Soaking in atmospheric electricity

June 15, 1999

While experts advise you to stay indoors to avoid lightning, few will tell you that you can't escape it altogether. In fact you're soaking in the return path for all the thunderstorms taking place across the world. Fortunately, the voltage is modest and the current is almost nothing, so the effect is almost impossible to measure.

"Fair weather electricity deals with the electric field and the electrical current in the atmosphere, and the conductivity of the air," explained Dr. Lothar Ruhnke of Airborne Research Associates in Weston, Mass. He recently retired from the Naval Research Laboratory but continues "doing research for fun."

The discovery of the fair weather circuit followed Ben Franklin's demonstration that lightning is caused by electricity. (Would-be experimenters take heed: Old Ben was exceptionally lucky. Others replicating his experiment have been killed, so don't try it.) Later experimenters showed that clear, calm air carries an electrical current which, it turns out is the return path for the electrical display we know as lightning.

Its importance is noted by the title of the International Conference on Atmospheric Electricity. Much of it has focused on lightning and its effects. Thursday morning's session, co-chaired by Ruhnke, however, dealt with fair weather electricity.

Atmospheric electricity is like a massive photographic flash. An electrical charge is built up, a switch is closed, and electrons barge across a gas, ionizing it and producing light. But a flash is a complete circuit. In the case of the Earth, Ruhnke explained, the atmosphere completes the circuit.

Thunderstorm charge generation happens inside clouds. Current flows out of the tops of clouds - blue jets and red sprites may play a role - and connects with the upper atmosphere and the ionosphere. Ultimately, the current returns to Earth through the clear atmosphere. Because it's diffused over most of the globe, it's also quite weak at any given point.

"All three values are very difficult to measure," Ruhnke said. The current is 10-12 amps per square meter - "almost nothing." The field is about 100 volts per meter, meaning that the electric potential increases by about 200 volts from the ground to the top of Michael Jordan's head when he's standing still. Finally, air is an excellent insulator, so its conductivity is close to zero.

"Because of all this, you don't feel anything," Ruhnke said. Even though you're standing in an electric field, your hair does not stand on end. (If you were outdoors, and it did, it would mean you're about to be hit by lightning, so tuck into a tight crouch right away.)

"When you measure the fair weather electric field, you're measuring the effects of all the thunderstorms on the Earth," he said.

But it's not evenly felt. Ruhnke said that scientists once thought the effect was evenly spread across the planet, so a measurement in Tokyo was as good as one in Kansas. It turns out that local turbulence, winds, and other fluctuations also cause small variations in the fair weather electric field.

"It's a great challenge to sort out local variations and sources from thunderstorms," Ruhnke continued. "If you could separate these effects, you could monitor the total thunderstorm activity locally."

Such a measure is important to various environmental studies, including the production of nitrous oxides (NOx), so the relative natural and industrial contributions can be measured.

Another is global warming. Ruhnke noted that Ralph Markson, a colleague at Airborne Research, examined 45 years of fair weather data taken by balloons from the surface of the Earth to the stratosphere. One of the factors that affects the existence of thunderstorms in the atmosphere is temperature.

"If there is any global warming, you should see an increase in thunderstorms and the electric field," Ruhnke said. "Markson sees no changes in the ionospheric potential." Because of variations in instruments, the growth of cities, and other effects, Ruhnke said that direct measures of global warming are quite difficult.

"People are looking for indirect methods," he continued "The fair weather field is one."

The fair weather field also becomes a sensor for air pollution, he noted, because aerosols - droplets and dust particles - attract and effectively neutralize ions.

He has recorded the difference in the air of Greenland and Antarctica where, 30 years ago, it was "nearly perfect" as shown by its ion content. "Now the air is fairly polluted around the globe."

In 1996, the first-ever coordinated simultaneous measurements were made for two days over Darwin, Australia, and Weston, Mass., - opposite sides of the world - to demonstrate that a single reliable measurement can be globally representative.

Ruhnke noted that fair weather conditions also are affected by the magnetosphere in the polar regions where the Earth's magnetic field leaves the upper atmosphere exposed to space. While many people have tried to link solar activity to terrestrial weather, he noted that no conclusive link has been found.

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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