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The Global Electrical Circuit: A Review

The Global Electrical Circuit: A Review. Author: Earle R. Williams (2009) Date: 14 February 2017 Presented By: Julie Barnum. Diurnal Variation of the Global Circuit in Universal Time. Carnegie Curve. Williams and Satori (2004) theory

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The Global Electrical Circuit: A Review

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  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 (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 14-15 UT (Africa/Europe) • This max however, is not just due to South American dominance over Africa… Thunder Day Curve (Whipple and Scrase, 1936)

  4. Surface-Based Measurements of the Global Circuit at High Latitude Vostok • 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 Burns et al. (2005) 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

  6. Semiannual Variation of the Global Circuit • 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 Markson (2007) Adlerman and Williams (1996) Christian et al. (2003)

  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. 2004) • Little correlation found • Their contribution to the global circuit is small, due to the rarity of occurrence compared to other current sources

  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!)

  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)

  10. Is the Global Circuit Declining with Time? • This long section starts with Harrison (2002) saying the E-field in the atmosphere was declining based on E-field records at Eskdalemuir, Scotland (1911-1981) • Backed up by Märcz and Harrison (2003) whosee a long-termdecline in E-field at Nagycenk, Hungary • Whilenotvalidatingthedeclineearlier in the 20th c, Harrison and Ingram (2005) foundJz at Kew to decline (’57-’78) • Williams (2003b) said of Harrison (2002)’s conclusionthatthiswas a localaerosoleffect • As for Märcz and Harrison (2003), thiswas a result of a shieldingeffect of trees Harrison (2002) Märcz and Harrison (2003) 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)

  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

  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)

  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%), whenaccording 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 (1954-2004) 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)

  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)

  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

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