In a nice piece of cross-pollination between disciplines, scientists have proposed a new method for measuring the Earth’s magnetic field strength using technology developed for ground-based observational astronomy. As it turns out, the laser guide stars astronomers use to sense the turbulence high up in the atmosphere can be used as cheap and efficient magnetometers.

To correct for the blurring effect of atmospheric turbulence in real time, astronomers make use of adaptive optical systems in telescopes that continually sense the distortions from a reference star and apply the necessary correction over the course of the observation. A recent development in this technology is the use of artifically generated guide stars when no natural one is available near the target of an observation: a powerful laser is fired into the atmosphere at the exact wavelength that will cause sodium molecules in the mesosphere, around 90-100 km up in the atmosphere, to fluoresce, creating a bright reference spot.

The altitude of the sodium layer is well suited to studying the Earth’s magnetic field at length scales of several 10s to 100s of kilometres, which gives information on, for example, ionic currents from ocean circulation (important for climate change research) and the behaviour of the Earth’s upper mantle. Studying the field at these scales normally requires expensive air- or space-borne instruments, and the data are therefore sparse.

The idea behind this new measurement technique (and I’m desperately trying to remember my university lectures on magnetic fields here) is to fire a circularly polarised laser tuned to the sodium D lines frequency and letting the resulting atomic spin polarization precess naturally along the magnetic field. The laser signal is modulated near the Larmor frequency – the characteristic spin precession frequency along a magnetic field line, which is directly related to the strength of the field at that location. When the modulation and Larmor frequencies coincide, a resonance occurs in the sodium atoms’ fluorescence and that change can be easily spotted with a CCD.

While this setup can only measure the scalar strength of the magnetic field (i.e. no direction) at a specific location, the relative simplicity of the required equipment means that several measurement stations can be placed some 100s of km apart, giving a map of the field strength on those scales. The authors even suggest that a movable platform such as a ship or a truck can be used.

It’s nice to see astronomers (Domenico Bonaccini Calia is an astronomer and AO specialist at ESO) working with atomic physicists on this. Adaptive optics is one area of astronomy that has provided immense benefits to other disciplines, from vision science to agricultural monitoring, and now geomagnetism studies and possibly climate change. Adaptive optics for astronomy of course was only made possible by the declassification of military research in the 1980s, but astronomers have done an excellent job at spreading their knowledge far and wide.

J. M. Higbie, S. M. Rochester, B. Patton, R. Holzlöhner, D. Bonaccini Calia, & D. Budker (2009). Magnetometry with Mesospheric Sodium arXiv:0912.4310v1 [physics.atom-ph] arXiv: 0912.4310v1

Image: Higbie et al, as above