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11 One problem in understanding the global electric circuit is identifying the active elements in the circuit. There is agreement that tropospheric weather systems are the dominant energy source—the difficulty is in determining the details.
12 The conductivity of the atmosphere has an important impact on the electrical behavior of charged clouds, especially tall cumulus clouds. The top of a thunderstorm is positively charged and is at a typical altitude of about 12 km, where the conductivity of air is ten times greater than it is just above the surface. The cloud's negative charge is near about 5 km altitude but is concentrated inside the cloud, where the conductivity is decreased by the cloud droplets. It is helpful to make some rough estimates of resistances in this system. For a cylinder of clear air 20 km in radius and extending from Earth's surface to a height of 10 km, the resistance between the top and the ground is roughly 400 МΩ. In cloudy air, however, the resistance is several gigaohms. In contrast, a cylinder of the same radius extending from 10 km altitude to the ionosphere would have a resistance of 40 МΩ, and the return path to the surface through the global atmosphere with its resistance of about 200 Ω is a comparative dead short.
13 Placing a thunderstorm in the atmosphere will necessarily produce Wilson currents from the cloud top into the ionosphere, back down through the global atmosphere, along the surface and underneath the thunderstorm. The values of the conduction currents observed above
individual thunderstorms were found in one study to vary between 0.09 and 3.4 A, with an average value of 1.7 A. Approximately half of this current flows through the overlying magnetosphere to the magnetically conjugate region in the opposite hemisphere. (See figure 2.) Because the time constant for discharge through air from the top of a thunderstorm is an order of magnitude smaller than, that of the global capacitor, some of the positive charge ends up being stored in the global capacitor.
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14 When a lightning flash occurs from the bottom of the cloud to the ground, it is completing the global circuit. All charged clouds exhibit this behavior to some extent, but very tall clouds contribute a greater share to the global circuit, Within active thunderstorms, strong updrafts transport cloud particles upward to the growing tops of the thunderstorms. Heavier precipitation particles lag behind and eventually fall out of the updrafts. The small ice particles at the cloud tops have a net positive charge and the larger particles lower in the cloud carry a net negative charge.
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Figure 3. CONDUCTIVITY PROFILE.
The electrical conductivity of Earth’s surface and
atmosphere is plotted as a function of altitude.
| I I I I I I I -14 -12 -10 -8 -6 -4 -2 0 04 -2 0 LOG OF CONDUCTIVITY IN SIEMENS PER METER Figure 3. Conductivity profile. The electrical conductivity of Earth's surface and atmosphere is plotted as a function of altitude. |
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15 Thunderstorms deliver negative charge
to Earth in several ways: negative cloud-to-
ground lightning flashes; quasi-DC point
discharge currents such as St. Elmo's fire;
conduction current; and negatively
charged precipitation. Attention has focused
on the first two mechanisms because they
are consistently larger. The most frequent
form of lightning is the intracloud
discharge, which connects the cloud's upper
and lower charged regions, temporarily
short-circuiting the charging mechanism.
The usual negative cloud-to-ground
lightning discharge occurs between the
lower negative charge and the surface, and
is a primary participant in the global circuit. Occasionally there will be a positive cloud-to
-ground discharge that conveys positive
charge from the upper charged region to
the ground.
| Figure 4. Launch of a high-altitude research balloon from South Pole Station on 21 December 1985. This balloon was launched by a University of Houston team as part of the 1985-86 South Pole Balloon Campaign. |
16 Prior to the discovery of the magnetosphere, the standard paradigm treated the ionosphere as an equipotential surface. Modern space research
has shown that this assumption is incorrect at all latitudes. A dawn-to- dusk horizontal potential drop of 20-100 kV is impressed upon the geomagnetic polar cap owing to the dynamo interaction of the solar wind and Earth's magnetic field. The cross-polar-cap potential is directly coupled through the currents driven across the polar cap to the global circuit and is one of the three main sources for the circuit, along with thunderstorms and thermospheric winds. Horizontal electric fields with scale sizes on the order of 500 km map down to the ground from the ionosphere and change the air-Earth current and vertical electric field by + 20% at high latitudes during periods of geomagnetic quiet and by greater amounts during geomagnetic storms.
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Monitoring the global electric circuit
17 Measuring the fair-weather electric field is a challenging task. Atmospheric electricity measurements are often made with "field mills," using electrostatic induction. The detecting plate is placed perpendicular to the direction of the field. Gauss's law tells us that a layer of electric charge will be induced by the action of the field on the outer surface of a conductor such that
Ơ
En = Є0
where En is the normal component of the electric field, ợ is the surface charge density and Є0 is the permittivity of free space. A rotating grounded disk with holes in it is used to alternately shield and expose the plate. This arrangement produces an alternating voltage on the plate that is proportional to the field.
18 Continuous measurement of the atmospheric electric field was begun by Kelvin at Kew Observatory in 1861. If there is a global circuit, then one should detect the same fair-weather electric field time variation anywhere on the globe. Local topography will amplify the field on mountain tops, but the shape of the temporal variations should be the same and should have a Universal time (UT) dependence that follows global thunderstorm activity. The thunderstorm rate is not a constant because continents are irregularly distributed in longitude. However, the records obtained at continental observatories show that variations in cloud cover, humidity, total aerosols and vertical convective air currents combine to produce varying
electric fields of local origin that can completely mask the electric field of global origin. It is only by careful data rejection that one can obtain hints of the gobal circuit at such stations.
19 On the other hand, data found over open ocean waters by the research vessel Carnegie during 1915-29 established that a common, UT-dependent variation in the
electric field can be detected at any oceanic site if enough data are taken to give a good signal-to-noise ratio. The diurnal variation of the global electric field that was observed is now called the Carnegie curve. It showed a daily maximum at 1900 UT, a time that corresponds to mid-afternoon in the Amazon basin and noon in the central US. This response can also be observed from mountaintop sites at altitudes above the planetary boundary layer, particularly on oceanic islands.
20 During the 1985-86 South Pole Balloon Campaign, the average diurnal variation of the electric field in geomagnetic quiet times was a 24-hour amplitude modulation of 30-40% of the DC average, with a minimum absolute value near 0300 UT, and a peak near 1800 UT. The diurnal modulation was reduced by about 25% on geomagnetically disturbed days, in agreement with predictions of magnetospheric coupling models and in disagreement with the assumptions of the standard paradigm of the pre-space age. In contrast, a balloon launched at Siple Station, Antarctica (at geomagnetic latitude 62° S), at the start of the intense magnetic storm of 19-20 December 1980, found geomagnetic perturbations in E vertical that were so large that there was no diurnal variation at all, a result that was not predicted by any model.
21 The best place on Earth for making ground-level observations of the global circuit is arguably on the Antarctic plateau.That is because the weather on the plateau is clear about 70% of the time, local atmospheric convection is suppressed by a deep temperature inversion and the topography results in an amplification factor of two or more in the electric field strength and current density.
