JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 101, NO. D23, PAGES 29,679-29,688, DECEMBER 27, 1996 Seasonal variation of the global electrical circuit Edwin J. Adlerman School of Meteorology and Center for Analysis and Prediction of Storms, University of Oklahoma,Norman Earle R. Williams Parsons Laboratory, Massachusetts Institute of Technology, Cambridge Abstract. The effects of boundary layer aerosol particles on the electricfield measurement of the DC globalcircuit are considered. Aitken (condensation) nuclei concentrations are found to have systematic local seasonal variations which obscure the globalbehaviorof the DC circuit.These local variations appear to be the result of several seasonal factors, including variations in atmospheric mixedlayer heights, variations in the productions rates of anthropogenic aerosols, and variations in surface wind speed. Air- Earth conduction currentmeasurements made by W. Cobb at Mauna Loa (1977-1983), a site remote from sources of pollution and mostlyabovethe boundary layer, appear to be relatively free of aerosol particle effects. The Mauna Loa data are examined and the air- Earth current is found to peak in the northern hemisphere summer, consistent with the peak of the globalthunderstorm activity in the sameseason. A reanalysis of the entire Carnegie and Maud oceandata set as well as ongoing Schumann resonance results support this finding.However,the generalabsence of a distinct semiannual signal remains unresolved. 1. Introduction Accordingto the classical picture of atmospheric electricity the electricalstructure of the Earth-ionosphere system can be modeled as a leaky spherical capacitor [Chalmers, 1967; Dolezalek, 1972]. The surface of the Earth and the highlycon- ductive ionosphere (above50 km) form the inner and outer shells, with the atmosphere functioningas a leaky dielectric. Worldwideweather activity maintains a potential difference of about 250 kV between these two electrical conductors, result- ing in a potential gradient of about 120 V/m at the Earth's surface. As a result of the finite conductivity of air, a vertical conduction current is also present. This configuration is re- ferred to as the DC global circuit. The results of the Carnegie and Maud cruises of 1910-1929 established the existence of a fairly universaldiurnal variation in the surface potential gradient over the oceans[Parkinson and Torreson, 1931]. These results were subsequently verified in many other similar observations [Paramonov, 1950a]. This variation correlated well with early estimatesof global thun- derstorm activity[Whipple, 1929a;Whipple and Scrase, 1936]. Analysis of the Carnegieand Maud data alsodemonstrated that the electricfield undergoes seasonal fluctuations in inten- sity.Parkinson and Torreson [1931] showed that the field in- tensity is almost30% greater during the northern hemisphere winter than in the summer. Further measurements at northern hemisphere stations confirmed these results [Paramonov, 1950b; Chauveau, 1922]. Previous attempts to explainthe ob- served winter maximum in field have focused on the seasonal distribution of thunderstorm activity [Markson, 1986]and local variables whichmightaffect the observations [Israel, 1973].The greatestobstacleto a complete understanding of the global Copyright1996 by the American Geophysical Union. Paper number 96JD01547. 0148-0227/96/96JD-01547509.00 circuit has been the uncertaintyinvolvedin an assessment of global thunderstormactivity. Seasonal fluctuations in thunderstorm activity and global electrical parameters are ultimately governedby variations in short-wave radiation from the Sun. Insolationvariations[List, 1968]have both a semiannual component (maxima at the equi- noxes) and an annual component (maximum in January). Wil- liams [1994] compiled global surface air temperatures over both the tropicallatitudes(_+25 ø ) and the temperatelatitudes (_+60ø). A distinct semiannual signal with a large maximum in April and a smaller maximumin October is apparentover the tropical belt, a result of the semiannualsignal in insolation. Thunder day data [World Meteorological Organization (WMO), 1956] for the samelatitude range showstrongmaxima during these two months.A distinct annual signalin temperature is dominant over the temperate latitudeswith a large maximum in the northern hemisphere summer, an apparentresult of the imbalanced distribution of land mass between the hemi- spheres, an effectthat dominates over the variation due to the eccentricity of the Earth's orbit around the Sun. Thunder day data for this latitude range show small maxima in April and October, with an annual component with a maximum in the northern hemisphere summer. Williams [1994]finds that these surface air temperature variations correlate well with global lightningactivityon both the semiannual and the annualtime- scales. In light of recent upward trends in surfaceair temperature [Hansen and Lebedeff, 1987] andpredictions of global warming [e.g., Houghton e! al., 1990, 1992] the indirect monitoring of lightning (and therefore temperature) through global circuit measurements takes on newmeaning [Williams, 1992]. Toward testingthe hypothesis that the global circuit is responsive to temperature on timescales longer than the diurnal, certain long-standing inconsistencies in the seasonal behavior of the global circuit [Dolezalek,1972; Israel, 1973] are in need of resolution. Since the establishment of baseline global circuit 29,679