Brief communication: Earthquake–cloud coupling through the global atmospheric electric circuit Article 

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Harrison, R. G., Aplin, K. L. and Rycroft, M. J. (2014) Brief communication: Earthquake–cloud coupling through the global atmospheric electric circuit. Natural Hazards and Earth System Science, 14 (4). pp. 773­777. ISSN 1684­9981 doi: https://doi.org/10.5194/nhess­14­773­2014 Available at http://centaur.reading.ac.uk/36745/ 

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Brief Communication: Earthquake–cloud coupling through theglobal atmospheric electric circuit

R. G. Harrison1, K. L. Aplin 2, and M. J. Rycroft3

1Department of Meteorology, University of Reading, Earley Gate, Reading RG6 6BB, UK2Clarendon Laboratory, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK3CAESAR Consultancy, 35 Millington Road, Cambridge, CB3 9HW, UK, and Centre for Space, Atmospheric and OceanicSciences, University of Bath, Bath BA2 7AY, UK

Correspondence to:K. L. Aplin (karen.aplin@physics.ox.ac.uk)

Received: 11 November 2013 – Published in Nat. Hazards Earth Syst. Sci. Discuss.: 6 December 2013Revised: 14 March 2014 – Accepted: 19 March 2014 – Published: 10 April 2014

Abstract. We illustrate how coupling could occur betweensurface air and clouds via the global electric circuit –through Atmospheric Lithosphere–Ionosphere Charge Ex-change (ALICE) processes – in an attempt to develop aphysical understanding of the possible relationships betweenearthquakes and clouds.

1 Introduction

From time to time, papers are published suggesting thatthere are visible changes in the atmosphere that are associ-ated with earthquakes, or that could even provide an earth-quake precursor. Because of the widespread availability ofhigh-quality satellite imagery, such optical possibilities areclearly compelling. Recently, for example, Guangmeng andJie (2013) considered the potential for using observations ofcloud changes as an earthquake precursor, by examining se-quences of satellite images around the times of earthquakes.A full statistical climatology of the cloud behaviour in anyearthquake region is essential before any cloud feature can betruly regarded as anomalous, but, more importantly, a plau-sible physical mechanism able to connect earthquakes andclouds has also been lacking.

Generating a physical mechanism linking earthquakes andclouds is troublesome because there is no clear agreementon what constitutes an “earthquake cloud”, and a wide rangeof disparate cloud-related phenomena have been attributedto the effects of earthquakes. For example, there are sev-eral reports of anomalous cloud formations over fault zones

near earthquakes (e.g. Guangmeng and Jie, 2013; Guo andWang, 2008), although the height of the clouds affected isnot consistent. In some other cases the clouds described havebeen iridescent, implying that detailed droplet properties,such as size, might be affected by an underlying physicalprocess. There are also observations of enhanced clear-skyemission in the thermal infrared radiation detected by satel-lites. Typically these are equivalent to a temperature changeof a few Kelvin, beyond the natural variability (Tramutoli etal., 2005), appearing some days before the earthquake (e.g.Saraf et al., 2008; Guo and Wang, 2008).

At the simplest level, clouds require water vapour, whichcould be released from the Earth’s crust by seismic changes.Whilst such plumes of water vapour might be initially buoy-ant from geothermal heating, mixing processes in the natu-rally variable lower atmosphere seem likely to remove theidentity of the seismically generated water vapour (or wa-ter vapour fluctuations) as it ascends to cloud levels. (Thereis also little prospect of high-altitude clouds being affected inthis way, such as those formed from ice). The thermal anoma-lies identified could also be generated by similar surface out-gassing, such as the infra-red absorbing gases of carbon diox-ide and methane (Saraf et al., 2008).

Changes in cloud features and associated thermal anoma-lies have been attributed to water condensation ontoseismically-released ions (Pulinets and Ouzounov, 2011;Freund, 2013), which may exceed the existing natural back-ground ionisation. A major difficulty with the ion-inducednucleation proposal is that condensation of water dropletson ions – a process exploited in the laboratory Wilson cloud

Published by Copernicus Publications on behalf of the European Geosciences Union.

774 R. G. Harrison et al.: Earthquake–cloud coupling through the global atmospheric electric circuit

chamber to visualise cosmic ray tracks – requires extremelevels of water super-saturation of 400 % or more. This levelof super-saturation is more than two orders of magnitudegreater than that naturally occurring in the terrestrial atmo-sphere. Changes in ion properties through hydration havebeen reported, and nucleation to yield ultrafine particles hasbeen observed, for example, in experiments using optimisedgas mixtures (e.g. reviewed in Harrison and Carslaw, 2003).However, these molecular cluster ions or ultrafine particlesremain much smaller than the minimum size of particle re-quired for cloud droplet formation, which is typically a fewhundred nanometers in diameter. Growth of ion clusters tothese sizes in clean air with sufficient supply of condens-able vapour requires many hours (e.g. Harrison and Carslaw,2003). Hence, even if this growth process can occur with-out interruption in the real atmosphere where conditions aremuch more variable, the duration of the growth will allow thecluster ions concerned to be displaced by a considerable dis-tance from their point of generation (∼ 100 km for 10 h, witha small surface wind speed of 3 ms−1). A rapid but continu-ous process, such as the flow of electric current, is thereforerequired if surface information is to be imprinted in cloudsdirectly above seismic activity.

Changes in the surface structure of rocks under stress havealso been suggested to lead to infra-red emission and subse-quent thermal anomalies (Freund, 2013). Furthermore, seis-mically released atmospheric ions may themselves directlyabsorb infra-red radiation (e.g. Rycroft et al, 2012). Finally,a link between enhanced ionisation from radioactivity andclouds was suggested from the long-range cloud dissipationapparent in a satellite image of the Chernobyl reactor plume(Brandli and Leuck, 1987)1.

2 Global circuit coupling

An alternative route for earthquake coupling to clouds seemspossible through atmospheric electricity. Previously, Harri-son et al. (2010) argued that enhanced ionisation in thelower atmosphere (and, specifically, in the planetary bound-ary layer), will increase the vertical current flow alwayspresent in fair weather from the global atmospheric electriccircuit (e.g. Rycroft et al., 2012). The importance of the ver-tical current density – denotedJc – is that it links surfaceair ionisation changes directly to the ionosphere, unlike sur-face electric field changes, which are insufficient to causeionospheric electrical changes (Denisenko et al., 2013). Thismechanism of Atmospheric Lithosphere–Ionosphere ChargeExchange (subsequently referred to here by the acronym

1Ionospheric changes during two major nuclear reactor ac-cidents are inconsistent. Fuks et al. (1997) observed an iono-spheric response following the Chernobyl incident, but Kakinamiet al. (2011) did not consider the ionospheric changes around theFukushima event to be unambiguously linked with the nuclear ac-cident.

9

Figure 1 314 315

316 317 318 Caption for figure 1: 319 320 Coupling of surface ionisation changes to layer clouds through the global circuit. The 321 conduction current flowing (density Jc), is related to the vertical columnar resistance 322 Rc and the globally-established ionospheric potential VI. The boundary layer in the 323 base of the lower atmosphere contributes the majority of the columnar resistance. 324 Hence, ionisation released into this region by rock fracturing (shown on the left) or 325 radioactivity (shown on the right) will reduce Rc and increase Jc for fixed VI, from 326 Ohm’s Law. The charge accumulating on cloud droplets at the upper and lower cloud 327 boundary is proportional to Jc and, therefore, in turn, to the surface ionisation. This 328 charge may influence the cloud microphysical processes, e.g., via droplet interactions. 329 330

ionosphere

boundary layer

Jc

V=VI

total columnar resistance

Rc

V=0

+ ++ + + ++ + + + + + +

- - - - - - - - - - - - -

extensive layer cloud

+

+

++ + + +

+ +----

Fig. 1. Coupling of surface ionisation changes to layer cloudsthrough the global circuit. The conduction current flowing (densityJc) is related to the vertical columnar resistanceRc and the glob-ally established ionospheric potentialVI . The boundary layer in thebase of the lower atmosphere contributes the majority of the colum-nar resistance. Hence, ionisation released into this region by rockfracturing (shown on the left) or radioactivity (shown on the right)will reduceRc and increaseJc for fixed VI , from Ohm’s Law. Thecharge accumulating on cloud droplets at the upper and lower cloudboundary is proportional toJc and, therefore, in turn, to the sur-face ionisation. This charge may influence the cloud microphysicalprocesses, e.g. via droplet interactions.

ALICE) provides an explanation for satellite observationsof pre-earthquake changes in natural radio waves in non-disturbed weather. Encouraged by the agreement between thepostulated changes and those now observed across a range ofearthquakes (Piša et al., 2013), the ALICE mechanism is ex-tended here to consider effects on simple cloud structures,horizontal layer clouds of water droplets (such as extensivelow-level stratus clouds) in semi-fair weather, through whichthe vertical current must pass in overcast conditions.

The consequence of vertical current flow through thehorizontal edge of a layer cloud is the local generationof charge at the horizontal cloud–air boundary (Fig. 1),which has already been directly observed within clouds usingballoon-carried instrumentation (Nicoll and Harrison, 2010).Charging of the water drops at the upper and lower cloudboundaries is anticipated to influence, in some cases, theevaporation–condensation of drops, and also the collisionalinteractions between small droplets (Rycroft et al., 2012).These effects result from the charge obtained by the dropletsafter they have formed. The droplet charging is proportionalto the vertical current flowing and the vertical gradient of the

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R. G. Harrison et al.: Earthquake–cloud coupling through the global atmospheric electric circuit 775

10

Figure 2 331 332

333 Caption for figure 2: 334 335 Calculated response in atmospheric electric and cloud parameters to changes in the 336 surface air volumetric ion production rate q for low pollution air (solid lines) 337 assuming 2000 particles per cm3 of radius 0.25 µm, and polluted air (dashed lines), 338 assuming 15000 particles per cm3. Variations are shown for (a) the vertical 339 conduction current Jc and (b) changes in the observed cloud base height. 340

1 2 5 10 20 50 100

-40

-20

020

40

q(cm−3s−1)

% J

c ch

ange

(a)

1 2 5 10 20 50 100

-300

-100

0100

300

q(cm−3s−1)

clou

d ba

se h

eigh

t (m

)

(b)

Fig. 2. Calculated response in atmospheric electric and cloud pa-rameters to changes in the surface air volumetric ion production rateq for low pollution air (solid lines) assuming 2000 particles cm−3

of radius 0.25 µm, and polluted air (dashed lines), assuming 15 000particles cm−3. Variations are shown for(a) the vertical conductioncurrentJc and(b) changes in the observed cloud base height.

cloud to clear air transition, with the charge per unit volumeρ at the cloud boundary given by

ρ = −ε0Jcd

dz

(1

σ

), (1)

in which the conductivityσ varies vertically with heightzacross the horizontal cloud–air boundary, andε0 is the per-mittivity of free space.

Equation (1) shows that properties of the cloud – specif-ically, the charge per unit volume at the cloud edge – arelinked to the vertical current flow. Should the current flowbe modified by ionisation changes near the surface, such asthrough the release of radon or the fracture of rocks (e.g. Fre-und, 2013), the cloud droplet charge would also vary in re-sponse. The global circuit current therefore provides a linkbetween surface changes and the cloud directly above.

3 Quantitative considerations

Changes in the vertical conduction current from surface ion-isation variations can be calculated by considering the bal-ance between ion generation and loss to atmospheric parti-cles, for a unit area column of atmosphere. The full method-ology was discussed in the description of the ALICE mecha-nism given in Harrison et al. (2010). For an ionospheric po-tential VI , which is assumed to be an equipotential regionsince the ionosphere’s conductivity is many orders of mag-nitude greater than that of the air’s conductivity below it, theconduction current densityJc is given by

Jc =VI

Rc≈ VI/

[k

σs+ RFT

], (2)

whereRc is the resistance of a unit area column betweenthe surface and the ionosphere. This resistance can be esti-mated using an approximate model based on the total sur-face layer conductivityσs, which is considered to representthe resistance in a layer of scale heightk (∼ 100 to 500 m)together with the resistance of the upper (“free troposphere”)part of the columnar resistanceRFT. The surface air conduc-tivity σs depends on the concentration of small ions presentn and their mean mobility µ, from

σs = 2nµe, (3)

wheree is the magnitude of the elementary charge (Harrisonand Carslaw, 2003). The surface air conductivity can be de-termined in terms of the ion production rateq and loss rateby ion–ion recombination and ion–aerosol attachment as

σs = µe

[√(β2Z2 + 4αq

)− βZ

]α

, (4)

where α is the ion–ion recombination coefficient(1.6× 10−12 m3 s−1), Z the monodisperse aerosol numberconcentration andβ the ion–aerosol attachment coefficient,which is ∼ 4× 10−11 m3 s−1 for 0.2 µm radius aerosol.Harrison et al. (2010) also pointed out that the responsedepends on the background aerosol concentration. This isbecause the sensitivity of the vertical conduction current tosurface ionisation change is greater in polluted air, as ionloss to aerosol particles is less effective at removing ionsthan, in clean air, the annihilation of ions by recombinationof a positive ion with a negative ion.

It can be seen from Eq. (4) that the air conductivity varieswith the ion production rateq, and that, through Eq. (2), theconduction current will respond, leading, through Eq. (1), toa change in the cloud edge charge. Hence a long-range re-lationship exists between the surface ion production and thecloud properties.

Calculation of the sensitivity of the vertical conductioncurrent requires some estimate of the likely changes in theionisation rate associated with earthquakes, from both radon

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776 R. G. Harrison et al.: Earthquake–cloud coupling through the global atmospheric electric circuit

Table 1. Summary and quantitative estimates of mechanisms by which rock stresses can produce excess ionisation in surface air beforeearthquakes.

Mechanism, example Ionisation changes Reference

Radon emissionbefore 1995 Kobeearthquake

Radon increase measured, conservatively estimated at 10 Bqm−3. Ion balance equationused to infer a 40 % change in ion concentration. Measured increases in ion concentra-tion before the earthquake of 1000–1400 cm−3 were consistent with changes inferredfrom the radon data. This implies a change in volumetric ionisation rate of 40–100 %,i.e. a rate of 14–20 cm−3s−1.

Omori et al. (2007) andreferences thereinYasuoka et al. (2010) andreferences therein

Radon emission in generalbefore earthquakes

Volumetric ionisation rate = 100–10 000 cm−3s−1 Liperovsky et al. (2005)

Fractoemission duringearthquakes andvolcanic eruptions

Ions per unit area at rock surface given as 109 cm−2s−1; vertical dimension of regioninto which the ions were released not available

Freund (2013)

release and fractoemission (Table 1). Measurements of ioni-sation rate anomalies before earthquakes are sparse and it isdifficult to trace some of the quoted figures to reliable mea-surements. Nevertheless, there are good data available fromthe time of the Kobe earthquake in Japan in 1995 (e.g. Ya-suoka et al., 2010), where radon concentrations increased forabout 2 months before the earthquake, reaching a peak (atwell over three standard deviations above the previous back-ground) 17 days before the earthquake, which was sustaineduntil the earthquake. When electricity supplies to the instru-ments resumed after the earthquake, radon levels had re-turned to the previous background level. Based on the radonincrease of 10 Bqm−3, and concurrent ion measurements,Omori et al. (2007) estimated a 40 % change in ion con-centration. In aerosol-free air, the ion concentration is pro-portional to the square root of the ion production rate and,in polluted urban air, the ion concentration scales linearlywith the ionisation rate (e.g. Harrison and Carslaw, 2003).The enhanced radon concentrations observed therefore cor-respond to a change in the ion production rate of 40–100 %,i.e.q = 14−20 cm−3 s−1, assuming a background ionisationrate ofq = 10 cm−3 s−1.

These values ofq are relatively conservative when com-pared to other suggested values, but they are based on fieldmeasurements rather than models. (Liperovsky et al. (2005)estimate a much greater ionisation rate enhancement, of 100–10000 cm−3 s−1.) Fractoemission mechanisms proposed byFreund (2013) do not yet provide an adequate estimateof the volumetric ionisation rate, so a combination of theradon-enhanced ionisation rates presented in Table 1 hasbeen used to estimate the enhanced ionisation rate as up to100 cm−3 s−1.

4 Estimate of cloud response

Some indications of the modulation of cloud propertieswhich might be expected from conduction current changesare available from studies of polar night clouds, showing an

averaged response in the cloud base consistent with conduc-tion current variations (Harrison and Ambaum, 2013). Po-lar night clouds were chosen for this analysis to remove theusual dominating influence of diurnal variations from solarheating, and the response was observed in the cloud baseheight. The determination of cloud base height is essentiallya measurement of vertical visibility, which can be regarded asindicating a change in the cloud base droplet properties, suchas droplet size or concentration. In this study, which averagedthe polar night cloud base measurements made, a similar re-sponse was found for both the Arctic and Antarctic of about4 m change in cloud base height for a unit percentage changein the conduction current density.

If this sensitivity is appropriate to semi-fair weather layerclouds in general (and a similar sensitivity was found throughan entirely different approach at a mid-latitude continentalsite by Harrison et al., 2013), then the possible cloud re-sponse to earthquake-induced changes can be estimated insimilar terms. Figure 2b applies this response to the calcu-lations of current density change from ionisation, obtainedfrom Fig. 2a. The sensitivity of the change in cloud propertiesis, as expected, greater in the polluted case, although it mustbe emphasised this is a highly idealised calculation which ne-glects any additional interactions between the cloud and pol-lution and indeed any other sources of variability. However,it still serves to illustrate the potential link between surfaceionisation changes and cloud properties aloft.

5 Conclusions

In reality, there is always considerable variability presentin the atmosphere and in clouds. Consequently there aremany competing sources of cloud variability, such as thatassociated with local orography, which may obscure effectssolely resulting from surface ionisation changes. Neverthe-less, there may also be conditions in which a cloud responseis observable, or indeed has possibly already been observed.Our purpose here is merely to suggest that there is a possible

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R. G. Harrison et al.: Earthquake–cloud coupling through the global atmospheric electric circuit 777

physical mechanism which can provide earthquake–cloudcoupling based on the ALICE ideas presented previously,which should be explored further. An appealing feature ofthis mechanism is that, rather than requiring transport of thesurface ionisation up to the cloud despite appreciable ionloss processes (with uncertain or indeed unlikely responsesin the cloud properties), the global circuit conduction currentdirectly, and rapidly, connects surface air ionisation changesto the properties of the cloud above in semi-fair weather.Many details clearly remain to be worked out, which wehope can be achieved experimentally and theoreticallydespite the traditional discipline boundaries between atmo-spheric and Earth sciences.

Edited by: A. CostaReviewed by: two anonymous referees

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