Acta Geophysica
vol. 59, no. 1, Feb. 2011, pp. 183-204
DOI: 10.2478/s11600-010-0035-4
________________________________________________
© 2010 Institute of Geophysics, Polish Academy of Sciences
Estimation of Global Lightning Activity
and Observations of Atmospheric Electric Field
Marek GOŁKOWSKI
1
, Marek KUBICKI
2
, Morris COHEN
3
,
Andrzej KUŁAK
4
, and Umran S. INAN
3,5
1
Department of Electrical Engineering, University of Colorado Denver,
Denver, CO, USA, e-mail: mark.golkowski@ucdenver.edu
2
Institute of Geophysics, Polish Academy of Sciences, Warszawa, Poland
e-mail: swider@igf.edu.pl (corresponding author)
3
Department of Electrical Engineering, Stanford University, Stanford, CA, USA
e-mails: mcohen@stanford.edu, inan@stanford.edu
4
Astronomical Observatory, Jagiellonian University, Kraków, e-mail: radiol1@wp.pl;
Department of Electronics, Academy of Mining and Metallurgy, Kraków, Poland
5
Department of Electrical Engineering, Koç University Sariyer, Istanbul, Turkey
Abstract
Variations in the global atmospheric electric circuit are investigated
using a wide range of globally spaced instruments observing VLF
(~10 kHz) waves, ELF (~300 Hz) waves, Schumann resonances
(4-60 Hz), and the atmospheric fair weather electric field. For the
ELF/VLF observations, propagation effects are accounted for in a novel
approach using established monthly averages of lightning location pro-
vided by the Lightning Image Sensor (LIS) and applying known fre-
quency specific attenuation parameters for daytime/nighttime ELF/VLF
propagation. Schumann resonances are analyzed using decomposition
into propagating and standing waves in the Earth-ionosphere waveguide.
Derived lightning activity is compared to existing global lightning detec-
tion networks and fair weather field observations. The results suggest
that characteristics of lightning discharges vary by region and may have
diverse effects upon the ionospheric potential.
Key words: global electric circuit, lightning discharges, ELF/VLF
waves, Schumann resonances.
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1. INTRODUCTION
The relationship between atmospheric currents and ionospheric potentials on
a planetary scale is known as the global electric circuit. The global nature of
the phenomena were first prominently documented in the reproducible
worldwide diurnal variation of electric potential in the lower atmosphere.
Wilson (1921) proposed a connection between the variation of atmospheric
potential and thunderstorm activity in which the latter plays the role as a
generator and current source. Although the global electric circuit has been
the subject of study for almost 100 years, many questions remain and even
fundamental assertions are often contended. While in the last few decades
many authors have asserted that global thunderstorms and lightning directly
drive the global circuit (Rycroft et al. 2000, Kartalev et al. 2006), several re-
cent works suggest a contrary view. Rycroft et al. (2007) employed circuit
simulation software to conclude that lightning discharges only contribute to
~1% of ionospheric potential changes and Williams and Sátori (2004) em-
phasize the importance of electrified shower clouds. It is worthy to note that
Wilson (1921) initially postulated the importance of both thunderclouds and
shower clouds although for many years the latter assertion was often over-
looked. It is clear that the link between the DC phenomena of the equipoten-
tial global ionosphere and the AC phenomena of lightning discharges and
their related effects are still poorly understood.
One of the challenges in investigating the global electric circuit is the
difficulty in obtaining simultaneous measurements on a global scale. Obser-
vations of the “fair-weather” electric field as a measure of ionospheric poten-
tial have been recorded for many years but the notion of an equipotential
global ionosphere is often muddled by effects of cosmic rays, energetic par-
ticle precipitation (Rycroft et al. 2000) and aerosol content in the stratos-
phere and troposphere (Tinsley and Zhou 2006). Even greater uncertainty
exists in quantitative evaluation of global lightning activity despite recent
technological advances in this area. The Optical Transient Detector (OTD)
onboard the MicroLab-1 satellite and its successor, the Lightning Image
Sensor (LIS) onboard the TRMM Observatory, have extended lightning de-
tection into space (Christian et al. 2003). Both instruments detect lightning
flashes optically with high efficiency but are limited to observing a finite
viewing area providing global coverage only with multiple passes. Thus,
although both LIS and OTD instruments provide extremely valuable long
term averages of lightning frequency and distribution, they are unable to
quantify lightning activity at any given instant.
Quantification of global lightning activity using radio measurements
often involves the ELF and VLF bands because of the low attenuation and
long distance propagation of lightning induced radiation at these frequencies.
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GLOBAL LIGHTNING ACTIVITY AND ATMOSPHERIC ELECTRIC FIELD
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In the ELF band, Schumann resonances are correlated with global lightning
activity (Füllekrug and Fraser Smith 1996), but the determination of
lightning location from Schumann resonances alone is difficult and requires
special analysis (Kułak et al. 2006). Global lightning location networks in
the VLF band have been constructed of which the World Wide Lightning
Location Network (WWLLN) (Dowden et al. 2002, Lay et al. 2004, Rodger
et al. 2006) is currently the most prominent example. However, WWLLN is
only able to locate major storms accurately, the detection efficiency for
cloud to ground lightning events is less than a few percent (Rodger et al.
2005). Utilization of WWLLN data to estimate global lightning activity is
thus strongly biased to large lightning events.
In the absence of a network that can accurately locate the majority of
lightning events worldwide, there is a need for accurate proxy measurements
of global lightning activity. Use of a smaller number of receiving sites (1-3)
to quantify global lightning activity (but not locate events) and comparison
with atmospheric electric field observations was carried out by Füllekrug et
al. (1999) and Troshichev et al. (2004). While energy in the ELF/VLF bands
is directly related to global lightning activity, the propagation effects of the
Earth-ionosphere waveguide, especially the day versus night differences,
make this data difficult to interpret. Neither Füllekrug et al. (1999) nor Tro-
shichev et al. (2004) take propagation effects into account.
A significant issue which recent work has exposed is the specific charac-
teristics of lightning in different regions of the globe. The intensity of
lightning events is often categorized by its peak current I
p
(the highest elec-
trical current in the return stroke), or the total charge moment Qℓ (total
charge transfer multiplied by altitude). In particular, ocean lightning has
been shown to be less prevalent than continental lightning but to host the
most intense discharges (Biswas and Hobbs 1990). Füllekrug et al. (2002)
confirm that high peak current discharges are more likely to occur over the
ocean and show evidence that such intense ocean discharges are also more
likely to be negative rather than positive CG strokes. Chen et al. (2008) ana-
lyze global distributions of lightning in the context of transient luminous
events (TLE) and find that sprites are more often triggered by continental
and coastal lightning with the Congo Basin in Subsaharan Africa a key “hot
zone” for such observations. In contrast, elve type TLE observations are
more prominently produced by ocean lightning with high concentrations in
the Carribean Sea, Central and Southwest Pacific Ocean and Indian Ocean.
Elves are a product of the electromagnetic pulse (EMP) emitted by lightning
strokes (Fukunishi et al. 1996) making them an indicator of high peak cur-
rent of lightning discharges. Sprites, on the other hand, are produced by the
quasi-static electric fields (Pasko et al. 1997) which are associated with
lightning charge moment. Thus, in comparing lightning characteristics
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M. GOŁKOWSKI et al.
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across the globe it seems that African lightning is characterized by relatively
higher charge moment, while lightning from the regions of the Pacific and
Indian Ocean can be expected to be dominant in the metric of peak current.
It is worth noting that such a conclusion is supported by earlier work by
Boccippio et al. (2000) who utilized OTD and LIS measurements to investi-
gate regional differences in lightning distributions. The authors find that
lightning in Africa and in particular the Congo Basin yielded the greatest
flash rates with Central America and Southeast Asia (the Maritime Conti-
nent) ranking second and third, respectively. However, for mean flash ra-
diance and optical emissions of lightning discharges, ocean regions and
Central America clearly supersede the African continent.
Taking into account the disparate properties of global lightning can shed
light on the role of these discharges in the global electric circuit, including
their effect on diurnal changes in ionospheric potential. Key to making
progress in this field is the integration of diverse measurements that have in
the past been treated largely in isolation. We present a preliminary study ex-
amining ELF/VLF radiation, Schumann resonances, lightning localization,
and ionospheric fair weather potential for 14 days during March and May of
2007.
2. SETUP AND METHODOLOGY
2.1 ELF/VLF measurements and analysis
Stanford University operates a network of global receiving sites that record
data in the band from 300 Hz to 47 kHz using aircore magnetic field anten-
nas. Cohen et al. (2009) provide a description of the receiver hardware. In this
study, data from 4 sites are used. The location and abbreviation of each site
is given in Table 1. Recordings at each site were made periodically 1 minute
out of every 5 minutes or 1 minute out of every 15 minutes for 21 or 23
hours per day depending on the site. For the purpose of estimating lightning
Table 1
Description of receiver stations
Site Abbreviation Latitude Longitude Observation
Adelaide, Australia AD 34.32°S 138.46°E ELF/VLF
Chistochina, USA CH 62.61°N 144.62°W ELF/VLF
Palmer, Antarctica PA 64.05°S 64.77°W ELF/VLF
Taylor, USA TA 40.46°N 85.51°W ELF/VLF
Świder, Poland SW 52.07°N 21.16°E E
z
Hornsund, Spitsbergen HO 72.00°N 15.50°E E
z
Hylaty, Poland HY 49.28°N 22.48°E SR
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GLOBAL LIGHTNING ACTIVITY AND ATMOSPHERIC ELECTRIC FIELD
187
activity two frequency bands known to be dominated by lightning radiation
were chosen for analysis, 9-11 kHz in the VLF band and 310-340 Hz in the
ELF band. In this context, further use of the terms VLF and ELF in this work
is taken to mean these restricted frequency bands. Hourly amplitude values
for each band were calculated at each site by averaging the appropriate fre-
quency spectrum over the synoptic minutes in the hour.
In the initial analysis, the hourly averages were examined directly as was
done by Troshichev et al. (2004) and it became immediately clear that the
day-night propagation effects of the Earth-ionosphere waveguide dominate
the variations. Figure 1 shows diurnal averages for 6 days in March 2007 for
the ELF and VLF bands recorded at PA as well as maps showing the posi-
tion of the day-night terminator at various hours during the day. The times
corresponding to darkness over the station witness increased amplitudes of
ELF/VLF activity. Moreover, the amplitude quickly decreases with the onset
of sunrise. The strong influence of the day/night transitions on the observa-
tions are due to the higher attenuation of propagating modes by the daytime
ionospheric boundary as compared to the nighttime boundary and also ref-
lection of radiation from the terminator boundary itself. Naturally, the prop-
agation effect depends not only on the location of the receiving station with
respect to the day-night terminator but also on the location of the lightning
Fig. 1. Maps showing day/night locations at 85 km altitude for various UT times.
VLF and ELF hourly amplitudes observed at PA closely correspond to sunrise/sun-
set conditions at the receiving station. Such local diurnal variations need to be taken
into account when interpreting ELF/VLF observations. Colour version of this figure
is available in electronic edition only.
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