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Spectral characteristics of lightning dart leader propagating in long path
Atmospheric Research 164–165 (2015) 95–98
Contents lists available at ScienceDirect
Atmospheric Research journal homepage: www.elsevier.com/locate/atmos
Spectral characteristics of lightning dart leader propagating in long path Jianyong Cen, Ping Yuan ⁎, Simin Xue, Xuejuan Wang Key Laboratory of Atomic and Molecular Physics and Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 29 April 2015 Accepted 30 April 2015 Available online 8 May 2015 Keywords: Lightning Dart leader Spectrum
a b s t r a c t Cloud-to-ground lightning with six return strokes has been recorded with high-speed slitless spectrograph at a recording rate of 9110 frames per second. The dart leaders propagate along the same channel which consists of a long horizontal path and a vertical path, and its speed at the horizontal path is faster than that at the vertical path. The first-order spectrum of dart leader is recorded with an exposure time of 110 μs. The wavelength observed in dart leader spectrum is firstly extended to near infrared range. The spectral characteristics indicate that all the emission lines in the 400–700 nm range are radiated from singly ionized nitrogen except the Hα line at 656.3 nm, and in the range of 700–1000 nm the emission lines are radiated from neutral nitrogen and oxygen. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lightning spectrum has been used to study the physical processes of lightning discharge for over a century (Rakov and Uman, 2003). Since 1960, the techniques of slitless spectroscopy have been applied successfully to obtain lightning spectrum, and numerous spectra were obtained (Uman and Orville, 1965). Although there are many works on the lightning spectrum (Wallace, 1960; Orville and Henderson, 1984; Weidman et al., 1989), most of them are focused on the return stroke because the leader spectrum is generally too weak to be captured. Many studies on lightning leaders (Qie and Kong, 2007; Kong et al., 2008; Zhang et al., 2009; Yoshida et al., 2012; Jiang et al., 2013) have yielded important progresses, but the spectra of lightning leaders are only reported in few literatures up to now. Orville (1968) reported the first spectrum of stepped leader in the 560 to 660 nm range with a slitless spectrograph. The second work on stepped leader spectrum was reported forty years later. Warner et al. (2011) captured the spectrum of a cloud-to-ground (CG) lightning stepped leader at a recording rate of 10,000 frames per second (fps) in the wavelength range from 600 to 1050 nm, and the 2D speed of the stepped leader was also calculated using the OI 777.4 nm line. The only work on spectrum of lightning dart leader has been presented by Orville (1975). The spectra of five dart leaders have been recorded by a high-speed slitless spectrograph with the time resolution of 20 μs and the exposure time of 9 μs in the range from 395 to 510 nm, and the temperature of dart leader channel was calculated to be approximately 20,000 K by singly ionized nitrogen emissions at 444.7 and 463.0 nm.
⁎ Corresponding author. E-mail address: [email protected] (P. Yuan).
http://dx.doi.org/10.1016/j.atmosres.2015.04.019 0169-8095/© 2015 Elsevier B.V. All rights reserved.
As an effective way to determine the physical parameters of lightning processes, more spectra with accurate relative intensities of spectral lines, longer wavelength range and higher wavelength resolution of lightning leaders are needed for investigation. In this paper the wavelength range of dart leader spectrum is extended to 1000 nm. Meanwhile, its propagation speeds along the horizontal path are presented based on the successive images of channel. 2. Instrumentation The data presented in this work were from the experiment which was conducted in the summer of 2012 in Qinghai Plateau of China. The altitude of observation site is about 2530 m. The slitless spectrograph was formed by adding a plane grating with 600 lines/mm in front of a 20 mm f/2.8D lens of a high-speed camera. It has a wavelength range from 400 to 1000 nm with a wavelength resolution of about 1.1 nm (Cen et al., 2014). The camera was used to record images with recording rate of 9110 fps. It was triggered with a pre-trigger recording time of about 1 s, and the total recording times ranged around 1.8 s. Images were GPS synchronized and time stamped. 3. Results The entire process of a cloud-to-ground lightning has been recorded at 17:06:33, 22 August 2012 (Beijing Time). The distance from the observation site to the CG lightning is about 7.14 km. The CG lightning consists of six return strokes which define as R1, R2, R3, R4, R5 and R6, respectively. Its entire duration is about 360 ms. The spectra of the six return strokes are all clearly recorded by slitless spectrograph. The light intensity of the stepped leader is too weak to be recorded. Images for channel of all dart leaders preceding subsequent return strokes are captured. Several parameters of the dart leaders are summarized in
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J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
Table 1 Parameters of the five dart leaders. DL1 represents the dart leader that precedes return stroke R2. The same is true of DL2, DL3, DL4 and DL5. V stands for the propagation speed, t for the time interval between the beginning of the dart leader and the ending of the previous return stroke. I is the light intensity of the return stroke which is caused by the corresponding dart leader. It is calculated by the sum of intensity (gray value) of each pixel in the picture of return stroke channel. Name
V (106 m/s)
t (ms)
I (arb.units)
DL1 DL2 DL3 DL4 DL5
4.46 1.49 6.11 15.58 4.36
59.4 169.0 17.8 44.4 61.6
4334 4244 3447 10,475 5725
Table 1. The average propagation speeds range from 106 to 107 m/s, which are close to the typical speed value of dart leader (Cooray et al., 2010). The speed of DL2 is the smallest while the corresponding
interstroke interval is the longest. This result is consistent with the statistically significant tendency reporting that the lower speed of dart leader was associated with a long previous interstroke interval (Jordan et al., 1992). However, the contrary tendency could not be found in Table 1. The smallest time interval is 17.8 s, which is between R3 and DL3, but the speed of DL3 is not the maximum. DL4 has the maximum speed. The developing process of DL1 is selected here for investigating the variation of speed. Fig. 1 presents the developing channel of the DL1 and the variation of the speed during propagation process. It can be seen from Fig. 1(a) that the entire channel can be viewed as two parts: horizontal propagation from A to B and vertical propagation from B to C. As shown in Fig. 1(b), the propagation speed of dart leader is in the order of 106 m/s. The speeds along the AB path are greater than the BC path, indicating that the propagation of dart leader along horizontal path is faster than that in vertical direction. In addition, the
Fig. 1. (a) The entire channel developed by DL1. (b) The speed of DL1 varying with time.
Fig. 2. Three sequential images showing the spectra of DL4 and return stroke R5.
J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
speed decreases as the dart leader approaches the ground, which is consistent with general case (Orville and Idone, 1982; Campos et al., 2014). Light intensity of dart leader is positively correlated with that of the following return stroke (Orville, 1975). As seen in Table 1, the light intensity of return stroke R5 is the largest. Correspondingly, the channel of DL4 is the brightest among all the leaders. Although the optical channel of all dart leaders are recorded, only the first-order spectrum of the dart leader DL4 is intense enough to be recorded. Two sequential images of the DL4 are captured before the leader terminated on the ground. The original spectra of the DL4 and the R5 are given in Fig. 2. The spectral lines in spectrum of the DL4 at clock time of 17:06:33.884 767.00 are very weak, but at 17:06:33.884 877.00, these lines are strong enough to be clearly recognized. The three original spectra are transformed into spectral graphs which are shown in Fig. 3. The abscissa represents wavelength in nanometer and the vertical ordinate represents spectral intensity. In the early stage of the dart leader at 17:06:33.884 767.00, only lines from neutral hydrogen (Hα), nitrogen (NI) and oxygen (OI) can be distinguished as seen in Fig. 3. In the later stage, at 17:06:33.884 877.00, intense lines from singly ionized nitrogen (NII) in the 400–700 nm range are clearly recorded. Meanwhile, the intensities of lines in near infrared range (700–1000 nm) radiated from neutral nitrogen and oxygen are obviously increased. Compared with the previous work (Orville, 1975) presenting NII lines at 399.5, 444.7, 463.0 and 500.1 nm, more NII lines and the lines of Hα, NI and OI have been recorded in our work. In the near infrared range, the emission lines and their profile appeared in the dart leader spectrum are very similar to those appeared in the stepped leader spectrum (Warner et al., 2011). The OI line at 777.4 nm is the most intense line and can be recorded throughout lightning, which is in accord with the spectrum of lightning stepped leader and return stroke (Orville and Henderson, 1984; Zhao et al., 2013; Wang et al., 2014). The spectrum of return stroke R5 is also shown in Fig. 3. All the emission lines appeared in dart leader are recorded with stronger intensity in the spectrum of R5. Besides, the lines at 517.9, 715.7, 906.1 and 926.6 nm with higher excitation energies are also recorded in the spectrum of R5. It indicates that the temperature of return stroke is higher than that of the preceding dart leader. In addition, only the weak emission lines of Hα, NI and OI with comparatively lower excitation energies could be seen in the spectrum of DL4 at the clock time of 17:06:33.884 767.00, and the strong emission lines of NII are recorded at the next time of 17:06:33.884 877.00, also indicating that the temperature increases quickly as the dart leader propagates downward to ground. It is proved that the spectral intensity is positively correlated to the light intensity of the lightning channel [Orville, 1975]. The ratio of intensities in the dart leader to that in the return stroke at a particular wavelength is given in Table 2. It ranges from 0.47 to 0.618 with the mean value of 0.55. The ratios calculated by the ion (NII) lines with high excitation energies are greater than those calculated by atom (NI and OI) lines with low excitation energies. The total intensity of spectrum for the dart leader (DL4 17:06:33.884 877.00) and R5 is also given in Table 2. The intensity ratio of DL4 to R5 is 0.63, which is close to the report of Jordan et al. (1997) showing the ratio is about 0.3–0.5. But the value is larger than that reported by Orville (1975) finding the ratio is about 0.11–0.13. Idone and Orville (1985) inferred that the largest ratios were associated with the largest return stroke currents and relative light intensities, which means that the ratios are largely associated with the luminous and discharge characteristics of the return stroke. Thus, the reason of the large value of the ratio analyzed here is probably due to the large discharge current of the return stroke R5. 4. Discussion and conclusion Orville (1975) predicted that the neutral emissions in the dart leader spectrum will be photographically recorded when the red and infrared
97
Fig. 3. Spectral graphs of dart leader and return stroke R5 showing intensity versus wavelength.
regions are observed. Here, the neutral emissions from Hα, NI and OI are recorded in the near infrared range, confirming this prediction. Usually, optical observations could only record the vertical path of the CG lightning channel as lightning channel emerges from the cloud base. The recorded horizontal propagation of lightning is even few due to the obstruction of the cloud. The dart leader propagating along a long horizontal path has been recorded in our observation. The speed of dart leader at the horizontal path is faster than that at the vertical
98
J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
Table 2 The intensity ratio of dart leader to return stroke at different wavelength. E is the upper excitation energy. ID stands for the intensity of spectral line of dart leader DL4, IR for the intensity of spectral line of return stroke R5. The value of intensity for each spectral line is obtained from vertical ordinate of the spectral graphs in Fig. 3. r is the intensity ratio of dart leader to return stroke. Wavelength (nm)
E (eV)
ID (arb.units)
IR (arb.units)
r (arb.units)
NII 444.7 NII 463.0 NII 568.0 NII 594.2 NII 616.8 NI 746.8 OI 777.4 NI 821.6 NI 868.0 Total intensity of spectrum
23.1 21.1 20.6 23.2 25.1 11.9 10.7 11.8 11.7
3.91 3.84 3.98 3.55 3.68 3.98 5.04 4.11 3.79 2083
6.35 6.21 7.66 6.25 6.74 8.0 10.72 8.04 7.21 3257
0.616 0.618 0.519 0.568 0.545 0.497 0.47 0.511 0.525 0.63
path. Since the horizontal path is close to the cloud base (Fig. 1a), inferring that the faster speed of dart leader in horizontal path may be associated with the large number of charges in the cloud. Lightning dart leader is a comparatively weak luminous process. In addition, the intensity of light from original channel will reduce a lot when it is split into spectrum by the slitless spectrograph. So it is difficult to record the spectrum of dart leader, and it is even more difficult to record both spectra of lead and return stroke for one lightning process. In our observations, optical channels of five dart leaders are recorded, but only the spectrum of the DL4 is intense enough to be recorded. In the early stage of the dart leader, only the weak neutral lines in the near infrared range can be recorded. With the propagation of the dart leader, the neutral lines become more and more intense and the ionic lines from nitrogen are beginning to appear. These characteristics indicating that many physical parameters (such as temperature, electron density, electrical conductivity, resistance) of the dart leader are changed obviously in the propagation process. More ionic lines of nitrogen with stronger intensities appeared in the spectrum of return stroke also inferring that the physical parameters of return stroke are different from those of the dart leader. The physical parameters of the dart leader and return stroke will be calculated in the next work and we will get more information about the difference between the spectral characteristics of the dart leader and that of the return stroke. In summary, the lightning dart leader has been captured by a highspeed slitless spectrograph. Its speed at horizontal path is faster than that at vertical path and it decreases as the dart leader approaches the ground. The spectrum of dart leader is extended to near infrared range and the whole wavelength is in the range from 400 to 1000 nm. The spectral components in the near infrared range are the atomic lines of nitrogen, oxygen and hydrogen, which is firstly reported. In the propagation process of the dart leader, the atomic lines of nitrogen, oxygen and hydrogen are firstly radiated. Then, as the dart leader approaches
the ground, the intensities of the atomic lines are obviously increased, and the ionic lines of nitrogen are appeared in the spectrum of dart leader.
Acknowledgment This work is supported by National Natural Science Foundation of China under Grant Nos. 11475139 and 11365019. The authors thank the Datong county education bureau and Taer town center school for their help in field experiment. References Campos, L.Z.S., Saba, M.M.F., Warner, T.A., Pinto Jr., O., Krider, E.P., Orville, R.E., 2014. Highspeed video observations of natural cloud-to-ground lightning leaders—a statistical analysis. Atmos. Res. 135–136, 285–305. Cen, J., Yuan, P., Xue, S., 2014. Observation of the optical and spectral characteristics of ball lightning. Phys. Rev. Lett. 112, 035001. Cooray, V., Dwyer, J., Rakov, V., Rahman, M., 2010. On the mechanism of X-ray production by dart leaders of lightning flashes. J. Atmos. Sol. Terr. Phys. 72, 848–855. Idone, V.P., Orville, R.E., 1985. Correlated peak relative light intensity and peak current in triggered lightning subsequent return strokes. J. Geophys. Res. 90, 6159–6164. Jiang, R., Qie, X., Wang, C., Yang, J., 2013. Propagating features of upward positive leaders in the initial stage of rocket-triggered lightning. Atmos. Res. 129–130, 90–96. Jordan, D.M., Idone, V.P., Rakov, V.A., Uman, M.A., Beasley, W.H., Jurenka, H., 1992. Observed dart leader speed in natural and triggered lightning. J. Geophys. Res. 97, 9951–9957. Jordan, D.M., Rakov, V.A., Beasley, W.H., Uman, M.A., 1997. Luminosity characteristics of dart leaders and return strokes in natural lightning. J. Geophys. Res. 102, 22025–22032. Kong, X., Qie, X., Zhao, Y., 2008. Characteristics of downward leader in a positive cloud-toground lightning flash observed by high-speed video camera and electric field changes. Geophys. Res. Lett. 35, L05816. Orville, R.E., 1968. Spectrum of the lightning stepped leader. J. Geophys. Res. 73, 6999–7008. Orville, R.E., 1975. Spectrum of the lightning dart leader. J. Atmos. Sci. 32, 1829–1837. Orville, R.E., Henderson, R.W., 1984. Absolute spectral irradiance measurements of lightning from 375 to 880 nm. J. Atmos. Sci. 41, 3180–3187. Orville, R.E., Idone, V.P., 1982. Lightning leader characteristics in the Thunderstorm Research International Program (TRIP). J. Geophys. Res. 87, 11177–11192. Qie, X., Kong, X., 2007. The progression features of a stepped leader process with four grounded leader branches. Geophys. Res. Lett. 34, L06809. Rakov, V.A., Uman, M.A., 2003. Lightning: physics and effects. Cambridge University Press. Uman, M.A., Orville, R.E., 1965. The opacity of lightning. J. Geophys. Res. 70, 5491–5497. Wallace, L., 1960. Note on the spectrum of lightning in the region 3670 to 4280 Å. J. Geophys. Res. 65, 1211–1214. Wang, X., Yuan, P., Cen, J., Liu, J., Li, Y., 2014. The channel radius and energy of cloud-toground lightning discharge plasma with multiple return strokes. Phys. Plasmas 21, 033503. Warner, T.A., Orville, R.E., Marshall, J.L., Huggins, K., 2011. Spectral (600–1050 nm) time exposures (99.6 μs) of a lightning stepped leader. J. Geophys. Res. 116, D12210. Weidman, C., Boye, A., Crowell, L., 1989. Lightning spectra in the 850 to 1400 nm nearinfrared region. J. Geophys. Res. 94, 13249–13257. Yoshida, S., Akita, M., Morimoto, T., Ushio, T., Kawasaki, Z., 2012. Propagation characteristics of lightning stepped leaders developing in charge regions and descending out of charge regions. Atmos. Res. 106, 86–92. Zhang, Y., Lu, W., Li, J., Dong, W., Zheng, D., Chen, S., 2009. Luminosity characteristics of leaders in natural cloud-to-ground lightning flashes. Atmos. Res. 91, 326–332. Zhao, J., Yuan, P., Cen, J., Liu, J., Wang, J., Zhang, G., 2013. Characteristics and applications of near-infrared emissions from lightning. J. Appl. Phys. 114, 163303.
Contents lists available at ScienceDirect
Atmospheric Research journal homepage: www.elsevier.com/locate/atmos
Spectral characteristics of lightning dart leader propagating in long path Jianyong Cen, Ping Yuan ⁎, Simin Xue, Xuejuan Wang Key Laboratory of Atomic and Molecular Physics and Materials of Gansu Province, College of Physics and Electronic Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 17 January 2015 Received in revised form 29 April 2015 Accepted 30 April 2015 Available online 8 May 2015 Keywords: Lightning Dart leader Spectrum
a b s t r a c t Cloud-to-ground lightning with six return strokes has been recorded with high-speed slitless spectrograph at a recording rate of 9110 frames per second. The dart leaders propagate along the same channel which consists of a long horizontal path and a vertical path, and its speed at the horizontal path is faster than that at the vertical path. The first-order spectrum of dart leader is recorded with an exposure time of 110 μs. The wavelength observed in dart leader spectrum is firstly extended to near infrared range. The spectral characteristics indicate that all the emission lines in the 400–700 nm range are radiated from singly ionized nitrogen except the Hα line at 656.3 nm, and in the range of 700–1000 nm the emission lines are radiated from neutral nitrogen and oxygen. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Lightning spectrum has been used to study the physical processes of lightning discharge for over a century (Rakov and Uman, 2003). Since 1960, the techniques of slitless spectroscopy have been applied successfully to obtain lightning spectrum, and numerous spectra were obtained (Uman and Orville, 1965). Although there are many works on the lightning spectrum (Wallace, 1960; Orville and Henderson, 1984; Weidman et al., 1989), most of them are focused on the return stroke because the leader spectrum is generally too weak to be captured. Many studies on lightning leaders (Qie and Kong, 2007; Kong et al., 2008; Zhang et al., 2009; Yoshida et al., 2012; Jiang et al., 2013) have yielded important progresses, but the spectra of lightning leaders are only reported in few literatures up to now. Orville (1968) reported the first spectrum of stepped leader in the 560 to 660 nm range with a slitless spectrograph. The second work on stepped leader spectrum was reported forty years later. Warner et al. (2011) captured the spectrum of a cloud-to-ground (CG) lightning stepped leader at a recording rate of 10,000 frames per second (fps) in the wavelength range from 600 to 1050 nm, and the 2D speed of the stepped leader was also calculated using the OI 777.4 nm line. The only work on spectrum of lightning dart leader has been presented by Orville (1975). The spectra of five dart leaders have been recorded by a high-speed slitless spectrograph with the time resolution of 20 μs and the exposure time of 9 μs in the range from 395 to 510 nm, and the temperature of dart leader channel was calculated to be approximately 20,000 K by singly ionized nitrogen emissions at 444.7 and 463.0 nm.
⁎ Corresponding author. E-mail address: [email protected] (P. Yuan).
http://dx.doi.org/10.1016/j.atmosres.2015.04.019 0169-8095/© 2015 Elsevier B.V. All rights reserved.
As an effective way to determine the physical parameters of lightning processes, more spectra with accurate relative intensities of spectral lines, longer wavelength range and higher wavelength resolution of lightning leaders are needed for investigation. In this paper the wavelength range of dart leader spectrum is extended to 1000 nm. Meanwhile, its propagation speeds along the horizontal path are presented based on the successive images of channel. 2. Instrumentation The data presented in this work were from the experiment which was conducted in the summer of 2012 in Qinghai Plateau of China. The altitude of observation site is about 2530 m. The slitless spectrograph was formed by adding a plane grating with 600 lines/mm in front of a 20 mm f/2.8D lens of a high-speed camera. It has a wavelength range from 400 to 1000 nm with a wavelength resolution of about 1.1 nm (Cen et al., 2014). The camera was used to record images with recording rate of 9110 fps. It was triggered with a pre-trigger recording time of about 1 s, and the total recording times ranged around 1.8 s. Images were GPS synchronized and time stamped. 3. Results The entire process of a cloud-to-ground lightning has been recorded at 17:06:33, 22 August 2012 (Beijing Time). The distance from the observation site to the CG lightning is about 7.14 km. The CG lightning consists of six return strokes which define as R1, R2, R3, R4, R5 and R6, respectively. Its entire duration is about 360 ms. The spectra of the six return strokes are all clearly recorded by slitless spectrograph. The light intensity of the stepped leader is too weak to be recorded. Images for channel of all dart leaders preceding subsequent return strokes are captured. Several parameters of the dart leaders are summarized in
96
J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
Table 1 Parameters of the five dart leaders. DL1 represents the dart leader that precedes return stroke R2. The same is true of DL2, DL3, DL4 and DL5. V stands for the propagation speed, t for the time interval between the beginning of the dart leader and the ending of the previous return stroke. I is the light intensity of the return stroke which is caused by the corresponding dart leader. It is calculated by the sum of intensity (gray value) of each pixel in the picture of return stroke channel. Name
V (106 m/s)
t (ms)
I (arb.units)
DL1 DL2 DL3 DL4 DL5
4.46 1.49 6.11 15.58 4.36
59.4 169.0 17.8 44.4 61.6
4334 4244 3447 10,475 5725
Table 1. The average propagation speeds range from 106 to 107 m/s, which are close to the typical speed value of dart leader (Cooray et al., 2010). The speed of DL2 is the smallest while the corresponding
interstroke interval is the longest. This result is consistent with the statistically significant tendency reporting that the lower speed of dart leader was associated with a long previous interstroke interval (Jordan et al., 1992). However, the contrary tendency could not be found in Table 1. The smallest time interval is 17.8 s, which is between R3 and DL3, but the speed of DL3 is not the maximum. DL4 has the maximum speed. The developing process of DL1 is selected here for investigating the variation of speed. Fig. 1 presents the developing channel of the DL1 and the variation of the speed during propagation process. It can be seen from Fig. 1(a) that the entire channel can be viewed as two parts: horizontal propagation from A to B and vertical propagation from B to C. As shown in Fig. 1(b), the propagation speed of dart leader is in the order of 106 m/s. The speeds along the AB path are greater than the BC path, indicating that the propagation of dart leader along horizontal path is faster than that in vertical direction. In addition, the
Fig. 1. (a) The entire channel developed by DL1. (b) The speed of DL1 varying with time.
Fig. 2. Three sequential images showing the spectra of DL4 and return stroke R5.
J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
speed decreases as the dart leader approaches the ground, which is consistent with general case (Orville and Idone, 1982; Campos et al., 2014). Light intensity of dart leader is positively correlated with that of the following return stroke (Orville, 1975). As seen in Table 1, the light intensity of return stroke R5 is the largest. Correspondingly, the channel of DL4 is the brightest among all the leaders. Although the optical channel of all dart leaders are recorded, only the first-order spectrum of the dart leader DL4 is intense enough to be recorded. Two sequential images of the DL4 are captured before the leader terminated on the ground. The original spectra of the DL4 and the R5 are given in Fig. 2. The spectral lines in spectrum of the DL4 at clock time of 17:06:33.884 767.00 are very weak, but at 17:06:33.884 877.00, these lines are strong enough to be clearly recognized. The three original spectra are transformed into spectral graphs which are shown in Fig. 3. The abscissa represents wavelength in nanometer and the vertical ordinate represents spectral intensity. In the early stage of the dart leader at 17:06:33.884 767.00, only lines from neutral hydrogen (Hα), nitrogen (NI) and oxygen (OI) can be distinguished as seen in Fig. 3. In the later stage, at 17:06:33.884 877.00, intense lines from singly ionized nitrogen (NII) in the 400–700 nm range are clearly recorded. Meanwhile, the intensities of lines in near infrared range (700–1000 nm) radiated from neutral nitrogen and oxygen are obviously increased. Compared with the previous work (Orville, 1975) presenting NII lines at 399.5, 444.7, 463.0 and 500.1 nm, more NII lines and the lines of Hα, NI and OI have been recorded in our work. In the near infrared range, the emission lines and their profile appeared in the dart leader spectrum are very similar to those appeared in the stepped leader spectrum (Warner et al., 2011). The OI line at 777.4 nm is the most intense line and can be recorded throughout lightning, which is in accord with the spectrum of lightning stepped leader and return stroke (Orville and Henderson, 1984; Zhao et al., 2013; Wang et al., 2014). The spectrum of return stroke R5 is also shown in Fig. 3. All the emission lines appeared in dart leader are recorded with stronger intensity in the spectrum of R5. Besides, the lines at 517.9, 715.7, 906.1 and 926.6 nm with higher excitation energies are also recorded in the spectrum of R5. It indicates that the temperature of return stroke is higher than that of the preceding dart leader. In addition, only the weak emission lines of Hα, NI and OI with comparatively lower excitation energies could be seen in the spectrum of DL4 at the clock time of 17:06:33.884 767.00, and the strong emission lines of NII are recorded at the next time of 17:06:33.884 877.00, also indicating that the temperature increases quickly as the dart leader propagates downward to ground. It is proved that the spectral intensity is positively correlated to the light intensity of the lightning channel [Orville, 1975]. The ratio of intensities in the dart leader to that in the return stroke at a particular wavelength is given in Table 2. It ranges from 0.47 to 0.618 with the mean value of 0.55. The ratios calculated by the ion (NII) lines with high excitation energies are greater than those calculated by atom (NI and OI) lines with low excitation energies. The total intensity of spectrum for the dart leader (DL4 17:06:33.884 877.00) and R5 is also given in Table 2. The intensity ratio of DL4 to R5 is 0.63, which is close to the report of Jordan et al. (1997) showing the ratio is about 0.3–0.5. But the value is larger than that reported by Orville (1975) finding the ratio is about 0.11–0.13. Idone and Orville (1985) inferred that the largest ratios were associated with the largest return stroke currents and relative light intensities, which means that the ratios are largely associated with the luminous and discharge characteristics of the return stroke. Thus, the reason of the large value of the ratio analyzed here is probably due to the large discharge current of the return stroke R5. 4. Discussion and conclusion Orville (1975) predicted that the neutral emissions in the dart leader spectrum will be photographically recorded when the red and infrared
97
Fig. 3. Spectral graphs of dart leader and return stroke R5 showing intensity versus wavelength.
regions are observed. Here, the neutral emissions from Hα, NI and OI are recorded in the near infrared range, confirming this prediction. Usually, optical observations could only record the vertical path of the CG lightning channel as lightning channel emerges from the cloud base. The recorded horizontal propagation of lightning is even few due to the obstruction of the cloud. The dart leader propagating along a long horizontal path has been recorded in our observation. The speed of dart leader at the horizontal path is faster than that at the vertical
98
J. Cen et al. / Atmospheric Research 164–165 (2015) 95–98
Table 2 The intensity ratio of dart leader to return stroke at different wavelength. E is the upper excitation energy. ID stands for the intensity of spectral line of dart leader DL4, IR for the intensity of spectral line of return stroke R5. The value of intensity for each spectral line is obtained from vertical ordinate of the spectral graphs in Fig. 3. r is the intensity ratio of dart leader to return stroke. Wavelength (nm)
E (eV)
ID (arb.units)
IR (arb.units)
r (arb.units)
NII 444.7 NII 463.0 NII 568.0 NII 594.2 NII 616.8 NI 746.8 OI 777.4 NI 821.6 NI 868.0 Total intensity of spectrum
23.1 21.1 20.6 23.2 25.1 11.9 10.7 11.8 11.7
3.91 3.84 3.98 3.55 3.68 3.98 5.04 4.11 3.79 2083
6.35 6.21 7.66 6.25 6.74 8.0 10.72 8.04 7.21 3257
0.616 0.618 0.519 0.568 0.545 0.497 0.47 0.511 0.525 0.63
path. Since the horizontal path is close to the cloud base (Fig. 1a), inferring that the faster speed of dart leader in horizontal path may be associated with the large number of charges in the cloud. Lightning dart leader is a comparatively weak luminous process. In addition, the intensity of light from original channel will reduce a lot when it is split into spectrum by the slitless spectrograph. So it is difficult to record the spectrum of dart leader, and it is even more difficult to record both spectra of lead and return stroke for one lightning process. In our observations, optical channels of five dart leaders are recorded, but only the spectrum of the DL4 is intense enough to be recorded. In the early stage of the dart leader, only the weak neutral lines in the near infrared range can be recorded. With the propagation of the dart leader, the neutral lines become more and more intense and the ionic lines from nitrogen are beginning to appear. These characteristics indicating that many physical parameters (such as temperature, electron density, electrical conductivity, resistance) of the dart leader are changed obviously in the propagation process. More ionic lines of nitrogen with stronger intensities appeared in the spectrum of return stroke also inferring that the physical parameters of return stroke are different from those of the dart leader. The physical parameters of the dart leader and return stroke will be calculated in the next work and we will get more information about the difference between the spectral characteristics of the dart leader and that of the return stroke. In summary, the lightning dart leader has been captured by a highspeed slitless spectrograph. Its speed at horizontal path is faster than that at vertical path and it decreases as the dart leader approaches the ground. The spectrum of dart leader is extended to near infrared range and the whole wavelength is in the range from 400 to 1000 nm. The spectral components in the near infrared range are the atomic lines of nitrogen, oxygen and hydrogen, which is firstly reported. In the propagation process of the dart leader, the atomic lines of nitrogen, oxygen and hydrogen are firstly radiated. Then, as the dart leader approaches
the ground, the intensities of the atomic lines are obviously increased, and the ionic lines of nitrogen are appeared in the spectrum of dart leader.
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