Ionospheric current source modeling and global geomagnetic induction using ground geomagnetic observatory data

Abstract

Long‐period global‐scale electromagnetic induction studies of deep Earth conductivity are based almost exclusively on magnetovariational methods and require accurate models of external source spatial structure. We describe approaches to inverting for both the external sources and three‐dimensional (3‐D) conductivity variations and apply these methods to long‐period (T≥1.2 days) geomagnetic observatory data. Our scheme involves three steps: (1) Observatory data from 60 years (only partly overlapping and with many large gaps) are reduced and merged into dominant spatial modes using a scheme based on frequency domain principal components. (2) Resulting modes are inverted for corresponding external source spatial structure, using a simplified conductivity model with radial variations overlain by a two‐dimensional thin sheet. The source inversion is regularized using a physically based source covariance, generated through superposition of correlated tilted zonal (quasi‐dipole) current loops, representing ionospheric source complexity smoothed by Earth rotation. Free parameters in the source covariance model are tuned by a leave‐one‐out cross‐validation scheme. (3) The estimated data modes are inverted for 3‐D Earth conductivity, assuming the source excitation estimated in step 2. Together, these developments constitute key components in a practical scheme for simultaneous inversion of the catalogue of historical and modern observatory data for external source spatial structure and 3‐D Earth conductivity.

Stochastic Inversion of Geomagnetic Observatory Data Including Rigorous Treatment of the Ocean Induction Effect With Implications for Transition Zone Water Content and Thermal Structure

“…The presence of a large conductivity anomaly underneath Northern Asia has also been reported by Kelbert et al (2009) and more recently confirmed by Sun et al (2015). The presence of a large conductivity anomaly underneath Northern Asia has also been reported by Kelbert et al (2009) and more recently confirmed by Sun et al (2015).…”

“…In this regard, geophysical techniques such as seismic, geodetic, gravimetric, and electromagnetic (EM) studies play prominent roles because of their ability to sense structure at depth. For instance, global-scale (Kelbert et al, 2009;Semenov & Kuvshinov, 2012;Sun et al, 2015;Tarits & Mandéa, 2010) and semiglobal-scale (e.g., Koyama et al, 2006Koyama et al, , 2014Shimizu et al, 2010) three-dimensional (3-D) EM inversions of long-period geomagnetic data reveal the presence of large-scale lateral heterogeneities in the mantle. Electrical conductivity is sensitive to temperature, chemical composition, oxygen fugacity, water content, and the presence of melt (e.g., Karato & Wang, 2013;Park & Ducea, 2003;Yoshino, 2010).…”

“…Hence, mapping electrical conductivity in turn enables constraining chemistry, mineralogy, and physical structure of the lithosphere and mantle (e.g., Deschamps & Khan, 2016;Dobson & Brodholt, 2000;Fullea et al, 2011;Khan et al, 2006;Pommier, 2014;Toffelmier & Tyburczy, 2007;Xu et al, 2000). For instance, global-scale (Kelbert et al, 2009;Semenov & Kuvshinov, 2012;Sun et al, 2015;Tarits & Mandéa, 2010) and semiglobal-scale (e.g., Koyama et al, 2006Koyama et al, , 2014Shimizu et al, 2010) three-dimensional (3-D) EM inversions of long-period geomagnetic data reveal the presence of large-scale lateral heterogeneities in the mantle.…”

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

“…For example, source biases due to Pc3-4 geomagnetic pulsations, indicative of bulk resistive Earth, are ubiquitous in MT transfer functions in the SEUS (Murphy & Egbert, 2018). Resistive upper mantle beneath the SEUS also appears in the global geomagnetic depth sounding (GDS) results of Sun et al (2015). Although lithospheric structure in those GDS results is poorly resolved due to the period range of the data set, this observation provides conditional but independent (as GDS relies on a different set of assumptions than MT) support for the presence of highly resistive lithosphere beneath the SEUS.…”

Rapid prediction of electric fields associated with geomagnetically induced currents in the presence of three‐dimensional ground structure: Projection of remote magnetic observatory data through magnetotelluric impedance tensors

“…While there are a limited number of EarthScope MT stations that were specifically designed to obtain MT impedance data to frequencies as low as 10 −5 Hz, it is technically very challenging to obtain stable electric field measurement at such low frequencies, so these were exceptional installations and do not provide a general solution to this issue. Another approach is to make use of deeper, global‐scale 3‐D mantle conductivity models that have been obtained through methods involving analyses of magnetic field data exclusively [ Kelbert et al ., ; Sun et al ., ]. While having effectively no spatial resolving power in the upper ~350 km of the crust and mantle, these models typically describe deep Earth conductivity variations for mantle depths of ~350 km–~1200 km in terms of an underlying radially symmetric (i.e., 1‐D model) compromising a series of radial shells of finite thickness, with 3‐D variations in conductivity represented as a set of spherical harmonic coefficients about the underlying baseline model.…”