Session 5: Higher level products (Internal)

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

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

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

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

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

Mantle conductivity Time domain approach, estimating both 1D and 3D variations Best resolution at 500-1500 km depth

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

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

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

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, …)

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

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