The velocities in the outer core

1971 ◽  
Vol 61 (4) ◽  
pp. 1051-1059
Author(s):  
A. L. Hales ◽  
J. L. Roberts
Keyword(s):  
Velocity Distribution ◽  
Lower Limit ◽  
Outer Core ◽  
Time Of Arrival ◽  
Travel Times ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Arc Distance ◽  
The Difference

abstract Earlier studies of the velocity distribution in the outer core have been based on the travel times of SKS.SKS arrivals can only be observed satisfactorily for arc distances at the surface greater than 85°. This lower limit of observation of SKS corresponds to an arc distance of 40.2° within the core. Thus the velocities in the outermost 250 km of the core are based upon an extrapolation. We have used observations of the difference in time of arrival of SKKS and SKS to obtain core travel times extending the range of observation down to a Δ within the core of about 14°. The velocity distribution thus found is significantly lower than those of Jeffreys (Bullen, 1963) and Randall (in press) near the core mantle boundary.

Multiple inner core reflections from a Novaya Zemlya explosion

1972 ◽  
Vol 62 (4) ◽  
pp. 1063-1071 ◽  
Author(s):  
R. D. Adams
Keyword(s):  
Lower Bound ◽  
Inner Core ◽  
Nuclear Explosion ◽  
Outer Core ◽  
Low Velocity ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
Novaya Zemlya ◽  
The Core ◽  
Velocity Channel

Abstract The phases P2KP, P3KP, and P4KP are well recorded from the Novaya Zemlya nuclear explosion of October 14, 1970, with the branch AB at distances of up to 20° beyond the theoretical end point A. This extension is attributed to diffraction around the core-mantle boundary. A slowness dT/dΔ = 4.56±0.02 sec/deg is determined for the AB branch of P4KP, in excellent agreement with recent determinations of the slowness of diffracted P. This slowness implies a velocity of 13.29±0.06 km/sec at the base of the mantle, and confirms recent suggestions of a low-velocity channel above the core-mantle boundary. There is evidence that arrivals recorded before the AB branch of P2KP may lie on two branches, with different slownesses. The ratio of amplitudes of successive orders of multiple inner core reflections gives a lower bound of about 2200 for Q in the outer core.

scholarly journals Persistent westward drift of the geomagnetic field at the core–mantle boundary linked to recurrent high-latitude weak/reverse flux patches

2020 ◽  
Vol 222 (2) ◽  
pp. 1423-1432
Author(s):  
Andreas Nilsson ◽  
Neil Suttie ◽  
Monika Korte ◽  
Richard Holme ◽  
Mimi Hill
Keyword(s):  
Northern Hemisphere ◽  
High Latitude ◽  
Geomagnetic Field ◽  
Outer Core ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Past ◽  
Convection Rolls ◽  
Westward Drift

SUMMARY Observations of changes in the geomagnetic field provide unique information about processes in the outer core where the field is generated. Recent geomagnetic field reconstructions based on palaeomagnetic data show persistent westward drift at high northern latitudes at the core–mantle boundary (CMB) over the past 4000 yr, as well as intermittent occurrence of high-latitude weak or reverse flux patches. To further investigate these features, we analysed time-longitude plots of a processed version of the geomagnetic field model pfm9k.1a, filtered to remove quasi-stationary features of the field. Our results suggest that westward drift at both high northern and southern latitudes of the CMB have been a persistent feature of the field over the past 9000 yr. In the Northern Hemisphere we detect two distinct signals with drift rates of 0.09° and 0.25° yr−1 and dominant zonal wavenumbers of m = 2 and 1, respectively. Comparisons with other geomagnetic field models support these observations but also highlight the importance of sedimentary data that provide crucial information on high-latitude geomagnetic field variations. The two distinct drift signals detected in the Northern Hemisphere can largely be decomposed into two westward propagating waveforms. We show that constructive interference between these two waveforms accurately predicts both the location and timing of previously observed high-latitude weak/reverse flux patches over the past 3–4 millennia. In addition, we also show that the 1125-yr periodicity signal inferred from the waveform interference correlates positively with variations in the dipole tilt over the same time period. The two identified drift signals may partially be explained by the westward motion of high-latitude convection rolls. However, the dispersion relation might also imply that part of the drift signal could be caused by magnetic Rossby waves riding on the mean background flow.

Problems related to PcP and the core-mantle boundary illustrated by two nuclear events

1973 ◽  
Vol 63 (6-1) ◽  
pp. 2047-2070 ◽  
Author(s):  
Goetz G. R. Buchbinder ◽  
Georges Poupinet
Keyword(s):  
Inner Core ◽  
Wave Form ◽  
Model Parameters ◽  
Travel Times ◽  
P Waves ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Order Of Magnitude

Abstract Two large nuclear explosions produced a considerable number of PcP phases. Analysis of the P and PcP travel times shows a scatter of ±2 sec. It is pointed out that PcP and P times must be considered together to eliminate gross upper mantle effects on the travel times. On a worldwide basis, the PcP peak-to-peak amplitudes exhibit a scatter of up to one order of magnitude, and, thus, the reflection coefficient of the core-mantle boundary (cmb) may not be determined with any significance from them. Comparing the wave form of PcP and the wave form of P waves convolved with thin-layered models of the cmb suggests that the cmb may be approximated by a thin high-impedance liquid layer of several kilometers in thickness embedded between the mantle and the core. Such a model can explain observed dilatational arrivals and a small decrease in amplitude near Δ ≈ 30°. The data do not permit exact determination of the model parameters because of uncertainty in the data and insensitivity of the method and because the cmb also may be laterally inhomogeneous. The frequency-dependence of the reflection and transmission coefficients of a layered cmb would have serious effects on the determination of inner core parameters.

GRACEFUL: Probing the deep Earth interior by synergistic use of observations of the magnetic and gravity fields, and of the rotation of the Earth

2020 ◽  
Author(s):  
Mioara Mandea ◽  
Veronique Dehant ◽  
Anny Cazenave
Keyword(s):  
Magnetic Field ◽  
Gravity Field ◽  
Time Scales ◽  
Earth Rotation ◽  
Outer Core ◽  
Time Dependent ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Earth

<div> <p>To understand the processes involved in the deep interior of the Earth and explaining its evolution, in particular the dynamics of the Earth’s fluid iron-rich outer core, only indirect satellite and ground observations are available. They each provide invaluable information about the core flow but are incomplete on their own:</p> <p>-        The time dependent magnetic field, originating mainly within the core, can be used to infer the motions of the fluid at the top of the core on decadal and subdecadal time scales.</p> <p>-        The time dependent gravity field variations that reflect changes in the mass distribution within the Earth and at its surface occur on a broad range of time scales. Decadal and interannual variations include the signature of the flow inside the core, though they are largely dominated by surface contributions related to the global water cycle and climate-driven land ice loss.</p> <p>-        Earth rotation changes (or variations in the length of the day) also occur on these time scales, and are largely related to the core fluid motions through exchange of angular momentum between the core and the mantle at the core-mantle boundary.</p> <p>Here, we present the main activities proposed in the frame of the GRACEFUL ERC project, which aims to combine information about the core deduced from the gravity field, from the magnetic field and from the Earth rotation in synergy, in order to examine in unprecedented depth the dynamical processes occurring inside the core and at the core-mantle boundary.</p> </div>

scholarly journals GRACE—Gravity Data for Understanding the Deep Earth’s Interior

2020 ◽  
Vol 12 (24) ◽  
pp. 4186
Author(s):  
Mioara Mandea ◽  
Véronique Dehant ◽  
Anny Cazenave
Keyword(s):  
Gravity Data ◽  
Outer Core ◽  
Space Mission ◽  
Last Deglaciation ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
Isostatic Adjustment ◽  
The Earth ◽  
Fluid Core

While the main causes of the temporal gravity variations observed by the Gravity Recovery and Climate Experiment (GRACE) space mission result from water mass redistributions occurring at the surface of the Earth in response to climatic and anthropogenic forces (e.g., changes in land hydrology, ocean mass, and mass of glaciers and ice sheets), solid Earth’s mass redistributions were also recorded by these observations. This is the case, in particular, for the glacial isostatic adjustment (GIA) or the viscous response of the mantle to the last deglaciation. However, it has only recently been shown that the gravity data also contain the signature of flows inside the outer core and their effects on the core–mantle boundary (CMB). Detecting deep Earth’s processes in GRACE observations offers an exciting opportunity to provide additional insight into the dynamics of the core–mantle interface. Here, we present one aspect of the GRACEFUL (GRavimetry, mAgnetism and CorE Flow) project, i.e., the possibility to use gravity field data for understanding the dynamic processes inside the fluid core and core–mantle boundary of the Earth, beside that offered by the geomagnetic field variations.

scholarly journals Mantle Electrical Conductivity and the Magnetic Field at the Core–Mantle Boundary

Fluids ◽  
2021 ◽  
Vol 6 (11) ◽  
pp. 403
Author(s):  
John V. Shebalin
Keyword(s):  
Magnetic Field ◽  
Electrical Conductivity ◽  
Normal Component ◽  
Outer Core ◽  
Electrically Conductive ◽  
Mantle Boundary ◽  
Core Mantle Boundary ◽  
The Core ◽  
The Magnetic Field ◽  
Theoretical Results

The Earth’s magnetic field is measured on and above the crust, while the turbulent dynamo in the outer core produces magnetic field values at the core–mantle boundary (CMB). The connection between the two sets of values is usually assumed to be independent of the electrical conductivity in the mantle. However, the turbulent magnetofluid in the Earth’s outer core produces a time-varying magnetic field that must induce currents in the lower mantle as it emerges, since the mantle is observed to be electrically conductive. Here, we develop a model to assess the possible effects of mantle electrical conductivity on the magnetic field values at the CMB. This model uses a new method for mapping the geomagnetic field from the Earth’s surface to the CMB. Since numerical and theoretical results suggest that the turbulent magnetic field in the outer core as it approaches the CMB is mostly parallel to this boundary, we assume that this property exists and set the normal component of the model magnetic field to zero at the CMB. This leads to a modification of the Mauersberger–Lowes spectrum at the CMB so that it is no longer flat, i.e., the modified spectrum depends on mantle conductance. We examined several cases in which mantle conductance ranges from low to high in order to gauge how CMB magnetic field strength and mantle ohmic heat generation may vary.

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