1. Session 5: Higher level products (Internal)

2. Summary
New data processing and modelling strategies facilitated/necessitated by data, and internal (core) field changes
3 presentations on core field and its temporal changes, and core surface flow
2 presentations on mantle electrical conductivity
1 presentation on the out-of-cycle World Magnetic Map update
1 presentation on the 3D Earth project

3. Core field/flow
Research issues: define the core field and its temporal evolution (especially secular acceleration), infer the core flow responsible for the temporal field changes, and understand better the dynamics of the core and the geodynamo responsible
Issues: fast core dynamics, especially secular acceleration pulses

4. Field models at short timescales
Discrepancy between torsional waves with period ~6 years inferred by Gillet et al. (2015), not present in model of Rapp and Lesur (2018)
Requires a sequential approach, with realistic priors on temporal and spatial variations/correlations
Sequence of monthly models

5. Secular acceleration pulses
Duration 1-2 years, spatially localized
Associated with – unpredictable – geomagnetic ‘jerks’ or impulses
Tools for studying them
Temporally-dependent spherical harmonic models
Point-by-point estimates of the core surface field (recently developed SOLA method)
Virtual observatories (VOs) – time series of field and its temporal derivatives at grids of points at satellite altitude by reducing data to a point at the centre of a volume, mimicking (and validated by) geomagnetic observatories on ground
Understand origins of features similar to observed from geodynamo simulations in a different numerical regime from standard

6. Virtual observatories
Original and modified method for producing monthly VO time series suggest estimates biased, probably from data gaps and local time effects
Now produce 4-monthly time series, stringent data selection criteria, fewer VO points, data only contribute to one VOs (each independent), data reduced to single point assuming cubic spatial potential dependence
Very little core flow temporal variation required to explain secular variation
Residuals to older VO data – larger values, spatially coherent patterns suggesting contamination
Newer VO data residuals smaller and less obviously biased

7. Mantle conductivity
Time domain approach, estimating both 1D and 3D variations
Best resolution at km depth

8. 3D time domain inversion
Specified external field spherical harmonics constituting the inducing signal
Persistence of features mimicking signatures in mantle tomography models

9. Alternative approach
What is the optimal (minimal) set of field coefficients specifying the inducing field?
Rank them by decreasing importance and determine number required
Varies according to external field activity, and for satellite vs ground data
Axial dipole dominates during storm, other coefficients important at quiet times

10. World Magnetic Model
Gone out of specification owing to geomagnetic jerks violating linear SV assumption
Prompt availability of satellite data crucial to produce accurate update
Original dF/dt
Updated dF/dt

11. 3D Earth – Swarm for solid Earth modelling
Use Swarm to investigate depth to Curie isotherm at regional scale
Compare to lithosphere thickness
Requires a number of assumptions (only magnetite, magnetization purely induced, …)

12. Incorporate multiple data sources to investigate:
Crustal structure (density, seismic velocities, heat production and thickness)
Mantle structure (LAB depth and composition and temperature distribution within the sub-lithosphere) and seismic radial anisotropy
Geologically useful models require aeromagnetic data
Simple model: affected by thermal and crustal thickness constraints
More details than in advanced model
Advanced model: loses key features such as mid-ocean ridges
Smoothed due to seismological constraints

13. Summary Already announced: VO product
Several new data processing/modelling approaches using Swarm data
Expect to get better mantle conductivity models and understanding of core dynamics, especially on the sub-decadal timescale
Rationale for 3D Earth approach needs clarifying