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    $\begingroup$ I had a hunch there was some real physics here, thank you! Yes, with a dispersive medium and a broadband transient, what timing the speed of the "burst" means deserves some thought! I'll give that thesis a looks this week, thank you! $\endgroup$ Commented Aug 3, 2018 at 9:06
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    $\begingroup$ @uhoh: Yes speed of wave bursts is a difficult topic with subtle nuances. BTW the thoughts of famous physicists on speed of propagation even fill a full book "Wave Propagation and Group Velocity" by Léon Brillouin. $\endgroup$ Commented Aug 3, 2018 at 9:55
  • $\begingroup$ @Andreas H. Since you mention Brillouin (& Sommerfeld)'s work, why don't you detail the last part of your answer to say that even the group velocity is not the speed at which information/ signal is propagated (in all generality,en.wikipedia.org/wiki/Front_velocity)? It would be a good addition to show that it is indeed a complex problem. Another nice reference: aapt.scitation.org/doi/10.1119/1.1976551 $\endgroup$ Commented Aug 3, 2018 at 12:14
  • $\begingroup$ The cited thesis is not relevant to this question - it measures the influence of the ionosphere (thus the day-night difference), but we're strictly interested in light and radio waves travelling in a straight line from lightning to observer. $\endgroup$ Commented Aug 3, 2018 at 12:36
  • $\begingroup$ @JonaChristopherSahnwaldt: Well, I would say that this depends on the receiver or antenna one uses. But yes it is a good point. The multi-path propagation will also influence the dispersion (in fibers this is known as modal dispersion). So the velocity should also be dependent on the antenna pattern, and how well one points the antenna to the source. So for practical reasons one would perhaps use an omidirectional (or something close to that) antenna and accepts the contributions from ionosphere. $\endgroup$ Commented Aug 3, 2018 at 12:51