1. The Global Electrical Circuit: A Review
Author: Earle R. Williams (2009)
Date: 14 February 2017
Presented By: Julie Barnum

2. Diurnal Variation of the Global Circuit in Universal Time
Carnegie Curve
Williams and Satori (2004) theory
Electrified shower clouds as important as thunder clouds for supplying current to global DC circuit
Kartalev et al. (2006)
Reason for the Carnegie curve favoring the Americas has to do with the influence from the magnetic dip equator
Comments from Williams (2009)
Evidence that lightning isn’t the primary current source for global DC circuit
Fate of high-altitude current from electrified clouds uncertain…
Thunder Day Curve
In Carnegie Curve, the Americas dominate
In the thunder day curve, Africa and Europe dominate (something supported by several satellite observations from OTD, LIS, and even the record we do have of Schumann Resonance obs)
W & S: South America exhibits less lightning, however, it has more rain than Africa…thus a larger abundance of electrified shower clouds and a thus a greater contribution overall to the Carnegie curve!
K: Thought that the magnetic dip equator guides the surface current and that the Carnegie curve is dominated by sources within +/- 11 degrees od the dip equator…where most of the thunderstorms and rain in Africa do NOT reside!
Comments by williams: 1 – yet, Kartelev et al. used lightning as measure for source current
2 – N/A
(Whipple and Scrase, 1936)

3. Diurnal Variation of the Global Circuit in Universal Time
Carnegie Curve
R. Blakeslee (International Conference on Atmos. Electricity, Beijing)
In optical observations, see the max in lighting around 19 UT (Americas), and not UT (Africa/Europe)
This max however, is not just due to South American dominance over Africa…
Thunder Day Curve
Rather this is due to a tail of lightning activity over the Maritime Continent…result of oceanic flashes due to landbreezes, which obviously doesn’t show up in the thunderday observations because this happened over the ocean, too far away from observing stations to be heard!
(Whipple and Scrase, 1936)

4. Surface-Based Measurements of the Global Circuit at High Latitude
Vostok
Burns et al. (2005)
Measurements of the GEC have had more success at high latitudes
With their low temps, have stratified boundary layer
Efforts to measure atmos. Electricity at high latitude pioneered by Simpson
Work to monitor the global circuit performed in Vostok, Antarctica and at the South Pole
Presence of second generator made obtaining results similar to Carnegie Curve difficult at first
This problem has managed to be corrected, and a residual signal (when multiple days are averaged) is quite similar to Carnegie Curve in amplitude/phase
Big emphasis made on the fact that these results were due to use of “clean” days and averaging of many days to get the Carnegie Curve look
Stratified boundary layer eliminates/severely reduces local fluctuations common at temperate lats (something that interferes with the global signal…)
Simpson plotted a curve that, when replotted in universal time was very similar to Carnegie Curve in phase!
Interaction of solar wind and earth’s magnetic field causes a dawn to dusk potential difference a/c polar cap….which contributes to the E-field at the surface
South Pole
Reddell et al. (2004)

5. Annual Variation of the Global Circuit
Before, found a NH winter maximum in potential gradient by Lord Kelvin, C.T.R. Wilson, and G.C. Simpson
Contradicted first by Whipple (1929), who saw the maximum being in NH summer, not winter
Modern measurements of satellite obs and Schumann Resonance intensity back up Whipple (whose findings were based on thunder day obs)
Whipple’s findings are also backed by reanalyzed Carnegie obs over oceans, surface E-field measurements at high latitudes, and a reanalysis of ionospheric potential obs
Further investigation by Adlerman and Williams (1996) showed the NH winter maximum in potential gradient was a local effect of enhanced aerosol
This locally-enhanced aerosol  diminished conductivity  increased e-field to guarantee continuity of fair weather conduction current!

6. Semiannual Variation of the Global Circuit
Adlerman and Williams (1996)
Markson (2007)
Tropical convection gets enhanced 2x a year, especially over continental areas
Despite small surface air temp change, see it prominently in air-earth current surface measurements, E-field measurements at Vostok, and in ionospheric potential climatology
Global lightning, integrated over the year does not have the semi-annual signal near equinoxes
Can only be seen if low-latitude lightning is integrated
Semiannual signal in traditional measurements of global DC circuit (air-earth current/ionospheric potential) show source currents more concentrated in tropics
Christian et al. (2003)
Over continental areas b/c that’s where more electrified wx occurs…2x per year, because that’s due to the Sun’s equinoxial crossings of the equatorial region!
Temp change is ~1 deg C

7. Role of Lightning in the DC Global Circuit
Plenty of papers propagate the idea that lightning is the main current source for the global circuit (despite Wilson (1920)’s theory)
Lots of evidence now, however, for the need of electrified shower clouds to explain aspects of the global circuit (e.g. American dominance over Africa in Carnegie Curve or the semiannual signal)
See a correlation between WWLLN and vertical current density from balloons in the stratosphere (over South Pole) (Holzworth et al. 2005)
Balloon measurements and phase information on diurnal time scale not addressed…and amplitude variations in Jz bigger than expected for globally representative signals
Comparisons have been made between E-field measurements at Vostok and global measurements of TLE events detected by ELF methods (Troshichev et al )
Little correlation found
Their contribution to the global circuit is small, due to the rarity of occurrence compared to other current sources
Apparently Ez and Jz are very complicated in high lats, while being less so in lower lats
Could use the little correlation found to say that lightning is indeed NOT the main current source for the global circuit…TLEs have very different diurnal variations than ordinary lightning over land in the afternoon…

8. Global Effects of Nuclear Weapons Tests
Ionospheric potential represents an integral of electrified tropospheric convection worldwide
Evidence that nuclear weapons testing in mid-1900s had a big impact on the ionospheric potential (Markson 2007)
Based on fallout in the stratosphere, not at the surface, where residence time ~3-4 years
Enhanced conductivity over electrified clouds at high levels  bigger supply current to global circuit, much less of an effect in the fair weather region
Increases in the global circuit backed up by Muhleisen (1977), found some values of 500 kV (~2x the mean value) for ionospheric potential
This theory not backed up by model calculations
Source current necessary to make these kinds of values falls short (factor of 3!)
High ionospheric potential values well correlated with timing of stratospheric nuclear weapons testing

9. Global Effects of Nuclear Weapons Tests
Another strike against this theory – stratospheric conductivity measurements in Australia (Paltridge 1965) found no anomalously high values
Studies of air-earth current at Athens (Märcz and Harrison, 2005) and at Kew Observatory (Harrison and Ingram, 2005) validate the impact from the weapons testing
Air-earth current more globally representative than surface E-field
Increases in this at both stations > factor of two
Markson’s (2007) plot of stratospheric burden of radioactivity decreases by two orders of magnitude from the 60’s to the 80’s (topic of the next section)
Markson (2007)
BULLET POINT 1: At a period when it was during the downturn in the inferred global stratospheric radioactivity burden from Markson (2007)
These increases at the two stations are also well timed with the stratospheric emplacement of radioactivity

10. Is the Global Circuit Declining with Time?
Harrison (2002)
This long section starts with Harrison (2002) saying the E-field in the atmosphere was declining based on E-field records at Eskdalemuir, Scotland ( )
Backed up by Märcz and Harrison (2003) who see a long-term decline in E-field at Nagycenk, Hungary
While not validating the decline earlier in the 20th c, Harrison and Ingram (2005) found Jz at Kew to decline (’57-’78)
Williams (2003b) said of Harrison (2002)’s conclusion that this was a local aerosol effect
As for Märcz and Harrison (2003), this was a result of a shielding effect of trees
Märcz and Harrison (2003)
This local aerosol effect reduced electrical conductivity and thus increased the E-field in the polluted earlier portion of the 20th c. (actually validated later by harrison with a smoke concentration study and looking at the seasonal variability of it)
The trees grew throughout the years, which act as good conductors, and shielded some of the electric field, thus making it so that it appeared as though the E-field was decreasing w/ time!
Harrison and Ingram (2005)

11. Is the Global Circuit Declining with Time?
Harrison (2007) then went into defending the use of measurements at the Eskdalemuir, Scotland site
Looked at Jz values (calculated from conductivity and E- field values) compared to a definitely polluted site (Kew), and to clean ocean air values
Except, need to look at conductivity, not Jz to determine this (Williams 2009)!
Looking at total conductivity at various locations, see that almost all land values are much lower than the ocean (AKA, Eskdalemuir is polluted)
Also…Harrison (2007) didn’t cite work by Pierce (1972a,b) at the same Scottish station, where Pierce found that the site was too polluted to study global variations
Williams (2009)
It had been said that that site was not good for measurement taking due to pollution from aerosols
Saw that the conductivity was large enough to show that the Scottish site was an unpolluted site
Hence, based on the table, we see that the large E-field at Eskdalemuir can be explained by the large differences in conductivity b/w clean days and polluted ones!
So…actually the conductivity values that were used in Harrison (2007) go to prove Pierce’s point more…

12. Is the Global Circuit Declining with Time?
Märcz and Harrison (2005) looked at more atmospheric electricity records to verify the decline in the global circuit…
In order of priority for studying this decline…ionospheric potential, air-Earth current density, potential gradient
Several studies that show alternate explanations not cited
Use potential gradient to show a decline, instead of contributing it to aerosols or tree shielding, but other surface E-field records at NASA Kennedy Space Center do not see this decline
But, don’t make any reference to the long-term stability of the global circuit in ionospheric potential measurements
But in the secondary measurement, don’t cite a slew of different literature that have alternate explanations for the decline

13. Is the Global Circuit Declining with Time?
Back to trees again… Märcz and Harrison (2006) argue that the tree configuration in Williams et al. (2005) in MA doesn’t work for Nagycenk, and show corrected results
Williams (2009) admits the trees could’ve been better represented in the model, so a correction is made here…with the same conclusion
Spoiler alert: it’s still that the trees dominate the E-field decline. Oh my.
The model to prove that the trees do in fact invalidate the declining E-field claim:
Electrostatic model based on a numerical solution of Laplace’s eqn by finite element method
3-D, symmetry in the N-S direction about E-W axis
Grove of trees shown as two lines
Extended forest to west modelled as a vertical ”step” in conductive medium
Uniform E-field imposed on this 3-D tree configuration
Williams (2009)
Where in these results we see that the impact of the trees is much smaller than previously shown by Williams
Basically, in the Nagycenk, there is a forest of trees to the west of the station, and a grove of trees to the east that contribute greatly to a screening of the fair weather E-field at the measurement location.
Two lines are each 1 m wide and 16 m tall, extending perpendicularly to the x-axis in the N-S direction +/ m
Forest had heights of 18 m, goes to infinity in N-S direction and W

14. Is the Global Circuit Declining with Time?
Model calculations say if the trees are removed, the E-field at PG1 is E0
If trees grow to their final heights, PG1 E-field no greater than 61 % of its initial value
“This reduction is within a few percent of the overall decline documented at Nagycenk since 1962 (59%), when according to Märcz and Harrison (2003) the “trees…hardly disturbed the measurements””
All this explains why their station’s E-field values in 2002 was so low (47 V/m), when fair weather field over oceans is more like 130 V/m
Markson (2007) found that when global data of ionospheric potential ( ) is corrected for the weapons testing, no decrease wrt time is evident
The last killing blow to the various papers supporting this decline in the global circuit was via a 14-year record of Schumann resonance intensity
Williams (2009)
Märcz and Harrison (2006) tried to emphasize max value of E-field at PG2 as evidence they’re beyond the influence of the trees…get access to true fair-weather field measurement…but because of the forest to the west, they don’t do this! If their measurements were extended westward, they wouldn’t get a flat E-field, but a declining one to the edge of the forest!
See no statistical significance of a decline in the SR data…

15. Global Circuit Response to Climate Change
Evidence that as global mean surface air temps increase, so should the global electric circuit
Carnegie Curve evidence on a diurnal time scale
Semiannual signal in global circuit evidence on intraseasonal time scales
Annual signal in global circuit is evidence on the seasonal time scale
ENSO signal in global lightning activity evidence on the interannual time scale
But, what about global circuit response on longer time scales?
Warming in tropics currently 0.1℃/decade
If temperature response of ionospheric potential is 10 % ℃/decade, would only see a 1 %/decade increase in this quantity
Bigger increase in high lats would make one want to investigate variations in storm properties there
Complications with lightning detection networks this make long- term trend analysis problematic
Could instead use thunder day obs and surface temperature however!
Williams (2009)
Carnegie Curve increases as continental zones dominating the GEC are warmed in succession by zonal motion of the sun
Tropics are warmed by the meridional motion of the Sun
NH land is warmed selectively in NH summer asymmetrically wrt land-sparse SH
Tropical “chimney” regions undergo temperature variations in response to the E-W oscillation of temperature in the Pacific Ocean
With the thunder day obs and temps (for summer in Fairbanks, AK), see an upward trend in summertime temps, and possibly an upward trend as well in thunder days…

16. Summary
Have learned much of the global electric circuit over time, where this paper focused on the global DC circuit for the most part
Diurnal variations, semiannual variations, annual variations
Different contributions of source current to the global electric circuit
Theories on how nuclear weapons testing in the stratosphere may have affected the global electric circuit
A controversial theory, many people in both camps
Ultimately led to the long discussion on whether or not the global electric circuit is indeed declining with time (seems no)
Finally, discussed the possibility that the global electric circuit may be affected with global climate change…but unsure of the long-term trends
In the end, have learned a lot, yet there are still many uncertainties