Time evolution of the fluid flow at the top of the core. Geomagnetic jerks (original) (raw)
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Earth, Planets and Space, 2000
Andrew Jackson's comment is sensible and draws attention to the fact that the lack of magnetic field intensity data prior to 1832 introduces some non-uniqueness in the computed fluid flow at the core-mantle boundary (CMB) in the frozen flux approximation. The problem of modeling the field from directional values is not trivial. Proctor and showed that in some situations even the morphology may not be recovered if one lacks the intensity measurements. However, the only uncertainty experienced by the ufm2 model (Bloxam and Jackson, 1992; thereafter B& J) is on the amplitude of the geomagnetic field, and not on its morphology because of the mainly dipolar structure of the field .
Geomagnetic jerks from the Earth’s surface to the top of the core
Earth, Planets and Space, 2007
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.
Physics of the Earth and Planetary Interiors, 2012
At observatory locations, the Earth's magnetic field displays an almost constant secular acceleration, except at times of geomagnetic jerks when this quantity suddenly changes its value. The 2003 geomagnetic jerk was the first to have been recorded globally and uniformly over the Earth, and is well captured by time-varying spherical harmonic models of the geomagnetic field, based on satellite data. We jointly derive instantaneous estimates of the core surface flow and acceleration accounting for the field secular variation and acceleration given by such models. We consider epochs just before and after the 2003 event and show that no steady flow, no matter how general, can simultaneously account for the observed secular variation and acceleration at either epoch. Assuming the flow to be tangentially geostrophic, we next show that even purely toroidal zonal flow accelerations cannot account for the observed secular acceleration and conclude that more general tangentially geostrophic flow accelerations are required by the data. These flow accelerations, however, may well be equatorially symmetric, as predicted by theoretical considerations. Investigating the flow acceleration change throughout the 2003 event shows that these also may be equatorially symmetric, but definitely not purely toroidal zonal. This unambiguously confirms earlier suggestions that the 2003 geomagnetic jerk was a more complex phenomena than a simple consequence of torsional oscillations.
The 1991 geomagnetic jerk as seen at the Earth's surface and the core-mantle boundary
Geophysical Journal International, 2010
We determine the occurrence times of geomagnetic impulses (jerks) around the year 1991 in the three geomagnetic secular variation components for the Earth's surface by a simple optimization algorithm. The geomagnetic field models we use are the low-degree parts of the models CM4 (Sabaka et al, GJI 159(2004)) and C 3 FM (Wardinski and Holme, JGR 111 ). We find that the temporal jerk pattern can be detected in fields (n ≤ 4), from which the spherical harmonic degrees n=2 or n=3 (tangential) and n=4 (radial) are representative. To calculate the secular variation components at the core-mantle boundary (CMB) we apply the non-harmonic downward continuation method (Ballani et al, GJI 149(2002), Greiner-Mai et al, GJI 158(2004)). For the mantle conductivity, three estimates, dependent on the radius, are assumed with conductances between 10 7 S and 2 · 10 8 S. The knowledge of the secular variation components at the CMB allows us to track the global distributions of jerk occurrence times in dependence on the mantle conductivity estimate. We find for each component a typically shaped, global topology for the location of the jerk occurrence times at the Earth's surface and the CMB. For the tangential (ϑ, ϕ)-components, these global topologies show always the well-known temporal bimodality on each surface. Another characteristic feature is found for the jerk of the r-component. It displays a double jerk centered around 1991 consisting of a v-shaped and a reversely v-shaped part, which are significantly correlated. Between the CMB and the Earth's surface, we find time delays in the range of one to two years for the tangential and less than one year for the radial jerk components. To understand these time delays, comparisons at fixed locations are carried out to check the influence of the respective conductivity function. For studying the time delay effects, we apply the inversion set-up of the non-harmonic downward continuation to calculate simulated temporal oscillations and derive analytical expressions approximating the phase shifts. We find that jerk occurrence time delays and simulated phase shifts of temporal oscillations have a similar behaviour with respect to the influence of the conductivity, and for the radial, and the tangential components, respectively. In addition, a new concept for determining a jerk amplitude is presented briefly. This so-called dynamical jerk morphology, which forms a portion of the geomagnetic secular acceleration, is defined for each component by a time function on the considered surface. Its temporal motion patterns at the CMB are likely related with jerk originating processes in the fluid outer core.
Variability of the topographic core-mantle torque calculated from core surface flow models
Physics of the Earth and Planetary Interiors, 2006
With the prospect of studying the relevance of the topographic core-mantle coupling to the variations of the Earth's rotation and also its applicability to constraining the core surface flow, we investigate the variability of the topographic torque estimated by using core surface flow models accompanied by (a) uncertainty due to the non-uniqueness problem in the flow inversion, and (b) variance originating in that of geomagnetic secular variation models employed in the inversion. Various flow models and their variances are estimated by inverting prescribed geomagnetic models at the epoch 1980. The subsequent topographic torque is then calculated by using a core-mantle boundary topography model obtained by seismic tomography. The calculated axial and equatorial torques are found subject to the variability of order 10 19 and 10 20 Nm, respectively, on which (b) is more effective than (a). The variability of the torque is attributed even to (a) and (b) of the large-scale flows (degrees 2 and 3). Yet, it still seems unlikely for the decadal polar motion with the observed amplitude to be excited exclusively by the equatorial topographic torque associated with any of reasonable core surface flow models. It is also confirmed that, with the topography model adopted here, the axial topographic torque on a rigid annulus in the core (coaxial with the Earth's rotation axis) associated with any of reasonable flow models is larger by two orders of magnitude than the plausible inertial torque on such cylinders. This implies that any core surface flow model consistent with the topographic coupling does not exist, unless the topography model is appropriately modified. Nevertheless, the topographic coupling might provide not only a weak constraint for explaining the decadal LOD variations, but also the possibility to probe the core surface flow and the core dynamics. been envisaged that the Earth's rotation has some connection to the fluid motion at the core surface which is considered responsible for the decadal variations of the geomagnetic field.
Toroidal fluid motion at the top of the Earth's core
Geophysical Journal International, 1990
Geomagnetic secular variation is caused by flow of liquid iron in the core. Geomagnetic observations can be used to determine properties of the flow but such calculations in general have non-unique solutions. We prove a uniqueness theorem: the flow is determined uniquely if it is toroidal (zero horizontal divergence), the mantle is an insulator, the core a perfect conductor (the frozen-flux hypothesis), and there is no surface current in the boundary layer at the top of the core, and provided the magnetic field satisfies a simple point condition. The condition of no surface current allows use of the horizontal components of secular variation; previous studies have used only the radial component. Horizontal components allow simultaneous determination of the shear (radial derivatives of horizontal components of velocity).
Geomagnetic Jerks: Rapid Core Field Variations and Core Dynamics
Space Science Reviews, 2010
The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of M. Mandea ( ) Helmholtz-Zentrum
Rapidly changing flows in the Earth’s core
Nature Geoscience, 2008
A large part of the Earth's magnetic field is generated by fluid motion in the molten outer core 1 . As a result of continuous satellite measurements since 1999, the core magnetic field and its recent variations can now be described with a high resolution in space and time 2 . These data have recently been used to investigate small-scale core flow 3,4 , but no advantage has yet been taken of the improved temporal resolution, partly because the filtering effect of the electrically conducting mantle was assumed to mask short-period magnetic variations 5 . Here we show that changes in the magnetic field occurring over only a few months, indicative of fluid flow at the top of the core, can in fact be resolved. Using nine years of magnetic field data obtained by satellites as well as Earth-based observatories, we determine the temporal changes in the core magnetic field and flow in the core. We find that the core flow is spatially localized and involves rapid variations over a few months, with surprisingly large local accelerations. Our results suggest that short-term fluctuations of the core magnetic field are robust features of rapid core dynamics and should be considered in the development of future numerical models of the geodynamo.
Fluid motions in the Earth’s core inferred from time spectral features of the geomagnetic field
Physical Review E, 2002
The aim of this work is to investigate the time spectral features of the main geomagnetic field fluctuations as measured on the Earth's surface in connection with a nontraditional turbulent dynamics of the fluid motions in the outer layers of the Earth's liquid core. The average geomagnetic field spectrum is found to be a power law, characterized by a spectral exponent ␣ϷϪ 11 3 , on time scales longer than 5 yr. We discuss the spectral exponent in connection with an intense magnetic field in the Earth's core and with a vortex coalescence process in a regime of drift-wave turbulence.