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Atmospheric physics

Seeing the light

Credit: E. KENNEDY/NAVAL RESEARCH LAB.

An aurora is not just a visual treat for Earth's latitudinally advantaged residents. These events also provide natural laboratories for physicists investigating the complex interplay of electromagnetic waves and ionized particles in plasmas. And we are not merely passive observers of such phenomena — we have the capability of manipulating these processes from the ground, as shown to striking effect by Todd R. Pedersen and Elizabeth A. Gerken elsewhere in this issue ( Nature 433, 498–500; 2005 ).

The playground for these luminous processes is the ionosphere — the ionized upper reach of the atmosphere that stretches from a height of around 100 km to the base of the magnetosphere far above. In Earth's polar regions, the geometry of the geomagnetic field is such that electrons and ions can occasionally be driven down from on high: an aurora (pictured) is the visual manifestation of the collision of these energetic particles with gases in the upper atmosphere and ionosphere.

With this basic understanding of the mechanism in place, researchers have shown previously that it is possible to induce such optical processes artificially. By pumping high-power radio waves into the ionosphere (the frequency of the waves being tuned to the local plasma environment), electrons can be locally energized to collide with atmospheric gases in a manner analogous to the natural auroral process. But the resulting optical effects are small, with emission intensities falling well below the detection limit of the human eye.

Now Pedersen and Gerken have shown that, if ionospheric conditions are just right, much stronger emissions can be generated by this approach — so strong, in fact, that they are in principle visible to the naked eye. The ‘trick’ underlying this demonstration was to choose a time when a natural aurora was already active (previous efforts were directed at quiet regions of the ionosphere). And rather than targeting the main ionospheric layer, the researchers tuned the radio waves to excite a much lower layer, which had been transiently ionized by the inbound charged particles.

These observations raise many questions about the processes involved. For example, is an active aurora really a prerequisite for generating such bright emissions, or is it instead largely a picturesque bystander? Should the answer turn out to be the latter, we are left with the tantalizing (some would say disconcerting) possibility that such radio-fuelled emissions could form the basis of a technology for urban lighting, celestial advertising, and more...

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Ziemelis, K. Seeing the light. Nature 433, 471 (2005). https://doi.org/10.1038/433471b

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