We're really starting to understand how the heart of our planet works

Dr John Tarduno

The discovery about the strength of the field has been made possible by a new research technique. It could help scientists better understand the ancient Earth and how its molten core behaved millions of years ago.

The field interacts with particles streaming from the Sun - it is this that gives us the Northern and Southern Lights.

The field also protects us from much of the Sun's harmful radiation. So, understanding how it works, and if it might temporarily vanish, could help scientists predict potentially harmful fluctuations.

North-South flips

It is known, for example, that the Earth's magnetic poles have flipped many times in the past. A compass used 100,000 years ago would have pointed South instead of North.

Squid allows researchers to get past the contamination problems

The evidence for these polarity changes is captured in tiny magnetic particles in new rock. As lava cools around the particles, they align themselves with the ambient magnetic field like a compass.

Great bands of rock displaying North-South flips are laid across the ocean floors, showing how new rock produced at volcanic ridges spreads out over time to make the sea bed. The ages of the rock bands date the great changes in the Earth's magnetic field.

"We know a lot about the directions of the Earth's magnetic field," said John Tarduno, professor of geophysics at the University of Rochester, US. "It is how we unravel plate tectonics and learn something about the Earth's core.

"But to understand the way the field works, you also need to know the field's strength in the past, and we don't know nearly enough about that."

New technique

The traditional way to measure the ancient Earth's magnetic field strength (called paleointensity) was developed more than four decades ago. It requires a piece of igneous rock to be heated and cooled in a chamber that is shielded from any outside magnetic field.

A feldspar crystal from India contains relic magnetism

In this way, the magnetism in the rock's particles can be "drained". Scientists can then increase the magnetic field and measure how much magnetism the particles in the rock can hold. However, because of contamination, scientists do not regard the technique as being particularly accurate.

To develop an improved method, scientists at the University of Rochester used a superconducting quantum interference device (Squid). This equipment is normally employed in computer chip design, and is extremely sensitive to the tiniest magnetic fields.

"With the Squid, we realised that we could start measuring the magnetic fields in single crystals instead of whole rocks," said Dr Tarduno. "That let us use samples we knew had no contamination."

Early tests have worked well on feldspar, the most common mineral on the Earth's surface. The mineral has a microscopic shell around its slivers of magnetic crystal that protect it from contamination.

Inside the core

"We can now measure paleointensity in places we could never measure anything before," said Dr Tarduno. "And the results are more reliable than ever before."

Measurements have been made on 100-million-year-old rock samples from India. Scientists are particularly interested in them because they come from a period when the Earth's magnetic field was not reversing.

It was found that during this time the Earth's magnetic field was three times stronger than the old method suggested.

Researchers say that besides possibly giving T. rex a better display of the Northern and Southern Lights, the field strength indicates what was happening inside the Earth's hot, molten core - the geodynamo that drives the magnetic field.

"Our findings suggest that there is a relationship between magnetic reversals and paleointensity," Dr Tarduno said. "Such a relationship fits very well with supercomputer models. It's an exciting time. We're really starting to understand how the heart of our planet works."

The research is published in the journal Science.