Global Modeling
On the basis of magnetic measurements on the Earth's ground surface, a low-precision global geomagnetic model was produced in the form of a time-dependent (spanning 400 years from 1590 to 1990) spherical harmonic expansion up to degree and order 14. In the satellite era, on the basis of combined surface and satellite magnetic measurements, a global geomagnetic model (CHAOS- 6), for example, was produced spanning from 1999 to 2017 with a much higher global spatial resolution, being able to reveal changes of the geomagnetic field in various temporal-spatial scales. In particular, the global models unveil rapid changes of the South Atlantic Anomaly which may be caused by a reverse magnetic flux patch at the Earth's core beneath Southern Africa, where charged particles in the innermost Van Allen belt can reach the altitude of Earth's upper atmosphere. It is worth mentioning that, since both surface and satellite magnetic measurements are limited only at some particular time or space, providing the direction and amplitude of a geomagnetic field at any time and space in the exterior of the Earth requires constructing a global geomagnetic model. A high-precision and high-resolution geomagnetic model can be usually validated by the measurements of such as ground observatories and geomagnetic satellites. The observed geomagnetic field consists of contributions from various sources that need to be carefully disentangled, representing a highly challenging task in constructing an accurate global geomagnetic model.
There exist mainly two different approaches in global geomagnetic field modeling that determine the Gauss coefficients of a model. The first is fully comprehensive, which attempts to include all the major magnetic contributions (the core geodynamo, lithospheric, ionospheric, magnetospheric, the mantle/crust induced, the oceans induced) by including the magnetic field data at all local times at all latitudes together with the parameterisations of all different sources and the simultaneous inversion of all parameters. The second is largely restrictive, which attempts to mainly focus on the large-scale and slowly-changing components (the core geodynamo, lithospheric and magnetospheric ring current) of the Earth's magnetic field. With the new geomagnetic constellation and its unprecedentedly accurate measurements, a higher-precision, higher-resolution and fully comprehensive geomagnetic model is expected to be constructed by an integration of satellite and ground-based magnetic data, facilitated by a careful sampling of the temporal and spatial data to disentangle different magnetic sources. Designing an innovative algorithm that is capable of reducing the non-uniqueness of the inverse problem in a geophysically reasonable way also represents an essential key to deriving a higher-precision and higher-resolution geomagnetic field model.
Below is a brief description of our progress on geomagnetic field modelling, derived by Swarm satellite (Alpha and Brave) during 2013-2014.
Figure 1. Core magnetic field map of radial component Br at Earth surface (left) and Core-Mantle Boundary (CMB) (right), given by model up to degree 13.
Figure 2. secular variation (left) and secular acceleration (right) of radial component at Earth surface.
Figure 3. Spatial power spectrum of the core field at the Earth surface (red) and core surface (blue) as described by model for epoch 2013-2014.