Fig. 4: Properties of sferics ground wave amplitudes.
From: Whistler echo trains triggered by energetic winter lightning
a Intensity of the magnetic field of the sferics ground waves measured at the receiving station as a function of the distance from the causative strokes. The different symbols in shades of pink-purple colour correspond to the strength of the first echo whistlers (extra strong: max Power Spectral Density (PSD) = apx 10−4 pT2/Hz, large dark purple open circles; strong: max PSD = apx 5 ⋅ 10−5 pT2/Hz, middle size purple full circles; middle intense: max PSD = apx 10−5 pT2/Hz, middle size dark pink open circles; weak: max PSD = apx 5 ⋅ 10−6 pT2/Hz, small light pink full circles). b Intensity of the magnetic field of the sferics ground waves normalized to account for an ideal propagation over a distance of 1000 km plotted as a function of the distance from the causative lightning stroke. The polarity of causative stroke is represented by the different symbols (dark purple full circles and light pink crosses respectively for negative and positive causative cloud-to-ground (CG) strokes). Coloured ovals denote the storm locations (Northern thunderstorm – blue, Central Mediterranean – green, and Eastern and African Mediterranean – orange). Source data are provided as a Source Data File.
