Electromagnetic Atmosphere-Plasma Coupling: The Global Atmospheric Electric Circuit
Summary (3 min read)
1. Introduction
- This paper is concerned with atmospheric electrical coupling from near the Earth's surface up into -and down from -the ionosphere at ~ 80 km altitude and higher.
- Fig. 2 shows the broad range of temporal scales that are involved in the many phenomena of importance.
- The consequent large electrostatic field above the thundercloud exceeds the conventional threshold field for electrical breakdown, creating a sprite from about 80 km at the base of the ionosphere down to about 55 km (Fullekrug et al.
2. Properties of the global electric circuit
- (a) Generator and load regions Fig. 3 is a schematic representation of the circuit taken from Rycroft et al. (2000) .
- The lower part of Fig. 3 shows the equivalent circuit.
- A very recent paper by Mach et al. (2011) has, for the first time, used experimental data from aircraft and satellites to deduce that thunderstorms over the land contribute 1.1 kA to the global circuit and, over the oceans, 0.7 kA.
- The conductivity and the electric current density J z flowing in the fair weather regions determine the vertical electric field E. Near the Earth's surface away from aerosol pollution in fair weather E = -130 V/m; the minus sign indicates that the electric field is directed downwards.
(b) Vertical variations
- The cosmic ray fluxes at different altitudes and for different rigidities (i.e. for different momenta) have been reported by Bazilevskaya et al.
- These results are in good agrement with those presented by Rycroft et al. (2007) and Rycroft and Odzimek (2010) .
- Fig. 3 of the Rycroft and Odzimek (2010) model considered the effect on the global circuit of reducing the conductivity within the thundercloud by a factor ranging from 2 to 29.
- With the increase in conductivity with height, the vertical electric field becomes so small with increasing height that the potential in the model atmosphere at 60 km is only 24 V less than the 250,000 kV ionospheric potential (Rycroft et al. 2007) .
- Stansbery et al. (1993) estimate that half of the current that reaches the ionosphere flows into the geomagnetically conjugate hemisphere.
3. Recent findings concerning the global circuit
- (a) Source term Fig. 7 shows the position of the RHESSI satellite when it detected a TGF (taken from Smith 2009; more results are given by Smith et al. 2010 ).
- Whilst it is not proven that TGFs play a role in maintaining the global circuit, it is important to consider the impact of relativistic processes taking place above thunderstorms on the global circuit.
- They note that such "relativistic electron beams are a new form of impulsive energy transfer between thunderclouds and the middle atmosphere which need to be considered as a novel element in the global atmospheric electric circuit".
- Globally, there are about 44 flashes per second, of which less than one is over the oceans.
- Price (2006) plotted (his Fig. 8 ) the Universal Time variation of the thunderstorm area of the three tropical continental regions.
(b) Current flow and ionospheric potential
- Integrating the vertical electric field profile measured aboard aircraft or balloons up to the troposphere essentially determines the ionospheric potential.
- The curve closely resembles the Carnegie Curve (Markson 1986 ), which is one of the "confirming ideas" (Aplin et al, 2008) supporting the behaviour of the Earth-atmosphere global circuit system.
- Harrison and Bennett (2007) considered the observations made on some days, in different years from 1966 to 1971, using electric field sensors carried on balloons launched from Weissenau, Germany, from which the ionospheric potential was derived by vertical integration.
- The results were compared with fair weather observations made on the same days at the Kew observatory, on the outskirts of London.
- This indicates that global circuit concept holds over an area at least as great as the size of Europe.
(c) Global circuit modulation
- Next, the authors investigate studies of the response of the global atmospheric electric circuit to changes of the flux of cosmic rays in order to test their understanding of the global circuit.
- Measurements of the conduction current show a positive response to cosmic ray changes, driven by the solar cycle (Markson, 1980; Harrison and Usoskin, 2010) .
- It shows as individual symbols the ionospheric potential observed from several different investigators on specific days; this usually lies between 150 and 300 kV.
- This indicates the flux of galactic cosmic rays with rigidities > 3 GV.
- The atmospheric conductivity is less at solar maximum than at solar minimum and the ionospheric potential is accordingly less.
(d) Global circuit cloud coupling
- This modulation of the fair weather current density by solar activity and associated cosmic ray changes provides a potential mechanism whereby the properties of clouds at low heights in fair weather regions could be changed by the currents passing through them, with implications, currently not quantified, for changes in weather and climate as a result.
- Theory indicates that, at the edge of a horizontal layer cloud, the transition from low conductivity air within a cloud to air of greater conductivity outside the cloud will be accompanied by a region of enhanced space charge, when the current flows vertically through the cloud boundary.
- This result is not inconsistent with a cosmic ray effect on the global circuit which also influences clouds through a conduction current mechanism, and the analysis indicates rapid time scales of ~1day or less.
- A similar periodicity is apparent in surface PG data at Nagycenk Observatory, Hungary (Harrison and Marcz 2007) , during fair weather conditions but absent during disturbed weather when global circuit influences would be masked.
- Fig. 12 shows a combination of neutrons (a) and Lerwick cloud data (c), filtered as in Harrison (2008) , but with the short period of conduction current data available from Lerwick around the same time (Harrison and Nicoll, 2007) , panel (b).
(e) Applications of global atmospheric electricity
- The potential gradient (PG) in surface air is sensitive to aerosol pollution, because of the removal of ions by aerosol particles.
- In severely polluted air, the PG can be substantially raised and, since historically many measurements were made in urban regions, this provides a method by which past urban pollution information can be reconstructed (Harrison and Aplin 2002, 2003; Harrison, 2006 Harrison, , 2009)) .
- This reduces the columnar resistance to the ionosphere and, in a fair weather region, the current to the ionosphere increases.
- It would be interesting to observe the cut-off frequency of "tweeks" (Reeve and Rycroft 1972) , signals from distant lightning propagating at night over the earthquake-affected region in order to test the prediction of this mechanism where precursors to major earthquakes occurring over land affect the ionosphere.
- Pulinets and Ouzounov (2010) have presented a more complex mechanism relating these observable phenomena, and others.
4. Concluding remarks
- In this paper the authors have outlined the physical processes operating in the D.C. and A.C. global atmospheric electric circuit which rapidly couple phenomena occurring near the Earth's surface with the ionosphere and the near-Earth space environment.
- Finally, the authors suggest some studies which could advance the subjects discussed further.
- It seems to be desirable to: (i) investigate in more detail the relative contributions made by thunderstorm generators and by rain/shower cloud generators as drivers of the global electric circuit (Liu et al.
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Citations
141 citations
Cites background from "Electromagnetic Atmosphere-Plasma C..."
...3(b); the topic is covered briefly in Section 3.1 of Rycroft and Harrison (2011)....
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...Combining these data sets with optical lightning counts made from space (see Christian et al., 2003; Rycroft and Harrison, 2011), Mach et al. (2011) concluded that the mean contributions to the global electric circuit from land and ocean thunderstorms are 1.1 kA and 0.7 kA, respectively....
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...…mass ejections (CMEs) and other space weather effects, (e) auroral activity and (f) gigantic jets (as discussed briefly, but with more references, in Rycroft and Harrison (2011)), (viii) the effects of gigantic jets (see Chou et al., 2010) on the global circuit—they short circuit the 5% of the…...
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...The topic of the global atmospheric electric circuit has been recently reviewed by Rycroft and Harrison (2011), which thoroughly discussed the background to the subject, and gave a large number of key references to the literature going back more than a hundred years....
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...3(a) (Rycroft and Harrison, 2011)....
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109 citations
82 citations
Cites background from "Electromagnetic Atmosphere-Plasma C..."
...[2] Understanding of lightning effects in near Earth’s space has progressed substantially by studying associated transient luminous events (TLEs) in the mesosphere [Rodger, 1999; Pasko et al., 2011; Rycroft and Harrison, 2011]....
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82 citations
References
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"Electromagnetic Atmosphere-Plasma C..." refers background in this paper
...The D.C. global atmospheric electric circuit has been considered in the context of the changing climate of planet Earth by Williams (1992), Price (1993), Tinsley et al. (1994), Gray et al. (2010) and Siingh et al. (2011)....
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981 citations
"Electromagnetic Atmosphere-Plasma C..." refers background in this paper
...It is even true through the ionospheric dynamo field region at ~ 100 to ~ 130 km altitude (Rishbeth and Garriott 1969; Kelley 2009)....
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Frequently Asked Questions (16)
Q2. What is the contribution of rain/shower clouds to the global circuit?
The contributions to the global circuit made by rain/shower clouds are 0.22 kA for ocean storms and 0.04 kA for storms over the land.
Q3. What are the promising simulations of lightning?
Carlson et al. (2009, 2010) suggest that TGF production is associated with current pulses (~ 1 ms) in lightning leader channels and runaway processes, and they have produced promising simulations.
Q4. What is the way to identify periodicities?
Other than cosmic ray step changes, an alternative method of identifying cloud and global circuit responses to cosmic rays is to use spectral analysis methods to identify periodicities which are unique to cosmic rays.
Q5. What is the role of the global electric circuit in climate change?
The global electric circuit may be involved in climate change via non-linear electrical effects on cloud microphysical processes (Aplin et al.
Q6. What is the gp of the cosmic ray flux at high latitudes?
In the troposphere the cosmic ray flux at high latitudes is typically ~ 20% larger in solar minimum conditions that near solar maximum; it is ~ 10% larger at 33 degrees magnetic latitude.
Q7. What is the difficulty in determining the sensitivity of clouds to such changes?
The difficulty in determining the sensitivity of clouds to such changes, empirically at least, is the need to remove the substantial natural variability commonly present in cloud and clearly evident from satellite images of planet Earth.
Q8. What is the ion-pair production rate at different altitudes?
The ion-pair production rate at different altitudes varies by ~ 2.5 as one moves from the geomagnetic equator to the magnetic poles.
Q9. Why is the resistance near the surface of the earth so small?
Most of the resistance is near the surface, due to the exponential distribution of the atmospheric density with a scale height H of ~ 7 km.
Q10. How is the conductivity of a thundercloud measured?
Inside an active thundercloud, the electrical conductivity is not well-constrained, but observations discussed by Rycroft et al. (2007) show that it is at least a factor of six less than its value in the clear air surrounding the thundercloud; Rycroft et al. (2007, 2008) showed values for a model thundercloud.
Q11. How much less is the conductivity at solar maximum?
If the conductivity varies as the square root of the ion production rate (Rycroft et al. 2008), as expected in marine air where there is no radon contribution, nor appreciable ion removal by aerosol, it will be ~ 6% less near solar maximum.
Q12. What are the three processes that occur when a lightning discharge is made?
These are leader processes which occur as a lightning discharge progresses in steps from a thundercloud towards the ground, the cloud-to-ground (CG) return stroke which is a large (~ 30 kA) current to the cloud, and intra-cloud (IC) discharges (Rakov and Uman 2003).
Q13. What is the effect of the modulation of the fair weather current density by solar activity?
This modulation of the fair weather current density by solar activity and associated cosmic ray changes provides a potential mechanism whereby the properties of clouds atlow heights in fair weather regions could be changed by the currents passing through them, with implications, currently not quantified, for changes in weather and climate as a result.
Q14. What are the mechanisms to account for such effects?
Mechanical (i.e. acoustic or gravity wave, or tidal, mechanisms to account for such effects have been considered by Hayakawa (2011).
Q15. What is the common type of radiation created by the electrons?
This is believed to be created as upward-going Bremsstrahlung radiation when the electrons collide with the nuclei of atmospheric atoms.
Q16. How many kA do thunderstorms contribute to the global circuit?
A very recent paper by Mach et al. (2011) has, for the first time, used experimental data from aircraft and satellites to deduce that thunderstorms over the land contribute 1.1 kA to the global circuit and, over the oceans, 0.7 kA.