The Earth’s magnetic field is a surprisingly rich source of information about the structure of the planet. Measurements of the field on the scale of a few metres can reveal buried objects such as landmines or archaeological fragments. Magnetic field maps on the kilometre scale can help locate geological formations that indicate the presence of oil or other minerals. And on the largest scale, the Earth’s field reveals details about the geodynamo that generates it.

But there is an intermediate scale over lengths of tens to hundreds of kilometres that is relatively poorly studied. In theory, this should reveal important details about the behaviour of the outer mantle, the solar-quiet dynamo in the ionosphere and ionic currents in salt water that could be used to measure ocean circulation, a major factor in models of climate change.

The only way to measure the field on this scale is to fly the apparatus sensitive enough to detect it on an aeroplane or satellite. But the huge expense of such missions largely explains the sparsity of data.

Today, James Higbie at Bucknell University in Pennsylvania and a few buddies, outline an entirely new way to measure the Earth’s magnetic field. This new technique has the potential to generate magnetic field maps at a fraction of the cost of conventional techniques.

Their idea is to exploit the naturally occuring layer of sodium atoms in the mesosphere, some 50 to 80 kilometres above the Earth’s surface. The plan is to make these atoms line up with the Earth’s magnetic field as they precess, something that can be done relatively easily by zapping them with a circularly polarised laser beam.

When the atoms and the field are exactly aligned, a resonance effect kicks in which modifies the fluorescence from the sodium D1 and D2 spectral lines. That’s something that can be easily picked up on the ground with a camera.

This gives a measurement of the strength of the field at that point. So making a map is just a question of measuring the field strength at many points.

Higbie and co are currently building a 20 Watt laser test the idea. Once the technique is perfected, it should be a straightforward job to make measurements all over the planet.

That’s because the infrastructure to do the job is largely in place. Many telescopes with adaptive optics already use lasers to make sodium atoms in the mesosphere fluoresce. They then use these atoms as “guide stars” to calibrate their scope’s adaptive optics.

The data this produces should provide a new way of studying the structure of the Earth at a scale that has been largely ignored until now. A good time to be a geophysicist.

Ref: arxiv.org/abs/0912.4310: Magnetometry with Mesospheric Sodium

Note: I’m taking a break over the holiday period, back on 4 January. In the meantime, the blog will be showing the highlights from what has been an interesting year for the Physics arXiv Blog.
All the best,
Kentucky