On flow magnitude and field-flow alignment at Earth's core surface

We present a method to estimate the typical magnitude of flow close to Earth's core surface based on observational knowledge of the geomagnetic main field (MF) and its secular variation (SV), together with prior information concerning field-flow alignment gleaned from numerical dynamo models. An expression linking the core surface flow magnitude to spherical harmonic spectra of the MF and SV is derived from the magnetic induction equation. This involves the angle γ between the flow and the horizontal gradient of the radial field. We study γ in a suite of numerical dynamo models and discuss the physical mechanisms that control it. Horizontal flow is observed to approximately follow contours of the radial field close to high-latitude flux bundles, while more efficient induction occurs at lower latitudes where predominantly zonal flows are often perpendicular to contours of the radial field. We show that the amount of field-flow alignment depends primarily on a magnetic modified Rayleigh number Raη=αg0ΔTD/ηΩ, which measures the vigour of convective driving relative to the strength of magnetic dissipation. Synthetic tests of the flow magnitude estimation scheme are encouraging, with results differing from true values by less than 8 per cent. Application to a high-quality geomagnetic field model based on satellite observations (the xCHAOS model in epoch 2004.0) leads to a flow magnitude estimate of 11-14 km yr−1, in accordance with previous estimates. When applied to the historical geomagnetic field model gufm1 for the interval 1840.0-1990.0, the method predicts temporal variations in flow magnitude similar to those found in earlier studies. The calculations rely primarily on knowledge of the MF and SV spectra; by extrapolating these beyond observed scales the influence of small scales on flow magnitude estimates is assessed. Exploring three possible spectral extrapolations we find that the magnitude of the core surface flow, including small scales, is likely less than 50 km yr−

Tests of core flow imaging methods with numerical dynamos

We test the quality of a new core flow imaging method that incorporates constraints on flow helicity, using synthetic magnetic secular variation data from 3-D self-consistent numerical dynamo models. Comparison with the dynamo model flows reveals that our imaging method delineates most of the main large-scale flow features, both in pattern and magnitude. The dynamo model flows are characterized by high-latitude vortices, some equatorial symmetry, columnar convection and a significant amount of flow along radial magnetic field contours. Our inversion method correctly images these aspects of the flows. The correlation coefficient between the dynamo velocity and the imaged velocity exceeds 0.5 in cases with large-scale flow and magnetic field pattern, but degrades substantially in more complex cases when the scale of the secular variation is small. The magnitude of the imaged velocity depends on the a priori-assumed ratio of tangential divergence to radial vorticity k, in some resemblance to the damping parameter in spectral methods, although with our method the misfit is insensitive to k-values. Including tangential magnetic diffusion in core flow inversion improves the quality of the imaged velocity patte

Effects of Earth's magnetic field variation on high frequency wave propagation in the ionosphere

The ionosphere is an anisotropic, dispersive medium for the propagation of radio frequency electromagnetic waves due to the presence of the Earth?s intrinsic magnetic field and free charges. The detailed physics of electromagnetic wavepropagation through a plasma is more complex when it is embedded in a magnetic field. In particular, the ground range of waves reflecting in the ionosphere presents detectable magnetic field effects. Earth?s magnetic field varies greatly, with the most drastic scenario being a polarity reversal. Here the spatial variability of the ground range is analyzed using numerical ray tracing under possible reversal scenarios. Pattern changes of the ?spitze?, a cusp in the ray path closely related to the geomagnetic field, are also assessed. The ground range increases with magnetic field intensity and ray alignment with the field direction. For the present field, which is almost axial dipolar, this happens for Northward propagation at the magnetic equator, peaking in Indonesia where the intensity is least weak along the equator. A similar situation occurs for a prevailing equatorial dipole with Eastward ray paths at the corresponding magnetic equator that here runs almost perpendicular to the geographic equator. Larger spitze angles occur for smaller magnetic inclinations, and higher intensities. This is clearly observed for the present field and the dipole rotation scenario along the corresponding magnetic equators. For less dipolar configurations the ground range and spitze spatial variabilities become smaller scale. Overall, studying ionospheric dynamics during a reversal may highlight possible effects of dipole decrease which is currently ongoing.Fil: Fagre, Mariano. Universidad Nacional de Tucumán. Facultad de Ciencias Exactas y Tecnología. Departamento de Electricidad, Electrónica y Computación. Laboratorio de Telecomunicaciones; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; ArgentinaFil: Zossi, Bruno Santiago. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; ArgentinaFil: Yiğit, Erdal. George Mason University; Estados UnidosFil: Amit, Hagay. Cnrs- S, Laboratoire de Planetologie Et de Geodyn; FranciaFil: Elias, Ana Georgina. Universidad Nacional de Tucumán. Instituto de Física del Noroeste Argentino. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet Noa Sur. Instituto de Física del Noroeste Argentino; Argentin

Geomagnetic Dipole Changes and Upwelling/Downwelling at the Top of the Earth's Core

The convective state of the top of Earth's outer core is still under debate. Conflicting evidence from seismology and geomagnetism provides arguments for and against a thick stably stratified layer below the core-mantle boundary. Mineral physics and cooling scenarios of the core favor a stratified layer. However, a non-zero secular variation of the total geomagnetic energy on the core-mantle boundary is evidence for the presence of radial motions extending to the top of the core. We compare the secular variation of the total geomagnetic energy with the secular variation of the geomagnetic dipole intensity and tilt. We demonstrate that both the level of cancellations of the sources and sinks of the dipole intensity secular variation, as well as the level of cancellations of the sources and sinks of the dipole tilt secular variation, are either larger than or comparable to the level of cancellations of the sources and sinks of the total geomagnetic energy secular variation on the core-mantle boundary, indicating that the latter is numerically significant hence upwelling/downwelling reach the top of the core. Radial motions below the core-mantle boundary are either evidence for no stratified layer or to its penetration by various dynamical mechanisms, most notably lateral heterogeneity of core-mantle boundary heat flux

Geomagnetic field dynamics on various time scales

International audienc

Can downwelling at the top of the Earth’s core be detected in the geomagnetic secular variation?

International audienc

On magnetic diffusion and energy cascade in core dynamics,

International audienc

Time-average and time-dependent parts of core flow

International audienc

Helical core flow from geomagnetic secular variation

International audienc