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UC Berkeley Press Release

Tropical thunderstorms affect space weather

– The persistant, day-after-day formation of clouds and thunderstorms over the tropics has a surprisingly big effect on the Earth's ionosphere some 200 miles above the cloud layer, according to new results from two of NASA's satellites published recently by physicists at the University of California, Berkeley.

Plasma bands encircling the earth
Two plasma bands encircle the Earth some 250 miles above the equator, seen here in a false-color composite image built up from 30 days of ultraviolet observations with NASA's IMAGE satellite. Bright, blue-white areas are where the plasma is densest. The continents are outlined by solid white lines, while the dotted lines mark regions where rising tides of hot air indirectly create the bright, dense zones in the bands. (UC Berkeley/NASA graphic)

Data from NASA's Imager for Magnetopause to Aurora Global Exploration (IMAGE) satellite show that two parallel bands of ionized particles that encircle the Earth in the tropics are altered by persistent storms over the Amazon Basin in South America, the Congo Basin in Africa, and Indonesia. The net effect of these repeated storms is to create a denser region of ionospheric plasma over these areas that glows more brightly in ultraviolet light than does the rest of the two plasma bands.

"This discovery will help improve forecasts of turbulence in the ionosphere, which can disrupt radio transmissions and the reception of signals from the Global Positioning System," said Thomas Immel, an assistant research physicist at UC Berkeley's Space Sciences Laboratory and lead author of a paper on the research published Aug. 11 in Geophysical Research Letters.

The ionosphere is formed by solar x-rays and ultraviolet light, which break apart atoms and molecules in the upper atmosphere, creating a layer of ionized, or electrically-charged, gas known as plasma. The densest part of the ionosphere forms two bands of plasma close to the equator at a height of almost 250 miles.

From March 20 to April 20, 2002, a period when the IMAGE satellite flew low over the tropics, on-board sensors recorded these bands, which glow in ultraviolet light. The pictures showed four pairs of bright bulges in the bands where the ionosphere was almost twice as dense as the average. Three of the bright pairs were located over tropical rainforests with lots of thunderstorm activity - the Amazon Basin in South America, the Congo Basin in Africa, and Indonesia. A fourth pair appeared over the Pacific Ocean.

The connection between thunderstorms and plasma bands in the ionosphere at first seemed unlikely, because the gas in the ionosphere is simply too thin for atmospheric tides to directly affect the much higher ionosphere. Thunderstorms develop in the lower atmosphere, or troposphere, which extends almost 10 miles above the equator. The gas in the plasma bands, 250 miles up, is about 10 billion times less dense than in the troposphere. The tide would have to collide with atoms in the atmosphere above to propagate upward, but the ionosphere where the plasma bands form is so thin, atoms rarely collide.

To get an idea of what might be happening, Immel and his UC Berkeley colleagues modeled the atmospheric tides using a computer simulation, called the Global Scale Wave Model, developed by the National Center for Atmospheric Research in Boulder, Colo.

The simulation showed that the tides could affect the plasma bands indirectly by modifying a layer of the atmosphere below the bands that shape them. Below the plasma bands, a layer of the ionosphere called the E-layer becomes partially electrified during the day. This region creates the plasma bands above it when high-altitude winds blow plasma in the E-layer across the Earth's magnetic field. Since plasma is electrically charged, its motion across the Earth's magnetic field acts like a generator, creating an electric field. This electric field shapes the plasma above into the two bands.

Anything that would change the motion of the E-layer plasma would also change the electric fields it generates, which would then reshape the plasma bands above.

The Global Scale Wave Model indicated the tides should dump their energy, released when water condenses into clouds, about 62 to 75 miles above the Earth in the E-layer. This is high enough to disrupt the plasma currents there, altering the electric fields and creating dense, bright zones in the plasma bands above.

"The single pair of bright zones over the Pacific Ocean that is not associated with strong thunderstorm activity shows the disruption is propagating around the Earth, making this the first global effect on space weather from surface weather that's been identified," said Immel. "We now know that accurate predictions of ionospheric disturbances have to incorporate this effect from tropical weather."

"This discovery has immediate implications for space weather, identifying four sectors on the Earth where space storms may produce greater ionospheric disturbances. North America is in one of these sectors, which may help explain why the U.S. suffers uniquely extreme ionospheric conditions during space weather events," he added.

Measurements made by NASA's Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite from March 20 to April 20, 2002, have confirmed that the dense zones exist in the plasma bands. Researchers now want to understand whether the effect changes with seasons or large events, like hurricanes.

The research was funded by NASA. The National Center for Atmospheric Research is sponsored by the National Science Foundation.