Double Diffusion in the Core
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Transcript of Double Diffusion in the Core
Double Diffusion in
the Core
Double Diffusion in
the Core
David GubbinsSEE, University of
Leeds
CIDER Community Workshop Marconi Center 2009
David GubbinsSEE, University of
Leeds
CIDER Community Workshop Marconi Center 2009
Double Diffusion in the Oceans
Double Diffusion in the Oceans
In the oceans, solar heating warms the top producing hot (light), salty (heavy) water
Heat diffuses faster than salt This leads to an instability, loss of heat increases the density and carries salty water downwards, producing an instability (salt fountain)
This process can produce “fingering” (salt fingers)
…and establish layering, stratified layers separated by thin zones of turbulent mixing
In the oceans, solar heating warms the top producing hot (light), salty (heavy) water
Heat diffuses faster than salt This leads to an instability, loss of heat increases the density and carries salty water downwards, producing an instability (salt fountain)
This process can produce “fingering” (salt fingers)
…and establish layering, stratified layers separated by thin zones of turbulent mixing
Double Diffusion in the Core
Double Diffusion in the Core
If both heat and composition are unstable, they combine
If a layer is compositionally stratified (e.g. at top or bottom of the core) we have the opposite situation to the oceans
The top is cold (heavy) and high in light elements (light)
This produces an overstability This can also produce a layered structure
The time scale is about 100 Myr
If both heat and composition are unstable, they combine
If a layer is compositionally stratified (e.g. at top or bottom of the core) we have the opposite situation to the oceans
The top is cold (heavy) and high in light elements (light)
This produces an overstability This can also produce a layered structure
The time scale is about 100 Myr
Possible Stratified Layers
Possible Stratified Layers
1991: Souriau & Poupinet find decrease in Vp of 0.1% in bottom 150 km of inner core, deduce density stratification
1992, 1995: Song & Helmberger find similar anomalous structure from amplitudes and waveforms -> PREM2
1995: Similar structure incorporated into model AK135…
…confirmed by others, eg Zou, Koper & Cormier (2008)
At the top: slow diffusion of light elements from the mantle may produce ~100km light layer (Asahara et al, 2007; Hernlund)
1991: Souriau & Poupinet find decrease in Vp of 0.1% in bottom 150 km of inner core, deduce density stratification
1992, 1995: Song & Helmberger find similar anomalous structure from amplitudes and waveforms -> PREM2
1995: Similar structure incorporated into model AK135…
…confirmed by others, eg Zou, Koper & Cormier (2008)
At the top: slow diffusion of light elements from the mantle may produce ~100km light layer (Asahara et al, 2007; Hernlund)
In the oceans, evaporation at the surface leads to hot salty water above cold fresh water, leading to a “perpetual salt fountain”. In the core the solute is light and the gradients are opposite, as on the right, leading to overstable oscillations.
DOUBLY-DIFFUSIVE CONVECTION
Parameters for the Core
Parameters for the Core
Two Rayleigh numbers: Ra, Rs Ra=gTTd3/Ratio Ra/Rs = TT/cxFor compositional stratification x = 0.02
For a layer on the liquidus T = 500 KThen Ra/Rs = 0.02
Two Rayleigh numbers: Ra, Rs Ra=gTTd3/Ratio Ra/Rs = TT/cxFor compositional stratification x = 0.02
For a layer on the liquidus T = 500 KThen Ra/Rs = 0.02
In the oceans double diffusion leads to a stack of stably stratified layers separated by thin, turbulent zones
After J. S. Turner, “Buoyancy in Fluids”, 1979
CONCLUSIONS: DYNAMICAL STABILITY
CONCLUSIONS: DYNAMICAL STABILITY
A partially stratified layer may be subject to doubly-diffusive instabilities
This may lead to layering within the stable zone, as in oceanic double diffusion
Layering could explain the discrepancy between seismic travel times and amplitudes
Worth a look!
A partially stratified layer may be subject to doubly-diffusive instabilities
This may lead to layering within the stable zone, as in oceanic double diffusion
Layering could explain the discrepancy between seismic travel times and amplitudes
Worth a look!