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We investigate sequential processes underlying the initial development of in‐cloud lightning flashes in the form of initial breakdown pulses (IBPs) between 7.4 and 9.0 km altitudes, using a 30–250 MHz VHF interferometer. When resolved, IBPs exhibit typical stepped leader features but are notably extensive (>500 m) and infrequent (∼1 millisecond intervals). Particularly, we observed four distinct phases within an IBP stepping cycle: the emergence of VHF sources forming edge structures at previous streamer zone edges (interpreted as space stem/leader development), the fast propagation of VHF along the edge structure (interpreted as the main leader connecting the space leader), the fast extension of VHF beyond the edge structure (interpreted as fast breakdown), and a decaying corona fan. These measurements illustrate clearly the processes involved in the initial development of in‐cloud lightning flashes, evidence the conducting main leader forming, and provide insights into other processes known to occur simultaneously, such as terrestrial gamma ray flashes. Plain Language Summary The initial development of a lightning flash inside a cloud has long been a mystery. This study utilizes state‐of‐the‐art lightning imaging techniques with a 30–250 MHz VHF interferometer, providing clear images of the processes involved in the initial development of in‐cloud lightning flashes. New radio features suggest distinct development phases, including what we interpret as space stems, space leaders, connection between the main leader and the space leader, fast breakdown, and corona fan development within an initial breakdown pulse stepping cycle. This provides evidence of the conducting main leader in the initial breakdown stage. These observations showcase the intricate streamer discharge phenomena during initial lightning development, and shed light on other processes known to occur simultaneously, including Terrestrial Gamma ray Flashes. Key Points We observed four distinct VHF processes in the development of 300–1,000 m long initial breakdown pulses (IBPs) in in‐cloud lightning flashes These four processes appear to map to the known processes in a conventional stepped leader, including space stem and space leader formation During an initial breakdown step, fast extension over several hundred meters indicates that fast breakdown may be an essential part of in‐cloud flash

The work presented in this dissertation is dedicated to the investigation of leader discharge mechanisms in lightning and transient luminous events. We introduce two new numerical models of the leader process that capture its onset and propagation, as briefly described below. The first theoretical model simulates air heating and streamer-to-leader transition in gas discharges. The model accounts for all physical processes known to play a role in the conversion of a streamer corona to a leader channel. Detailed discussion on the role of electron detachment in the development of the thermal-ionizational instability that triggers the spark development in short air gaps is presented. The dynamics of fast heating by quenching of excited electronic states is discussed and the scaling of its main channels with ambient air density is quantified. The developed model is employed to study the streamer-to-leader transition process and to obtain its scaling with ambient air density. The introduced methodology for estimation of leader speeds is based on the assumption that the propagation of a leader is limited by the air heating of every newly-formed leader section. It is demonstrated that the streamer-to-leader transition time has an inverse-squared dependence on the ambient air density at near-ground pressures, in agreement with similarity laws for Joule heating in a streamer channel. Model results indicate that a deviation from this similarity scaling occurs at very-low air densities, where the rate of electronic power deposition is balanced by the channel expansion, and air heating from quenching of excited electronic states is very inefficient. These findings place a limit on the maximum altitude at which a hot and highly-conducting lightning leader channel can be formed in the Earth’s atmosphere. This result is important for understating of the gigantic jet discharges between thundercloud tops and the lower ionosphere. Our simulations of leader propagation at stratospheric altitudes demonstrate that initial speeds of gigantic jets are consistent with the leader propagation mechanism, and that the observed acceleration in gigantic jets can be attributed to the expansion of its streamer zone in a medium of exponentially-decreasing air density. This process defines the existence of an altitude at which the streamer zone becomes so long that it dynamically extends all the way to the ionosphere. We extend the air heating model to simulate the effects of strong currents flowing in sprites in the mesosphere. We show that fast air heating (due to quenching of excited electronic states) in sprite cores can produce > 0.01 Pa pressure perturbations on the ground, observed in association with sprites. The second theoretical model simulates the electromagnetic radiation generated during the initial breakdown stage of lightning discharges. We use this model to describe in detail how the leader discharge dynamics generates the so-called initial breakdown pulses (IBPs) and narrow bipolar events (NBEs) observed with multi-station electric field change sensors on the ground. IBPs have been correlated with individual bursts of light that appear to be illuminations of a lightning leader channel; as a flash evolves the location of IBP sources inside the cloud coincides with the position of negative leaders as determined by a VHF lightning mapping system. NBEs are electric field signatures with broadband waveforms resembling classic IBPs in both amplitude and duration. Nonetheless, NBEs are quite peculiar in the sense that very infrequently they are followed by conventional lightning processes. Only a small fraction of positive-polarity NBEs happen as the first event in an otherwise regular intracloud lightning discharge. Typically, the initiator-type of NBEs has no difference with other NBEs that did not start lightning, except for the fact that they occur deeper inside the thunderstorm (i.e., at lower altitudes). In this dissertation we propose that IBPs and NBEs are the electromagnetic transients associated with the sudden (i.e., stepwise) elongation of the initial negative leader extremity in the thunderstorm electric field. To demonstrate our hypothesis we use a novel model of the leader electrodynamics. It consists of a generalization of electrostatic and transmission-line approximations existing in the refereed literature. We demonstrate how the IBP and NBE waveform characteristics directly reflect the properties of the bidirectional lightning leader (such as step length, for example) and amplitude of the thunderstorm electric field.

A kinetic model of electron cascade growth in the electromagnetic field of a focused intense laser pulse as used for laser spark generation in gases has been numerically implemented in Visual C code. The effects considered comprise Drude absorption, diffusive kinetic and inelastic losses as well as (three-particle) electron recombination. The objectives were to illustrate the dynamic process of gas ionization, and to clarify the pressure dependence of known breakdown thresholds within a range of about 2 × 104 to 2 × 106 Pa of initial pressure. Two-dimensional (cylindric coordinates) simulations of the optical breakdown in nitrogen were conducted on a commercial PC, using constant values for the collision cross section (2 × 10−19 m2), prevalent electronic excitation states (~4.8 eV), and a laser wavelength of 1064 nm. A certain aerosol concentration on the order of 3 ppb was assumed in order to provide initial electrons for cascade growth. Exemplary results with laser pulse energy of 26 mJ, pulse duration of 14 ns and an 18 µm focal spot size illustrate the dynamic process of ionization within a very short time period of less than 0.5 ns. The kinetic energy of the electrons is found to increase sharply up to more than 100,000 K on breakdown. A series of simulations considered the minimum pulse energy of breakdown (MPE) under variation of initial pressure. Identical laser parameters as in experiments conducted previously were used and the results are in excellent agreement with respect to curve shapes, i.e., MPE ~1/p0.4 in the first experiment and MPE ~1/p0.3 in the second one. The absolute values lie within a factor of two, which is explained by model abstraction and input data uncertainties.