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Monitoring global lightning with Schumann resonances

Acknowledging that his work seems “a little oddball for CEE,” senior researcher Earle Williams is interested in global lightning activity as measured with Schumann resonances. “Schumann resonance is basically an electromagnetic wave, maintained by lightning in the three major zones of electrical activity: South America, Africa, and Southeast Asia. The wave resonates at eight Hz (cycles/sec) around the earth,” he notes. (In comparison, orchestras tune to A at 440 Hz.)

The major zones of lighting activity are roughly 90° apart on the globe. Williams and Bob Boldi (Lincoln Labs), Stan Heckman, Vadim Mushtak, and David Lowenfels monitor the lightning from a station in the woods of the Univ. of Rhode Island’s Alton Jones Campus. Sufficiently sensitive to pick up distant activity, the equipment gets saturated by thunderstorms even a few hundred kms away, says Williams. “The best place to listen to lightning around the planet would be at the north or south pole, but we have to make compromises.”

The most prominent piece of monitoring equipment is a ball atop a 10-m-high (39 ft) pole, used for measuring the electric field in the global electromagnetic resonance. Williams explains, “There is a 10th of a millivolt difference between the sphere and the ground, but it’s still a large signal compared to local noise sources. We built a wide band antenna designed to catch both the Schumann resonances in the range of 3 Hz to 50 Hz, and the higher frequency ‘sferics’ (electromagnetic radiation from severe weather formations) components of lightning. So far the ball has never taken a direct hit even in some violent storms, although the group has lost other equipment when lightning hit the ground and burned the cables which run 600 ft across the ground to a recording hut in the woods.

About 100 lighting flashes a second occur in ordinary storms around the planet, causing a quasi-continuous background signal. Accompanying the fundamental mode of the Schumann resonances at 8 cycles/sec. are five to six higher harmonics. Williams specifies, “There are many modes for a resonance, just as there are many modes [different harmonics] for a violin string. We can record these modes as background information, but it’s hard to interpret because all the major lightning regions are simultaneously active. From Rhode Island we can see lightning activity in Africa on the north/south coil of our magnetometer, and in South America on the east/west coil.”

Another aspect of Schumann resonances are transients, the extraordinarily large lightnings sometimes seen in the late stage of a thunderstorm that single-handedly excite the resonances. Often a thunderstorm begins with a vigorous lightning show, only to quiet down during the period of widespread stratiform rain. Occasionally a particularly strong lightning propagates with many spidery legs branching off over a very large horizontal distance. “Those lightnings by themselves ring the whole earth’s ionospheric cavity at 8 cycles/sec. On the oscilloscope you can see one lightning go off and then ring down for about 500 milliseconds. During that period, that one lightning dominates all the energy of all the lightning flashes on the planet, enabling us to locate it from Rhode Island,” says Williams. “We can make global maps and associate lightnings with various regions very clearly.”

Tracking down lightning depends very much on the season. Australia, South Africa, and Brazil are loaded with storms in January. When the sun moves to the northern hemisphere, the thunderstorms follow. Tropical zones are most active at equinox in March/April and September/October when the sun is over the equator.

At times the storms in North America can dominate the tropical zones. “The storms in the Midwest are unparalleled in terms of total flash rate. Even with all their lightning, tropical storms don’t have the same violence and electrical vigor, and they’re not accompanied by big hail or tornadoes. We think the most dangerous storms are in North America, but they occur only about every four days. In the tropics the storms occur every day, all year, because the tropics are transporting heat through convection into higher latitudes. The three zones of convection— South America, Africa, and the Maritime continent (Oceania, northern Australia and Southeast Asia)—are a natural response to thermal equilibrium of the planet.”

While studying the background component of Schumann resonances, Williams’ group became interested in a phenomenon called sprites, which are big luminous glows hovering high above large thunderstorms. “Transients cause dielectric breakdown in the upper atmosphere, which lead to these brief luminous sprites. It’s the same principal as a neon sign, where a very low density gas is excited with high voltage. The air at 80 km above the earth is about four orders of magnitude less dense than surface air. Inside a thunderstorm, a huge lightning suddenly subjects that region to a field big enough to cause a glow, and we see it as a sprite.”

The typical cloud-to-ground flash in an ordinary thunderstorm has negative polarity (takes negative charge to ground). For poorly understood reasons, the transients which create sprites have positive polarity and take positive charges to ground. These huge positive flashes occur in mesoscale convective systems characterized by rainfall spread over a much larger region than an ordinary compact thunderstorm. Such large storms predominate in the Midwest and Great Plains states, the subtropics, Brazil, and the Congo River basin in Africa.

Williams’ group shifted its concentration from the background resonances to the transients so they could study sprites, and they began to make global maps to mark the big transients. Their interest in Africa was sparked by two students, Danny Castro ‘99 and Akash Patel ‘01. “Studying rainfall over Africa in relation to the transients we detected from Rhode Island, Danny found a five-day periodicity in both rainfall and big lightnings. When rainfall increased or decreased, lightning followed the same pattern,” describes Williams.

Puzzled about the underlying cause for this periodicity, Williams’ group  suggested two explanations. One is easterly waves in the tropical atmosphere, which are about 3000 km long and propagate westward across Africa with a three- to five-day period. When the waves leave Africa, they spawn the tropical storms which become hurricanes. Sometimes every wavelength of an easterly wave will propel a storm off the coast of Africa to drift toward the coast of the US in a whole chain, looking like a pearl necklace in satellite photos.

In the early 1970s, Roland Madden and Paul Julian discovered a natural wave in the atmosphere (the global five-day wave) -which propagates westward around the world every five days. Williams calls it the “meteorological equivalent of our Schumann resonance wave, which is also one wavelength around the world. Each of these long wave lengths allows us to look at big regions simultaneously.”

Akash Patel analyzed surface pressure variations around the world and found that when there was a maximum pressure someplace, a minimum pressure was diametrically opposite it. When the pressure in this wave is falling over Africa, the MIT group noticed a burst of transient lightning discharges and rainfall scattered randomly across the continent. “Evidently there’s a coupling between this big planetary wave and the activity in Africa,” says Williams. “The five-day wave goes on almost all the time. It’s much more robust than the Madden-Julian oscillation, which is the most widely studied tropical wave.” While the planetary wave doesn’t always couple to the lightning, they found a very clear-cut relationship for one particular three-month period in 1997.

But there are mysteries about this relationship. “It’s always a chicken-or-egg problem: is the wave responsible for the lightning and rainfall, or are the lightning and rainfall really responsible for the wave?” ponders Williams. “The planetary wave has been extensively studied by theoreticians who claim that the wave is all horizontal motion, without any vertical component. There must be vertical motions in the wave to help trigger the convection and subsequently stimulate the lightning activity.”

If this pressure wave circles the globe every five days and stimulates African convection, why doesn’t it strongly modulate the other two chimney regions (areas of major heat convection to the atmosphere) in South America and the Maritime continent, so that they show up as clearly in the Schumann data from Rhode Island? “Something about Africa is particularly susceptible to that wave action,” proposes Williams. “Global observations from space have examined the bright light which comes off the lightning, and there’s no question that Africa is the strongest chimney. At times, the lightning in Africa dominates both South America and Oceania by as much as a factor of two. ”

“We got interested in Schumann resonances because of the possibility of using them as a kind of tropical thermometer,” says Williams. “Lightning activity increases where it’s warmer, and two of every three lightnings are within 23° of the equator. If that’s true globally, then with our $20,000 of equipment in Rhode Island, we can sense these changes on a coarse scale over the whole planet. On short timescales we know that lightning responds to temperature, but it’s an unresolved issue how the global lightning responds to temperature on very long time scales.”

The two global waves—the easterly waves and the five-day waves— seem to be strongly interacting and very closely related. “We want to see if they’re good predictors for the convection over all of Africa, since Africa affects our weather downstream. Studying these waves requires a global measurement system, which we can achieve by looking at Schumann resonances or at global pressure data.

“Knowing where all the lightning is should help us better understand the very largest scale patterns of weather on the planet. The traditional view is that the tropics are always the same, but studies show that these chimneys in Africa can change by as much as a factor of two in rainfall and more than a factor of two in lightning on a five-day time scale. Those large swings must influence extratropical weather appreciably! The zone in central Africa has not received much attention, even though it is a very important player in the earth’s general heat circulation,” says Williams.

Studying long-range changes on the global thermometer is a difficult problem.  “Over our six-year record, we do not see any trend. It’s possible that the atmosphere adjusts to temperature changes on very long time scales. On short time scales, the sun warms the earth’s surface every day before the upper atmosphere has a chance to respond, and clouds begin to build by late morning. Warmer conditions cause more lightning and more convection on short time scales. However, looking beyond those timescales is difficult not only because of the physics, but because of our limited data set on Schumann resonances.”

All people who study global warming complain about insufficient data,  and the natural variations on all time scales that compete with anthropogenic signals. “It’s really ridiculous how much gets blamed on global warming,” says Williams, citing Boston’s notoriously variable snowfall as an example. “Some years we get 8 ft (240 cm) and other winters we get 10 in. (25 cm). With so many complicated factors, interpretations about manmade effects are premature until we understand the whole system much better.”