Oceanic Magnetic Fields

Robert Tyler

-Planetary Geodynamics Branch, NASA Goddard Space Flight Center -Astronomy Department,University of Maryland College Park

Academia SinicaTapei, 2012

• Emphasis on connection with ocean/climate studies and the potential for magnetic remote sensing of large-scale ocean heat transport and variability

• Brief review of “modern” studies + work in progress

If not for confounding “noise,” magnetic remote sensing should be the method of choice for remote sensing large-scale ocean flow

•The ocean flow generated magnetic fields pass unharmed through ice •These magnetic fields represent natural integrals of flow (i.e. flow transport) we “see” all the way to the sea floor •The flow involved is in fact weighted by electrical conductivity, which is a function of temperature The transport involved therefore represents heat transport, a primary climate variable (about half of the poleward heat transport is carried in the ocean)•Studies indicate that the task of inversion (i.e. obtaining unique flow transports from the magnetic signals) is easy

• The dynamic ocean is observationally under sampled (hopelessly so) by in situ methods

• The ocean is relatively opaque to most remote sensing methods (compared to the atmosphere, say) we usually “see” only the surface

• Sometimes the surface is covered with ice

But there is “noise”

• The ocean flow magnetic signals are weak (though above detection and a substantial part of unmodeled residuals)

• There is substantial overlap of space/time scales with other signals

Review: There are various electrodynamic processes in ocean

MHD in thin ocean is a different process (and involves different scaling) than the process described in the usual description of induction.

MHD in thin ocean is a different process (and involves different scaling) than the process described in the usual description of induction.

Unlike typical cases of MHD, direct path of EM propagation is relatively unimportant

The Lorentz forces on ocean can usually be neglected (except on millennial climate scales and/or for past stronger geomagnetic fields)

2) Magnetic field signature of Earth’s ocean tides (Tyler, et al., Science, 2003)

Eddy electric currents in ocean

Magnetic field observed by Galileo indicates presence of conducting ocean

1) Magnetic induction response of Europa (Khurana, et al., Nature, 1998)

Magnetic field observed by CHAMP satellite describes global ocean tides

observed:

predicted:

observed:

predicted:

Review: Examples of induction vs. motional induction

F

Jupiter’s background field

But there is “noise”

• The ocean flow magnetic signals are weak (though above detection and a substantial part of unmodeled residuals)

• There is substantial overlap of space/time scales with other signals

Let’s return to…

Magnetic field of ocean circulation: toroidal component (within ocean)

Magnetic field of ocean circulation: poloidal component (satellite altitude)

Magnetic detection of tsunami flow:

Tyler, 2005: predicted that magnetic signals of tsunamis are appreciable and easy to relate to tsunami flow. Case of Sumatra

Manoj et al. 2011: Magnetic signal of Chilean tsunami clearly seen at Easter Island

Toh, et al. 2011

Utada et al., 2011

Hamano et al., 2011a, 2011b

Larsen, 1970’s

Klausner et al., 2012: newest, most sophisticated treatment using wavelet decomposition shows onset and properties of tsunami

Vivier et al, 2004: Observations of the oceanic magnetic field at satellite altitude would be easy to invert to monitor the Antarctic Circumpolar Transport

A crude but very simple inversion of M2 tide is obtained from magnetic signal at satellite altitude:

Real Imaginary

Tide obtained from inversion of magnetic data (the crude approximation used in this simple inversion is not valid near mag. equator nor coastline).

Tide which is correct

But there is “noise”

• The ocean flow magnetic signals are weak (though above detection and a substantial part of unmodeled residuals)

• There is substantial overlap of space/time scales with other signals

Many of the oceanic magnetic fields have space and time scales overlapping with other geomagnetic sources

Need to open a new dimension of classification in order to separate the oceanic components

As examples, tides and tsunamis have been successfully separated because of their extreme positions along the dimension of statistical stationarity

If not for confounding “noise,” magnetic remote sensing should be the method of choice for remote sensing large-scale ocean flow

•The ocean flow generated magnetic fields pass unharmed through ice •These magnetic fields represent natural integrals of flow (i.e. flow transport) we “see” all the way to the sea floor •The flow involved is in fact weighted by electrical conductivity, which is a function of temperature The transport involved therefore represents heat transport, a primary climate variable (about half of the poleward heat transport is carried in the ocean)•Studies indicate that the task of inversion (i.e. obtaining the flow transports from the magnetic signals) is easy

• The dynamic ocean is observationally under sampled (hopelessly so) by in situ methods• The ocean is relatively opaque (compared to the atmosphere, say) to most remote sensing methods we

usually “see” only the surface• Sometimes the surface is covered with ice

Let’s return to the opening statement:

Conclusion: Magnetic remote sensing of ocean flow is highly motivated, and the unique inversion of magnetic signals seems easy; the challenge is to devise sophisticated approaches for extracting/separating the oceanic components

Work in progress at GSFC to address this:

•Development of theoretical relationships and objective analyses of observations to illuminate the relationships and correlation structure between oceanographic and geomagnetic variables

•Development of similar relationships describing the expected correlation between the oceanic signals as seen in geomagnetic vs. other remote observations

•Assessment of the level and uniqueness of oceanographic information content in these magnetic signals

•Use of these relationships to devise long-term strategies for optimally extracting ocean/climate information from geomagnetic surveys

• Primary Focus is on extracting the oceanic information which is of most importance to the ocean/climate community (e.g. ocean heat transport, heat content, tidal mixing)

• Secondary focus is on reducing the unresolved “residuals” in Comprehensive Geomagnetic modeling

• As a specific example, a major new innovation occurring in the development of CM5 is the introduction of Q matrices describing induced electric currents in the realistic ocean. As a logical extension, it makes sense to extend this by including the motionally induced electric currents due to ocean flow as well