Meteorological condition of the parent thunderstorm. On the evening of 7 August 2019, during a short time period between 1305:56 and 1306:32 UTC, MMIA recorded an outbreak of 12 blue emissions over a compact thunderstorm near the coastline of South China. We compared the trigger time of blue emissions with the data recorded by a ground-based very-low-frequency/low-frequency (VLF/LF) network of multiple stations (detailed information of time match is provided in Methods and Table S1). It is found that nine of these 12 blue emissions are associated with negative NBEs, and the other three are with positive NBEs. To the best of our knowledge, it is the first ever report of NBEs with both polarities produced in one thunderstorm observed from space and ground-based E-field sensors at a closest distance of about 100 km. Among these events, the height of six negative NBEs and one positive NBE can be determined through the ionospheric reflection pair6,31. The source altitude of negative NBEs was located at about 18 km (above mean sea level, MSL) near the cloud top, while the positive NBE occurred at an altitude of about 14 km inside the thundercloud, which is consistent with previous studies on the height distribution of NBEs8,12,15, indicating that the higher negative NBEs are inferred to initiate between upper positive charge region and screening negative charge layer12,14.
Fig. 1a shows the lightning activity of the thunderstorm overlapped on the infrared (IR) brightness temperatures at 10.4 μm derived from the geostationary meteorological satellite Himawari‐832 at 1310 UTC. We see that all NBEs occurred near the region of coldest cloud top with a temperature of 190 K. Fig. 1b presents the vertical proﬁle of radar echo of the thunderstorm. The highest altitude of reflectivity reaches nearly 18 km, showing the penetration of thundercloud into stratosphere. NBEs were produced under the intense convective surges. The negative NBEs occurred in the region with 15 dBZ reflectivity at the cloud top, while the positive NBE was located relatively deeper in the thundercloud.
Optical and electrical observations of positive NBEs. Fig. 2 presents the optical blue emissions recorded by MMIA at 1306:02.691042 UTC and the associated positive NBE waveform detected by four VLF/LF stations at range of 105 km to 1228 km. The height of this NBE is determined to be about 15 km (MSL) by calculating the time difference between the ground wave and two ionospheric reflections. It can be seen that the positive NBE is associated with the blue emissions centered at 337 nm, which is consistent with Soler30 who reported that seven positive NBEs located between 8 and 15 km inside thunderclouds are associated with the 337 nm emissions. This suggests that positive NBEs are corona discharges formed by numerous streamers, confirming previous work by Tilles et al10 and Liu et al11.
Additionally, this positive NBE also gave off weak 777.4 nm emission. The context of these events plotted in Fig. 2c shows that the main pulse of NBE signals is followed by some extra sferic-radiating activity, indicating that the NBE was not the only discharging activity. Thus, it is likely that the 777.4 nm emission is equally related to the additional discharging activity but not related to the NBE. Jacobson et al28 examined the optical emission of 193 positive and 24 negative NBEs based on the measurements of FORTE and LASA. It was found that no obvious optical emissions were associated with NBEs except for two positive NBEs. They examined the context of these two NBE events and found that their main sferic pulses were both preceded by some earlier sferic activity. Therefore, the 777.4 nm optical emission is likely related to additional sferic activity accompanying the NBE itself. The remaining two positive NBEs shown in Fig. S2 generally exhibit similar optical features that include a distinct 337 nm emission and no 777.4 nm signal above the noise level. The distant stations failed to record the signal of two positive NBEs due to the relatively small amplitude, and their height is also not calculatable from the E-field signal recorded at the GZ station due to the weak ionospheric reflection.
Optical and electrical observations of negative NBEs. Fig. 3 compares the blue emissions recorded by MMIA at 1306:20.968870 UTC and the associated negative NBE waveform measured at five VLF/LF stations located at range of 105 km to 1300 km. The waveforms recorded at four relatively distant stations (>800 km) exhibit the obvious ionospoheric reflection that can be used to determine a source height of about 18 km (MSL). It can be seen that this negative NBE was also associated with the 337 nm emission, but there was no significative 777.4 nm signal. The emissions at 777.4 nm are from atomic oxygen and are one of the major lines of the lightning leader spectrum, suggesting that negative NBEs are associated with fast streamer breakdown, similar to the positive NBE. An evaluation, based on optical radiation transfer, of the streamers of these NBEs is reported in a complementary publication, which presents the streamer-like structure of negative NBEs that typically involves around 109 streamer initiation events33. Here it is emphasized that the negative NBE produced the distinct 337 nm optical signature with rise time of <0.05 ms, showing the much sharper optical emission of this negative NBE than positive NBEs. In addition, we can see that the VLF/LF signal of negative NBEs corresponds to the rise stage of 337 nm emissions. For the remaining eight negative NBEs shown in Fig. S2, our analyses generally obtain the similar optical feature, namely the rise time of the 337 nm emissions for the negative NBEs is much shorter than that of positive NBEs.
In order to characterize the waveform parameters of the optical signals, we define the rise time, duration, and signal-to-noise ratio (SNR) (definition of rise time, duration, and SNR is given in Fig. S1 of the Supplementary information). Table 1 summarized the detailed parameters of optical signature and VLF/LF sferics for these NBEs. The observed negative NBEs have a wide range of strength. Previous observations show that the fast breakdown of NBEs occurs with an extremely wide range of strength, both in VHF and VLF/LF bands, while still initiating ordinary IC discharges4,10. We can see that the optical rise time of negative NBEs is in the range of 0.03 ms-0.08 ms, which is much shorter than the positive counterparts (>0.2 ms in all three cases). For the whole optical duration, there is also a considerable difference that in all cases positive NBEs endure longer than the negative ones. The reason why negative NBEs appear much sharper than positive NBEs in the optical emission is discussed below.
Comparison between blue emission and E-signal of NBEs. As the negative NBEs in our obervations occur close to the cloud top, its optical radiation is less affected by the thundercloud scattering34,35. This allows us to compare by means of radio observations. As shown in Fig. 3, we see that the VLF/LF signal of negative NBEs corresponds to the onset of 337 nm blue emission, but the E-field signal of NBEs in the VLF/LF band is much shorter than their optical blue emission. The E-field signal of NBEs is usually shorter than 0.03 ms, while the optical duration of the associated blue emissions is usually longer than 3 ms. This suggests that the lifetime of current source is much shorter than the optical pulses.
To characterize the source current further from optical measurements and compare the VLF/LF signal of negative NBEs with the 337 nm emission in association, we implement the first- and second-order differential on the original 337 nm optical signal shown in Fig. 4a. Since the sampling rate of the optical signal is 100 kHz, its maximum bandwidth is 50 kHz. To eliminate the influence of bandwidth, we apply a 50-kHz lowpass filter to the VLF/LF signal. It is seen that the second-order differential of 337 nm emission is very similar to the 50-kHz lowpass filtered waveform of negative NBEs. The second-order differential of 337 nm emission can be divided into two parts. The first part is the main narrow bipolar waveform, which resembles the sferic waveform of negative NBEs in the VLF/LF band; the second part is the one ensuing small oscillation pulses, and these small pulses are also discernible on the VLF/LF signal owing to the close distance (105 km) of the GZ station (the VLF/LF waveform for other cases is given in Fig. S2-S10). These small pulses after the main bipolar pulse of the VLF/LF signal suggest that there still exists some weak current pulses after the fast breakdown4, which corresponds to the slow descent stage of 337 nm blue emissions. The E-ﬁeld changes observation by Karunarathne et al36 suggest that NBE has static offset in its bipolar pulse and was also followed by a slow electrostatic change lasting about several ms, which is similar to our optical observation. Therefore, the duration of NBEs could be as long as several ms as observed by the optical detector, but on the VLF/LF waveform, the subsequent small oscillation pulses may be attenuated due to distance.
The question is why the second-order differential of the optical signal exhibits a similarity with the lowpass-filtered VLF/LF waveform. The luminosity is roughly proportional to the electrical current intensity (I)37. The VLF/LF signal at the GZ station is the time derivative of the vertical E-ﬁeld (d /dt). The measurement distance of about 105 km for the VLF/LF signal might be very critical for finding this waveform similarity. As the E-field is also proportional to the temporal derivative of I at such a distance38,39, it is reasonable that the second-order differential of the optical signal is similar to the lowpass-filtered VLF/LF signal of negative NBEs.