by Henry Lansford

Acting as a giant spherical capacitor, the earth maintains an electric field in the atmosphere. But air is a leaky

insulator, and an electric current flows constantly from the ionosphere to the surface, in areas of fair weather all around the world. This current would neutralize the earth's negative charge in less than an hour if there were no generator to recharge it. Worldwide thunderstorm activity is the principal process that maintains an elec­trical potential difference of hundreds of thousands of volts between the surface of the earth and the highly conductive region of the upper atmosphere known as the ionosphere.

Some 60 years ago, the British physicist C. T. R. Wilson proposed that the earth's thunderstorms, several thousand of which are in progress at any given moment, collectively constitute a generator that maintains the earth's negative charge. In papers published in 1916 and 1920, Wilson outlined his hypothesis of a global electric circuit completed by a positive current flowing up from the earth into thunder­storm clouds, out of the tops of the thun­derstorms into the highly conductive ionosphere. There it would be rapidly distributed around the globe, then back down to the earth through the lower atmo­sphere in fair-weather regions.

Wilson's hypothesis provided a unifying theory that tied together many observations of atmospheric electrical phenomena, and it offered a plausible solution to the riddle of how the earth maintains its constant negative charge. (See "Amber, thunder, and lightning," accompanying this article.) Although the upward current above thun­derstorms has been measured directly only a few times, its existence is generally accepted. As Bernard Vonnegut of the Atmospheric Sciences Research Center at the State University of New York puts it, "Wilson's suggestion is the only game in town. People sometimes stand up at meetings and say it hasn't really been established, but nobody

16 MOSAIC May/June 1983

There is general agreement that a system exists for the maintenance of the earth's electric budget; questions about its nature still strike sparks.

Global circuit. Thunderstorms generate the exchange of electric charge.

has published anything that proposes a reasonable alternative."

Measurements made by research aircraft flying over thunders torms indicate that a cu r ren t of rough ly one ampere flows upward from an average storm. Estimates of the number of thunderstorms in progress around the world at any given time range from 1,500 to 3,000. The total fair-weather current worldwide is somewhere in the neighborhood of 1,500 amperes, so the current generated by thunderstorms and the fair-weather current appear to match to at least an order of magnitude.

Measurements that have been made of the ionospheric potential indicate that it does not vary much with geographic loca­t ion. However , the global ionospher ic potential increases during the hours when the sun is shining over the earth's major land masses and many thunderstorms are in progress. Its value drops when dark­ness falls over the con t inen ts and the thunders torms dissipate. This correlation supports the hypothesis that thunderstorms play a major role in maintaining the iono­spheric potential.

Fair weather and foul

A l t h o u g h there is general agreement abou t the b road out l ines of Wi l son ' s hypothesis, there is some uncertainty about the way the various components of the global circuit work. For example, Wilson assumed that thunders torms carry a posi­tive charge upward, maintaining a negative charge on the earth 's surface. In the past, it was generally accepted that lightning carries a negative charge downward, bu t recent data from lightning detection and location networks in the United States and Europe indicate that some lightning flashes carry a positive charge to the ground.

A network operated by the department of atmospheric sciences of the State University of New York at Albany recently recorded a large number of these positive lightning strokes during late spring and early sum-

MOSAIC May/June 1983 17

Amber, thunder, and lightning

Scientists first became aware of atmospheric electricity in the early eighteenth century, when they noticed the similarity between lightning and the spark produced by the discharge of static electricity from a lump of amber rubbed briskly with a cloth. One scientist wrote that the spark that crackled from the amber to his finger "seems in some degree to represent thunder and lightning."

The notion that thunderstorms are electrified was tested independently but almost simultaneously by several investi­gators in the mid-1700s. Benjamin Franklin's famous kite experiment was preceded by about a month by one by the French physicist Thomas-Francois Dalibard. He erected a 12-meter vertical iron rod that was supported by wooden masts and insulated from the ground at its base by a glass bottle, and sparks were seen jumping across a gap between the rod and a grounded wire. The apparatus had collected electricity from the clouds overhead by a process known as point discharge, a silent, nonluminous electrical discharge from a conductor that projects into a gas that has a different potential. Georg Wilhelm Richmann achieved even more dramatic experimental results. He was struck dead by lightning that hit a conductor he had erected atop his labora­tory in St. Petersburg.

Another French scientist, Pierre Charles Lemonnier, strung a wire into his laboratory from a pointed iron rod atop a ten-meter pole. He observed that dust particles were attracted to the wire even when it held a charge too weak to produce a spark. When dust was drawn to the end of the wire on a cloudless June day in 1752, Lemonnier realized that the atmosphere carries an electric charge in fair weather as well as when thunderstorms are present. He also found that the fair-weather charge varied with the time of day. Three years later, Giovanni Battista Beccaria in Turin confirmed this fluctuation and established that the fair-weather charge is positive while the charge from thunderstorms is negative.

The discovery by Charles Augustin de Coulomb and others that air is an electrical conductor, albeit a poor one, raised a fundamental question. The earth clearly maintains a perpetual negative charge. Since the air is a conductor, how is this possible? Why doesn't the fair-weather positive current that flows downward through the atmosphere neutralize the earth's negative charge? Some 60 years ago, the British physicist C. T. R. Wilson proposed a simple but elegant geophysical answer to that question. Wilson's hypothesis provided a conceptual framework within which research efforts in the field continue to contribute more detailed knowledge, •

mer. During the peak thunderstorm season in midsummer, however, most of the light­ning flashes carried a negative charge to the g round . Richard Orville of the State

Un ive r s i t y of N e w York, Albany , who designed the multistate network, says that he cannot yet explain this phenomenon, which clearly is important in the role of l ightning in the global circuit.

It was assumed for a long time that point discharge from trees, bushes, and other objects beneath thunders torms accounted for as much as 5 to 20 times as much transfer of negative charge from clouds to the g round as lightning did. But E. Philip Krider of the University of Arizona, a leader in the field of lightning research, says that work that he and his colleagues have been doing suggests that lightning plays a more impor tan t role. He says a preliminary data analysis suggests that "early in the devel­opment of a thunderstorm, when the light­ning flashing rates are high and the storm has m o s t of its electrical s t r eng th , the l ightning t ransport dominates everything else by a very large margin ."

Krider, who jokingly refers to himself as a " l i g h t n i n g c h a u v i n i s t , " says that , instead of point discharge accounting for four or five times as much charge transfer

Lansford is a free-lance science writer who specializes in geophysical subjects.

as l ightning, "our data suggest that it's the other way around, and that lightning dominates everything else by four to one." Kr ider , w h o wi th o thers developed the i n s t r u m e n t s that are used in l igh tn ing detection and location networks like the one run by Richard Orville's group, says that such networks can provide answers to many questions about the role of cloud-t o - g r o u n d l ightn ing in ma in ta in ing the global circuit. " H o w much lightning is there? What is its geographical distribution? H o w does l igh tn ing act ivi ty vary wi th t ime?" Krider believes lightning detection ne tworks can provide such information directly on a regional scale, and " they can be used to calibrate satellite systems that can survey lightning globally."

Charging a storm

T h e very fundamental question of how a thunderstorm becomes electrified has been the subject of intense debate. Most atmo­spher ic scientists agree tha t a posi t ive charge tends to build up at the top of a thunders torm cloud and a negative charge at the bot tom, and that most of the electric charge mus t be transferred between dif­ferent regions of the cloud by the movement of electrified water droplets or ice particles. Beyond that, there is not much agreement. " W e d o n ' t have a good idea wha t the charged particle populat ion is in a cloud,"

Vonnegut says, "so we're free to speculate. And you can' t describe the movement of the charged particles unless you understand the convective structure of the thunder­storm, which we don ' t . "

There is much debate over whether the d o m i n a n t mechan i sm in t h u n d e r s t o r m electrification is the downward movement of electrified precipitation particles under the influence of gravity or the movement both upward and downward of electrified cloud particles carried by convective u p -drafts and downdrafts .

In the early 1960s, two British cloud physicists, B. J. Mason and John Latham, presented evidence that most thunderstorm electrification results from the production of charged ice splinters during the growth of hailstones. Mason and Latham became o u t s p o k e n s u p p o r t e r s of the view that precipitation is the key factor in thunder­storm electrification, with ice playing a major role. Vonnegut , Charles Moore of the New Mexico School of Mining and Technology, and some other United States scientists argued that the convective mech­anism may be more important .

As r ecen t ly as 1 9 8 2 , L a t h a m and Vonnegut were still arguing in the pages of The Quarterly Journal of the Royal Meteorological Society, Latham concluded his rebuttal by pointing out that he and Vonnegut had first debated this issue 17

18 MOSAIC May/June 1983

MOSAIC May/June 1983 19

Electrified. Lightning from a summer evening thunderstorm flashes across the sky in Boulder, Colorado. Either positive or negative charge can be involved.

From the sun. Solar flare activity (above) sends out bursts of electromagnetic radiation and electrified particles. Flare incidence is associated with thunderstorm frequency.

years before in the same journa l "The question remains open , however / ' Latham wrote. "Perhaps, w h e n it is reassessed in 1999, it can be resolved."

Others hope that a resolution will come sooner. Large-scale thunders torm research projects like the Cooperat ive Convective Precipitation Experiment (see "Ice in the Summer S k y / ' Mosaic, Volume 13, Num­ber 4) are des igned to s tudy dynamic processes such as convect ion along with the microphysical and electrical aspects of thunderstorms. This integrated approach promises to provide a more complete under­standing of all of the complex processes involved in thunders torm behavior than can be obtained by s tudying any single aspect by itself.

Role of the sun

Ralph Markson of the Center for Space Research at the Massachuset t s Institute of Technology has addressed the question of the current that flows from the tops of thunders torms up into the ionosphere and its connection to solar activity. Reinhold Reiter of the West German Institute for Atmospher ic Environmental Research has observed a rapid rise in the current of the global circuit within a day of outbreaks of solar flares—huge explosions on the surface of the sun . Solar f lares send burs t s of electromagnetic radiat ion and electrified particles out from the sun. The stream of charged particles, known as the solar wind, increases in density and velocity when solar flares occur, producing so-called geomag­netic storms when they interact with the earth 's magnetic field.

Markson believes that charged particles from solar flares also ionize the atmosphere above thunderstorms, reducing its electrical resistance and increasing the flow of current in the global circuit. This would account for the increase that has been observed by Reiter and others. As Markson describes it, the column of air above a thunderstorm "becomes in effect a valve controlling the current from the generator that maintains the ionospheric potential and fair-weather electric field."

Solar activity controls this valve in two ways, Markson believes: through the effect of charged particles from solar flares and t h r o u g h the effect of less p ronounced fluctuations in the solar wind on incoming cosmic rays from sources outside the solar system. This is accepted by many scientists, bu t Markson has also proposed a mech­anism to account for solar effects on the earth 's weather that has stirred up a good deal of controversy. (See "Solar activity

20 MOSAIC May/June 1983

and terrestrial w e a t h e r / ' accompanying this article, and "Is land Earth in a Solar Sea / ' Mosaic, Volume 9, Number 5.)

Al though Wilson 's simple but elegant hypo thes i s of the global electric circuit provides a broad unifying theory within which all of this research fits loosely together, some scientists see a need for a more detailed unders tanding of the earth's electrical budget. Raymond Roble of the National Center for Atmospheric Research and Paul Hays of the University of Mich­igan have been work ing with numerical models of global atmospheric electricity. These models are roughly comparable to the general circulation models that atmo­spheric scientists use to study the physical behavior of the lower atmosphere. "There's a lot of interest in the details of atmospheric electricity, but not much work on the effects of the global generator on its total environ­m e n t / ' says Roble. " W e decided that a

global model of a tmospher i c electricity might give us some new insights ."

The consensus, Roble says, is that thun­derstorms are the main generator in the global circuit, but that they may not be the sole source of the current that flows be tween the ear th and the ionosphere . "Local per turbat ions may be produced by generators in the upper stratosphere and the ionosphere, especially at high latitudes, where in tense aurora l cu r r en t sys tems flow," Roble says.

Whole systems

To investigate global phenomena and the electrical coupling between the upper and lower a tmosphere, Roble feels that it is necessary to model the whole system rather than study individual pieces of it. "We tried to make our model as realistic as we could," he says. " W e have latitudinal variations, cosmic rays, magnetic field lines to couple

upper atmospheric phenomena, and moun­tains. W e treat thunders torms as simple dipole current generators. . . individual storms are on too small a scale to deal with in our gr id ." The grid is based on five degrees of latitude and longitude.

Roble says he and Hays are still in fairly early stages of their model ing work, but that they have identif ied a number of interesting factors: " In the model, a thun­derstorm over a mountain seems to produce more current output than one at sea level, because of the higher conduct ivi ty main­tained at higher altitudes by the ionizing effects of cosmic rays . C loud iness is a resistive element in the circuit-—fluctua­tions in cloudiness produce fluctuations in resistence."

The Roble/Hays model indicates that air pollution has an effect on the conduc­tivity of the atmosphere, as ions become a t tached to pol lu t ion par t ic les . " A s air pollution increases over the decades," Roble says, "it will probably change the electrical properties of the a tmosphere . "

Better information on the global distri­but ion and freouencv of cloud-to-^round l ightning would improve the model sig­nificantly, Roble says. He is enthusiastic about plans for using satellite systems to monitor lightning globally. He also notes that Krider 's observations about the role of l ightning and point discharge in charge transfer between thunders torms and the ground are important. T h e model, Roble says, can be used to identify key factors in the global electric circuit. It could provide field researchers with guidance to the kinds of empirical data that will be most useful in refining Wilson's basic unifying theory of atmospheric electricity.

T h e uncertainty about the details of the global electric circuit has been described by one meteorologist as "a sort of classical, turn~of-the-century geophysical problem." He says that , a l t h o u g h n o b o d y has a research project ent i t led " T h e Global Electric Circuit," many scientists are fitting together pieces of the puzzle. "Everyone is contr ibut ing something, an ingredient of the total picture," he says. Such a total picture should ultimately reveal the details of the ear th 's electrical budget within the framework of Wilson's global circuit. •

National Science Foundation support for

its part of the research discussed in this

article comes principally from its Mete­

orology, Solar Terrestrial and Experimental

Meteorology and Weather Modification

Programs.

Solar activity and terrestrial weather For many years, atmospheric scientists have observed a number of apparent

correlations between solar activity and terrestrial weather. For example, a few days after large flares on the side of the sun that faces the earth, an increase in thunderstorm frequency has been noted over Europe and North America.

However, the amount of energy that reaches the earth's upper atmosphere from such solar activity is so small compared to the quantity involved in even a single thunderstorm that it has been difficult to conceive of a mechanism that would explain a cause-and-effect relationship. Ralph Markson of MIT believes that the mechanism is electrical.

Scientists generally have no problem with Markson's conclusion that the solar wind modulates the earth's electric field by altering the resistance of the air above thunderstorms. But some find it harder to go a step further with him to the conclusion that this electrical mechanism links solar activity with terrestrial weather.

Markson believes that cloud electrification is sensitive to ambient atmospheric electrical conditions, and he suggests that the meteorological consequences may include:

• Increased rainfall from existing thunderstorms, caused by the influence of an increased electric field on the growth rate of ice crystals and coalescence of raindrops.

• Intensification of thunderstorm dynamics, triggered by additional latent heat released from more rapid formation of ice crystals and liquid water drops; the heat would be enhanced by positive feedback as additional cloud growth caused more freezing and condensation.

• Creation of more thunderclouds through increased electrification of smaller clouds by induction from the enhanced fair-weather electric field.

Model ing work done by J. Doyne Sartor of the National Center for Atmospheric Research shortly before his death in 1979 supported Markson's ideas. Sartor concluded that the effect of solar flares on the earth's electric field could determine whether thunderstorm electrification is weak or strong.

Some scientists remain skeptical, simply because so little is understood about thunderstorm electrification and the physics of precipitation. Even though Markson's theory is supported by statistical evidence of solar activity's effects on terrestrial weather, they demand more evidence of detailed physical mechanisms. •

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