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Inner Core and Its Libration Under Gravitational Equilibrium: Implications to Lower‐Mantle Density Anomaly.
Inner Core and Its Libration Under Gravitational Equilibrium: Implications to Lower‐Mantle Density Anomaly.
- Source :
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Journal of Geophysical Research. Solid Earth . Jan2021, Vol. 126 Issue 1, p1-12. 12p. - Publication Year :
- 2021
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Abstract
- The inner core's (IC) axial torsional libration (ATL) is a manifestation of the mantle‐inner core gravitational rotational normal mode. Its natural period is critically determined by the interior sectoral quadrupole q22(Vex) (of mass distribution of spherical‐harmonic degree 2 and order 2) belonging to the mantle plus the core‐mantle boundary. We derive the (multipole) formalism to calculate q22(Vex) based on two physical postulations: (i) Postulation I that equates the observed 6‐year variation in Earth's length‐of‐day to ATL as laid out in Chao (2017, https://doi.org/10.1002/2016JB013515); (ii) Postulation II requiring that the shape of IC, thanks to its finite viscosity, conforms to the gravitational equipotential surface in the long term. q22(Vex) being a complex value, its phase relation with ATL asserts that the major axis of the IC aligns with that of q22(Vex) subject to the lowest‐energy state. The magnitude of q22(Vex) thus evaluated by Earth parameters is |q22(Vex)|≈0.78–1.71kgm−3 (the two endmember values corresponding respectively to IC's nominal polar moment of inertia vs. the effective value under 100% tangent‐cylinder coupling). The corresponding equatorial elliptical shape of the IC under Postulation II has peak‐to‐peak topography of 75–164 m. The acquired q22(Vex) can serve as source information for inversion on, among others, the density anomaly of the large low shear velocity provinces constructs in the lower mantle. Key Points: Equatorial ellipticity of inner core (IC) under gravitational equilibrium is derived in terms of spherical‐harmonic density multipolesInterior sectoral quadrupole determined by equating IC's axial libration to 6‐year oscillation found in Earth's length‐of‐dayThe obtained quadrupole is useful for inferring large low shear velocity provinces‐like density anomaly in lower mantle + core‐mantle boundary [ABSTRACT FROM AUTHOR]
Details
- Language :
- English
- ISSN :
- 21699313
- Volume :
- 126
- Issue :
- 1
- Database :
- Academic Search Index
- Journal :
- Journal of Geophysical Research. Solid Earth
- Publication Type :
- Academic Journal
- Accession number :
- 148305714
- Full Text :
- https://doi.org/10.1029/2020JB020541