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. "This is the first really good number we've had based on observations, not inference," said author Bruce A. Buffett, professor of earth and planetary science at UC Berkeley. "The result is not controversial, but it does rule out a very weak magnetic field and argues against a very strong field." The results are published in the Dec. 16 issue of the journal Nature. A strong magnetic field inside the outer core means there is a lot of convection and thus a lot of heat being produced, which scientists would need to account for, Buffett said. The presumed sources of energy are the residual heat from 4 billion years ago when the planet was hot and molten, release of gravitational energy as heavy elements sink to the bottom of the liquid core, and radioactive decay of long-lived elements such as potassium, uranium and thorium. A weak field - 5 Gauss, for example - would imply that little heat is being supplied by radioactive decay, while a strong field, on the order of 100 Gauss, would imply a large contribution from radioactive decay. "A measurement of the magnetic field tells us what the energy requirements are and what the sources of heat are," Buffett said. About 60 percent of the power generated inside the earth likely comes from the exclusion of light elements from the solid inner core as it freezes and grows, he said. This constantly builds up crud in the outer core. The Earth's magnetic field is produced in the outer two-thirds of the planet's iron/nickel core. This outer core, about 1,400 miles thick, is liquid, while the inner core is a frozen iron and nickel wrecking ball with a radius of about 800 miles - roughly the size of the moon. The core is surrounded by a hot, gooey mantle and a rigid surface crust. SD