New Clues to Fleeting Flashes High In the Atmosphere
Lightning Interacts with Space, Electrons Rain Down
By Robert Roy Britt
Senior Science Writer
posted: 02:49 pm ET
16 December 1999


Energetic byproducts of lightning known as whistler waves streak thousands of miles above Earth's atmosphere into the magnetosphere, where they engage in a near-space dalliance that could be called the electron shuffle.

The whistler waves interact with already gyrating electrons, then fling them off onto new paths. Some of the electrons rain back into the atmosphere a mere second later and a thousand miles away.

This energy exchange in the magnetosphere tempers the sun's periodic bursts of electrical energy, new details of which appear in this month's issue of Geophysical Research Letters. Understanding how the exchange occurs, and where the electrons go, could help researchers better gauge the dangers faced by satellites.

By setting up monitoring devices across the United States, Stanford University electrical engineering professor Umran Inan and his colleagues observed the phenomenon during a lightning storm over Austin, Texas in late 1998. Roughly one second after a single lightning flash, the researchers detected electrons raining into the atmosphere as far north as South Dakota. Similar conditions were observed in other storms.

"This may be important in understanding how the very large number of particles trapped in the radiation belts are kept in control," said Michael Johnson, a graduate student who worked on the project with Inan. "The sun periodically sends out a burst of (particles), and this is one possible way for them to not build up indefinitely."

A meeting in space

Johnson explained that energy from lightning moves in all directions. A small portion, travelling as whistler waves, heads into the magnetosphere, where invisible lines of radiation run from one of the planet's magnetic poles to the other.

Meanwhile, the sun spews a constant stream of energy our way. These charged particles, including electrons, are known as the solar wind. The electrons become trapped in the magnetosphere's lines of radiation, a nifty feature that helps protect the planet. There, they bounce back and forth between the north and south magnetic poles.

The energy of the whistler waves, Johnson says, is able to interact with these trapped electrons, an effect that may extend 15,000 miles or more above Earth.

"If the wave interacts with lots of electrons, some will change their orbit by enough so that the next time they hit the north or south magnetic pole area, they'll fall into the atmosphere," Johnson said. "There are around 100 lightning strikes on the Earth every second, so all these interactions may have a big impact on trapped electron populations."

The raining electrons are not like any conventional rain. They don't even make it to the ground. And their energy levels are so low that they cannot be seen with the eye.

"There are around 100 lightning strikes on the Earth every second, so all these interactions may have a big impact on trapped electron populations."

But around 80 kilometers up (50 miles) the electrons spread out in the increasingly dense atmosphere, producing low-level light, X-rays and enhanced areas of ionization, Johnson said.

The scientists measured the ionization by monitoring very low-frequency radio waves. The measurements confirm theories that had predicted the process, and also show that it may be more prevalent than thought, especially at night.

"It also must occur during the day, but the effect on the ionosphere is relatively small compared to solar ionization," said lead researcher Inan. "At these mid-latitudes, we now think the nighttime ionosphere may be dominated by these lightning effects."

Johnson said this newly studied form of space lightning is related to the aurora, bright and beautiful light patterns commonly known as the Northern or Southern Lights. But the two phenomena are distinct, he said. The aurora occurs only at the very far northern or southern latitudes and involves electrons at different altitudes with different energies.


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