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1. Studying Sequences of Initial Breakdown Pulses in Cloud‐to‐Ground Lightning Flashes

   https://doi.org/10.1029/2019JD032104  

   Karunarathne, Nilmini ; Marshall, Thomas C. ; Karunarathne, Sumedhe ; Stolzenburg, Maribeth ( January 2020 , Journal of Geophysical Research: Atmospheres)

   The properties of the first 5–12 classic initial breakdown pulses (IBPs) of three cloud‐to‐ground (CG) lightning flashes were determined using a modified transmission line model. As part of the modeling, the current with respect to time of each IBP was determined from the measured electric field changes at multiple sites using three theoretical methods called Hilbert transform, Hertzian dipole, and matrix inversion. In the transmission line modeling the length of each IBP was estimated from high‐speed video data of the IBPs. The modeling provided the following properties of the larger classic IBPs in each flash: peak current, velocity, total charge, charge moment, radiated power, and total energy dissipated for successive IB pulses in three developing lightning flashes. For the main initial leader channel in the three CG flashes (and for one long branch), IBP peak current was largest for the first or second classic IBP and declined mostly monotonically with successive IBPs. For the same channels, IBP current velocity was smallest for the first classic IBP and increased mostly monotonically with successive IBPs. The smallest velocities were (2.0, 2.5, 2.5, 3.0) × 10⁷m/s, respectively, while the largest velocities were (9.2, 12.2, 11.5, 12.0) × 10⁷m/s, respectively. These data support earlier hypotheses that it is the classic IB pulses during initial leader of normal negative CG flashes that change the nonconductive air into an ionized path that is sufficiently long and conductive to start the stepped leader.

    

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2. The Mechanism of the Origin and Development of Lightning From Initiating Event to Initial Breakdown Pulses (v.2)

   https://doi.org/10.1029/2020JD033191  

   Kostinskiy, Alexander Yu. ; Marshall, Thomas C. ; Stolzenburg, Maribeth ( November 2020 , Journal of Geophysical Research: Atmospheres)

   Based on experimental results of recent years, this article presents a qualitative description of a possible mechanism (termed the Mechanism) covering the main stages of lightning initiation, starting before and including the initiating event, followed by the initial electric field change (IEC), followed by the first few initial breakdown pulses (IBPs). The Mechanism assumes initiation occurs in a region of ~1 km³with average electric fieldE > 0.3 MV/(m·atm), which contains, because of turbulence, numerous small “E_thvolumes” of ~10⁻⁴–10⁻³ m³withE ≥ 3 MV/(m·atm). The Mechanism allows for lightning initiation by either of two observed types of events: a high‐power, very high frequency (VHF) event such as a Narrow Bipolar Event or a weak VHF event. According to the Mechanism, both types of initiating events are caused by a group of relativistic runaway electron avalanche particles (where the initial electrons are secondary particles of an extensive air shower) passing through manyE_thvolumes, thereby causing the nearly simultaneous launching of many positive streamer flashes. Due to ionization‐heating instability, unusual plasma formations (UPFs) appear along the streamers' trajectories. These UPFs combine into three‐dimensional (3‐D) networks of hot plasma channels during the IEC, resulting in its observed weak current flow. The subsequent development and combination of two (or more) of these 3‐D networks of hot plasma channels then causes the first IBP. Each subsequent IBP is caused when another 3‐D network of hot plasma channels combines with the chain of networks caused by earlier IBPs.

    

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3. Observations of the Origin of Downward Terrestrial Gamma‐Ray Flashes

   https://doi.org/10.1029/2019JD031940  

   Belz, J. W. ; Krehbiel, P. R. ; Remington, J. ; Stanley, M. A. ; Abbasi, R. U. ; LeVon, R. ; Rison, W. ; Rodeheffer, D. ; Abu‐Zayyad, T. ; Allen, M. ; et al ( December 2020 , Journal of Geophysical Research: Atmospheres)

   In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.

    

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4. Efficient modeling of electron kinetics under influence of externally applied electric field in magnetized weakly ionized plasma

   https://doi.org/10.1088/1361-6595/acdaf1  

   Janalizadeh, Reza ; Pervez, Zaid ; Pasko, Victor P. ( July 2023 , Plasma Sources Science and Technology)

   We present a theory based on the conventional two-term (i.e. Lorentzian) approximation to the exact solution of the Boltzmann equation in non-magnetized weakly ionized plasma to efficiently obtain the electron rate and transport coefficients in a magnetized plasma for an arbitrary magnitude and direction of applied electric field$\vec{E}$and magnetic field$\vec{B}$. The proposed transcendental method does not require the two-term solution of the Boltzmann equation in magnetized plasma, based on which the transport parameters vary as a function of the reduced electric field$E/N$, reduced electron cyclotron frequency${\omega }_{\mathrm{c}\mathrm{e}}/N$, and angle$\mathrm{\angle }\vec{E},\vec{B}$between$\vec{E}$and$\vec{B}$vectors, whereNis the density of neutrals. Comparisons between the coefficients derived from BOLSIG+’s solution (obtained via the two-term expansion when$\vec{B}\ne 0$) and coefficients of the presented method are illustrated for air, a mixture of molecular hydrogen (H₂) and helium (He) representing the giant gas planets of the Solar System, and pure carbon dioxide (CO₂). The new approach may be used in the modeling of magnetized plasma encountered in the context of transient luminous events, e.g. sprite streamers in the atmosphere of Earth and Jupiter, in modeling the propagation of lightning’s electromagnetic pulses in Earth’s ionosphere, and in various laboratory and industrial applications of nonthermal plasmas.

    

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5. Towards limited-domain full waveform ambient noise inversion

   https://doi.org/10.1093/gji/ggae091  

   Tsai, Victor C. ; Sager, Korbinian ; Bowden, Daniel C. ( March 2024 , Geophysical Journal International)

   Ambient noise tomography is a well-established tomographic imaging technique but the effect that spatially variable noise sources have on the measurements remains challenging to account for. Full waveform ambient noise inversion has emerged recently as a promising solution but is computationally challenging since even distant noise sources can have an influence on the interstation correlation functions and therefore requires a prohibitively large numerical domain, beyond that of the tomographic region of interest. We investigate a new strategy that allows us to reduce the simulation domain while still being able to account for distant contributions. To allow nearby numerical sources to account for distant true sources, we introduce correlated sources and generate a time-dependent effective source distribution at the boundary of a small region of interest that excites the correlation wavefield of a larger domain. In a series of 2-D numerical simulations, we demonstrate that the proposed methodology with correlated sources is able to successfully represent a far-field source that is simultaneously present with nearby sources and the methodology also successfully results in a robustly estimated noise source distribution. Furthermore, we show how beamforming results can be used as prior information regarding the azimuthal variation of the ambient noise sources in helping determine the far-field noise distribution. These experiments provide insight into how to reduce the computational cost needed to perform full waveform ambient noise inversion, which is key to turning it into a viable tomographic technique. In addition, the presented experiments may help reduce source-induced bias in time-dependent monitoring applications.

    

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