Characterization of Pulses Superimposed on the Initial Continuous Current of Upward Lightning

We compare the characteristics of initial continuous current pulses (ICC pulses) in natural upward lightning as observed on the Gaisberg tower (100 m), the Peissenberg tower (160 m), and the Fukui chimney (200 m) with their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. The geometric mean (GM) values of the ICC pulse, magnitude, duration, risetime, Half Peak Width, and continuous current level just prior to the pulse, for rocket-triggered lightning are different from their counterparts for Gaisberg-tower flashes, Peissenberg-tower flashes, and Fukui-chimney flashes. The characteristics of ICC pulses in natural upward lightning (Gaisberg, Peissenberg, and Fukui) are similar approximately within a factor of two. INTRODUCTION Many researchers have studied lightning initiated by upward leaders [e. g., Uman 1987]. In upward lightning, the leader stage is followed by a continuous current with a duration of some hundred milliseconds and an amplitude of some tens to some thousands of amperes, after the leader reaches the cloud charge source region. There are usually current pulses superimposed on the continuous current. They are called Initial Continuous Current Pulses (ICC pulses). Waveforms of the ICC pulses were first shown by Berger [1967]. After that, many researchers compared the wavefrom of the ICC pulse with those of return stroke or M component pulses [Fisher et al. 1992, Wang et al. 1997, Heidler et al. 2000]. These studies suggest that the return stroke and the ICC pulses involve different mechanisms leading to the observed differences in their characteristics. While the ICC pulses in natural upward lightning are expected to be similar to the M component-type pulses, no quantitative confirmation of this inference is available at this time. In this international collaboration, we compare the characteristics of ICC pulses in rocket-triggered lightning in Florida with their counterparts in natural upward lightning as observed on the Gaisberg tower (100 m), the Peissenberg tower (160 m), and the Fukui chimney (200 m). In the comparison of ICC pulses, their magnitude, duration, risetime, half-peak width, and continuous current level just prior to the pulse of the ICC are considered. DATA DESCRIPTION The rocket-triggered lightning experiments were conducted at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida. The rocket launchers were located on flat ground, 20-30 m above sea level. The current waveforms of the initial stage were measured with a 1mΩ current viewing resistor [Rakov et al. 2001]. The data were recorded by a tape recorder with a bandwidth from dc to 400 kHz and a noise level of approximately 20 A. We used 45 negative flashes containing 290 pulses for this study. Natural upward lightning in Austria has been observed at the Gaisberg tower located at the top of the mountain (1270 m) near Salzburg. The height of the tower is 100 m. The overall current waveforms were measured at the base of the air terminal on the top of the tower, by a current viewing resistor of 0.25 mΩ having a bandwidth of 0 Hz to 3.2 MHz [Diendorfer et al., 2000]. Output signals were recorded by a 8 bit digitizing board (Bandwidth: 15 MHz, Memory: 16 MB) installed in a personal computer. The lower current measurement limit was 17 A. We used 74 negative flashes containing 370 pulses for this study. The current waveforms that exhibited large magnitude and short risetime were severely distorted. As a result, 26 pulses that exhibited large magnitudes (> 2 kA) and short risetimes were not suitable for this analysis. In Germany, the Peissenberg tower is located about 60 km southwest of Munich, on a ridge (about 950 m above sea level) called "Hoher Peissenberg". The height of the Peissenberg tower is about 160 m. The lightning current was measured with a current transformer (Pearson CT: 0.15 Hz – 200 kHz) [Heidler, 2000] installed on the top of the tower The signals were recorded by a digital oscilloscope with a storage capability of 1 million points using a sample interval of 1μs. The lower current measurement limit was 15 A. We used 21 negative flashes containing 124 pulses for this study. In Japan, the Fukui chimney is the chimney of the Mikuni cooperated power station located in the Fukui thermal plant station on the coast of the Japan Sea. The height of the chimney is 200 m. The current was measured with two coaxial shunts (2 mΩ:8-150 kA, 10 mΩ:0.2-12kA)[Asakawa, 1996]. The output signals from the shunts were recorded by an 10-bit digital recorder (2M words, 100 MHz). The lower current measurement limit was about 200 A. We used 36 negative flashes containing 231 pulses for this study. RESULTS The definitions of pulse characteristics examined in this study are as follows. The magnitude is the difference between the peak of the current pulse and the preceding continuous current level. The duration is the time interval from the beginning of the wave front to the somewhat subjectively selected point at which the trailing edge of the current pulse becomes indistinguishable from the overall continuous current waveform. The risetime is the time interval on the wave front between the 10% and 90% values of the magnitude. The half peak width is the time interval between the 50% values of the magnitude on the wave front and on the falling portion of the pulse. The continuous current level is the value of the continuous current just prior to the ICC pulse. Table 1 shows the GM values of magnitude, duration, risetime, half-peak width of the ICC pulse and continuous current level just prior to the pulse in negative flashes. In this table, parameters for natural upward lightning, except for the continuous current level, are similar to each other within a factor of two. This suggests that the characteristics of ICC pulses in natural upward lightning are independent of geographical location. On the other hand, they differ significantly from their counterparts in rockettriggered lightning. The parameters of ICC pulses in Florida rocket-triggered lightning are similar to those of M-component pulses in rocket-triggered lightning as shown by Wang et al [1999], (See Table 1). The GM value of continuous current level for Florida rocket triggered lightning is similar to those in natural upward lightning in Austria and in Germany approximately within a factor of two, while the GM of the continuous current level for Fukui is considerably larger than in all other data set analyzed here. Table 1 Characteristics of ICC pulses in upward lightning (geometric mean values)