Toward remote sensing of ocean variability using ocean generated magnetic fields

The ocean is an electrically conducting fluid moving through the earth's background magnetic field. Through magnetohydrodynamic interaction between the flow and the magnetic field, secondary electric and magnetic fields are generated which reach far outside of the ocean. The secondary magnetic fields can be used in principle to remotely monitor ocean flow, temperature, and salinity variations. Here we review recent developments in assessing this potential. Theory and simulations show that the ocean generated magnetic fields at low satellite altitudes can be easily converted into information about large-scale flow transport variability which is very important in ocean and climate studies. The primary challenge is in extracting the relatively small (typically less than 10 nT) ocean signals from the magnetic field records which are also influenced by a variety of other sources. We show examples (both in principle and practice) where oceanic magnetic signals are extracted from the magnetic records by using statistical constraints imposed by the behavior of the flow sources. Finally, we discuss paths toward assessing the practical potential of this remote sensing method in light of present and up-coming magnetic surveys such as CHAMP, OERSTED, SAC-C, and SWARM.