First Measurement of Magnetic Field In Earth's Core

An anonymous reader writes "A University of California, Berkeley, geophysicist has made the first-ever measurement of the strength of the magnetic field inside Earth's core, 1,800 miles underground. The magnetic field strength is 25 Gauss, or 50 times stronger than the magnetic field at the surface that makes compass needles align north-south. Though this number is in the middle of the range geophysicists predict, it puts constraints on the identity of the heat sources in the core that keep the internal dynamo running to maintain this magnetic field."

Comments

Neato

[wizardforce]

He used the precession effect on Earth's core caused by the moon to calculate how much the magnetic induction deviated the calculated value of precession from the measured value. Basically, the field imparts a force that counteracts the precession of the inner core that is measurable. It's pretty clever how he was able to calculate the strength of the magnetic field the way he did:

He realized, however, that the tug of the moon on the tilt of the earth's spin axis could provide information about the magnetic field inside. This tug would make the inner core precess – that is, make the spin axis slowly rotate in the opposite direction – which would produce magnetic changes in the outer core that damp the precession. Radio observations of distant quasars – extremely bright, active galaxies – provide very precise measurements of the changes in the earth's rotation axis needed to calculate this damping.

"The moon is continually forcing the rotation axis of the core to precess, and we're looking at the response of the fluid outer core to the precession of the inner core," he said.

By calculating the effect of the moon on the spinning inner core, Buffett discovered that the precession makes the slightly out-of-round inner core generate shear waves in the liquid outer core. These waves of molten iron and nickel move within a tight cone only 30 to 40 meters thick, interacting with the magnetic field to produce an electric current that heats the liquid. This serves to damp the precession of the rotation axis. The damping causes the precession to lag behind the moon as it orbits the earth. A measurement of the lag allowed Buffett to calculate the magnitude of the damping and thus of the magnetic field inside the outer core.