Post-Return Stroke VHF Electromagnetic Activity in North-Western Mediterranean CG Flashes

We investigate the electromagnetic activity following the first return stroke (RS), using data from the SLAVIA (Shielded Loop Antenna with a Versatile Integrated Amplifier) sensor and the lightning mapping array SAETTA (Suivi de l’Activité Electrique Tridimensionnelle Totale de l’Atmosphère). From data collected between September and December 2015 in the north-western Mediterranean region, we selected 69 negative cloud-to-ground (-CG) and 29 positive cloud-to-ground (+CG) flashes. Within SAETTA data, we observe a reduction in counts and power of VHF radiation sources immediately after the RS pulse in 62 -CG flashes. Contrastingly, all examined +CG flashes exhibit a rapid increase in the counts and power of VHF radiation sources immediately after the RS. We offer a hypothesis of a step-like propagation of a negative leader starting at the top end of the lightning channel inside the thundercloud, emitting electromagnetic radiation across a broad frequency spectrum.