the banded spectral structure could be due to multiple bounces between the ionosphere and ground. Under favourable conditions, consecutive steeper and steeper reflections can produce a "tweek", ie a pulsetrain with decreasing repetition rate. During the tail, there are distinct spectral maxima about 1.7 kHz apart, defined by the delay for near vertical travel to the D-layer and down again. Tweek formation would be favoured by high-altitude horizontal lightning, good ground reflectivity (water), and a steep ionization gradient at the bottom of the ionosphere at night.

 

As the phasing somewhat depends on the ground distance between the strike and the receiver, patterns from spatially extended storm areas may average out in a slow spectrogram, unless the lightning activity is concentrated in a small area or ring around the receiver. Attached is a plot from the Blitzortung archive from 1 to 3 UT. We could speculate that the region of activity in Western France was creating your tweeks and the Polish "arc" was responsible for Lubos', but it's hard to tell for sure.

 

Best 73,

Markus (DF6NM)

 

 

VLF, 

  

 Just discovered a strange QRN pattern on VLF this night. See attachment.  

 The origin cannot be local because the same was shown on the OK2BVG VLF  

 grabber. 

 What is it? 

  

 73, Stefan/DK7FC