Electromagnetic Atmosphere-Plasma Coupling: The Global Atmospheric Electric Circuit

TLDR: In this article, a description of the global atmospheric electric circuit operating between the Earth's surface and the ionosphere is given, with a huge range of horizontal and vertical spatial scales, ranging from 10−9 m to 1012 m, concerned with many important processes at work.

Abstract: A description is given of the global atmospheric electric circuit operating between the Earth’s surface and the ionosphere. Attention is drawn to the huge range of horizontal and vertical spatial scales, ranging from 10−9 m to 1012 m, concerned with the many important processes at work. A similarly enormous range of time scales is involved from 10−6 s to 109 s, in the physical effects and different phenomena that need to be considered. The current flowing in the global circuit is generated by disturbed weather such as thunderstorms and electrified rain/shower clouds, mostly occurring over the Earth’s land surface. The profile of electrical conductivity up through the atmosphere, determined mainly by galactic cosmic ray ionization, is a crucial parameter of the circuit. Model simulation results on the variation of the ionospheric potential, ∼250 kV positive with respect to the Earth’s potential, following lightning discharges and sprites are summarized. Experimental results comparing global circuit variations with the neutron rate recorded at Climax, Colorado, are then discussed. Within the return (load) part of the circuit in the fair weather regions remote from the generators, charge layers exist on the upper and lower edges of extensive layer clouds; new experimental evidence for these charge layers is also reviewed. Finally, some directions for future research in the subject are suggested.

Figures

Figure 12

Fig. 2. Diagram showing the enormous range of time scales involved in important processes (horizontal words) occurring at different altitudes. Vertical words represent different phenomena which occur on different temporal scales over different altitude ranges – see the text for a fuller discussion. The boundary between A.C. and D.C. variations is placed at ~ 200 s, the RC time constant of the poorly conducting atmospheric region between the good conducting Earth and the ionosphere.

Fig. 3. Taken from Rycroft et al. (2000), the upper panel illustrates a part of the global atmospheric electric circuit. On the left is shown a representative thunderstorm, one of the ~ thousand which are active at all times and which occur over only a small part of the Earth’s surface. Huge potential differences are generated inside thunderclouds; acting as giant batteries, they drive an upward (Wilson) current to the ionosphere at an altitude of ~ 80 km. The ionosphere is essentially an equipotenial surface at ~ + 250 kV with respect to the Earth’s surface, but small currents reach up into the magnetosphere. In fair weather regions remote from thunderstorms, over most of the Earth’s surface, downward currents ~ 2 pA/m 2 flow vertically (radially) to the ground. Much more positive charge resides near the Earth’s surface than in the stratosphere or mesosphere, as indicated by the density of the + symbols. The lower panel outlines a simple electrical engineering representation of the circuit. Over mountainous regions, the atmospheric columnar resistance is much less than it is elsewhere. Values for the RC (or Cr) time constant are shown at the lower right.

Fig. 3. Taken from Rycroft et al. (2000), the upper panel illustrates a part of the global atmospheric electric circuit. On the left is shown a representative thunderstorm, one of the ~ thousand which are active at all times and which occur over only a small part of the Earth’s surface. Huge potential differences are generated inside thunderclouds; acting as giant batteries, they drive an upward (Wilson) current to the ionosphere at an altitude of ~ 80 km. The ionosphere is essentially an equipotenial surface at ~ + 250 kV with respect to the Earth’s surface, but small currents reach up into the magnetosphere. In fair weather regions remote from thunderstorms, over most of the Earth’s surface, downward currents ~ 2 pA/m 2 flow vertically (radially) to the ground. Much more positive charge resides near the Earth’s surface than in the stratosphere or mesosphere, as indicated by the density of the + symbols. The lower panel outlines a simple electrical engineering representation of the circuit. Over mountainous regions, the atmospheric columnar resistance is much less than it is elsewhere. Values for the RC (or Cr) time constant are shown at the lower right.

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Full text

Electromagnetic atmosphere-plasma
coupling: the global atmospheric electric
circuit
Article
Accepted Version
Rycroft, M. J. and Harrison, R. G. (2012) Electromagnetic
atmosphere-plasma coupling: the global atmospheric electric
circuit. Space Science Reviews, 168 (1). pp. 363-384. ISSN
0038-6308 doi: https://doi.org/10.1007/s11214-011-9830-8
Available at https://centaur.reading.ac.uk/24952/
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1
Electromagnetic atmosphere-plasma coupling: The global electric
circuit
M.J. Rycroft
(1)
and R.G. Harrison
(2)
(1) CAESAR Consultancy, 35 Millington Road, Cambridge CB3 9HW, UK;
International Space University, 1 rue Jean-Dominique Cassini, 67400 Ilkirch-
Graffenstaden, France
(2) Department of Meteorology, University of Reading, Earley Gate, PO Box 243,
Reading, Berkshire RG6 6BB, UK
accepted for publication in Space Science Reviews doi:
10.1007/s11214-011-9830-8
Abstract
A description is given of the global atmospheric electric circuit operating between the
Earth’s surface and the ionosphere. Attention is drawn to the huge range of horizontal
and vertical spatial scales ranging from 10
-9
m to 10
12
m, concerned with the many
important processes at work. A similarly enormous range of time scales is involved from
10
-6
s to 10
9
s, in the physical effects and different phenomena that need to be
considered. The current flowing in the global circuit is generated by disturbed weather
such as thunderstorms and electrified rain/shower clouds, mostly occurring over the
Earth's land surface. The profile of electrical conductivity up through the atmosphere,
determined mainly by galactic cosmic ray ionisation, is a crucial parameter of the
circuit. Model simulation results on the variation of the ionospheric potential, ~ 250kV
positive with respect to the Earth's potential, following lightning discharges and sprites
are summarized. Recent experimental results comparing global circuit variations with the
neutron rate recorded at Climax, Colorado, are then discussed. Within the return (load)
part of the circuit in the fair weather regions remote from the generators, charge layers
exist on the upper and lower edges of extensive layer clouds; new experimental evidence
for these charge layers is also reviewed. Finally, some directions for future research in the
subject are suggested.
Keywords: global circuit; thunderstorms; electrified cloud generators; cosmic rays;
atmospheric electrical conductivity profile; fair weather regions; ionospheric potential;
lightning; sprites; layer cloud electrification
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. This
coupling takes place rapidly, at, or close to, the speed of light c (Rycroft 2006), as
opposed to coupling mechanisms involving mechanical waves of one type or the other
which propagate at speeds much slower than c, and which are discussed in other papers in
this volume. As outlined by Aplin et al. (2008), the subject of atmospheric electricity had
its origins in the eighteenth century, grew into the concept of the global atmospheric
electric circuit in the early twentieth century with the seminal papers of Wilson (1921,
1929, 1956), and matured considerably in the first decade of the twenty first century.

2
The global circuit is formed between the surface of the Earth and the ionosphere, both of
which are good electrical conductors in comparison with the insulating atmosphere
between them. D.C. and A.C. electric generators exist in the atmosphere, for example in
thunderstorms, creating currents flowing up to the ionosphere. The current circuit closes
through fair weather parts of the atmosphere that are remote from the generators. Useful
background papers on different aspects of the global atmospheric electric circuit have
been written by Vonnegut (1973), Makino and Ogawa (1984, 1985), Roble and Tzur
(1986), Volland (1987), Hays and Roble (1979), Roble aand Hays (1979), Roble (1991),
Bering et al. (1998), Rycroft et al. (2000, 2007, 2008), Williams (2002, 2009), Harrison
(2004), Siingh et al. (2005, 2007, 2011), Aplin (2006), Markson (2007), Tinsley et al.
(2007), Harrison et al. (2008) and Tinsley (2008). The present paper complements these
papers and those which they cite. 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).
When considering a subject in physics, and its mathematical representation, the treatment
conventionally begins simply, using linear theory. However, as the subject develops, it is
usually appreciated that a more complex representation is appropriate. At larger
amplitudes nonlinearities can arise and the system may even become chaotic on the small
scale, or on the local, moderate (regional) or largest (global) scales. An important
characteristic of such complex systems is that they simultaneously need to consider a
wide range of spatial scales and temporal scales. Up to the present, the treatment of
atmospheric electricity has remained linear, whereas nowadays climate studies often
involve multi-scale and nonlinear behaviour (Donner et al. 2009; Slingo et al. 2009;
Palmer and Williams 2010). The global electric circuit may be involved in climate
change via non-linear electrical effects on cloud microphysical processes (Aplin et al.
2008; Carslaw 2009; Harrison and Ambaum 2008, 2010; Nicoll and Harrison 2010), as
discussed later in this paper.
Fig. 1 shows the huge range of horizontal and vertical scales involved in the diverse
phenomena and processes of interest which occur in the atmosphere and in the near-Earth
space environment (Rycroft 2010). The horizontal scale extends over 18 orders of
magnitude, and the vertical scale over 12 orders. At the Earth’s surface, point discharge
currents (Chalmers 1962; Ette and Utah 1973; Marcz and Bencze 1998) emanate from
sharp-ended vertical conductors, such as grass and the spiky needles of coniferous trees,
which have scales of millimetre size. Pointed hills and ridges have scales of kilometres to
hundreds of kilometres, and the oceans longer scales, although sea spray has dimensions
of millimetres.
In the lowest part of the atmosphere over continental surfaces, ionisation is generated
from the escape of radon isotopes (Harrison et al. 2010) and by galactic cosmic rays
arriving from beyond the solar system (Bazilevskaya et al. 2000, 2008; Velinov et al.
2009). In clean air, water vapour condenses onto these ions to form cluster ions ~ 1 nm in
size (Aplin et al. 2008, Rycroft et al. 2008). Some cluster ions are removed by ion-ion
recombination and others are lost to aerosol particles (Harrison and Tammet 2008,

3
Hirsikko et al. 2011). In some circumstances in which condensable vapours such as
sulphuric acid are abundant, the cluster ions may eventually grow into ~ 100 nm sized
cloud condensation nuclei (CCN) onto which cloud droplets can form; raindrops, which
are up to three orders of magnitude larger, result from coalescence of the cloud droplets.
It is worth mentioning here that Enghoff et al. (2011) have recently studied sulphuric acid
aerosol nucleation in an atmospheric pressure reaction vessel where a 580 MeV electron
beam has ionised the gas. They found clear evidence for an ion-induced effect on aerosol
nucleation under conditions which resemble those of the Earth’s atmosphere.
Also shown in Fig. 1 are low level stratiform clouds (SCs) such as stratocumulus, where
the electric charges at the cloud edges are important (Nicoll and Harrison 2009, 2010),
and thunderstorm cells (TCs) which can grow into large thunderstorms (Williams and
Yair 2006), termed mesoscale convective systems (MCSs). Thunderstorms produce
lightning discharges which radiate electromagnetic waves across a broad range of
frequencies; these constitute the A.C. part of the global electric circuit, discussed later.
Above large thunderstorms transient luminous events (TLEs), such as sprites, elves and
blue jets (Fullekrug et al. 2006), may occur just below the ionosphere. The lower
ionosphere responds to activity from above, in the form of wave-particle interactions
between whistler-mode waves from lightning and energetic electrons trapped in the
magnetosphere (Rycroft 2010); extra ionization is then produced in the lowest ionosphere
(Rodger et al. 2001). The magnetosphere is stimulated by activity on the Sun, that
information travelling through interplanetary space in the form of coronal mass ejections
(CMEs); these phenomena are generically termed space weather (Bothmer and Daglis
2007). Such phenomena (Rycroft 2010) are important in terms of possible damage to
satellites and other assets in space and to humans aboard spacecraft.
Fig. 2 shows the broad range of temporal scales that are involved in the many phenomena
of importance. On the shortest time scales of microseconds are electrical discharge
phenomena. 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). Lightning radiates all radio frequencies from MHz (associated with
leader processes) to ~ 10 kHz (where the spectrum peaks (Smith et al. 2010)) to “slow
tails” (~ 100 Hz, Mullayarov et al. 2010), and to the longest wavelength electromagnetic
waves occurring in the Earth’s environment (~ 10 Hz). These latter waves excite
Schumann resonances of the spherical shell cavity between the good conducting Earth
and ionosphere, the fundamental of which is at 8 Hz (Williams 1992; Price et al. 2007;
Simoes et al. 2008; Yang et al. 2009; Shvets et al. 2010; Nickolaenko et al. 2010;
Golkowski et al. 2011).
A few stations around the world can record the radiation of various frequencies generated
by lightning and by sprites in order to find their location. Williams et al. (2010) did this
for radio signals produced over Africa, investigating their dependence on the charge
moment changes of the parent lightning. Whitley et al. (2011) have recently shown that
with four stations around the world sources can be geolocated to an accuracy of ~ 10 km,

Frequently asked questions

Q1. What are the contributions in "Electromagnetic atmosphere-plasma coupling: the global atmospheric electric circuit" ?

Model simulation results on the variation of the ionospheric potential, ~ 250kV positive with respect to the Earth 's potential, following lightning discharges and sprites are summarized. Finally, some directions for future research in the subject are suggested. 

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.