They appear at dawn at an altitude of about 150 kilometers: odd radar blips that descend from the sky over the course of the morning, growing stronger as they drop 20 km as the day progresses. Then they rise back to 150 km during the afternoon, before disappearing at sunset. These radar echoes have puzzled researchers for decades. And now a new study may finally have an explanation for what causes them.
At first glance on a radar screen, the echoes don't look much different than the zones of light and heavy precipitation that show up on weather radars, which operate at much shorter wavelengths. And although mysterious, the high-altitude radar echoes are somewhat predictable, says Meers Oppenheim, a space physicist at Boston University (BU). First, there's their daily routine of falling and rising. Then there's the fact that they weaken during a solar eclipse and strengthen during a solar flare. All this points to the sun as a possible trigger, but exactly how isn't clear, Oppenheim says. Adding to the mystery, rockets and satellites probing the upper atmosphere don't see the echoes.
Now, Oppenheim and colleague Yakov Dimant, also a space physicist at BU, have come up with a possible explanation. The phantom echoes begin, the researchers say, when extremely energetic ultraviolet (UV) radiation from the sun strikes gas molecules in a relatively narrow layer of the upper atmosphere. Oppenheim describes this layer, which lies at an altitude of about 150 km, as "a sweet spot of absorption." At higher altitudes, the air is so thin that the radiation doesn't interact strongly with gas molecules there; at lower altitudes, the radiation is largely blocked by overlying atmosphere. Within the layer, however, the UV radiation knocks electrons off the molecules, rendering the cold and remarkably sparse atmosphere a thin soup of charged particles, Oppenheim explains. (This layer of charged particles is somewhat akin to the atmosphere's ozone layer in that it blocks certain harmful wavelengths of radiation.)
Then, interactions among the free-ranging photoelectrons, the much heavier molecules from which they came, and the lines in Earth's magnetic field set up variations in atmospheric density that are something like sound waves, with denser-than-average concentrations of the heavier particles in some regions of space and low-density zones in others. That's what comes through as echoes on radar, the researchers report online in the latest issue of Geophysical Research Letters.
The team confirmed its idea using supercomputer simulations of the motions of billions of individual particles within a 10-meter-on-a-side slice of atmosphere. Results suggest that the free-ranging electrons start out at supersonic speeds but soon slow down and lose energy as they collide with heftier gas molecules.
So why did no one figure this out before? The wide variety of molecules in the radiation-fried atmosphere, as well as their varying lifetimes (some ephemeral, some long-lived), make computer simulations very difficult, Oppenheim says. "If it were simple, it would have been figured out in the 1960s."
The team's notion that high-energy photoelectrons play a major role in creating the mysterious echoes "is a very promising beginning" and "a step in the right direction," says Erhan Kudeki, a space physicist at the University of Illinois, Urbana-Champaign, who was not involved with the work. Plus, he adds, the sun's purported role in the process helps explain how the echoes turn on and off very quickly at dawn and dusk. Further analyses, including higher resolution models of the processes, should help clarify what's going on in this enigmatic slice of Earth's atmosphere, Kudeki says.
In the meantime, the daily up-and-down migrations of the radar echoes may help scientists track other motions such as atmospheric tides, the changes in air pressure influenced by the varying positions of the sun and moon. Most instruments can't detect such tides, but the radar echoes may serve as markers that make their motions apparent. Because other studies have linked those tides to variations in rainfall and temperature in the atmosphere near ground level, researchers could one day use the movements of the echoes to help fine-tune weather forecasts over broad areas.
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