3. Gravity Tides

The spatial gradient of the tide-generating potential is known as the tide-generating force (TGF), and the negative of the radial tide-generating force is used to compute the gravity tide [26, 73]. Gravity tides, as measured by a (superconducting) gravimeter at Earth’s surface, are related to but distinct from the solid Earth tide displacements described in Solid Earth Tides. They can be computed as the sum of several effects [26, 47, 74]:

  1. Direct: the tide-generating force between the Earth and the planetary body. This is the entire signal for a rigid, ocean-free and non-deforming Earth.

  2. Displacement: height changes (radial displacements) to/from the Earth’s center of mass

  3. Deformation: local changes in the Earth’s density due to the tidally induced deformation

  4. Self-attraction and loading effects: redistribution of ocean mass, which generates a tidal potential and induces deformation of the Earth’s crust

  5. Non-Equilibrium effects: ocean’s non-instantaneous and non-uniform response to the tide-generating potential (estimated from ocean tide models)

  6. Centripetal effects: centripetal acceleration from the Earth’s variable rotation (modulated by polar motion and ocean loading)

The first three effects are often combined to calculate a major component of the gravity tide based on the tide-generating force and the Earth’s elastic response [26, 27]. As with solid Earth tides, the Love and Shida Numbers are frequency-dependent (particularly in the diurnal band due to Free Core Nutation resonance) [76, 79].