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Geophysical Research Letters
Supporting Information for
Observation of a New Class of Electric Discharges within Artificial Clouds
of Charged Water Droplets and Its Implication for Lightning Initiation
within Thunderclouds
Alexander Yu. Kostinskiy1,2, Vladimir S. Syssoev1,3, Nikolay A. Bogatov1,
Evgeny A. Mareev1, Mikhail G. Andreev3, Leonid M. Makalsky3,
Dmitry I. Sukharevsky1,3, Vladimir A. Rakov1,4
1Institute
of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia
2National
3High-Voltage
Research University Higher School of Economics, Moscow, Russia
Research Center of the All-Russia Institute of Electrical Engineering, Istra, Moscow region,
Russia
4Department
of Electrical and Computer Engineering, University of Florida, USA
Contents of this file
Text S1
Figure S1
Text S2
Figure S2
Introduction
This supporting information provides a description of the experimental setup and an
example of simultaneous IR and visible-range images of the same event.
Text S1. Experimental Setup
The experimental setup is shown in Fig. S1. Clouds of charged water droplets (10–15 m3
in size) of either polarity can be created. Charged cloud (1) was created by steam
generator (2.1) and high-voltage source (2.2) coupled with the corona-producing sharp
point. The latter was located in the nozzle (2.3) through which the steam-air jet was
passing. The jet had a temperature of about 100–120 °С and a pressure of 0.2–0.6 MPa.
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It moved out at a speed of about 400–420 m/s with an aperture angle of 28°, forming a
jet-shaped cloud seen in Fig. 1. The nozzle was located at the center of a grounded
metal plane (3) of 2 m in diameter. As a result of rapid cooling, the steam condensed
into water droplets with the typical radius of about 0.5 μm. The cloud temperature was
practically the same as that of ambient air, except for its central part. The ions charging
water droplets were formed in the corona discharge between the sharp point and the
nozzle (2.3). A DC voltage of 10–20 kV was applied to the point. Current associated with
the charge carried by the jet was in the range of 60 to 150 μA. The density of water
droplets in the cloud was 106 – 3 x 107 cm-3, and the average droplet charge was 1 to 30
charges of electron. As the total charge accumulated in the cloud approached 60 μC or
so, spark discharges spontaneously occurred between the cloud and grounded objects
nearby. In the case of negatively charged cloud, most of the sparks developed from a
grounded metallic sphere (4), as seen in Fig. 1. The sphere was 5 cm in diameter and
located 0.8 m from the grounded plane center. Its uppermost point was 12 cm above the
plane. It is important to note that the artificial cloud described here only roughly
simulates a real thundercloud, which has different dimensions, temperature, ice crystals
with a wide range of sizes, etc.
Currents in sparks originating on the sphere (4) were measured with a 1-Ω resistive
shunt, the signal from which was relayed to a Tektronix digitizing oscilloscope (5). As the
current exceeded a preset value, the oscilloscope was triggered, which, in turn,
generated a pulse that was used to trigger the high-speed framing camera with image
enhancement operating in the visible range (4Picos) (6) and high-speed infrared camera
(FLIR 7700M) (7). The latter was typically operated at 115 frames per second (frame
duration of 8.7 ms and exposure time of 6.7 ms) with a resolution of 640×512 pixels. The
high-speed camera operating in the visible range produced 2 frames with 1360×1024
pixels each. Overall picture of the discharge was recorded using a Canon still camera
(8). All cameras were installed at a distance of about 3 m from the cloud. The distance
from the cameras to the instrumented 5-cm sphere varied from 2.5 to 3.5 m or so. The
image shown in Fig. 1 was obtained using the Canon camera (5-s exposure). The
images shown in Figs. 2–5 were obtained using the FLIR 7700M (6.7-ms exposure). The
larger and smaller images shown in Fig. S2 were obtained using the FLIR 7700M (7.7ms exposure) and 4Picos (1-μs exposure), respectively.
In order to monitor the dynamics of the charge of the cloud, we used an elevated copper
sphere (9), 50 cm in diameter, which was grounded through a 100-MΩ resistor. Signals
from the resistor, indicating variations of the electric potential induced on the sphere by
the cloud charge, were recorded by the oscilloscope (5). This sphere was located at a
distance of 6 m from the cloud. The electric field at the surface of the grounded plane
was measured by an electric field mill (fluxmeter) (10).
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Figure S1. Experimental setup: 1 – charged cloud; 2.1 – steam generator; 2.2 – highvoltage source with corona-producing sharp point; 2.3 – nozzle; 3 – grounded metal
plane; 4 – 5-cm grounded sphere equipped with current measuring shunt; 6 – highspeed visible-range framing camera; 7 – high-speed infrared camera; 8 – still camera; 9
– 50-cm sphere for monitoring variations of the cloud charge; 10 – electric field mill
(fluxmeter).
Text S2. Example of simultaneous IR and rare visible-range images of the same
UPF inside the cloud
In Figure S2, we show an example of simultaneous IR and rare visible-range images of
the same UPF inside the cloud. The latter image corresponds to a fragment of the
former, but main features of the IR image are clearly identifiable in the visible-range one.
It is likely that the visible-range imaging was made possible in this case by the proximity
of UPF to the edge of the cloud, where the optical density of the cloud was relatively low.
Exposure time of the IR camera was 7.7 ms vs. 1 µs for the visible-range camera. Note
that a considerably greater level of detail is provided by the IR camera, although some of
the additional features could be due to not only its frequency range, but also its much
longer exposure time. Based on the 1-µs exposure of the visible-range camera and
record timing, the UPF seen in the left (smaller) panel of Figure S2 was formed as early
as within 1.4 µs of the initial corona burst from the grounded sphere.
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Figure S2. Simultaneous IR (right) and visible-range (left) images of UPF inside the
cloud. The latter (smaller) image corresponds to a fragment of the former, but main
features of the IR image are clearly identifiable in the visible-range one. Exposure time
of the IR camera was 7.7 ms vs. 1 µs for the visible-range camera. Note the much
greater level of detail provided by the IR camera. AGP stands for “above the grounded
plane”.
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