STSE Swarm + Innovation Science Study: MTR Meeting Magnetic Tidal Signals and Their Use in Mapping the Electrical Conductivity of the Lithosphere and Upper Mantle Alexey Kuvshinov (ETH), Alexander Grayver (ETH), Neesha Schnepf (UoC), Chandrasekharan Manoj (UoC), Nils Olsen (DTU) and Terence Sabaka (NASA)

Agenda for the meeting 14: :30 Kuvshinov A: Overview of the Study 14: :00 Schnepf N.: Sensitivity Analysis and Building Laterally-Variable Conductivity of the Ocean 15: :40 Grayver A.: Rigorous Quasi-1D Inversion of Satellite-Detected Tidal Magnetic Signals Coffee break (10 minutes) 15: :20 Olsen N.: Recovery of Tidal Signals from CHAMP and Swarm Magnetic Satellite Data 16: :40 Kuvshinov A. (all): Plans for Future 16: :10 General discussion/AOB

New team members Dr. Alexander Grayver (ETH): Inversion of tidal signals Dr. Chandrasekharan Manoj (NGDC/UoC): Support in building laterally variable seawater conductivity and seafloor data processing

Only two techniques directly probe the Earth’s mantle SeismologyEM Velocity Wave propagation Conductivity Diffusion From surface to coreFrom surface to 1600 km Point-wise (well-known) source (earthquake, nuclear test) Spatially-distributed sources above the Earth Can probe the volume where there are no observations Very limited resolution beneath the regions without observations Sensitive to temperature, composition, melt/water content Sensitive to bulk mechanical properties

Main source for deep EM studies: ring current Distance: 2-9 radii above the Earth Geometry: simple (predominantly P10) Origin: solar wind Periods of variations: 4 days – 4 months Target depths: 600 – 1600 km Swarm L2 product: mantle conductivity at these depths (using this source)

From middle/lower mantle to upper mantle source: Sq source challenging! periods: 4 hours – 1 day source: magnetospheric ring current periods: 2 days – 4 months 10 km 600 km 1600 km upper mantle (a lot of interest; water, partial melting) Middle/lower mantle

Ionospheric Sq source Distance: 100 km above the Earth Periods: 24, 12, 8, 6 … hours (daily variations) Geometry: complicated (two whorls): challenge to recover Origin: solar heating of the ionized gas

Four facts to remember when working with Sq The source has complex spatial structure and thus standard approach based on response functions does not work The Sq source varies from day to day, from season to season and from year to year The Sq source has to be specified before data inversion With ground-based data the source can be recovered by e.g. potential method (which allows for separating external and induced parts)

Can we probe the Earth from space using Sq signal? No At ground (the source is above; 100 km) : At satellite altitude (the source is beneath; satellite flies at 450 km):

Alternative source in this period range? Tides Other than M2 and O1 are most probably not-detectable

Can we detect tidal signal from space? Recovered from satellite data* Predicted by 3-D modelling** * Sabaka T, N, Olsen, R. Tyler, A. Kuvshinov, CM5, a pre-Swarm comprehensive geomagnetic field model derived from over 12 yr of CHAMP, Ørsted, SAC-C and observatory data. Geophys. J. Int., 200, 1596–1626. Agreement is stunning! ** Schnepf N., A. Kuvshinov, T. Sabaka, Can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals? GRL, /2015GL

Sq versus Tides Source is global Sq source is determined from geomagnetic observatory data, thus it is generally poorly resolved e.g. in oceanic regions In the best scenario (quite time, equinoctial conditions) one can work with single day data Inductive excitation; more sensitive to conductive (deeper) structures Earth’s probing is not possible with satellite Sq data Sq Tides Source is confined to oceanic regions Tidal source is determined much more accurately (through velocity and main field models) Due to the closeness of the tidal periods to Sq periods one has to analyse long enough time series to detect tidal signals Galvanic excitation (DC like); more sensitive to resistive (shallower) structures Whether the Earth’s probing is possible with satellite data?

STSE Swarm + Innovation Science Study Title: Magnetic Tidal Signals and Their Use in Mapping the Electrical Conductivity of the Lithosphere and Upper Mantle Kick off: April 4, 2015 Duration: 18 months Institutions involved: ETH / DTU / NASA /UoC

Work breakdown structure

Schedule

WP1100/1150: Justification of velocity models for modelling We considered two global models of depth-integrated tidal velocities TPXO7.2 (Egbert and Erofeeva, 2002); of 0.25 degree resolution Egbert, G. D., and S. Y. Erofeeva (2002), Efficient inverse modeling of barotropic ocean tides, J. Atmos. Oceanic Technol., 19, 183–204. HAMTIDE (Taguchi et al., 2014); of degree resolution Taguchi, E., D. Stammer, and W. Zahel (2014), Inferring deep ocean tidal energy dissipation from the global high-resolution data- assimilative HAMTIDE model, J. Geophys. Res. Oceans, 119, 4573–4592, doi: /2013JC

Justification of velocity models for modelling, contd. TPXO 7.2 HAMTIDE Real and imaginary parts of M2 depth-integrated velocities

Justification of velocity models for modelling, contd. TPXO 7.2 HAMTIDE Amplitude of radial B component at satellite height (430km)

Justification of velocity models for modelling, contd. TPXO 7.2 minus HAMTIDE Difference in amplitude of radial B component at satellite height

Justification of velocity models for modelling, contd. Sabaka, T. J., N. Olsen, R. H. Tyler, and A. Kuvshinov (2015), CM5, a pre-Swarm comprehensive geomagnetic field model derived from over 12 yr of CHAMP, Orsted, SAC-C and observatory data, Geophys. J. Int., 200, 1596–1626.

Justification of velocity models for modelling, contd. Summing up: the models are very similar but we decided for further work to use HAMTIDE model The model has better resolution (and thus more appropriate if ground-based/sea bottom data will be exploited) Model is from Europe

WP1100/1150: Justification of the data, contd ERI group (Tokyo University) provided us with cable data (from 10 Pacific cables) U Cable Groundings Voltmeter + data seafloor MT surveys

WP1100/1150: Justification of the data, contd + data from 2 North Atlantic cables

WP1100/1150: Justification of the data, contd Ground-based magnetic data are readily available from Intermagnet Z: Valentia Z: Hartland

WP1100/1150: Justification of the data, contd More and more observatories have been equipping with electric field instruments (UK, Japan, Ireland, etc.). These data are also available. Valentia, May 2015

Extra “package” not foreseen at the beginning 14: :00 Schnepf N.: Sensitivity Analysis and Building Laterally-Variable Conductivity of the Ocean In previous studies we always used mean seawater conductivity to specify the source : In this study we construct (more realistic) laterally-variable conductivity:

Future plans Study the feasibility of 3-D inversion Study the feasibility of incorporating into inversion ground-based and seafloor data Quasi 1-D inversion of Swarm-detected tidal signals Accelerate/refactor 3-D forward problem code Process seafloor and ground-based data