Different Types of Corona Discharges Associated With High-Altitude Positive Narrow Bipolar Events Nearby Cloud Top

1. Introduction Blue LUminous Events (BLUEs) are special Transient Luminous Events associated with thunderclouds that radiate intense near-ultraviolet (UV)blue optical emissions dominated by 337nm with weak or absent signals in the atomic oxygen line at 777.4nm. They have also been termed as blue corona discharges in the recent studies (Dimitriadou etal.,2022; Husbjerg etal.,2022; Li etal.,2021; F. Liu, Lu, etal.,2021; F. Liu, Zhu, etal.,2021; Soler etal.,2020,2021,2022 ). They have similar features with different phenomena in other studies, such as blue starters/blue jets (Edens,2011; Kuo etal.,2005; Wescott etal.,1996,2001), BLEs (Chou etal.,2011,2018; F. Liu etal.,2018), glimpses (Chanrion etal.,2017) and gnomes (also called Pixies) (Lyons etal.,2003). These optical signals normally last a few to hundreds of milliseconds and appear either isolated or in groups in the active thunderstorms, especially those with overshooting cloud tops and they occurred at the global frequency about 11s −1 at local midnight (Chanrion etal.,2017; Chou etal.,2018; Dimitriadou etal.,2022; Edens,2011; Husbjerg etal.,2022; Li, Neubert, etal.,2022; Li etal.,2021; F. Liu, Lu, etal.,2021; F. Liu, Zhu, etal.,2021; Lyons etal.,2003; Soler etal.,2021). Abstract Singleand multi-pulse blue corona discharges are frequently observed in thunderstorm clouds. Although we know they often correlate with Narrow Bipolar Events (NBEs) in Very Low Frequency/Low Frequency radio signals, their physics is not well understood. Here, we report a detailed analysis of different types of blue corona discharges observed by the Atmosphere-Space Interactions Monitor during an overpass of a thundercloud cell nearby Malaysia. Both singleand multi-pulse blue corona discharges were associated with positive NBEs at the top of the cloud, reaching about 18km altitude. We find that the primary pulses of multi-pulse discharges have weaker current moments than the single-pulse discharges, suggesting that the multi-pulse discharges either have shorter vertical channels or have weaker currents than the single-pulse discharges. The subsequent pulse trains of the multi-pulse discharges delayed some milliseconds are likely from horizontally oriented electrical discharges, but some NBEs, correlated with both single-and multi-pulse discharges, include small-amplitude oscillations within a few microseconds inside their waveforms, which are unresolved in the optical observation and yet to be understood. Furthermore, by jointly analyzing the optical and radio observations, we estimate the photon free mean path at the cloud top to be ∼6m. Plain Language Summary Recent studies indicate that the blue corona discharges detected by the Atmosphere-Space Interactions Monitor onboard the international space station have close association with a special type of intracloud discharges named Narrow Bipolar Events (NBEs). In this study, we present a detailed analysis of different types of NBE-associated corona discharges detected by both optical and radio observations. All the detected corona discharges are found to be associated with unusual high-altitude positive NBEs, which located a few kilometers below the cloud top where the cloud droplets have low impact on the optical observation. This allowed us to infer the physical properties of them and their parent thundercloud by using theoretical models. The results can provide important reference to further investigate the physical mechanism of corona discharge and their role in lightning initiations. LI ETAL. © 2023. The Authors. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Different Types of Corona Discharges Associated With HighAltitude Positive Narrow Bipolar Events Nearby Cloud Top Dongshuai Li1,2 , Alejandro Luque1 , F. J. Gordillo-Vazquez1 , F. J. Pérez-Invernón1 , Lasse Skaaning Husbjerg2 , Torsten Neubert2 , Olivier Chanrion2 , Gaopeng Lu3 , Hongbo Zhang4 , Jing Han5, Nikolai G. Lehtinen6 , Nikolai Østgaard6 , and Víctor Reglero7 1Instituto de Astrofísica de Andalucía (IAA), CSIC, Granada, Spain, 2National Space Institute, Technical University of Denmark (DTU Space), Kongens Lyngby, Denmark, 3CAS Key Laboratory of Geospace Environment, University of Science and Technology of China, Hefei, China, 4Key Laboratory of Middle Atmosphere and Global Environment Observation (LAGEO), Institute of Atmospheric Science, Chinese Academy of Sciences, Beijing, China, 5Hainan Institute of Meteorological Sciences, Haikou, China, 6Birkeland Centre for Space Science, Department of Physics and Technology, University of Bergen, Bergen, Norway, 7Image Processing Laboratory, University of Valencia, Valencia, Spain Key Points: • Corona discharges are found to be associated with unusual high-altitude positive narrow bipolar events nearby cloud top • Corona discharges are classified into different types according to their different optical and radio features • The detailed features of corona discharges and their parent thundercloud are estimated using different theoretical models Supporting Information: Supporting Information may be found in the online version of this article. Correspondence to: D. Li and A. Luque, [email protected]; [email protected] Citation: Li, D., Luque, A., Gordillo-Vazquez, F. J., Pérez-Invernón, F. J., Husbjerg, L. S., Neubert, T., etal. (2023). Different types of corona discharges associated with high-altitude positive narrow bipolar events nearby cloud top. Journal of Geophysical Research: Atmospheres, 128, e2022JD037883. https://doi. org/10.1029/2022JD037883 Received 21 SEP 2022 Accepted 30 JAN 2023 10.1029/2022JD037883 RESEARCH ARTICLE 1 of 18 Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 2 of 18 Recently, corona discharges have attracted a lot of attention due to their close correlation with a special type of intracloud (IC) discharges named Compact Intracloud Discharges (CIDs) that emit strong Very High Frequency (VHF)radiationandhavethe short-duration (tens of microseconds) bipolar-shaped waveforms in the Very Low Frequency/Low Frequency (VLF/LF) band, termed as Narrow Bipolar Events (NBEs) or Narrow Bipolar Pulses (Leal etal.,2019; Le Vine,1980; Nag & Rakov,2010a,2010b; Rison etal.,2016; Smith etal.,1999). Based on the atmospheric electricity sign convention, NBE can be either positive or negative according to the polarity of first initial half cycle in its electric field waveform (Leal etal.,2019; Willett etal.,1989). The majority of positive NBEs are located at median heights about 13km between the main negative and upper positive charge regions (Karunarathne etal.,2015; Smith etal.,1999,2004; Wu etal.,2012,2014), while the negative NBEs predominantly occur at higher altitudes 14–20km between the main positive charge region and the screening negative layers (Ahmad etal.,2017; Leal etal.,2019; Smith etal.,1999,2004; Wu etal.,2012,2014). However, some negative NBEs are also found to occur at lower altitudes, from 4 to 8km (Bandara etal.,2019), and a few cases of positive NBEs are also reported to occur at lower altitudes from 5 to 10km (Wu etal.,2014). Additionally, the altitudes of positive NBEs might be even higher than 16km, when they are associated with convective surges overshooting the tropopause (Jacobson & Heavner,2005; Jacobson etal.,2007; Karunarathne etal.,2015; Nag & Rakov,2010a,2010b). NBEs can occur either individually isolated from other lightning discharges within tens of milliseconds (Kostinskiy etal.,2020; Le Vine,1980; Rison etal.,2016; Smith etal.,1999) or as the lightning initiation event (Karunarathne etal.,2015; López etal.,2022; Lyu etal.,2019; Nag & Rakov,2010a; Rison etal.,2016; Wu etal.,2011,2014), or sometime localized in groups (Bandara etal.,2021). The nature of NBEs, and their relation to the formation of the lightning leader is still poorly understood; however, it may provide further insight into the most important problem in lightning physics: the initiation of lightning inside thunderstorms (Rison etal.,2016). Recent observations connected NBEs with a new type of discharge, called fast breakdown (FB), suggesting that NBEs are produced by a system of streamer coronas without a conducting channel or hotleader involved (Lyu etal.,2019; Rison etal.,2016; Tilles etal.,2019), which is further supported by the recent studies of the NBEs-associated BLUEs detected by Atmosphere-Space Interactions Monitor (ASIM) (Li, Luque, Lehtinen, etal.,2022; Li, Neubert, etal.,2022; Li etal.,2021; F. Liu, Lu, etal.,2021; Soler etal.,2020). In this study, we present a detailed analysis of the different types of corona discharges observed by ASIM during its overpass of an active thundercloud near Malaysia. The BLUEs are found to be associated with unusual high-altitude positive NBEs nearby a deep convective cloud top where the cloud droplets have low impact on the optical observations. This allows us to estimate thedetailed features of the corona discharges by jointly analyzing the optical and radio observations. 2. Instruments and Observations Since 2 April 2018, the Modular Multispectral Imaging Array (MMIA) of the ASIM onboard the International Space Station has provided important insights into Earth thunderstorms from space (Chanrion et al., 2019; Neubert etal.,2019). It includes three photometers with temporal sampling rate at 10 5 samples/s including one in the UV band at 180–230nm, while the other two are associated with the cameras, in the near-UV at the strongest spectral line of the second positive system of Nitrogen, N22P (337nm) and in the strongest lightning emission band, OI (777.4nm), respectively. The spatial resolution of the cameras on the ground is around 400×400m with 12 frames per second. On the evening of 30 April 2020, 21 BLUEs were observed by ASIM when it passed over a thundercloud cell nearby Malaysia during the time period from 17:49:55 UTC to 17:50:55 UTC. All these BLUEs are only detected in the 337nm photometer and camera, with no or weak signals in the 180–230nm photometer nor in the 777.4nm photometer and camera. Among them, 16 BLUEs were captured by both photometers and their corresponding cameras of MMIA, other 5 BLUEs were only captured by the photometers of MMIA without the corresponding camera images. Figure1 shows the distribution of the cloud-to-ground (CG)/intracloud(IC) lightning and 21 BLUEs (16 with camera images (green square) and 5 without camera images (pink square)) superimposed on the Cloud Top Height (CTH, in km) provided by the Fengyun-4A (FY-4A) satellite (Yang etal.,2017) at the time 17:50:00 UTC (a) and the zoom of its black-dotted rectangular region (b), as well as the 337nm images detected by MMIA in the zoom region (c). During the time period when the BLUEs occurred, there were a total of 20 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 3 of 18 lightning events with 11 CGs (red dots) and 9 ICs (red crosses) reported by the ground-based Vaisala GLD360 global lightning network (Said & Murphy,2016) in the zoom region of Figure1b. The total number of lightning events at the zoom region, shown in Figure1d, started to increase around 15:00 UTC, then peaked at the time around 17:50 UTC when ASIM passed over. The BLUEs are accompanied by the highest concentration of IC and CG lightnings, which is consistent with previous studies (Chanrion etal.,2017; Husbjerg etal.,2022; Li, Neubert, etal.,2022). The geolocations (latitude and longitude) of the 16 BLUEs are based on the 337nm images detected by MMIA. For the 5 BLUEs without the corresponding camera images, we use the meta data of 337nm camera images to find their geolocations. Note that the final geolocations of all the BLUEs have been projected to the cloud top (about 18km) with a horizontal uncertainty of less than 10km (Bitzer etal.,2021; Husbjerg etal.,2022; Li, Neubert, etal.,2022). The broadband VLF/LF magnetic field sensor operates at 400Hz–400kHz located at Universiti Teknikal Malaysia Melaka, Malacca, Malaysia (Ahmad etal.,2017; Zhang etal.,2016) (see the yellow star in Figure1a). After the time of both MMIA and VLF/LF sensor being back-propagated to the BLUE source locations, we evaluate Figure 1. The distribution of 21 BLUEs (16 with camera images (green square) and 5 without camera images (pink square)) along with the cloud-to-ground (red dots)/ intracloud (red crosses) lightning on the Cloud Top Height (CTH)at 17:50:00 UTC (a), the zoom of its black-dotted rectangular region (b) and the projected images measured by the 337nm camera of MMIA in the zoom region (c). In (a), the ground-based Very Low Frequency/Low Frequency sensor at Malaysia is shown as yellow star. The footprints of Atmosphere-Space Interactions Monitor (ASIM) are shown in black dashed line. Numbers of lightning events from 15:00 UTC to 19:00 UTC in the zoom region are shown in (d): positive CGs (+CGs), negative CGs (−CGs), positive ICs (+ICs) and negative ICs (−ICs). The ASIM overpass time is marked in black line. 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 4 of 18 the time offsets based on the start time between the 337nm photometer signals and NBE radio pulses. In our case, the time shift for MMIA with respect to the ground-based VLF/LF measurements is within −15±0.6ms (see Figure S1 in Supporting InformationS1). 3. Methodology 3.1. Light-Scattering Model To simplify the modeling, we assume the corona discharges are impulsive and point-like sources inside a homogeneous isotropic cloud. We fit the 337nm photometer signal of MMIA based on the first-hitting-time model proposed by Soler etal.(2020) to infer the depth L (relative to the cloud top). The photon flux emitting from the cloud top with the time t0 being the moment of light emission: 𝑓𝑓 (𝑡𝑡)=𝐴𝐴 ( 𝜏𝜏 𝑡𝑡−𝑡𝑡0 )3∕2 exp(−𝜏𝜏∕(𝑡𝑡−𝑡𝑡0)−𝜈𝜈(𝑡𝑡−𝑡𝑡0)) , (1) where A is the fitting constant, ν is the collision rate, τ is the characteristic time of diffusion for the depth L between the source and the cloud top.By fitting the 337nm photometer signal of MMIA, one can obtain the values of the parameters A, t0, ν, and τ. The mean free path Λ with a uniform population of droplets is approximated according to the equations in Thomson and Krider(1982): Λ≈ 1 2 𝜋𝜋𝜋𝜋 2 𝑁𝑁𝑑𝑑 , (2) where r=20μm is the particle radius and Nd=1×10 8m −3 is the particle number density (Luque etal.,2020; Soler etal.,2020). The depth L can be estimated as: 𝐿𝐿≈√4Λ𝑐𝑐𝑐𝑐∕(3(1 − 𝑔𝑔)) (3) where g=0.87 is the scattering asymmetry parameter and c is the speed of light. 3.2. Electromagnetic Radiation Model In the simulation, we assume the source of corona discharge as a vertical dipole located at an altitude of H away from observer at a horizontal distance D. The ground is assumed to be perfectly conducting since the corona discharges in our case occurred above the ocean. The magnetic field dBϕ for a dipole source is proposed by Uman etal.(1975) and given by: 𝑑𝑑  𝐵𝐵𝜙𝜙(𝐷𝐷𝐷 𝐷𝐷)=𝜇𝜇0𝑑𝑑𝑑𝑑 ′ 4 𝜋𝜋sin 𝜃𝜃 [𝑖𝑖 ( 𝑑𝑑′𝐷𝐷𝐷 − 𝑅𝑅 ∕ 𝑐𝑐 ) 𝑅𝑅2+1 𝑐𝑐𝑅𝑅 𝜕𝜕𝑖𝑖 ( 𝑑𝑑′𝐷𝐷𝐷 − 𝑅𝑅 ∕ 𝑐𝑐 ) 𝜕𝜕𝐷𝐷 ] 𝑎𝑎 𝜙𝜙 (4) where dz′ is the size of the dipole source, c is the speed of light, μ0 is the magnetic permeability of free space, R is the observation distance between dz′ and the observer. θ is the angle between dz′ and the vector of the observation distance 𝐴𝐴 𝑅𝑅 , 𝐴𝐴sin ( 𝜃𝜃 )= 𝐷𝐷 ∕𝑅𝑅 . 𝐴𝐴 𝐴𝐴𝐴 𝜙𝜙 is the unit vector in ϕ direction. The current i(t) is assumed to be the bi-Gaussian function: 𝑖𝑖 (𝑡𝑡)=𝑖𝑖0 ( 𝑒𝑒−𝑡𝑡2∕𝜏𝜏12−𝑒𝑒−𝑡𝑡2∕𝜏𝜏22 ), (5) where i0 is the amplitude, τ1 and τ2 is the rise time and the fall time, respectively. The length-integrated current or current moment 𝑀𝑖(𝑡)=∫𝑖(𝑡)𝑑𝑙 , where 𝐴𝐴𝐴𝐴 is the length of the lightning current, can be inferred by solving the inverse convolution problem (Cummer,2003; Cummer & Inan,2000): 𝐵𝐵 (𝑡𝑡)= ∫∞ −∞ 𝑀𝑀𝑖𝑖(𝜏𝜏)ℎ(𝑡𝑡−𝜏𝜏) 𝑑𝑑𝜏𝜏𝑑 (6) 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 5 of 18 where B(t) is the measured magnetic field waveform and h(t) is the propagation response evaluated from the modeling results of Equation4. 4. Results In this study, we first classify the corona discharges into two groups based on their optical features: single-pulse BLUEs (Li etal.,2021; Soler etal.,2020) and multi-pulse BLUEs (Li, Luque, Lehtinen, etal.,2022; Soler etal.,2020). Both singleand multi-pulse BLUEs are statistically significant with their 337nm signals above μ±5σ level of the background noise, with absent or negligible signals in both the 180–230nm photometer and the 777.4nm photometer (see AppendixA for more details). For the multi-pulse BLUEs, we calculate the binned average of 15 data points (about 150µs) of their 337nm photometer signals (see AppendixB for further details). FigureB1 shows that the secondary optical peaks of all the multi-pulse BLUEs are statistically significant above the standard deviation of preceding signals. Both singleand multi-pulse BLUEs are associated with positive NBEs (+NBE), then by considering their corresponding radio features, we further classify the BLUEs into four different types, namely (a) single-pulse BLUEs associated with NBEs 𝐴𝐴( BLUE𝑆𝑆 ) , (b) single-pulse BLUEs associated with NBEs including secondary peaks and oscillations 𝐴𝐴( BLUE𝑆𝑆 𝑂𝑂𝑆𝑆𝑂𝑂 ) , (c) multi-pulse BLUEs associated with NBEs and their subsequent pulse trains 𝐴𝐴( BLUE𝑀𝑀 ) and (d) multi-pulse BLUEs associated with oscillated NBEs and their subsequent pulse trains 𝐴𝐴( BLUE𝑀𝑀 𝑂𝑂𝑂𝑂𝑂𝑂 ) . For the cases of NBEs with and without oscillations, we estimate the existence of oscillations when the amplitudes of the subsequent radio pulses with the same polarity of the ground wave are above the 3σ level of the background noise (see AppendixC for further details). The small-amplitude oscillations within a few microseconds inside NBE waveforms are marked as “OSC” in the corresponding cases of both singleand multiple-pulse BLUEs in FiguresC1 andC2, respectively. Table1 shows the detailed feature of the four different types of BLUEs. Among them, there are 10 single-pulse BLUEs and 11 multi-pulse BLUEs, including 4 𝐴𝐴BLUE𝑆𝑆 , 6 𝐴𝐴BLUE𝑆𝑆 𝑂𝑂𝑆𝑆𝑂𝑂 , 8 𝐴𝐴BLUE𝑀𝑀 , and 3 𝐴𝐴BLUE𝑀𝑀 𝑂𝑂𝑂𝑂𝑂𝑂 . All the BLUEs are found to be isolated from other lightning discharges with no 777.4nm emission identified by MMIA within at least 100ms. 5 BLUEs are detected by GLD360 with one identified as+CG and others as+ICs, which are also isolated from other lightning activities. The rise times of the BLUEs change from 0.04 to 0.19ms with the total time duration ranging from 0.96 to 3.54ms for the single-pulse BLUEs and to 6.91ms for the multi-pulse BLUEs. There is no obvious difference for the peak irradiance between the single-pulse and multi-pulse BLUEs. Further details for all the cases can be found in Figures S2–S22 in Supporting InformationS1 with two examples for both singleand multi-pulse BLUEs shown in Figures2 and3, respectively. As shown in Figure2, both 𝐴𝐴BLUE𝑆𝑆 and 𝐴𝐴BLUE𝑆𝑆 𝑂𝑂𝑆𝑆𝑂𝑂 are found to be associated with +NBEs. The waveforms of NBEs in Figures2c and2d include the ground wave followed by a 1-hop sky waves, first reflected from the surface of the earth and then from the ionosphere. In our case, since the polarity of the azimuthal magnetic field pointing in the anti-clockwise direction is defined to be positive (Krider etal.,1976; Zhang etal.,2016), based on the atmospheric electricity sign convention, the first initial half cycle of the ground wave of the azimuthal magnetic field is negative for+NBEs. For the 𝐴𝐴( BLUE𝑀𝑀 ) in Figure3, the primary BLUE is found to be associated with a +NBE pulse, but its subsequent optical pulse is found to be associated with several subsequent pulse trains within 3.1ms. The 𝐴𝐴BLUE𝑀𝑀 𝑂𝑂𝑂𝑂𝑂𝑂 is found to be similar to 𝐴𝐴( BLUE𝑀𝑀 ) with NBE pulse and two subsequent optical pulses within 1.4 and 4.4ms, respectively, but with secondary peaks and microsecond-scale oscillations inside the NBE waveform. The subsequent optical pulses of multi-pulse BLUEs, which followed the primary corona discharges a few milliseconds later, have comparable optical emissions but their associated radio signals are either accompanied by weaker radio emissions or buried in the background noise (see Figures S5, S8, S10, S12, and S19 in Supporting InformationS1). Li, Luque, Lehtinen, etal.(2022) discussed the multi-pulse corona discharges related to this study and noted that the subsequent pulse trains of the multi-pulse corona discharges include the electromagnetic pulse pairs that resemble 1-hop sky waves without the ground wave (the red dashed circle outlines the subsequent pulse trains in Figures3e and3f), which might emanate from the horizontally oriented corona discharges. As shown in Table1, the altitude H of the NBEs are evaluated using the propagation distance and the time delay between the ground wave and 1-hop sky waves in the VLF/LF radio signals based on the simplified ray-theory method (Smith etal.,1999,2004). The ray path is assumed to follow the rules of geometric optics by propagating in a straight path and being ideally reflected at the ionospheric reflection height in a spherical Earth geometry with an uncertainty about ±1km compared to the full-wave method (Li etal.,2020). Previous studies indicate 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 6 of 18 that the majority of +NBEs are located at a median height around 13km, between the main negative and upper positive charge regions (F. Liu, Zhu, etal.,2021; Smith etal.,2004; Wu etal.,2014). However, note that the +NBEs in our study are found to be located at relatively high altitudes, ranging from 15.5 to 18km near the CTH obtained from the FY-4A satellite (see Table1). To further understand the features of the BLUEs, we estimate the depths L (relative to the cloud top) and the current moments Mi based on the light-scattering model and the electromagnetic radiation model in Section3. In the fitting process, we only fit the BLUEs with clear impulsive pulses and considered as good fitting condition when the coefficient of determination R 2>0.6 (see green lines in Figures2a, 2b, and3a). The modeling light curves agree well with the 337nm photometer signals of MMIA, indicating the evaluated depths L for the BLUEs are from 1 to 3km below the cloud top (see Figures S2–S22 in Supporting InformationS1). Among them, 3 cases with ID 27206, ID 27243 and ID 27245 are too noisy to be fitted, as well as 3 cases with ID 27224, ID 27231 and ID 27236 contain a small pulse on the rising edge of light-curve that distorted the fitting process (see the footnote in Table1 for further details). Figure4 further shows the correlation between different parameters associated with the BLUEs. Two special cases, whose subsequent pulse trains seem to be “NBE-like” events with ID 27236 and ID 27238, are marked by Table 1 The Detailed Feature of All the BLUEs Occurred at the Time Period From 17:49:55 to 17:50:55 UTC ID Flux (μW/m 2)Bϕ (nT) Rise time a (ms) Time duration b (ms) Mi c(kA⋅km) H (km) Optical L d (km) CTH (km) Type 27206 2.00 10.84 – – 24.24 17.23 – 18.46 S with oscillations 27210 6.6 2.19 0.08 2.05 7.48 17.06 1.83 18.60 S 27211 4.54 1.36 0.07 1.56 2.64 17.68 1.61 18.65 M 27213 5.57 2.08 0.12 2.02 5.50 16.67 2.34 18.55 M 27214 10.81 2.75 0.04 0.96 10.08 17.11 1.31 18.55 S with oscillations 27215 13.50 2.58 0.11 2.49 14.89 16.30 2.09 18.67 S with oscillations 27218 5.56 0.69 – – 1.67 16.69 - 18.60 M 27222 12.42 1.94 0.08 2.73 4.21 17.03 1.82 18.27 M with oscillations 27224 10.28 1.75 – – 6.23 15.55 – 18.21 M 27225 14.05 3.42 0.19 3.54 7.66 16.65 2.84 18.21 S with oscillations 27231 4.54 1.39 – – 2.91 15.55 – 17.77 M 27234 6.60 4.49 0.13 2.85 9.18 16.32 2.34 17.50 S 27235 12.96 12.78 0.04 1.01 26.61 17.28 1.34 17.34 S 27236 e 8.69 1.38 – – 3.69 15.87 – 17.47 M 27237 24.77 15.85 0.04 0.99 37.07 17.98 1.27 17.88 S 27238 e 10.81 11.15 0.04 0.97 22.62 17.95 1.26 17.04 M 27239 5.56 0.71 0.14 6.91 5.02 16.34 2.42 17.09 M with oscillations 27241 4.54 0.29 0.18 5.69 – 16.54 3.05 17.09 M 27243 3.52 0.60 – – – 16.78 – 17.23 S with oscillations 27244 7.12 9.69 0.05 1.26 32.73 17.78 1.48 17.03 S with oscillations 27245 3.01 0.60 – – 3.59 17.33 – 16.73 M with oscillations Note. Note that the current moments (Mi) are inferred by solving the inverse convolution problem (Cummer,2003; Cummer & Inan,2000) based on the Uman's equation (Uman etal.,1975). The altitudes (H) are estimated using the simplified ray-theory method proposed by Smith etal.(1999,2004) based on the ground-based VLF/LF sferics. The depths (L) relative to the cloud tops are evaluated by using the first-hitting-time model proposed by Soler etal.(2020) based on the 337nm photometer signals of MMIA. The Cloud Top Heights (CTHs) are obtained from FY-4A satellite products. aRise time is the time taken for the amplitude of a fitted photometer signal to rise from 10% to 90% of the peak. bTime duration is the time interval for the amplitude of a fitted photometer signal to rise from 10% and fall to 10% of the peak. cThe current moment Mi for ID 27241 and ID 27243 cannot be estimated due to their complex radio signals (see Figures S19 and S20 in Supporting InformationS1 for details). dFor ID 27224, ID 27231, and ID 27236, there is a small pulse on the rising edge of light-curve that distorted the fit process (see Figures S10, S12, and S15 in Supporting InformationS1 for details). The photometer signal is too noisy to be fitted for ID 27206, ID 27243, and ID 27245 (see Figures S2, S20, and S22 in Supporting InformationS1 for details). eSpecial multi-pulse cases (see Figures S15 and S17 in Supporting InformationS1 for details). 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 7 of 18 green dots. They might be two NBE events that occurred closely in time (see Figures S15 and S17 in Supporting InformationS1), however, it is too noisy to identify them through the radio signals. As shown in Figure4a, the rise times of MMIA photometer signals have an obvious correlation with the altitudes H of NBEs. This might be due to the high-altitude+NBEs in our study are located only a few kilometers below the cloud top where the cloud droplets have relatively low impact on the MMIA measurements. Figure4b shows a linear correlation between the radio-signal inferred altitude H and the parameter 𝐴𝐴𝐴𝐴 = √ 4𝑐𝑐𝑐𝑐∕(3(1 − 𝑔𝑔 )) evaluated from the MMIA photometer signals. According to Equation3, the photon mean free path at the cloud top can be obtained by using Λ=1/(0.4) 2≈6m, where 0.4m 1/2 is the slope of the fitting line in Figure4b. This is consistent with the photon mean free path Λ≈4m assumed in the previous studies by considering the particle radius r=20μm and the number density Nd=1×10 8m −3 (Li etal.,2021; Luque etal.,2020; Soler etal.,2020). Figure 2. Examples of the single-pulse BLUEs associated with NBEs 𝐴𝐴( BLUE 𝑆𝑆) for ID 27235 (a,c,e) and thesingle-pulseBLUEsassociatedwithNBEs including secondary peaks and oscillations 𝐴𝐴( BLUE 𝑆𝑆 𝑂𝑂𝑆𝑆𝑂𝑂 ) for ID 27214 (b,d,f). MMIA photometer irradiance (blue: 337nm, black: 180–230nm, red: 777.4nm and green: modeling result of the first-hitting-time model) (a, b) and its corresponding radio signal detected from the ground-based Very Low Frequency/Low Frequency sensor nearby Malaysia (c, d). The 337nm images of MMIA are shown in the (e) and (f). The pink horizontal dashed line is the mean of the background noises with the pink shaded band μ±3σ in (c), (d). The oscillations are marked as OSC in (d). The ground wave and the ionospheric 1-hop sky waves are marked as G and 1-Hop in (c), (d), respectively. 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 8 of 18 Figure 3. Similar to Figure2, but for the multi-pulse BLUEs associated with NBEs and their subsequent pulse trains (marked in the red dashed circle region) 𝐴𝐴( BLUE 𝑀𝑀) for ID 27211 (a, c, e, g) and the multi-pulse BLUEs associated with oscillated NBEs and their subsequent pulse trains (marked in the red dashed circle region) 𝐴𝐴( BLUE 𝑀𝑀 𝑂𝑂𝑂𝑂𝑂𝑂 ) for ID 27245 (b, d, f, h). Note that (f) only shows the subsequent pulse trains after 4.4ms since the radio signals after 1.4ms are not obvious and might overlap with the multiple-hop ionospheric reflections of NBEs (see Figure S22 in Supporting InformationS1). The pink horizontal dashed line is the mean of the background noises with the pink shaded band μ±3σ in (c, d, e, f). The oscillations are marked as OSC in (d). The ground wave and the ionospheric 1-hop sky waves are marked as G and 1-Hop in (c), (d), respectively. 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 9 of 18 Moreover, as expected, the amplitude of the azimuthal magnetic field component Bϕ and the estimated current moment Mi show a tight linear relationship in Figure4c. Despite one special case, the current moments and themagnetic fields of the NBEs corresponding to the multi-pulse BLUEs (red dots) are found to be weaker than those related to the single-pulse BLUEs (blue dots). It suggests that the multi-pulse BLUEs either have shorter vertical channels or have weaker currents than the single-pulse BLUEs. 5. Discussion and Summary In this study, we first classify 21 BLUEs near the cloud top of a localized thunderstorm into two groups based on their optical features: Single-pulse BLUEs (10) and multi-pulse BLUEs (11). Then by considering their corresponding radio features, we further classify them into four different types including (a) the single-pulse BLUEs associated with NBEs 𝐴𝐴( BLUE𝑆𝑆 ) , (b) the single-pulse BLUEs associated with NBEs including secondary peaks and oscillations 𝐴𝐴( BLUE𝑆𝑆 𝑂𝑂𝑆𝑆𝑂𝑂 ) , (c) the multi-pulse BLUEs associated with NBEs and their subsequent pulse trains 𝐴𝐴( BLUE𝑀𝑀 ) and (d) the multi-pulse BLUEs associated with oscillated NBEs and their subsequent pulse trains 𝐴𝐴( BLUE𝑀𝑀 𝑂𝑂𝑂𝑂𝑂𝑂 ) . Figure 4. The correlation of (a) the rise time of 337nm photometer signal and the altitude of NBEs (H), (b) the altitude of NBEs (H) and the parameter 𝐴𝐴𝐴𝐴 = √ 4𝑐𝑐𝑐𝑐∕(3(1 − 𝑔𝑔 )) and (c) the current moment (Mi) and the magnetic field strength (Bϕ). The singleand multi-pulse BLUEs are shown in blue and red dots, respectively. The 2 special multi-pulse cases for ID 27236 and ID 27238 are marked as green dots. 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 16 of 18 Data Availability Statement The Modular Multispectral Imaging Array (MMIA) level 1 data and Global Lightning Detection Network GLD360 data were obtained from https://asdc.space.dtu.dk/. ASIM data is proprietary and not currently available for public release. Interested parties should direct their data request to the ASIM Science Data Centre ([email protected]). The Fengyun-4A (FY-4A) satellite data is public to the registered user by contacting (dataser[email protected]v.cn) and supplied by the FENGYUN satellite Data Center (http://satellite.nsmc.org.cn/ PortalSite/Data/Satellite.aspx?currentculture=en-US). The VLF/LF radio data that support the findings of this study are openly available at (https://doi.org/10.5281/zenodo.7096902). References Ahmad, M. R., Periannan, D., Sabri, M. H. M., Aziz, M. Z. A. A., Lu, G., Zhang, H., etal. (2017). Emission heights of narrow bipolar events in a tropical storm over the Malacca Strait. In 2017 international conference on electrical engineering and computer science (ICECOS) (pp.305–309). https://doi.org/10.1109/ICECOS.2017.8167155 Attanasio, A., da Silva, C., & Krehbiel, P. (2021). Electrostatic conditions that produce fast breakdown in thunderstorms. 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Journal of Geophysical Research, 85(C7), 4091–4095. https:// doi.org/10.1029/JC085iC07p04091 Acknowledgments This work was supported by the European Research Council (ERC) under the European Union H2020 programme/ ERC Grant agreement 681257. It also received funding from the European Union Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Grant agreement SAINT 722337. Additionally, this work was supported by the Spanish Ministry of Science and Innovation, MINECO, under project PID2019-109269RB-C43 and FEDER program. D.L. would like to acknowledge the Independent Research Fund Denmark (Danmarks Frie Forskningsfond) under Grant agreement 1026-00420B. D.L., A.L., F.J.G.V. and F.J.P.I. would like to acknowledge financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). G.L. is supported by the Chinese Meridian Project, and the International Partnership Program of Chinese Academy of Sciences (No.183311KYSB20200003). ASIM is a mission of the European Space Agency (ESA) and is funded by ESA and by national grants of Denmark, Norway and Spain. The ASIM Science Data Centre is supported by ESA PRODEX contracts C 4000115884 (DTU) and 4000123438 (Bergen). 21698996, 2023, 4, Downloaded from https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2022JD037883 by Csic Organización Central Om (Oficialia Mayor) (Urici), Wiley Online Library on [11/03/2024]. See the Terms and Conditions (https://onlinelibrary.wiley.com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License Journal of Geophysical Research: Atmospheres LI ETAL. 10.1029/2022JD037883 17 of 18 Li, D., Liu, F., Pérez-Invernón, F. J., Lu, G., Qin, Z., Zhu, B., & Luque, A. (2020). 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