It鈥檚 an invisible wall. For 50 years, physicists have puzzled over why some radar signals sent skywards bounce back 150 kilometres above the ground. It now seems it is down to vibrating electrons.
The bounceback was first seen in 1962, shortly after atmospheric scientists installed a large radar system outside Lima in Peru.
The echo appears daily in a well-defined pattern, turning up high in the sky at dawn and descending by about 20 kilometres during the morning. At noon, the phenomenon flips its course and ascends throughout the evening until the sun sets and it disappears entirely (see picture below).
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More recent research shows that it grows stronger during solar flares and fades during solar eclipses.
鈥淵ou get them every day. We have no idea what they鈥檙e there for. That鈥檚 unusual,鈥� says an atmospheric scientist at Cornell University, who was not involved in this research. 鈥淚f you make pictures of the echoes they鈥檙e incredibly detailed.鈥�
Frothy waves
To investigate these echoes, and of Boston University made a computer model of the upper atmosphere that used a thousand processors simultaneously and took the better part of a year to run.
The model showed how ultraviolet light from the sun strips electrons from oxygen and nitrogen molecules in the upper atmosphere. The loose electrons vibrate, generating waves that ripple through the electrons and the oxygen and nitrogen ions.
鈥淭hey鈥檙e not ferociously strong waves,鈥� says Oppenheim. 鈥淭hey are more like the froth on the top of water when a heavy wind blows over it.鈥� But they are strong enough to reflect radar beams. Given the changes seen during solar flares and eclipses, it seems obvious in hindsight that the waves are solar-driven, he says.
The results shed light on a part of the atmosphere that is difficult to monitor: above where planes and balloons can fly, but below where satellites can orbit.
Hysell is already thinking about experiments to test Oppenheim and Dimant鈥檚 simulation. For such a hard-to-reach region, the only option he sees is to launch a rocket 鈥� an experiment that is expensive and fun, he says. 鈥淚t鈥檚 really an adrenaline rush that you don鈥檛 get by any other kinds of experimentation in this field.鈥�
Journal reference: Geophysical Research Letters,
