Research ArticleGEOPHYSICS

Satellite tidal magnetic signals constrain oceanic lithosphere-asthenosphere boundary

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Science Advances  30 Sep 2016:
Vol. 2, no. 9, e1600798
DOI: 10.1126/sciadv.1600798
  • Fig. 1 Input used to derive the extraneous electric current due to tidal flow (compare Eq. 2).

    (A to C) Eastward, southward, and upward (radial) components of Earth’s main (core) magnetic field for epoch 2010 (23). (D) Depth-averaged ocean water conductivity calculated for the year 2009 using global data of ocean salinity, temperature, and pressure. (E and F) In-phase and quadrature parts of the depth-integrated horizontal water velocity of M2 tide (7). Note the scale in the right bottom corner.

  • Fig. 2 Amplitudes of the radial magnetic field components due to the M2 tide at an altitude of 430 km.

    (A) Extracted from satellite data. (B) Calculated on the basis of the recovered conductivity model (see structurally sparse model in Fig. 4).

  • Fig. 3 Difference between observed and predicted in-phase (left) and quadrature (right) radial magnetic field components due to the tidal flow at the satellite altitude (430 km).

    (A and B) For an insulating mantle. (C and D) For a homogeneous mantle of 0.2 S/m conductivity (used as an initial guess for the inversion). (E and F) For the structurally sparse model shown in Fig. 4.

  • Fig. 4 The recovered global radial electrical conductivity models compared with mineral physics constraints and local models derived from seafloor MT data.

    (A) Black solid and dashed lines represent the most probable models obtained by using structurally sparse (that is, permit conductivity jumps but allow as few features as possible) and smooth constraints in the inversion algorithm, respectively (35). The gray lines denote the 1000 models for which misfit differs no more than 10% from the most probable solutions. Conductivities of the dry and water-saturated olivine (1) are shown with red and orange lines, respectively. (B) The blue line represents the conductivity model from the Philippine Sea plate [<60 million years old (Ma)] (4), whereas the red curve represents that from the East Pacific region (70 Ma) (22) and the yellow line shows that from the West Pacific region (125 to 150 Ma) (4). The horizontal dash-dotted line marks the average oceanic LAB depth estimated from seismic data (21).

  • Fig. 5 Testing the sensitivity of the modeled responses to uncertainty in the extraneous current Embedded Image term.

    The amplitude difference of the radial magnetic field component between models using extraneous current constructed with (A) TPXO8-atlas and HAMTIDE tidal models, (B) a constant ocean electrical conductivity of 3.2 S/m and LVOC shown in Fig. 1D, (C) seasonally averaged conductivity (Fig. 1D) and Northern Hemisphere winter ocean conductivity, and (D) core field and core plus lithosphere fields. All fields are calculated at an altitude of 430 km. The recovered structurally sparse model (Fig. 4) was used as a mantle conductivity profile for these tests. Note the different scales.

  • Fig. 6 Workflow diagram showing principal steps and dependencies used to derive electrical conductivity models from satellite-detected tidal magnetic signals.

    φ, θ, and r are spherical coordinates. All quantities, except scalars p, β, and σ1···n, are defined on a global grid and depend on φ and θ. Note that integrals ∫…dr imply integration along water column, that is ∫aah(φ,θ)…dr.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/2/9/e1600798/DC1

    movie S1. Animation of the observed and predicted satellite magnetic tidal signals for Earth’s principal lunar semidiurnal tide.

    data file S1. A 1° × 1° map of depth-averaged ocean electric conductivity produced following the method of Fofonoff (25). The salinity and temperature data used are from National Oceanic and Atmospheric Administration’s 2009 World Ocean Atlas annual climatology, whereas local pressure was estimated using the method of Saunders (37).

  • Supplementary Materials

    This PDF file includes:

    • Legend for movie S1
    • Legend for data file S1

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    Other Supplementary Material for this manuscript includes the following:

    • movie S1 (.mp4 format). Animation of the observed and predicted satellite magnetic tidal signals for Earth?s principal lunar semidiurnal tide.
    • data file S1 (.txt format). A 1? ? 1? map of depth-averaged ocean electric conductivity produced following the method of Fofonoff (25). The salinity and temperature data used are from National Oceanic and Atmospheric Administration?s 2009 World Ocean Atlas annual climatology, whereas local pressure was estimated using the method of Saunders (37).

    Files in this Data Supplement:

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