Scientists may have finally found the cause of a mysterious disappearance of electrons dozens of miles above Earth.
It turns out that a layer of invisible meteor dust falling to Earth every day may be sucking up electrons coming from higher in the atmosphere, creating the so-called "D-region ledge," where the concentration of electrons suddenly plunges, Earle Williams, an atmospheric electrician at the Massachusetts Institute of Technology, said earlier this month at the annual meeting of the American Geophysical Union.
Physicists have long been hunting for the disappearing electrons, and had turned to everything from high-flying ice clouds to electrically charged water clusters in the atmosphere to explain the sudden drop-off in this region, he said. [Infographic: Earth's Atmosphere Top to Bottom]
"It's the most dramatic gradient anywhere in the ionosphere," Williams said, referring to the part of Earth's upper atmosphere where the D-region ledge is found. "It really is very conspicuous, so it's begging for an explanation."
The case of the disappearing electrons
Far above Earth's surface, ultraviolet rays from the sun interact with nitric oxide in the atmosphere to produce electrons that travel toward Earth. But since the 1960s, scientists have known that there is an sharp drop in the number of electrons present in the atmosphere at night. This drop was found when rockets first breached the upper atmosphere to sample its temperature, pressure and electron density. This "electron ledge" occurs within the ionosphere's D-layer, which stretches between 37 miles and 56 miles (60 km and 90 km) above the Earth's surface.
The D-region ledge plays a critical role in modern communication. The planet itself conducts electricity, as does the layer of the ionosphere above the ledge, but electromagnetic waves don't travel through nonconductive materials, like the electron-depleted region below the ledge. Below the ledge, the electron-depleted air acts as an insulator, forming a layer between the Earth and its atmosphere that enables radio waves and very low frequency electromagnetic waves to circle the globe. [Quiz: The Science of Electricity]
The D-region ledge appears most strongly at night and shows up equally in the atmosphere above the poles, the equator and everywhere in between. Yet no one knew why it was there.
Exhausting the possibilities
In the new study, Williams and his colleague, Joanne Wu, a doctoral candidate at the National Cheng Kung University in Taiwan, say that they and other colleagues had looked at many of the prevailing hypotheses to explain the ledge and found most lacking. For instance, some researchers had proposed that ice clouds could be absorbing the free electrons. But ice clouds tend to clump closer to Earth's high latitudes, whereas the D-region ledge is equally prominent everywhere from the poles to the equator.
Then they came upon a 1980 paper in the Journal of the Atmospheric Sciences, which suggested that another layer of the atmosphere, called the sodium layer, could be attributed to fine dust from meteoroids. In that explanation, as space rocks travel through the thin upper atmosphere, they jostle nitrogen and oxygen molecules, heating up in the process. As they fall farther, they collide with more atoms in the more densely packed atmosphere, becoming hot enough to boil, at which point individual sodium atoms peel off from the meteoroid.
That made the team wonder: Could the heating of tiny meteors also explain the D-region ledge?
In this new theory, minerals such as iron and silicon, which make up a much greater part of meteoroids than sodium, would also boil off the meteoroid, forming a cloud of smoke and dust. Free-floating silicon and iron atoms would then smash into oxygen and nitrogen in the atmosphere, knocking free electrons in the iron and silicon atoms' outer electron shells. The electrons from the meteor boiling would then transform into faint glimmers of light too small to see with the naked eye.
Meanwhile, the meteor dust itself would bind to the free electrons that were formed when the sun's ultraviolent rays interacted with the atmosphere. The reason the D-region ledge is so prominent at night is because ultraviolet radiation from the sun during the day is 100 times greater than at night, so the production of free electrons dwarfs the ledge effect during the day.
If the theory is right, then "you'll form a thick zone of dust descending very slowly due to gravity," Williams said. "Eventually all this dust comes to the Earth's surface. It's about 100 tons per day worldwide," but we can't easily detect it because the tiny particles are so small.
Lots of small rocks
But why does this ledge occur 53 miles (85 km) above Earth? In the researchers' theory, the meteoroids at this height are mostly the right size and are traveling at the right speeds to burn up at that height in the atmosphere. The meteors that could explain the ledge would need to be small: roughly about 10 micrograms. And they would need to be "slow," traveling about 29,000 to 33,500 miles per hour (13 to 15 km/s) — just above the escape velocity of Earth, Williams said.
There may be plenty of these small, slow meteors. Both radars, which show little pings of electricity when teensy meteoroids burn up in the atmosphere, as well as a few satellites, which have been pockmarked by the barrage of myriad tiny meteoroids, suggested that the vast majority of space rocks that bombard the Earth every day are these small, unimpressive specimens. Though they don't leave a stunning light trail like so-called shooting stars, small meteors could be a thousandfold more numerous than the cosmic debris that lights up the night sky, Williams said.
"It's a very plausible idea," said Morris Cohen, an ionospheric physicist at the Georgia Institute of Technology in Atlanta, who was not involved in the study. "There's a lot of circumstantial theory to back up to the idea, it's all consistent."
However, it will be tough to test the idea directly, as that region of the atmosphere is so inaccessible, Cohen said.
"It's too high to reach with balloons, and it's too low to hit with satellites," Cohen told Live Science.
The D-region ledge is not the only strange boundary in that portion of the atmosphere — the region between 50 and 62 miles (80 and 100 km) above the surface also has an "airglow" layer, caused by the sharp rise in the ionization of hydroxyl, as well as multiple sodium layers, said Steven Cummer, an electrical engineer at Duke University in North Carolina, who was not involved in the study.
"This region is so hard to measure that little work has been done to see if these boundaries are connected," Cummer told Live Science in an email. "But the idea that a fundamental feature of Earth's atmosphere is created by the continual deposition of material by meteors is pretty exciting."