Time‐dependent mapping of the magnetic field at the core‐mantle boundary

Bloxham, Jeremy; Jackson, Andrew

doi:10.1029/92jb01591

Cited by 314 publications

(308 citation statements)

References 34 publications

(9 reference statements)

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“…Various choices of regularization norm in time and space have been explored by different authors (e.g. Bloxham & Jackson 1992;Jackson et al 2007;Gillet et al 2007;Korte & Holme 2010;Finlay et al 2012). In this study we used the Ohmic dissipation norm at the CMB (Gubbins & Bloxham 1985;Jackson et al 2000) for the spatial regularization:…”

Section: Inversion Proceduresmentioning

confidence: 99%

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Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

Panovska

Korte

Finlay

et al. 2015

Geophysical Journal International

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S U M M A R YCharacterization of geomagnetic field behaviour on timescales of centuries to millennia is necessary to understand the mechanisms that sustain the geodynamo and drive its evolution. As Holocene paleomagnetic and archeomagnetic data have become more abundant, strategies for regularized inversion of modern field data have been adapted to produce numerous timevarying global field models. We evaluate the effectiveness of several approaches to inversion and data handling, by assessing both global and regional properties of the resulting models. Global Holocene field models cannot resolve Southern hemisphere regional field variations without the use of sediments. A standard data set is used to construct multiple models using two different strategies for relative paleointensity calibration and declination orientation and a selection of starting models in the inversion procedure. When data uncertainties are considered, the results are similar overall regardless of whether we use iterative calibration and reorientation, or co-estimation of the calibration and orientation parameters as part of the inversion procedure. In each case the quality of the starting model used for initial relative paleointensity calibration and declination orientation is crucial and must be based on the best absolute information available. Without adequate initial calibration the morphology of dipole moment variations can be recovered but its absolute value will be correlated with the initial intensity calibrations, an effect that might be mitigated by ensuring an appropriate fit to enough high quality absolute intensity data with low uncertainties. The declination reorientation mainly impacts regional field structure and in the presence of non-zonal fields will result in a non-zero local average. The importance of declination orientation is highlighted by inconsistencies in the West Pacific and Australian sediment records in CALS10k.1b model. Great care must also be taken to assess uncertainties associated with both paleomagnetic and age data and to evaluate the effects of poor data distribution. New consistently allocated uncertainty estimates for sediment paleomagnetic records highlight the importance of adequate uncertainties in the inversion process, as they determine the relative weighting among the data and overall normalized misfit levels which in turn influence the complexity of the inferred field models. Residual distributions suggest that the most appropriate misfit measure is the L 1 norm (minimum absolute deviation) rather than L 2 (least squares), but this seems to have relatively minor impact on the overall results. For future Holocene field modelling we see a need for comprehensive methods to assess uncertainty in individual archeomagnetic data so that these data or models derived from them can be used for reliable initial relative paleointensity calibration and declination orientation in sediments. More work will be needed to assess whether co-estimation or an iterative approach to inversion is more efficient overall...

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Section: Inversion Proceduresmentioning

confidence: 99%

“…To measure the temporal complexity of the model we follow Bloxham & Jackson (1992), Jackson et al (2000) and and adopt a norm based on the second time derivative of the radial magnetic field integrated over the CMB and over the time span of the model:…”

Section: Inversion Proceduresmentioning

confidence: 99%

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

Panovska

Korte

Finlay

et al. 2015

Geophysical Journal International

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“…(10). We have also made a test with the heat norm (Gubbins, 1975;Bloxham and Jackson, 1992). The subsequent non-advective SV still shows no siginificant differences, though it has slightly higher (resp.…”

Section: Robustness Of Resultsmentioning

confidence: 99%

Geomagnetic secular variation violating the frozen-flux condition at the core surface

2010

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We consider a method to extract the part of a given geomagnetic secular variation (SV) model that is not consistent with a frozen-flux condition. This condition is usually derived from the diffusionless radial induction equation at the core-mantle boundary (CMB), and is defined explicitly in the spatial domain: radial flux changes within closed null-flux curves at the core surface are not allowed at any instant. We study here this condition in the spherical harmonic (SH) domain, relying on the SH expansion of the diffusionless equation. SV models at a certain epoch are separated into advective and non-advective parts. The advective (resp. non-advective) part satisfies (resp. does not satisfy) the frozen-flux condition redefined in the SH domain. We show that this separation is not unique. In this work, we achieve a unique separation by assuming the orthogonality of the two parts in terms of the radial SV energy at the CMB. From the recent geomagnetic models, GRIMM and CM4, we find that the non-advective part shows up mainly in the small reverse patches of the radial magnetic field at the CMB. However, non-advective behaviors are also observed outside these patches. As far as no restriction is imposed on core flow configuration, time variations of the non-advective part are not correlated to those of the SV models. However, if the flow is restricted to be tangentially geostrophic, time variations of the SV models have to be partly non-advective. Furthermore, for this flow configuration, the secular decrease of the axial dipole has to be largely non-advective.

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“…For a more accurate global picture, it is necessary to resort to tracing of individual particle trajectories. In recent years, time-dependent models of the main geomagnetic ®eld going back to 1690 have been produced (Bloxham and Jackson, 1992), using which, long term (b 10 2 y) variations in cuto rigidities of cosmic rays can be computed. Enhanced cosmic ray production of 10 fe coincident with the Mono Lake and Laschamp geomagnetic excursions have been reported in recent literature (McHargue et al, 1995).…”

Section: Resultsmentioning

confidence: 99%

Changes in cosmic ray cut-off rigidities due to secular variations of the geomagnetic field

Bhattacharyya¹,

Mitra

1997

Ann. Geophys.

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Abstract. An analytical expression is derived for the cuto rigidity of cosmic rays arriving at a point in an arbitrary direction, when the main geomagnetic ®eld is approximated by that of an eccentric dipole. This expression is used to determine changes in geomagnetic cuto s due to secular variation of the geomagnetic ®eld since 1835. E ects of westward drift of the quadrupole ®eld and decrease in the e ective dipole moment are seen in the isorigidity contours. On account of the immense computer time required to determine the cuto rigidities more accurately using the particle trajectory tracing technique, the present formulation may be useful in estimating the transmission factor of the geomagnetic ®eld in cosmic ray studies, modulation of cosmogenic isotope production by geomagnetic secular variation, and the contribution of geomagnetic ®eld variation to long term changes in climate through cosmic ray related modulation of the current¯ow in the global electric circuit.

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Time‐dependent mapping of the magnetic field at the core‐mantle boundary

Cited by 314 publications

References 34 publications

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

Geomagnetic secular variation violating the frozen-flux condition at the core surface

Changes in cosmic ray cut-off rigidities due to secular variations of the geomagnetic field

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