Fig. 1: Frequency-time spectrograms with examples of whistler echo trains recorded on January 3, 2017.

The colour represents the logarithm of the Power Spectral Density (PSD) in nT²Hz⁻¹. The white vertical lines, spanning over the entire frequency band and initiating each group of dispersed signals, are attributed to sferics emitted by lightning return strokes, which triggered the echo trains. Weaker broadband impulsive signals extending from the top of the spectrograms correspond to sferics emitted by distant lightning. Two overlapping whistler echo trains can be recognized in panels a and c. It is noteworthy that the intensity of VLF noise in the background is diminished within the whistler trains in panels b and c. This intriguing phenomenon was explained by a modification of electron distribution in the noise generation region caused by the intense whistler trains⁵⁵.