Head‐on collisions between negative and positive streamer discharges have recently been suggested to be responsible for the production of high electric fields leading to X‐rays emissions. Using a plasma fluid approach, we model head‐on collisions between negative and positive streamers. We observe the occurrence of a very strong electric field at the location of the streamer collision. However, the enhancement of the field produces a strong increase in the electron density, which leads to a collapse of the field over only a few picoseconds. Using a Monte Carlo model, we have verified that this process is therefore not responsible for the acceleration of a significant number of electrons to energy >1 keV. We conclude that no significant X‐ray emission could be produced by the head‐on encounter of nonthermal streamer discharges. Moreover, we quantify the optical emissions produced in the streamer collision.
Future space missions (e.g., ASIM and TARANIS) are soon to be launched to observe transient luminous events (TLEs) from a nadir‐viewing geometry. The mission GLIMS already performed observations of TLEs from a nadir‐viewing geometry on board the International Space Station. Although this observation geometry is of first interest to study TLEs, it makes the determination of some quantities, such as streamer altitudes, very difficult. In this study, we propose a method to estimate the altitude of downward propagating sprite streamers using a spectrophotometric approach. Using a plasma fluid model, we simulate sprite streamers at different altitudes and quantify their optical emissions in the Lyman‐Birge‐Hopfield (LBH) (∼100–260 nm), the first positive (1PN2) (∼650–1070 nm), and the second positive (2PN2) (∼330–450 nm) bands systems of molecular nitrogen and the first negative ( 1NN2+) (∼390–430 nm) bands systems of N2+. The estimation of associated ratios allows to trace back the electric field in the streamer head as well as the altitude at which the streamer is propagating owing to different dependencies of quenching processes on the air density. The method takes into account the nonsteady state of the populations of some excited species and the exponential expansion of the streamer. The reported results could potentially be used for all TLEs but is of special interest in the case of column sprites or at the early stage of carrot sprites.
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