Atmospheric Electricity
Historical Development
The development of ideas on the DC global circuit received great impetus from three giants of research in atmospheric electricity: Benjamin Franklin, William Thomson (Lord Kelvin), and C.T.R. Wilson.
Their three contributions, which dominated each of three successive centuries, are discussed in turn.
Franklin can be credited with the first enunciation of a global flow of moist, electrified air.
His concept is best discussed around his own picture (Figure 1), as presented to the Royal Academy of Sciences in Paris in 1779. Warm, moist air ascends in the tropics and descends in the polar regions.
This cloudy air was believed to deliver electricity to the cold polar ice cap on snow, where it would accumulate until breakdown of the rarefied upper atmosphere occurred in the form of the aurora.
While this explanation for the aurora is now known to be flawed, the postulated role for the tropics and the picture of the large-scale circulation of the atmosphere were clearly prescient.
Figure 1. Benjamin Franklin's picture of airflow and transport of electricity from equatorial to polar regions.
Reproduced with permission from Silverman, S., 1970. Franklin's theory of the aurora. Journal of the Franklin Institute 290, 177.
Lord Kelvin, 100 years later developed potential theory, a mathematical tool needed for theoretically underpinning the global circuit. Remarkably, more than 40 years before the conductive ionosphere was postulated by A. Kennelly and O. Heaviside in 1902, Kelvin advanced the spherical capacitor picture for the global circuit.
His expectation for an outer conductor was based on his knowledge that rarefied air of the upper atmosphere was a poor insulator in comparison with air at the Earth's surface.
He also advocated organized measurements of the Earth's electric field, and this suggestion undoubtedly motivated subsequent electrical observations from the research vessels Carnegie and Maude by the Carnegie Institution. Kelvin undertook his own surface measurements of potential gradient, verified that the Earth carried a negative charge, and concluded that the global circuit peaked in winter, a result now believed to be dominated by local effects. He also prophesied the use of electrical measurements for purposes of weather prediction.
There can be no doubt that electric indications, when sufficiently studied, will be found important additions to our means for prognosticating the weather and the speaker hoped soon to see the atmospheric electrometer generally adopted as a useful and convenient weather glass.
Measurements by Wilson of the field changes associated with lightning in thunderclouds led him to conclude that the polarity of thunderclouds was systematically positive in upper levels and negative at lower levels.
Wilson was also engaged with measurements of the currents flowing to Earth during periods of fair weather.
The observation of the transatlantic propagation of radio waves in 1903 by G. Marconi verified the presence of the conductive ionosphere.
This collective information led Wilson in 1920 to formulate his famous hypothesis for the global electrical circuit: thunderstorms are batteries and drive current upward to the conductive ionosphere where it spreads out to return to Earth in fair weather regions, as illustrated in Figure 2.
Wilson's idea led F. J. W. Whipple to compare the universal time (UT) variation of electric field over the oceans, now referred to as the ‘Carnegie curve,’ with the UT diurnal variation of thunder areas on a global basis, as shown in Figure 3.
Three major tropical continental zones are activated sequentially by the surface heating associated with the passage of the Sun.
The similarity in phase between these two curves has long stood as key substantiating evidence for the Wilson's global circuit hypothesis.
Additional support came in 1950 when O. Gish and G. Wait measured upward currents over thunderstorms from an airplane. More recent measurements have shown upward currents over showerclouds.
Figure 2. Simple illustration of the operation of the DC global circuit with electrified clouds as generators and a return current to earth in fair weather regions.
Figure 3. Comparisons of the UT variation of electric field over the ocean (the Carnegie curve) and the UT variation of thunder areas worldwide.
Adapted from Whipple, J.N., 1929. On the association of the diurnal variation of electric potential gradient in fine weather with the distribution of thunderstorms over the globe. Quarterly Journal of the Royal Meteorological Society 55, 1–17.
Wilson's student T.W. Wormell later extended the surface measurements of current and formulated a statistical charge ‘balance sheet’ for the global circuit.
These results showed that point discharge current dominated over the lightning current in modulating the negative charge transfer to the Earth's surface by electrified storms.
The first coordinated measurements of the global circuit were made by R. Muhleisen, one set from Germany and another from a ship in the Atlantic Ocean.
These balloon measurements integrated the vertical electric field in the atmosphere to provide the so-called ionospheric potential of the global circuit. The simultaneous soundings at two locations agreed to within 5% in three-quarters of the measurements, providing considerable support for the worldwide nature of the global circuit response.
Extensive measurements of the DC circuit were also carried out by R. Markson using instrumented aircraft and with balloons from stations in Massachusetts and Australia with similar correlated results, and with diurnal variations that closely follow the classical Carnegie curve.
The first suggestion that the spherical capacitor of the DC global circuit also served as an electromagnetic waveguide appeared when in 1952 W. Schumann postulated the existence of electromagnetic resonances maintained by global lightning activity.
Partial experimental verification of the resonances was obtained by Schumann's student H. Koenig in Munich in the late 1950s.
M. Balser and C. Wagner of the MIT Lincoln Laboratory verified the multimodal resonances with the first spectral measurements in 1960.
Transient excitations of the Schumann resonances by single extraordinarily energetic flashes were reported in the early 1970s by D.L. Jones and his colleagues.
Telluric Currents
