constellation. The parent model's internal field time dependence for degrees 1 to 13 is represented by order 6 B-splines with knots at yearly intervals. The parent model's degree 1 external field time dependence is described by periodic functions for the annual and semi-annual signals and by dependence on the 20-min Vector Magnetic Disturbance index. Signals induced by these external fields are also parameterized. Satellite data are weighted by spatial density and by two different noise estimators: (a) by standard deviation along segments of the satellite track and (b) a larger-scale noise estimator defined in terms of a measure of vector activity at the geographically closest magnetic observatories to the sample point. Forecasting of the magnetic field secular variation beyond the span of data is by advection of the main field using core surface flows.

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The determination of fluid flow at the core surface from geomagnetic observations

Cited by 39 publications

References 30 publications

Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model

Recent geomagnetic secular variation from Swarm and ground observatories as estimated in the CHAOS-6 geomagnetic field model

Decadal variability in core surface flows deduced from geomagnetic observatory monthly means

The BGS magnetic field candidate models for the 12th generation IGRF

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