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©2020. The Authors. This is an open access article under the terms of the Creative Commons Attribution‐NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.https://creativecommons.org/licenses/by-nc/4.0/ ; Narrow bipolar events (NBEs) (also called narrow bipolar pulses [NBPs] or compact intracloud discharges [CIDs]) are energetic intracloud discharges characterized by narrow bipolar electromagnetic waveforms identified from ground-based very low frequency (VLF)/low-frequency (LF) observations. The simplified ray-theory method proposed by Smith et al. (1999, https://doi.org/10.1029/1998JD200045; 2004, https://doi.org/10.1029/2002RS002790) is widely used to infer the altitude of intracloud lightning and the effective (or virtual) reflection height of the ionosphere from VLF/LF signals. However, due to the large amount of high-frequency components in NBEs, the propagation effect of the electromagnetic fields for NBEs at large distance depends nontrivially on the geometry and the effective conductivity of the Earth-ionosphere waveguide (EIWG). In this study, we investigate the propagation of NBEs by using a full-wave Finite-Difference Time-Domain (FDTD) approach. The simulated results are compared with ground-based measurements at different distances in Southern China, and we assess the accuracy of the simplified ray-theory method in estimating the altitude of the NBE source and the effective reflection height of the ionosphere. It is noted that the evaluated NBE altitudes have a slight difference of about ±1 km when compared with the full-wave FDTD results, while the evaluated ionospheric reflection heights are found to be bigger than those obtained from FDTD model by about 5 km. ©2020. The Authors. ; This work was supported by the European Research Council (ERC) under the European Union H2020 Programme/ERC Grant 681257. The ground-based VLF/LF measurements related to this article can be obtained from CAS Key Laboratory of Geo-space Environment in the School of Earth and Space Sciences at University of Science and Technology of China in Hefei, China (http://222.195.83.28/) ; Peer reviewed

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. ; The physical mechanism of Narrow bipolar events (NBEs) has been studied for decades but it still holds many mysteries. Recent observations indicate that the fast breakdown discharges that produce NBEs sometimes contain a secondary fast breakdown that propagates back in the opposite direction but this has not been fully addressed so far in electromagnetic models. In this study, we investigate fast breakdown using different approaches that employ a Modified Transmission Line with Exponential decay (MTLE) model and propose a new model, named "rebounding MTLE model," which reproduces the secondary fast breakdown current in NBEs. The model provides new insights into the physics of the fast breakdown mechanism. © 2022 The Authors. ; This work was supported by the European Research Council (ERC) under the European Union H2020 programme/ERC grant agreement 681257. Additionally, this work was supported by the Spanish Ministry of Science and Innovation, MINECO, under project PID2019-109269RB-C43 and FEDER program. Dongshuai Li, Alejandro Luque, and F. J. Gordillo-Vázquez 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). ; Peer reviewed

This is an open access article under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits use, distribution and reproduction in any medium, provided the original work is properly cited. ; Narrow Bipolar Events (NBEs) are powerful radio emissions from thunderstorms, which have been recently associated with blue optical flashes on cloud tops and attributed to extensive streamer electrical discharges named fast breakdown. Combining data obtained from a thunderstorm over South China by the space-based Atmosphere Space Interactions Monitor, the Vaisala GLD360 global lightning network and very low frequency/low frequency radio detectors, here we report and analyze for the first time the optical emissions of blue luminous events associated with negative NBEs and located at the top edge of a thundercloud. These emissions are weakly affected by scattering from cloud droplets, allowing us to estimate the source extension and optical energy involved in the process. The optical energy in the 337-nm band emitted by fast breakdown is about 104 J, which involves around 109 streamer initiation events. © 2021. The Authors. ; 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 M. 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. Li, A. Luque and F. J. Gordillo-Vázquez 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). F. Liu, G. Lu and B. Zhu are supported by the National Key Research and Development Program of China (2017YFC1501501) and National Natural Science Foundation of China (41775004, 41875006, 42005068, U1938115). ; Peer reviewed

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes. ; Blue LUminous Events (BLUEs) are transient corona discharges occurring in thunderclouds and characterized by strong 337.0 nm light flashes with absent (or weak) 777.4 nm component. We present the first nighttime climatology of BLUEs as detected by the Modular Multispectral Imaging Array of the Atmosphere-Space Interaction Monitor showing their worldwide geographical and seasonal distribution. A total (land and ocean) of (Formula presented.) 11 BLUEs occur around the globe every second at local midnight and the average BLUE land/sea ratio is (Formula presented.) 7:4. The northwest region of Colombia shows an annual nighttime peak. Globally, BLUEs are maximized during the boreal summer-autumn, contrary to lightning which is maximed in the boreal summer. The geographical distribution of nighttime BLUEs shows three main regions in, by order of importance, the Americas, Europe/Africa and Asia/Australia. © 2021 The Authors. ; ASIM is a mission of ESA's SciSpace programme for scientific utilization of the ISS and non-ISS space exploration platforms and space environment analogues. ASIM and the ASIM Science Data Centre are funded by ESA and by National grants of Denmark, Norway and Spain. This work was supported by the Spanish Ministry of Science and Innovation under project PID2019-109269RB-C43 and the FEDER program. S. Soler acknowledges a PhD research contract through the project PID2019-109269RB-C43. F. J. Pérez-Invernón acknowledges the sponsorship provided by the Federal Ministry for Education and Research of Germany through the Alexander von Humboldt Foundation. A. Luque and D. Li were supported by the European Research Council (ERC) under European Union H2020 programme/ERC grant agreement 681257. S. Soler, F. J. Gordillo-Vázquez, A. Luque and D. Li 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). ; Peer reviewed

This Open Access article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. ; Narrow bipolar events (NBEs) are signatures in radio signals from thunderstorms observed by ground-based receivers. NBEs may occur at the onset of lightning, but the discharge process is not well understood. Here, we present spectral measurements by the Atmosphere‐Space Interactions Monitor (ASIM) on the International Space Station that are associated with nine negative and three positive NBEs observed by a ground‐based array of receivers. We found that both polarities NBEs are associated with emissions at 337 nm with weak or no detectable emissions at 777.4 nm, suggesting that NBEs are associated with streamer breakdown. The rise times of the emissions for negative NBEs are about 10 μs, consistent with source locations at cloud tops where photons undergo little scattering by cloud particles, and for positive NBEs are ~1 ms, consistent with locations deeper in the clouds. For negative NBEs, the emission strength is almost linearly correlated with the peak current of the associated NBEs. Our findings suggest that ground-based observations of radio signals provide a new means to measure the occurrences and strength of cloud-top discharges near the tropopause. © 2021, The Author(s). ; The authors would acknowledge financial support from the National Key Research & Development Program (2017YFC1501501), the Project of Stable Support for Youth Team in Basic Research Field, CAS (YSBR-018), the B-type Strategic Priority Program of the Chinese Academy of Sciences (XDB41000000), National Natural Science Foundation of China (41775004, 41831070, 41875006, 41974181, 42005068, and U1938115), and the Open Research Project of Large Research Infrastructures of CAS—"Study on the interaction between low/mid-latitude atmosphere and ionosphere based on the Chinese Meridian Project". F.F.L is also supported by Fundamental Research Funds for the Central Universities (WK2080000134). A.L. and D.L. is supported by the European Research Council (ERC) under the European Union H2020 programme/ERC Grant Agreement 681257. F.J.G.V. acknowledges funding by the Spanish Ministry of Science and Innovation (AEI) under project PID2019‐109269RB‐C43 and the FEDER program. F.J.G.V., A.L., and D.L. acknowledge financial support from the Spanish AEI through the Center of Excellence "Severo Ochoa" award for the Instituto de Astrofísica de Andalucía (SEV‐2017‐0709). ASIM and the ASIM Science Data Centre are funded by ESA and by national grants of Denmark, Norway and Spain. We thank Vaisala for the GLD360 lightning data. ; Peer reviewed