Auroras are spectacular light shows rarely seen outside the polar regions. Users of the world's largest radio telescope in Arecibo, Puerto Rico, have created their own artificial aurora by stimulating the lower ionosphere with intense radio waves.
Auroras are spectacular displays of multicoloured light (right), rarely seen at low latitudes. But users of the world's largest radio telescope in Arecibo, Puerto Rico, can now create their own light shows. As L. M. Kagan et al. describe in Physical Review Letters ( 85, 218–221; 2000), shining powerful radio waves into the night sky produces a green fluorescent glow. This provides us with the first ever pictures of clouds of metal ions in the lower ionosphere — a phenomenon that strongly disrupts radio and satellite communications.
The ionosphere is an ion-rich region of the Earth's atmosphere that reflects radio waves over long distances. The density of ions increases with altitude in response to the ionizing effects of the Sun's radiation. A faint airglow — caused by emission of green and red light from ionized oxygen and nitrogen — can be seen at higher altitudes by sensitive detectors. But the lower ionosphere, such as the E layer, doesn't glow.
Metal ions in the E layer, torn from passing meteorites, can build up into patches of stronger ionization that interfere with satellite signals. Intense radio waves from Arecibo were used to excite these metal ions, which in turn heat up free electrons. These energetic electrons collide with oxygen ions making them glow, as in a natural aurora. Who needs northern lights when you can have aurora equatoralis?
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Tomlin, S. Polar lights in the Caribbean. Nature 406, 141 (2000). https://doi.org/10.1038/35018208
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DOI: https://doi.org/10.1038/35018208
