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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. 1 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). 2 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. 3 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”. 4