Weijia Kuang

International Geomagnetic Reference Field: the 12th generation

The 12th generation of the International Geomagnetic Reference Field (IGRF) was adopted in December 2014 by the Working Group V-MOD appointed by the International Association of Geomagnetism and Aeronomy (IAGA). It updates the previous IGRF generation with a definitive main field model for epoch 2010.0, a main field model for epoch 2015.0, and a linear annual predictive secular variation model for 2015.0-2020.0. Here, we present the equations defining the IGRF model, provide the spherical harmonic coefficients, and provide maps of the magnetic declination, inclination, and total intensity for epoch 2015.0 and their predicted rates of change for 2015.0-2020.0. We also update the magnetic pole positions and discuss briefly the latest changes and possible future trends of the Earth’s magnetic field.

Performance benchmarks for a next generation numerical dynamo model

Numerical simulations of the geodynamo have successfully represented many observable characteristics of the geomagnetic field, yielding insight into the fundamental processes that generate magnetic fields in the Earths core. Because of limited spatial resolution, however, the diffusivities in numerical dynamo models are much larger than those in the Earths core, and consequently, questions remain about how realistic these models are. The typical strategy used to address this issue has been to continue to increase the resolution of these quasi-laminar models with increasing computational resources, thus pushing them toward more realistic parameter regimes. We assess which methods are most promising for the next generation of supercomputers, which will offer access to O(106) processor cores for large problems. Here we report performance and accuracy benchmarks from 15 dynamo codes that employ a range of numerical and parallelization methods. Computational performance is assessed on the basis of weak and strong scaling behavior up to 16,384 processor cores. Extrapolations of our weak-scaling results indicate that dynamo codes that employ two-dimensional or three-dimensional domain decompositions can perform efficiently on up to ∼106 processor cores, paving the way for more realistic simulations in the next model generation.

Understanding Time-variable Gravity due to Core Dynamical Processes with Numerical Geodynamo Modeling

On decadal time scales, there are three major physical processes in the Earth’s outer core that contribute to gravity variations: (i) the mass redistribution due to advection in the outer core, (ii) the mantle deformation in response to (i), and (iii) the (core) pressure loading on the core-mantle boundary. Except the last one, they cannot be evaluated from surface observations. In this paper we use MoSST core dynamics model and PREM model to understand the gravity anomalies from the three processes. Our numerical results show that, the gravity anomalies are comparable in magnitude, though that from the process (i) is in general the strongest. The gravity anomalies from the first two processes tend to offset each other (“mantle shielding”). Consequently the pressure loading effect contributes more to axisymmetric part of the net gravity variation, while the net effect from the first two processes is more important to non axisymmetric components.

Interpretation of Core Field Models

In this chapter we review several recent research results on the observed geomagnetic secular variation and secular acceleration, the core flow models inferred from these observations, and their implications, in particular those of the torsional oscillations, on short period secular variation and on the dynamical properties inside the core. We also provide a comprehensive review on the recent development in geomagnetic data assimilation, and its applications to predict future secular variation. Most of the reviewed research results are either reported in IAGA General Assembly in Soporan in 2009, or in the period between this and the previous IAGA conference.

High-Order Compact Implicit Difference Methods For Parabolic Equations in Geodynamo Simulation

A series of compact implicit schemes of fourth and sixth orders are developed for solving differential equations involved in geodynamics simulations. Three illustrative examples are described to demonstrate that high-order convergence rates are achieved while good efficiency in terms of fewer grid points is maintained. This study shows that high-order compact implicit difference methods provide high flexibility and good convergence in solving some special differential equations on nonuniform grids.

Dynamic Responses of the Earth's Outer Core to Assimilation of Observed Geomagnetic Secular Variation

Assimilation of surface geomagnetic observations and geodynamo models has advanced very quickly in recent years. However, compared to advanced data assimilation systems in meteorology, geomagnetic data assimilation (GDAS) is still in an early stage. Among many challenges ranging from data to models is the disparity between the short observation records and the long time scales of the core dynamics. To better utilize available observational information, we have made an effort in this study to directly assimilate the Gauss coefficients of both the core field and its secular variation (SV) obtained via global geomagnetic field modeling, aiming at understanding the dynamical responses of the core fluid to these additional observational constraints. Our studies show that the SV assimilation helps significantly to shorten the dynamo model spin-up process. The flow beneath the core-mantle boundary (CMB) responds significantly to the observed field and its SV. The strongest responses occur in the relatively small scale flow (of the degrees L≈30 in spherical harmonic expansions). This part of the flow includes the axisymmetric toroidal flow (of order m=0) and non-axisymmetric poloidal flow with m≥5. These responses can be used to better understand the core flow and, in particular, to improve accuracies of predicting geomagnetic variability in the future.

Decadal Polar Motion of the Earth Excited by the Convective Outer Core From Geodynamo Simulations

Long time geodetic observation records show that the orientation of the Earths rotation axis with respect to the terrestrial reference frame, or polar motion, changes on a broad range of time scales. Apart from external torques from the luni-solar tides, these changes are excited by interactions among different components of the Earth system. The convective fluid outer core has long been conjectured a likely contributor to the observed polar motion on time scales upwards of decades, such as the ∼30−year Markowitz wobble. We investigated the electromagnetic (EM) coupling scenario across the core-mantle boundary (CMB) via numerical geodynamo simulation for different geodynamo parameters (Rayleigh numbers and magnetic Rossby numbers). Our simulated polar motion varies strongly with the dynamo parameters, while its excitation on decadal time scales appear to converge asymptotically within the adopted range of numerical Rossby numbers. Three strongest asymptotic modes emerge from numerical results, with periods around 30, 40 and 60 years for the prograde excitation, and around 24, 30 and 60 years for the retrograde excitation. Their amplitudes are all larger than 5 × 10−8, or approximately 10 milliarcseconds. The results suggest that the electromagnetic core-mantle coupling could explain a substantial portion, if not all, of the observed decadal polar motion. In particular, the predicted 60-year polar motion deserves special attention for future observations and studies.