Electron acceleration above thunderclouds - Figure 2.
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Figure 2. Left. Upper panel. Electric field measurements from ~5–30 Hz at Nagycenk (NCK) are used to infer the charge moment change of the two consecutive lightning discharges (black dotted lines). The charge moment of the first positive lightning discharge exceeds the limit for sprite initiation. The second positive lightning discharge exhibits a much smaller charge moment. Middle panel. The recordings of the quasi-static current from ~1–50 Hz near Portishead (PTH) indicate that the first lightning discharge initiated a sprite. Lower panel. The low frequency radio noise from ~4–400 kHz near Orléans (ORL) and Bath (UOB) indicates radio emissions from sprite streamers (red dotted line) initiated by the first lightning discharge and a re-brightening of the remaining sprite streamers during the second lightning discharge. Right. Upper panel. The second lightning discharge exhibits the typical ~0.1–1 ms long ~5–15 kHz (VLF) electric field enhancement which is larger in LeQuartier (LQT) when compared to Bath as a result of the proximity to the lightning discharge. Lower panel. About ~8–9 ms after the second lightning discharge, a ~1 ms long ~270–400 kHz (LF/MF) radio pulse indicates the acceleration of electrons associated with an electron beam which is recorded by both radio receivers. Note that the leader steps recorded with the lightning mapping array from ~60–66 MHz (crosses in the lower panel) do not seem to be related to the VLF or LF/MF recordings.
The acceleration of electrons results in observable electromagnetic waves which can be used for remote sensing. Here, we make use of ~4 Hz–66 MHz radio waves emitted by two consecutive intense positive lightning discharges to investigate their impact on the atmosphere above a thundercloud. It is found that the first positive lightning discharge initiates a sprite where electrons are accelerated during the exponential growth and branching of the sprite streamers. This preconditioned plasma above the thundercloud is subsequently exposed to a second positive lightning discharge associated with a bouncing-wave discharge. This discharge process causes a re-brightening of the existing sprite streamers above the thundercloud and initiates a subsequent relativistic electron beam.