Observing the signature of the magnetic field's behaviour in the radial variation of inner core anisotropy

2020 ◽  
Author(s):  
Janneke de Jong ◽  
Lennart de Groot ◽  
Arwen Deuss
Keyword(s):  
Magnetic Field ◽  
Heat Flux ◽  
Seismic Anisotropy ◽  
Inner Core ◽  
Body Wave ◽  
Outer Core ◽  
Core Structure ◽  
Reversal Rate ◽  
Seismic Structure ◽  
The Magnetic Field

<p>The release of latent heat and lighter materials during inner core solidification is the driving force of the liquid iron flow in the outer core which generates the Earth's magnetic field. It is well known that the behaviour of the magnetic field varies over long time scales. Two clearly identifiable regimes are recognized, (i) superchrons and (ii) periods of hyperactivity (Biggin et al. 2012). Superchrons are characterized by an exceptionally low reversal rate of the magnetic pole and are associated with a low core mantle boundary (CMB) heat flux. Hyperactive periods are defined by a high reversal rate and have a high CMB heat flux.</p><p>Here we investigate whether the occurrence of these two regimes is related to radial variations in inner core seismic structure. Using seismic body-wave observations of compressional PKIKP-waves (Irving & Deuss 2011, Waszek & Deuss 2011, Lythgoe et al. 2013)., we construct a model of inner core anisotropy by comparing the difference between travel times for polar and equatorial rays. Anisotropy is the directional dependence of wave velocity and is determined by the structure of iron crystals in the inner core, hence changes in seismic anisotropy are due to changes in inner core crystal texture. We invert for radial changes in anisotropy while allowing for lateral variations and find that a model of the inner core containing five layers best fits our data. The model contains an isotropic uppermost inner core and four deeper layers with varying degrees of anisotropy.</p><p>Texture differences of the inner core iron crystals have been linked to changes in the solidification process of the inner core (Bergman et al. 2005), i.e. the motor of outer core flow. Therefore, the observed anisotropy variation, caused by variations of inner core solidification, might be related to changes in the behaviour of the magnetic field. Using an inner core growth model (Buffett et al. 1996) we convert depth to time for a range of inner core nucleation ages between 3.0 and 0.5 Ga (Olsen 2016). We find a remarkable correlation between the solidification time of the seismically observed layers and the occurrence of the magnetic regimes for two inner core ages; one with a nucleation at 1.4 Ga and one at 0.6 Ga, corresponding to an average CMB heat flux of 7.6 TW and 16.7 TW respectively.</p><p>Although we currently cannot differentiate between these two inner core ages considering our results alone, they do show that a relation between inner core structure and the behaviour of the magnetic field is possible, and suggest that seismic observations of inner core structure might provide new and independent insights into the magnetic field and its history.</p>

Exoplanetary magnetic fields as a tool for future interior characterization

2020 ◽  
Author(s):  
Irene Bonati ◽  
Marine Lasbleis ◽  
Lena Noack
Keyword(s):  
Magnetic Field ◽  
Solar System ◽  
Magnetic Fields ◽  
Iron Content ◽  
Inner Core ◽  
Outer Core ◽  
Magnetic Activity ◽  
Light Elements ◽  
The Core ◽  
The Magnetic Field

<p>Most planets located within the solar system display evidence of past and/or current magnetic activity. Magnetic fields of rocky bodies are thought to be driven by thermo-chemical convection taking place in an electrically conducting fluid in their deep interior (the liquid outer core for Earth), and are thus evidence of strong internal dynamics. Furthermore, magnetism is thought to play a crucial role for the development and the long-term stability of habitable surface conditions, as it shields the upper atmosphere from mass loss induced by stellar winds and extreme space weather events.<span class="Apple-converted-space"> </span></p> <p>The discovery of a large number of rocky exoplanets motivates the search and the study of magnetic fields beyond the solar system. While current observations are limited to providing the planetary radius and minimum mass, future missions aimed at the exploration of exoplanetary atmospheres will open up new avenues for the inversion of interior properties starting from atmospheric parameters. Such a goal requires knowledge of the planetary cores and the development of exoplanetary magnetic fields, as well as their influence on atmospheric evolution and its interaction with the surrounding stellar wind.<span class="Apple-converted-space"> </span></p> <p>The aim of the current study is to identify trends and parameter(s) controlling the core evolution and magnetic field sustainment in super-Earths. To do this we investigate the evolution of the cores of planets having different masses (0.8-2 Earth masses) and iron inventories (bulk iron content and mantle iron number). Starting out from the internal temperature profile after the complete solidification of a global magma ocean (Noack and Lasbleis, 2020), we determine the size and the structure of the core, and model its thermal and magnetic evolution during the subsequent 5 billion years. By taking into account the energy release resulting from the growth of a solid inner core, we compute the thermal and compositional buoyancy fluxes, as well as the generated magnetic field strengths and lifetimes.<span class="Apple-converted-space"> </span></p> <p>Our findings show that while the planetary mass is not a controlling parameter, both the bulk iron content and the mantle iron number strongly influence inner core growth and the lifetime of the magnetic field. Iron-rich planets having a high mantle iron number tend to start out and end up with solid inner cores that are substantially larger than iron-poor bodies, sometimes even reaching up to the radius of the outer core and thus shutting down magnetic activity. We therefore find that there is a “sweet spot” for longer-lasting magnetic fields, located at intermediate bulk iron contents and low mantle iron numbers. <span class="Apple-converted-space"> </span></p> <p>We also varied the content of light elements in the core and found that the addition of a small fraction of light elements helps keeping the magnetic field active for longer, even at high bulk iron contents. Field strengths can reach up to several times the one of Earth, even though such a signal might still be too weak to be detected by current radio telescopes. Nevertheless, the development of new observation techniques and the multiple future missions devoted to atmospheric exploration will provide useful insights on the presence and frequency of planetary magnetic fields.</p>

scholarly journals Comparison of the measured and modelled electron densities and temperatures in the ionosphere and plasmasphere during 20-30 January, 1993

2000 ◽  
Vol 18 (10) ◽  
pp. 1257-1262 ◽  
Author(s):  
A. V. Pavlov ◽  
T. Abe ◽  
K.-I. Oyama
Keyword(s):  
Magnetic Field ◽  
Heat Flux ◽  
Electron Temperature ◽  
Field Line ◽  
Electron Density ◽  
Magnetic Field Line ◽  
New Approach ◽  
Additional Heating ◽  
Vibrational Levels ◽  
The Magnetic Field

Abstract. We present a comparison of the electron density and temperature behaviour in the ionosphere and plasmasphere measured by the Millstone Hill incoherent-scatter radar and the instruments on board of the EXOS-D satellite with numerical model calculations from a time-dependent mathematical model of the Earth's ionosphere and plasmasphere during the geomagnetically quiet and storm period on 20–30 January, 1993. We have evaluated the value of the additional heating rate that should be added to the normal photoelectron heating in the electron energy equation in the daytime plasmasphere region above 5000 km along the magnetic field line to explain the high electron temperature measured by the instruments on board of the EXOS-D satellite within the Millstone Hill magnetic field flux tube in the Northern Hemisphere. The additional heating brings the measured and modelled electron temperatures into agreement in the plasmasphere and into very large disagreement in the ionosphere if the classical electron heat flux along magnetic field line is used in the model. A new approach, based on a new effective electron thermal conductivity coefficient along the magnetic field line, is presented to model the electron temperature in the ionosphere and plasmasphere. This new approach leads to a heat flux which is less than that given by the classical Spitzer-Harm theory. The evaluated additional heating of electrons in the plasmasphere and the decrease of the thermal conductivity in the topside ionosphere and the greater part of the plasmasphere found for the first time here allow the model to accurately reproduce the electron temperatures observed by the instruments on board the EXOS-D satellite in the plasmasphere and the Millstone Hill incoherent-scatter radar in the ionosphere. The effects of the daytime additional plasmaspheric heating of electrons on the electron temperature and density are small at the F-region altitudes if the modified electron heat flux is used. The deviations from the Boltzmann distribution for the first five vibrational levels of N2(v) and O2(v) were calculated. The present study suggests that these deviations are not significant at the first vibrational levels of N2 and O2 and the second level of O2, and the calculated distributions of N2(v) and O2(v) are highly non-Boltzmann at vibrational levels v > 2. The resulting effect of N2(v > 0) and O2(v > 0) on NmF2 is the decrease of the calculated daytime NmF2 up to a factor of 1.5. The modelled electron temperature is very sensitive to the electron density, and this decrease in electron density results in the increase of the calculated daytime electron temperature up to about 580 K at the F2 peak altitude giving closer agreement between the measured and modelled electron temperatures. Both the daytime and night-time densities are not reproduced by the model without N2(v > 0) and O2(v > 0), and inclusion of vibrationally excited N2 and O2 brings the model and data into better agreement.Key words: Ionosphere (ionospheric disturbances; ionosphere-magnetosphere interactions; plasma temperature and density)  

scholarly journals Hydromagnetic dynamos in rotating spherical fluid shells in dependence on the Prandtl number, density stratification and electromagnetic boundary conditions

2014 ◽  
Vol 44 (4) ◽  
pp. 293-312 ◽  
Author(s):  
Tomáš Šoltis ◽  
Ján Šimkanin
Keyword(s):  
Magnetic Field ◽  
Electrical Conductivity ◽  
Magnetic Fields ◽  
Prandtl Number ◽  
Inner Core ◽  
Polar Regions ◽  
Number Density ◽  
Magnetic Diffusion ◽  
Spherical Fluid ◽  
The Magnetic Field

Abstract We present an investigation of dynamo in a simultaneous dependence on the non-uniform stratification, electrical conductivity of the inner core and the Prandtl number. Computations are performed using the MAG dynamo code. In all the investigated cases, the generated magnetic fields are dipolar. Our results show that the dynamos, especially magnetic field structures, are independent in our investigated cases on the electrical conductivity of the inner core. This is in agreement with results obtained in previous analyses. The influence of non-uniform stratification is for our parameters weak, which is understandable because most of the shell is unstably stratified, and the stably stratified region is only a thin layer near the CMB. The teleconvection is not observed in our study. However, the influence of the Prandtl number is strong. The generated magnetic fields do not become weak in the polar regions because the magnetic field inside the tangent cylinder is always regenerated due to the weak magnetic diffusion.

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.

Viscomagnetic Heat Flux Experiments as a Test of Gas Kinetic Theory in the Burnett Regime

1978 ◽  
Vol 33 (7) ◽  
pp. 749-760 ◽  
Author(s):  
G. E. J. Eggermont ◽  
P. W. Hermans ◽  
L. J. F. Hermans ◽  
H. F. P. Knaap ◽  
J. J. M. Beenakker
Keyword(s):  
Magnetic Field ◽  
Boundary Layer ◽  
Heat Flux ◽  
External Magnetic Field ◽  
Room Temperature ◽  
Flow Direction ◽  
Rectangular Channel ◽  
Gas Kinetic Theory ◽  
The Magnetic Field ◽  
Good Agreement

In a rarefied polyatomic gas streaming through a rectangular channel, an external magnetic field produces a heat flux perpendicular to the flow direction. Experiments on this “viscom agnetic heat flux” have been performed for CO, N2, CH4 and HD at room temperature, with different orientations of the magnetic field. Such measurements enable one to separate the boundary layer contribution from the purely bulk contribution by means of the theory recently developed by Vestner. Very good agreement is found between the experimentally determined bulk contribution and the theoretical Burnett value for CO, N2 and CH4 , yet the behavior of HD is found to be anomalous.

The effect of a strongly stratified layer in the upper part of Mercury’s core on its magnetic field

2020 ◽  
Author(s):  
Patrick Kolhey ◽  
Daniel Heyner ◽  
Johannes Wicht ◽  
Karl-Heinz Glassmeier
Keyword(s):  
Magnetic Field ◽  
Dipole Moment ◽  
Magnetic Dipole ◽  
Magnetic Dipole Moment ◽  
Rotation Axis ◽  
Outer Core ◽  
Magnetic Field Generation ◽  
Fluid Core ◽  
Planetary Magnetic Fields ◽  
The Magnetic Field

<p>In the 1970’s the flybys of NASA’s Mariner 10 spacecraft confirmed the existence of an internally generated magnetic field at Mercury. The measurements taken during its flybys already revealed, that Mercury‘s magnetic field is unique along other planetary magnetic fields, since the magnetic dipole moment of ~190 nT ∙ R<sub>M</sub><sup>3 </sup>is very weak, e.g. compared to Earth’s magnetic dipole moment. The following MESSENGER mission from NASA investigated Mercury and its magnetic field more precisely and exposed additional interesting properties about the planet’s magnetic field. The tilt of its dipole component is less than 1°, which indicates a strong alignment of the field along the planet’s rotation axis. Additionally the measurement showed that the magnetic field equator is shifted roughly 0.2 ∙ R<sub>M</sub> towards north compared to Mercury‘s actual geographic equator.</p><p>Since its discovery Mercury‘s magnetic field has puzzled the community and modelling the dynamo process inside the planet’s interior is still a challenging task. Adapting the typical control parameters and the geometry in the models of the geodynamo for Mercury does not lead to the observed field morphology and strength. Therefore new non-Earth-like models were developed over the past decades trying to match Mercury’s peculiar magnetic field. One promising model suggests a stably stratified layer on the upper part of Mercury’s core. Such a layer divides the fluid core in a convecting part and a non-convecting part, where the magnetic field generation is mainly inhibited. As a consequence the magnetic field inside the outer core is damped very efficiently passing through the stably stratified layer by a so-called skin effect. Additionally, the non-axisymmetric parts of the magnetic field are vanishing, too, such that a dipole dominated magnetic is left at the planet’s surface.</p><p>In this study we present new direct numerical simulations of the magnetohydrodynamical dynamo problem which include a stably stratified layer on top of the outer core. We explore a wide parameter range, varying mainly the Rayleigh and Ekman number in the model under the aspect of a strong stratification of the stable layer. We show which conditions are necessary to produce a Mercury-like magnetic field and give a inside about the planets interior structure.</p>

3D Transdimensional Seismic Tomography of the Inner Core

2021 ◽  
Author(s):  
Henry Brett ◽  
Rhys Hawkins ◽  
Karen Lythgoe ◽  
Lauren Waszek ◽  
Arwen Deuss
Keyword(s):  
Seismic Tomography ◽  
Inner Core ◽  
Probability Distributions ◽  
Model Space ◽  
Core Structure ◽  
P Wave ◽  
Quality Data ◽  
Seismic Structure ◽  
Data Set ◽  
Detailed Interpretation

<p>The inner core contains strong seismic heterogeneity, both laterally and from the surface to the centre. Accurately resolving the seismic structure of the inner core is key to unravelling the evolution of the core. Seismic models of inner core structure are often limited by their parameterization, which means it is difficult to interpret which features of the inner core are real (e.g. hemispheres or the inner most inner core). To overcome this we conduct seismic tomography using transdimensional inversion on a high quality data set of 5296 differential and 2344 absolute P-wave travel times. By taking a transdimensional approach we allow the data to define how the model space is parameterized and this provides us with both the mean structure of the inner core but also the probability distributions of each model parameter. This allows us to identify which regions of the model space are well constrained and likewise which regions are poorly constrained. We compare results from a static MCMC model and a transdimensional MCMC model, this provides confidence in our results as both models show clear similarities in structure. From no prior assumptions on inner core structure we recover many first order observations: such as anisotropic hemispheres and an isotropic outer inner core (OIC) along with potential observations of an inner most inner core. With higher resolution than previous inner core tomography we can provide more detailed interpretation of inner core structure and draw conclusions with greater confidence. We also conduct transdimensional inversions on a subset of our data which does not contain South Sandwich Islands (SSI) events which are considered by many to be unreliable or contaminated with mantle structure. The overall inner core structure remains largely the same however, showing that the SSI data does not significantly alter our final interpretations.</p>

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