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A streamer is a kind of electric discharge in which there forms a column of ionized matter growing due to the amplified field at its tip. Streamers in air are an important stage of spark discharge, in particular lightning, and constitute sprites (mesospheric discharges). Streamers in large discharges (in sprites, in laboratory experiments with a meter-scale inter-electrode gap and megavolt potential difference, and in lightning) create large branched structures. Microscopic processes, which are responsible for propagation of streamers, are ionization, attachment, electron drift and diffusion, photoionization, and ion processes (detachment, recombination, etc.). Their numerical modeling reproduces the columnar shape of the ionization front, but does not allow to identify the physical and mathematical mechanisms of how the streamer radius and speed are chosen and of branching. A streamer may be described by a finite set of parameters, such as its speed and the radius of its head. The relations between these parameters are known, but do not constitute a complete set of equations: one free parameter (the radius) remains, which defines a streamer "mode" (the terminology is chosen by analogy with small transverse harmonic perturbations of a flat ionization front). As an additional requirement which would give a unique solution, we propose the choice of the most unstable mode that has the maximum speed. Thus, we treat the streamer as a nonlinear instability. This approach yields the results which agree with experimental measurements of streamer speeds, as well as of the streamer propagation threshold field. [ABSTRACT FROM AUTHOR]

During Airbus aircraft campaigns flying into thunderstorms in 2014–2016, X‐rays were observed during two stages of aircraft‐triggered lightning: nanosecond pulses of X‐rays associated with negative leader steps and bursts of X‐rays during recoil events. This work will focus on the observations of X‐ray bursts associated with recoils. Recoils are observed as microsecond‐fast changes in the local electric field, associated with large currents passing through the aircraft, and are found to sometimes be associated with bursts of X‐rays. From over 200 aircraft‐triggered lightning strikes, 54 recoil events were found to be associated with microsecond bursts of X‐rays. The majority of the bursts consist of 1–3 X‐ray pulses, with some bursts containing as many as 29 X‐ray pulses. We compare the observed superposed X‐ray spectrum with modeled spectra using a GEANT4 model of the detector and aircraft, to determine the source potential needed to accelerate the electrons that produce the observed X‐rays. A model of the recoil leader was made to determine the gap distance and gap potential between the recoil leader and the aircraft. From the modeling, we determine a solution space for the gap and leader lengths where the gap length is constrained by the observed minimum and maximum times between the onset of the X‐ray pulses and the onset of the current pulses detected at the aircraft (1 to 93 m). We also find two constraints from the fitting of the modeled spectra to the superposed spectrum, limiting the leader length to between 1 and ∼240 m. Key Points: Observations of 54 X‐ray bursts associated with recoil events during aircraft‐triggered lightning are presentedUsing three models, we find constraints to the gap and leader length of the recoil eventsWe find that the gap length is restricted from 1 to 93 m and the leader length is from 1 to 240 m [ABSTRACT FROM AUTHOR]

We investigate the attenuation of whistler waves generated by hypotetical Venusian lightning occurring at the altitude of the cloud layer under different ionospheric conditions. We use the Stanford full-wave method for stratified media of Lehtinen and Inan (2008) to model wave propagation through the ionosphere of Venus. This method calculates the electromagnetic field created by an arbitrary source in a plane-stratified medium (i.e., uniform in the horizontal direction). We see that the existence of holes in electronic densities and the magnetic field configuration caused by solar wind play an important role in the propagation of electromagnetic waves through the Venusian ionosphere. [ABSTRACT FROM AUTHOR]