The Global Electric Circuit

abridged

 

Edgar A Bering III, Arthur A. Few and James R. Benbrook

An electric current totaling one kiloanmp worldwide flows from thunderstorms in the troposphere into the ionosphere and magnetosphere eventually returing to the ground through the fair – weather atmosphere and closing via lightning.

 

1 On a clear day, there is "downward” electric field of 100 to 300 volts/meter at Earth's surface, although this field is not noticeable in daily life. That is, one does not encounter a 1 kV poten­tial difference when getting into a car on an upper floor in a parking garage, and elec­trocution is not the major hazard associated with jump­ing out of trees. The major reason why we don't notice

the fair-weather field is that virtually everything is a good conductor compared to air. Objects such as tree trunks and our bodies are excellent ionic conductors that short out the field and keep us from noticing it. But the field is there.

2   "To explain the fair-weather electric field, William Thomson (Lord Kelvin) proposed that the ionosphere be viewed as the positive plate of a spherical capacitor charged to a potential of about 260 kilovolts with respect to the ground, which is the negative plate. Today, we know that this capacitor discharges through the atmos­phere, with an average current of about 1 kiloampere integrated over the Earth. Three quasi-DC sources of electromotive force drive the global circuit: thunderstorms, a dynamo interaction between the solar wind and the magnetosphere, and the dynamo effect of atmospheric tides in the thermosphere.

3   '' Thunderstorms are thought to be the most powerful of these sources by a factor of three. The electric current that flows upward from thunderstorms into the ionosphere is known as the Wilson current, named for Charles T. R. Wilson, who in 1920 first suggested that thunderstorms play this role. This current spreads out over the globe through the ionosphere and also through the magnetosphere along magnetic field lines to the opposite hemi­sphere. The current returns to the surface of the Earth as the fair-weather air-Earth current. Cloud-to-ground lightning strokes, such as the one shown in figure 1, return the charge to the thunderstorms and close the global circuit. This global process is summarized in figure 2, which also shows the location of the relevant layers of the atmosphere. Many attempts have been made over the years to confirm the Wilson hypothesis.

4   In addition to the DC circuit, the neutral atmos­phere between Earth's sur­face and the ionosphere be­haves like a waveguide when excited with ultralow-frequency electromagnetic radiation. The elements of the discrete spectrum of trans­mission frequencies at 8, 14, 20,... Hz are called the Schumann resonances, after W. O. Schumann, who first proposed them in 1952.

These resonances are excited by electromagnetic emissions from lightning strokes and can be regarded as excitations of an AC global circuit. The Schumann spectrum is observed with induction coil magnetometers deployed at remote locations far from artificial electrical interference.

  5   The global electric circuit is an old subject that has recently experienced a renaissance. Thus, mature models exist for both the AC and DC global circuits. However, owing to the difficulty of making measurements, these models rest on a small database. New instruments have allowed a critical reexamination of atmospheric electricity, and the results have challenged the standard paradigm of the global circuit.

The conducting atmosphere

6   The "wires" in the global circuit are literally made of thin air. The exception

is the "ground wire," which is the ground itself. Thus, we look first at the conductivity of the atmosphere. Ions strongly influence the electrical properties of the atmosphere, because positive and nega­tive ions can be separated from each other to produce large-scale electric fields and because their presence in air produces conductivity. The principle source of ionization in the atmosphere below 30 km

 altitude is galactic cosmic rays. Collisions between cosmic rays and neutral molecules produce both positively and negatively charged molecules, mostly oxygen and nitrogen. (In the dense atmosphere, free electrons are almost nonexistent.) Within a few milliseconds, these O₂^+/- and N₂^+/- ions un­dergo ion chemical reactions and become hydrated with several water molecules (typically 6 to 8, but at cold temperatures as many as 20) to form "small ions" such as NO^-₃ (H₂O)₈ or H₃O⁺(H₂O)₆. In the lower atmosphere, the recombination lifetime for these ions is typically five minutes. If aerosols such as cloud or fog droplets, haze or pollution particles are present, small ions attach to them, forming "large ions," thereby reducing their mobility and the atmospheric conductivity. Measurements inside clouds have shown that the conductivity is reduced to 5% of its value at the same altitude outside the cloud.

7  Figure 3 shows the conductivity profile of the atmos­phere and ionosphere. The atmospheric density decreases with altitude with an exponential or e-folding scale height of 7 to 9 km. The mobility of small ions is governed by collisions with neutrals, and so the conductivity increases roughly exponentially with altitude, but with a smaller e-folding scale height of approximately 5 km, due to the increase in the cosmic-ray ionization rate with altitude in the lower atmosphere. In the stratosphere, conductivity and neutral density scale heights are the same—about 7 km. At the top of the middle atmosphere, above 65 km, the neutral density is so low that the lifetimes of the primary electrons become long enough for them to par­ticipate in conduction. This change produces a "knee" in the conductivity profile as the conductivity increases rap­idly up into the ionosphere. The air near the surface has a conductivity on the order of 10^-14 siemens per meter; at 100 km altitude the conductivity is 10^-3 S/m, eleven orders of magnitude greater.

8 The conductivity of the ground is approximately that of the lower ionosphere. Because there is a current flowing from the lower ionosphere to the ground, the altitude profile of the electric field is the inverse of the conductivity profile, which means it decreases exponen­tially with increasing altitude, implying a net space charge in the air. The model of the spherical capacitor is appro­priate if one recognizes that the "charge on the outer plate" of the capacitor is distributed throughout the atmosphere гаthег than being concentrated at a single level. The accumulation of charge in the fair-weather atmosphere above any point is inversely proportional to the local conductivity. Therefore, the distribution of charge on the "atmospheric plate" of the spherical capacitor is a mirror image of the conductivity profile in figure 3.

 

 

9   Vertically integrated resistivity is called the columnar resistance and is measured in units of Ω m². One conse­quence of the increase of conductivity with altitude is that the columnar resistance of the atmosphere is concentrated near the surface. Roughly one-half of the total columnar resistance occurs in the lower 3 km of the atmosphere. Hence, high-altitude surfaces such as the interior central plateau of Antarctica have smaller columnar resistances than do sea-level surfaces. Over the whole Earth, the resistance between the surface and the ionosphere is about 200 ohms. To calculate the capacitance of the spherical capacitor, a separation equal to the scale height (5-7 km) must be used rather than the height of the ionosphere. The result is a capacitance of 1 farad and a time constant for Earth's spherical capacitor in the range of 1000 to 3000 seconds:

 

 

               1

        T = f = 2π RC= 1200 s

 

Because this decay time is short, we know that the charging of the global capacitor must be nearly continuous or the global electric field would disappear.

 

 

 

10    Without radiation, Maxwell's equations can be reduced to a single equation, which expresses conservation of the total or Maxwell current. The Maxwell current is the sum of the mechanical currents (such as convection) that are the drivers of cloud and boundary layer charging, the conduction current and the displacement current. The boundary layer is the atmospheric layer adjacent to the surface. Turbulence in this layer causes the physical properties of the boundary layer air to be modified because of its contact with the surface. Because the lower atmos­phere has a net space charge, turbulent vertical air flows carry part of the current in this layer. Outside of the clouds and above the boundary layer, only the conduction and lightning-driven displacement currents are important. Conduction currents are driven by the large electrostatic fields generated by the charged clouds.

 

Figure 2. Flow of electric current in the global circuit. All of the unlabeled arrows represent current flow. The strongest batteries in the circuit are the thunderstorms indicated on the right. They produce the Wilson current. The fair-weather currents are indicated by downward-pointing arrows away from the thunderstorms. (Based on a diagram by Ray G. Roble.)