Island geomagnetic observatories and upper mantle electric conductivity profiles
Final Report Abstract
The magnetic field of the Earth results from a variety of sources down from the iron core up to the magnetospheric boundary. Therefore, the magnetic field contains information on the physical state and electrodynamic processes of the various source regions. However, it is difficult to separate the various contributions as they partly overlap both in space and in time. This project was aiming at separating the field induced in the lithosphere and upper mantle layers using geomagnetic observatory data, and to extract information on the electrical conductivity of these layers. Usually, additional electric field measurements are necessary to separate the induced field of the lithosphere and upper mantle, at least if only one observatory is available in a certain region. Here, we use a new method that allows to extract the inducing field with a single observatory and with magnetic field measurements alone, given that the subsurface conductivity structure varies only radially. This method works for observatories located on islands by making use of the fields induced by the conductivity contrast between seawater and subsurface rocks. These induced fields allow to sample the underlying 1-D conductivity profile of the subsurface, and can be analyzed with advanced numerical solvers and if bathymetry is known. Within this project, this method was successfully applied to geomagnetic observatory data for the first time. Tristan da Cunha in the South Atlantic Ocean is an active volcanic island and the dynamics and location of its supposed mantle plume are a topic of current research. We found that the electrical conductivity profile below the island is anomalously conductive, indicating a region of partial melt below the island. In contrast, results for Gan on the Maldives indicate a normal oceanic mantle that is fairly compatible with the known conductivity profile of ~150 Myr old Pacific mantle. For both stations, additional electric field measurements are available which we found to be compatible with the results based on magnetic field measurements alone, except for a constant offset at TDC. This offset most probably is a result of unresolved local conductivity anomalies, resulting in so-called galvanic distortions. Geomagnetic field data are recorded at about a dozen of geomagnetic observatories on remote islands. In a follow-up project, these data can be used to obtain better knowledge of the oceanic upper mantle at minimum additional cost. Also, the induced field variations can be modelled and subtracted from the geomagnetic field record if the surrounding conductivity distribution and bathymetry is known. Therefore, this research allows to study ionospheric and magnetospheric sources using data from remote island observatories.
Publications
- (2017): Magnetotellurics with geomagnetic observatory data influenced by the ocean effect: upper mantle conductivity under the islands of Gan and Tristan da Cunha. AGU Fall Meeting 2017 (New Orleans, USA)
Morschhauser, A., Grayver, A., Kuvshinov, A., Samrock, F., Matzka, J.
- (2017): Magnetotellurics with magnetic observatory data affected by the ocean effect: Methodology and Results. Joint Assembly 2017 IAPSO-IAMAS-IAGA (Cape Town, South Africa)
Morschhauser, A., Grayver, A., Kuvshinov, A.
- (2018): Tippers at island geomagnetic observatories constrain electrical conductivity of oceanic lithosphere and upper mantle. 24th EM Induction Workshop (Helsingør, Denmark)
Morschhauser A., Grayver, A., Kuvshinov, A., Samrock, F., Matzka, J.
- Magnetotellurics with magnetic observatory data affected by the ocean effect: Methodology and Result. In: Becken, M., Hölz, S. (Eds.) (2018): Protokoll über das 27. Schmucker-Weidelt-Kolloquium für Elektromagnetische Tiefenforschung: Breklum, 25.-29. September 2017, Potsdam : Deutsche Geophysikalische Gesellschaft e. V., ii-vi, p. 17-29
Morschhauser A., Grayver, A., Kuvshinov, A., Samrock, F., Matzka, J.
