scholarly journals Geomagnetic polar minima do not arise from steady meridional circulation

2018 ◽  
Vol 115 (44) ◽  
pp. 11186-11191 ◽  
Author(s):  
Hao Cao ◽  
Rakesh K. Yadav ◽  
Jonathan M. Aurnou
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Inner Core ◽  
Meridional Circulation ◽  
Background Field ◽  
Outer Core ◽  
Near Surface ◽  
Core Mantle Boundary ◽  
Core Conditions ◽  
Polar Magnetic Fields

Observations of the Earth’s magnetic field have revealed locally pronounced field minima near each pole at the core–mantle boundary (CMB). The existence of the polar magnetic minima has long been attributed to the supposed large-scale overturning circulation of molten metal in the outer core: Fluid upwells within the inner core tangent cylinder toward the poles and then diverges toward lower latitudes when it reaches the CMB, where Coriolis effects sweep the fluid into anticyclonic vortical flows. The diverging near-surface meridional circulation is believed to advectively draw magnetic flux away from the poles, resulting in the low intensity or even reversed polar magnetic fields. However, the interconnections between polar magnetic minima and meridional circulations have not to date been ascertained quantitatively. Here, we quantify the magnetic effects of steady, axisymmetric meridional circulation via numerically solving the axisymmetric magnetohydrodynamic equations for Earth’s outer core under the magnetostrophic approximation. Extrapolated to core conditions, our results show that the change in polar magnetic field resulting from steady, large-scale meridional circulations in Earth’s outer core is less than 3% of the background field, significantly smaller than the ∼ 100% polar magnetic minima observed at the CMB. This suggests that the geomagnetic polar minima cannot be produced solely by axisymmetric, steady meridional circulations and must depend upon additional tangent cylinder dynamics, likely including nonaxisymmetric, time-varying processes.

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 Torsional waves driven by convection and jets in Earth’s liquid core

2018 ◽  
Vol 216 (1) ◽  
pp. 123-129 ◽  
Author(s):  
R J Teed ◽  
C A Jones ◽  
S M Tobias
Keyword(s):  
Magnetic Field ◽  
Density Gradient ◽  
Inner Core ◽  
Liquid Core ◽  
Outer Core ◽  
Dynamo Action ◽  
Radial Magnetic Field ◽  
Torsional Waves ◽  
Plausible Mechanism ◽  
Rich Liquid

SUMMARY Turbulence and waves in Earth’s iron-rich liquid outer core are believed to be responsible for the generation of the geomagnetic field via dynamo action. When waves break upon the mantle they cause a shift in the rotation rate of Earth’s solid exterior and contribute to variations in the length-of-day on a ∼6-yr timescale. Though the outer core cannot be probed by direct observation, such torsional waves are believed to propagate along Earth’s radial magnetic field, but as yet no self-consistent mechanism for their generation has been determined. Here we provide evidence of a realistic physical excitation mechanism for torsional waves observed in numerical simulations. We find that inefficient convection above and below the solid inner core traps buoyant fluid forming a density gradient between pole and equator, similar to that observed in Earth’s atmosphere. Consequently, a shearing jet stream—a ‘thermal wind’—is formed near the inner core; evidence of such a jet has recently been found. Owing to the sharp density gradient and influence of magnetic field, convection at this location is able to operate with the turnover frequency required to generate waves. Amplified by the jet it then triggers a train of oscillations. Our results demonstrate a plausible mechanism for generating torsional waves under Earth-like conditions and thus further cement their importance for Earth’s core dynamics.

scholarly journals Approaching Earth’s core conditions in high-resolution geodynamo simulations

2019 ◽  
Vol 219 (Supplement_1) ◽  
pp. S137-S151 ◽  
Author(s):  
Julien Aubert
Keyword(s):  
Magnetic Field ◽  
High Resolution ◽  
Large Scale ◽  
Small Scale ◽  
Low Resolution ◽  
Earth’S Core ◽  
Earth's Core ◽  
Density Anomaly ◽  
Core Conditions ◽  
The Magnetic Field

SUMMARY The geodynamo features a broad separation between the large scale at which Earth’s magnetic field is sustained against ohmic dissipation and the small scales of the turbulent and electrically conducting underlying fluid flow in the outer core. Here, the properties of this scale separation are analysed using high-resolution numerical simulations that approach closer to Earth’s core conditions than earlier models. The new simulations are obtained by increasing the resolution and gradually relaxing the hyperdiffusive approximation of previously published low-resolution cases. This upsizing process does not perturb the previously obtained large-scale, leading-order quasi-geostrophic (QG) and first-order magneto-Archimedes-Coriolis (MAC) force balances. As a result, upsizing causes only weak transients typically lasting a fraction of a convective overturn time, thereby demonstrating the efficiency of this approach to reach extreme conditions at reduced computational cost. As Earth’s core conditions are approached in the upsized simulations, Ohmic losses dissipate up to 97 per cent of the injected convective power. Kinetic energy spectra feature a gradually broadening self-similar, power-law spectral range extending over more than a decade in length scale. In this range, the spectral energy density profile of vorticity is shown to be approximately flat between the large scale at which the magnetic field draws its energy from convection through the QG-MAC force balance and the small scale at which this energy is dissipated. The resulting velocity and density anomaly planforms in the physical space consist in large-scale columnar sheets and plumes, respectively, co-existing with small-scale vorticity filaments and density anomaly ramifications. In contrast, magnetic field planforms keep their large-scale structure after upsizing. The small-scale vorticity filaments are aligned with the large-scale magnetic field lines, thereby minimizing the dynamical influence of the Lorentz force. The diagnostic outputs of the upsized simulations are more consistent with the asymptotic QG-MAC theory than those of the low-resolution cases that they originate from, but still feature small residual deviations that may call for further theoretical refinements to account for the structuring constraints of the magnetic field on the flow.

scholarly journals Horizontal Transition of Turbulent Cascade in the Near-Surface Layer of Tropical Cyclones

2015 ◽  
Vol 72 (12) ◽  
pp. 4915-4925 ◽  
Author(s):  
Jie Tang ◽  
David Byrne ◽  
Jun A. Zhang ◽  
Yuan Wang ◽  
Xiao-tu Lei ◽  
...  
Keyword(s):  
Surface Layer ◽  
Tropical Cyclones ◽  
Inner Core ◽  
Outer Core ◽  
Core Region ◽  
Near Surface ◽  
Sources And Sinks ◽  
Available Energy ◽  
Energy Transportation ◽  
Turbulent Cascade

Abstract Tropical cyclones (TC) consist of a large range of interacting scales from hundreds of kilometers to a few meters. The energy transportation among these different scales—that is, from smaller to larger scales (upscale) or vice versa (downscale)—may have profound impacts on TC energy dynamics as a result of the associated changes in available energy sources and sinks. From multilayer tower measurements in the low-level (<120 m) boundary layer of several landing TCs, the authors found there are two distinct regions where the energy flux changes from upscale to downscale as a function of distance to the storm center. The boundary between these two regions is approximately 1.5 times the radius of maximum wind. Two-dimensional turbulence (upscale cascade) occurs more typically at regions close to the inner-core region of TCs, while 3D turbulence (downscale cascade) mostly occurs at the outer-core region in the surface layer.

scholarly journals Urban-Induced Changes on Local Circulation in Complex Terrain: Central Mexico Basin

Atmosphere ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 904
Author(s):  
Lourdes P. Aquino-Martínez ◽  
Arturo I. Quintanar ◽  
Carlos A. Ochoa-Moya ◽  
Erika Danaé López-Espinoza ◽  
David K. Adams ◽  
...  
Keyword(s):  
Large Scale ◽  
Meridional Circulation ◽  
Circulation Pattern ◽  
Heat Fluxes ◽  
Local Circulation ◽  
Pressure System ◽  
Near Surface ◽  
Zonal Circulation ◽  
Downslope Wind ◽  
Mexico Basin

Land use land cover (LULC) significantly impacts local circulation in the Mexico Basin, particularly wind field circulations such as gap winds, convergence lines, and thermally induced upslope/downslope wind. A case study with a high-pressure system over the Mexico Basin isolates the influence of large-scale synoptic forcing. Numerical simulations reveal a wind system composed of meridional circulation and a zonal component. Thermal pressure gradients between the Mexico basin and its colder surroundings create near-surface convergence lines as part of the meridional circulation. Experiments show that the intensity and organization of meridional circulations and downslope winds increase when LULC changes from natural and cultivated land to urban. Zonal circulation exhibits a typical circulation pattern with the upslope flow and descending motion in the middle of the basin. Large values of moist static energy are near the surface where air parcels pick up energy from the surface either as fluxes of enthalpy or latent heat. Surface heat fluxes and stored energy in the ground are drivers of local circulation, which is more evident in zonal circulation patterns.

scholarly journals Magnetic and velocity fields in a dynamo operating at extremely small Ekman and magnetic Prandtl numbers

2017 ◽  
Vol 47 (4) ◽  
pp. 261-276
Author(s):  
Ján Šimkanin ◽  
Juraj Kyselica
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Inner Core ◽  
Strong Field ◽  
Boussinesq Approximation ◽  
Velocity Fields ◽  
Small Scale ◽  
Prandtl Numbers ◽  
Core Rotation ◽  
Inner Core Rotation

AbstractNumerical simulations of the geodynamo are becoming more realistic because of advances in computer technology. Here, the geodynamo model is investigated numerically at the extremely low Ekman and magnetic Prandtl numbers using the PARODY dynamo code. These parameters are more realistic than those used in previous numerical studies of the geodynamo. Our model is based on the Boussinesq approximation and the temperature gradient between upper and lower boundaries is a source of convection. This study attempts to answer the question how realistic the geodynamo models are. Numerical results show that our dynamo belongs to the strong-field dynamos. The generated magnetic field is dipolar and large-scale while convection is small-scale and sheet-like flows (plumes) are preferred to a columnar convection. Scales of magnetic and velocity fields are separated, which enables hydromagnetic dynamos to maintain the magnetic field at the low magnetic Prandtl numbers. The inner core rotation rate is lower than that in previous geodynamo models. On the other hand, dimensional magnitudes of velocity and magnetic fields and those of the magnetic and viscous dissipation are larger than those expected in the Earth’s core due to our parameter range chosen.

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>

Attenuation in the earth at low frequencies

1983 ◽  
Vol 308 (1504) ◽  
pp. 479-522 ◽  
Keyword(s):  
Large Scale ◽  
Inner Core ◽  
Outer Core ◽  
Quality Data ◽  
Low Frequencies ◽  
The Earth ◽  
Relative Standard ◽  
Average Shear ◽  
Standard Deviations ◽  
Apparent Attenuation

The introduction of global, digitally recording, seismic networks has provided the seismological community with a large quantity of high quality data. At low frequencies the IDA (International Deployment of Accelerometers) network provides the best available data and, in this report, over 500 IDA records have been carefully analysed giving nearly 4000 reliable measurements of centre frequency and apparent attenuation of fundamental spheroidal modes. The attenuation rate of a normal mode of free oscillation of the Earth is measured in terms of its or quality factor and mean Q values for the modes 0 S 8 - 0 S 46 are presented with standard deviations of 2-9% . Mean centre frequencies have relative standard deviation of 5 x 10- 5 to 5 x 10- 4 . The distribution of the centre frequencies reveals a large-scale aspherical structure in velocity and density but the distribution of the apparent attenuation measurements does not reveal a corresponding structure. A total of 26 new measurements of the mean Q of overtone modes with standard deviations of 5-30 % have also been obtained by using single-record and multiple-record techniques. Combining the new data with reliable Q measurements from the literature gives a total of 71 data with which we can infer the radial structure of attenuation inside the Earth. This structure is not well constrained in detail and very simple models are capable of fitting the data. Experiments with synthetic data show that an improvement of an order of magnitude in both the number and quality of the measurements is required to make detailed inferences about the structure of attenuation. The data do constrain the average shear Q- 1 in the inner core to be 1/3500 ( ± 60 %) and the average shear Q- 1 the mantle to be 1/250 ( ± 4 %). These values are appropriate for frequencies less than 5 mHz. Comparison with published values at higher frequencies indicates there is a measurable frequency dependence of attenuation between 3 and 30 mHz. Very little can be inferred about bulk dissipation in the Earth beyond that it must exist to satisfy the attenuation of the radial modes. Experiments show that the data can be satisfied if bulk attenuation is an average 1.3%, or more, of the shear attenuation. Constraining bulk attenuation to be no greater than 2 % of the shear attenuation, and constraining the outer core to have no attenuation, forces bulk attenuation to be concentrated in the upper mantle.

scholarly journals Electrodynamics of turbulent fluids with fluctuating electric conductivity

2020 ◽  
Vol 86 (3) ◽  
Author(s):  
G. Rüdiger ◽  
M. Küker ◽  
P. J. Käpylä
Keyword(s):  
Magnetic Field ◽  
Large Scale ◽  
Mean Field ◽  
Outer Core ◽  
Cycle Times ◽  
Solar Convection ◽  
Magnetic Diffusivity ◽  
Large Scale Magnetic Field ◽  
Alpha Effect ◽  
Turbulent Fluids

Consequences of fluctuating microscopic conductivity in mean-field electrodynamics of turbulent fluids are formulated and discussed. If the conductivity fluctuations are assumed to be uncorrelated with the velocity fluctuations then only the turbulence-originated magnetic diffusivity of the fluid is reduced and the decay time of a large-scale magnetic field or the cycle times of oscillating turbulent dynamo models are increased. If, however, the fluctuations of conductivity and flow in a certain well-defined direction are correlated, an additional diamagnetic pumping effect results, transporting the magnetic field in the opposite direction to the diffusivity flux vector $\langle \unicode[STIX]{x1D702}^{\prime }\boldsymbol{u}^{\prime }\rangle$ . In the presence of global rotation, even for homogeneous turbulence fields, an alpha effect appears. If the characteristic values of the outer core of the Earth or the solar convection zone are applied, the dynamo number of the new alpha effect does not reach supercritical values to operate as an $\unicode[STIX]{x1D6FC}^{2}$ -dynamo but oscillating $\unicode[STIX]{x1D6FC}\unicode[STIX]{x1D6FA}$ -dynamos with differential rotation are not excluded.

scholarly journals Assimilation of Himawari-8 Imager Radiance Data with the WRF-3DVAR system for the prediction of Typhoon Soulder

2020 ◽  
Author(s):  
Dongmei Xu ◽  
Aiqing Shu ◽  
Zhankui Zhang
Keyword(s):  
Inner Core ◽  
Surface Wind ◽  
Three Dimensional ◽  
Surface Wind Speed ◽  
Background Field ◽  
Japan Meteorological Agency ◽  
Near Surface ◽  
Frequency Space ◽  
Geostationary Meteorological Satellite ◽  
Typhoon Season

Abstract. Himawari-8 is a new generation geostationary meteorological satellite launched by Japan Meteorological Agency (JMA). It carries the Advanced Himawari imager (AHI) onboard, which can continuously monitor high-impact weather events with high frequency space and time. The assimilation of AHI was implemented with the framework of the mesoscale numerical model WRF and its three-dimensional variational assimilation system (3DVAR) for the analysis and prediction of typhoon Soudelor in the Pacific Typhoon season in 2015. The effective assimilation of AHI Imager data in tropical cyclone with rapid intensify development has been realized. The results show that after assimilating the AHI imager data under clear sky conditions, the typhoon position in the background field in the model is effectively corrected compared with the control experiment without AHI data. It is found that assimilation of AHI imager data is able to improve the analyses of the water vapor and wind in typhoon inner-core region. The analyses and forecast of the typhoon minimum sea level pressure, the maximum near-surface wind speed, and the typhoon track are further improved．

Sign in / Sign up

Export Citation Format

Share Document