Hemispherical asymmetry in core dynamics induces degree-1 pressure variations at the core mantle boundary （CMB）, which in turn deforms the overlaying elastic mantle, at the same time keeps center of mass of the whol...Hemispherical asymmetry in core dynamics induces degree-1 pressure variations at the core mantle boundary （CMB）, which in turn deforms the overlaying elastic mantle, at the same time keeps center of mass of the whole Earth stationary in space. We develop a systematic procedure to deal with the degree-1 CMB pressure loading. We find by direct calculation a surprisingly negative load Love number h1=-1.425 for vertical displacement. Further analysis indicates that the negative hi corre- sponds to thickening above the positive load that defies intuition that pressure inflation pushes over- laying material up and thins the enveloping shell. We also redefine the pressure load Love numbers in general to enable comparison between the surface mass load and the CMB pressure load for the whole spectrum of harmonic degrees. We find that the gravitational perturbations from the two kinds of loads at degrees n〉l are very similar in amplitude but opposite in sign. In particular, if the CMB pressure variation at degree 2 is at the level of -1 hpa/yr （1 cm water height per year）, it would perturb the variation of Earth＇s oblateness, known as the J2, at the observed level.展开更多
The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core. We use the MoSST core dynamics model to simulate the Martian historical dynamo, focusing on the variation of the dynamo...The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core. We use the MoSST core dynamics model to simulate the Martian historical dynamo, focusing on the variation of the dynamo states with the Rayleigh number Ra (a non-dimensional parameter describing the buoyancy force in the core). Our numerical results show that the mean field length scale does not vary monotonically with the Rayleigh number, and the field morphology at the core mantle boundary changes with Rayleigh number. In particular, it drifts westward with a speed decreasing with Rayleigh number.展开更多
Observed Martian crustal magnetism shows that the Mars does not possess a global-scale,dynamo-driven intrinsic magnetic field.In addition,the remnant field at the surface is hemi-spherically asymmetric.Our earlier sim...Observed Martian crustal magnetism shows that the Mars does not possess a global-scale,dynamo-driven intrinsic magnetic field.In addition,the remnant field at the surface is hemi-spherically asymmetric.Our earlier simulation results suggest that the Martian dynamo could be sub-critical near its end(the energy required to sustain a subcritical dynamo is less than that to excite the dynamo)and the generated field morphology is non-dipolar.We further the study to examine the characteristics of the magnetic field via Empirical Orthogonal Function(EOF)analysis on the subcritical dynamo solutions with the Rayleigh number Rth = 2480(below the critical point for the onset of the Martian dynamo).Our results show that the magnetic field is dominantly equatorial dipolar.Reversals and excursions occur frequently,and the magnetic dipole moment does not vary monotonically in time.展开更多
Constraining numerical geodynamo models with surface geomagnetic observationsis very important in many respects:it directly helps to improve numericalgeodynamo models,and expands their geophysical applications beyond ...Constraining numerical geodynamo models with surface geomagnetic observationsis very important in many respects:it directly helps to improve numericalgeodynamo models,and expands their geophysical applications beyond geomagnetism.A successful approach to integrate observations with numerical models isdata assimilation,in which Bayesian algorithms are used to combine observationaldata with model outputs,so that the modified solutions can then be used as initialconditions for forecasts of future physical states.In this paper,we present the firstgeomagnetic data assimilation framework,which comprises the MoSST core dynamicsmodel,a newly developed data assimilation component(based on ensemble covarianceestimation and optimal interpolation),and geomagnetic field models basedon paleo,archeo,historical and modern geomagnetic data.The overall architecture,mathematical formulation,numerical algorithms and computational techniques of theframework are discussed.Initial results with 100-year geomagnetic data assimilationand with synthetic data assimilation are presented to demonstrate the operation of thesystem.展开更多
文摘Hemispherical asymmetry in core dynamics induces degree-1 pressure variations at the core mantle boundary （CMB）, which in turn deforms the overlaying elastic mantle, at the same time keeps center of mass of the whole Earth stationary in space. We develop a systematic procedure to deal with the degree-1 CMB pressure loading. We find by direct calculation a surprisingly negative load Love number h1=-1.425 for vertical displacement. Further analysis indicates that the negative hi corre- sponds to thickening above the positive load that defies intuition that pressure inflation pushes over- laying material up and thins the enveloping shell. We also redefine the pressure load Love numbers in general to enable comparison between the surface mass load and the CMB pressure load for the whole spectrum of harmonic degrees. We find that the gravitational perturbations from the two kinds of loads at degrees n〉l are very similar in amplitude but opposite in sign. In particular, if the CMB pressure variation at degree 2 is at the level of -1 hpa/yr （1 cm water height per year）, it would perturb the variation of Earth＇s oblateness, known as the J2, at the observed level.
基金Supported by National Natural Science Foundation of China (Grant No. 40328006)
文摘The observed Mars remnant magnetism suggests that there was an active dynamo in the Martian core. We use the MoSST core dynamics model to simulate the Martian historical dynamo, focusing on the variation of the dynamo states with the Rayleigh number Ra (a non-dimensional parameter describing the buoyancy force in the core). Our numerical results show that the mean field length scale does not vary monotonically with the Rayleigh number, and the field morphology at the core mantle boundary changes with Rayleigh number. In particular, it drifts westward with a speed decreasing with Rayleigh number.
文摘Observed Martian crustal magnetism shows that the Mars does not possess a global-scale,dynamo-driven intrinsic magnetic field.In addition,the remnant field at the surface is hemi-spherically asymmetric.Our earlier simulation results suggest that the Martian dynamo could be sub-critical near its end(the energy required to sustain a subcritical dynamo is less than that to excite the dynamo)and the generated field morphology is non-dipolar.We further the study to examine the characteristics of the magnetic field via Empirical Orthogonal Function(EOF)analysis on the subcritical dynamo solutions with the Rayleigh number Rth = 2480(below the critical point for the onset of the Martian dynamo).Our results show that the magnetic field is dominantly equatorial dipolar.Reversals and excursions occur frequently,and the magnetic dipole moment does not vary monotonically in time.
基金This research is supported by NSF Mathematical Geosciences program under the grants EAR-0327875 and EAR-0327843,NASA Solid Earth and Natural Hazard Program,NASA Mars Fundamental Research Program.We also thank GSFC NPPCS and NASA NAS for computation support.
文摘Constraining numerical geodynamo models with surface geomagnetic observationsis very important in many respects:it directly helps to improve numericalgeodynamo models,and expands their geophysical applications beyond geomagnetism.A successful approach to integrate observations with numerical models isdata assimilation,in which Bayesian algorithms are used to combine observationaldata with model outputs,so that the modified solutions can then be used as initialconditions for forecasts of future physical states.In this paper,we present the firstgeomagnetic data assimilation framework,which comprises the MoSST core dynamicsmodel,a newly developed data assimilation component(based on ensemble covarianceestimation and optimal interpolation),and geomagnetic field models basedon paleo,archeo,historical and modern geomagnetic data.The overall architecture,mathematical formulation,numerical algorithms and computational techniques of theframework are discussed.Initial results with 100-year geomagnetic data assimilationand with synthetic data assimilation are presented to demonstrate the operation of thesystem.
