On the consequences of strong stable stratification at the top of Earth's outer core

Bloxham, Jeremy

doi:10.1029/gl017i012p02081

Cited by 32 publications

(27 citation statements)

References 28 publications

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“…This has been undertaken several times in the past (Whaler 1980(Whaler , 1984(Whaler , 1986Bloxham 1990) but the results have not been sufficiently conclusive to definitively support, or reject, the hypothesis. One of the major difficulties to overcome is the quality of the magnetic field model used.…”

Section: Introductionmentioning

confidence: 97%

Are geomagnetic data consistent with stably stratified flow at the core–mantle boundary?

Lesur¹,

Whaler

Wardinski³

2015

Geophysical Journal International

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S U M M A R YRecent first principles calculations of the Earth's outer core thermal and electrical conductivities have raised their values by a factor of three. This has significant implications for geodynamo operation, in particular, forcing the development of a stably stratified layer at the core-mantle boundary (CMB). This study seeks to test the hypothesis of a stably stratified layer in the uppermost core by analysing geomagnetic observations made by the CHAMP satellite. An inversion method is utilized that jointly solves for the time-dependent main field and the core surface flow, where we assume the temporal variability of the main field, its secular variation (SV), to be entirely due to advective motion within the liquid outer core. The results show that a large-scale pure toroidal flow, consistent with a stably stratified layer atop the outer core, is not compatible with the observed magnetic field during the CHAMP era. However, allowing just a small amount of poloidal flow leads to a model fitting the observations satisfactorily. As this poloidal flow component is large scale, within a predominantly toroidal, essentially tangentially geostrophic flow, it is compatible with a stably stratified upper outer core. Further, our assumption of little or no diffusive SV may not hold, and a small amount of SV generated locally by diffusion might lead to a large-scale pure toroidal flow providing an acceptable fit to the data.Key words: Magnetic field; Rapid time variations; Satellite magnetics. I N T RO D U C T I O NMagnetic data are one of the rare sources of information about the motion of the iron-rich liquid forming the outer core of the Earth. It is generally accepted that the Earth's magnetic field is generated mainly in the outer core by convective flows associated with heat and light element release due to the slow freezing of the Earth's inner core. A better understanding of the evolution of the Earth's deep interior is therefore closely linked to our ability to describe this outer core flow. The magnetic field and core flow are related through the induction equation, derived directly from Maxwell's equations.Most approaches begin by deriving magnetic field models from measurements at the Earth's surface and satellite altitude. These models are then used to deduce the flow in the core, even if the modelling step somehow filters out part of the information available in the data. Several difficulties limit further our ability to describe the core flow.First, the magnetic field is measured at the Earth's surface and, after neglecting mantle electrical conductivity to allow the field to be downward continued to the core-mantle boundary (CMB), only the radial component of the poloidal part of the magnetic field is continuous across the conductivity jump there. This component can reasonably be estimated at the top of the free stream, just under the liquid viscous boundary layer (Roberts & Scott 1965;Jault & Le Mouël 1991). However, it is available only for the longest wavelengths, because the field generated...

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Section: Introductionmentioning

confidence: 97%

Are geomagnetic data consistent with stably stratified flow at the core–mantle boundary?

Lesur¹,

Whaler

Wardinski³

2015

Geophysical Journal International

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“…An internal origin has been supported by a variety of analyses and authors (e.g. Malin and Hodder, 1982;Courtillot et al, 1984;Malin et al, 1983;Courtillot and Le Mouël, 1985;McLeod, 1985;Gavoret et al, 1986;Golovkov et al, 1989;Alexandrescu et al, 1995Alexandrescu et al, , 1996Bellanger et al, 2001;Bloxham et al, 2002). Although an internal origin is now widely accepted, the processes causing geomagnetic jerks as a whole are not yet wellunderstood.…”

Section: Introductionmentioning

confidence: 98%

“…The determination of core flow is nonetheless highly non-unique, and more assump-tions have to be made in order to reduce or resolve this non-uniqueness. Different assumptions have been used so far to reduce this non-uniqueness: the flow is constrained to be purely toroidal (Whaler, 1980;Bloxham, 1990), to be tangentially geostrophic (Hills, 1979;Le Mouël et al, 1985;Gire et al, 1986;Backus andLe Mouël, 1986, 1987) or to be steady over a definite time span (Gubbins, 1982;Voorhies, 1984Voorhies, , 1986Voorhies and Backus, 1985).…”

Section: Introductionmentioning

confidence: 99%

Geomagnetic jerks from the Earth’s surface to the top of the core

2007

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Rapid changes in the magnetic field characterised by an abrupt change in the secular variation have been named “secular variation impulses” or “geomagnetic jerks”. Three of these events, around 1968, 1978 and 1990, occurred during the time-span covered by the comprehensive model CM4 (Sabaka et al., 2002, 2004). This model, providing the best temporal resolution between 1960 and 2002 as well as a fine separation of the different magnetic sources, can be used to study rapid phenomena of internal origin. In order to analyse these events all over the globe, synthetic time series were obtained from the CM4 model between 1960–2002. Geomagnetic jerks are detected here as a rapid movement of the zero isoline of the second field derivative. Analysis of the area swept out by this isoline as a function of time allows us to map the spatial extent of jerks though time, and to identify an event around 1985 that is localized in the Pacific area. At the core surface, we compute the fluid flows under the frozen-flux and tangentially geostrophic assumptions. The flows do not exhibit any special pattern at jerk times, but instead show a smooth temporal evolution over the whole time period. However, the mean amplitude of the dynamical pressure associated with these flows present maxima at each jerk occurrence and helps to confirm the identification of a jerk in 1985.

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“…It is considered that the SV can be attributed to the PT flow, if a stratification layer strong enough to inhibit vertical fluid motion has at least a thickness of electromagnetic skin depth d c beneath the CMB [e.g., Gubbins, 1991]. In such a layer, the leading order force balance necessarily involves the Lorentz force to form the magnetostrophic state (1), otherwise the toroidal flow would consist of zonal components alone and the magnetic observations could not be explained [Bloxham, 1990]. This Lorentz force is very severely regulated by the TM constraint in the absence of zonal poloidal flows such as with PT flow models.…”

Section: Difficulty Of Pt Flow To Meet the Tm Constraintmentioning

confidence: 99%

“…Indeed, the PT flow assumption is motivated by a magnetostrophic balance near the core surface, involving only toroidal flows consisting conceivably of (purely zonal) geostrophic flows and horizontally polarized slow MAC-waves [Bloxham, 1990]. Likewise, a helical flow model may well satisfy the RV equation.…”

Section: Introductionmentioning

confidence: 99%

Radial vorticity constraint in core flow modeling

Asari

Lesur

2011

J. Geophys. Res.

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[1] We present a new method for estimating core surface flows by relaxing the tangentially geostrophic (TG) constraint. Ageostrophic flows are allowed if they are consistent with the radial component of the vorticity equation under assumptions of the magnetostrophic force balance and an insulating mantle. We thus derive a tangentially magnetostrophic (TM) constraint for flows in the spherical harmonic domain and implement it in a least squares inversion of GRIMM-2, a recently proposed core field model, for temporally continuous core flow models (2000.0-2010.0). Comparing the flows calculated using the TG and TM constraints, we show that the number of degrees of freedom for the poloidal flows is notably increased by admitting ageostrophic flows compatible with the TM constraint. We find a significantly improved fit to the GRIMM-2 secular variation (SV) by including zonal poloidal flow in TM flow models. Correlations between the predicted and observed length-of-day variations are equally good under the TG and TM constraints. In addition, we estimate flow models by imposing the TM constraint together with other dynamical constraints: either purely toroidal (PT) flow or helical flow constraint. For the PT case we cannot find any flow which explains the observed SV, while for the helical case the SV can be fitted. The poor compatibility between the TM and PT constraints seems to arise from the absence of zonal poloidal flows. The PT flow assumption is likely to be negated when the radial magnetostrophic vorticity balance is taken into account, even if otherwise consistent with magnetic observations.

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On the consequences of strong stable stratification at the top of Earth's outer core

Cited by 32 publications

References 28 publications

Are geomagnetic data consistent with stably stratified flow at the core–mantle boundary?

Are geomagnetic data consistent with stably stratified flow at the core–mantle boundary?

Geomagnetic jerks from the Earth’s surface to the top of the core

Radial vorticity constraint in core flow modeling

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