Post-Return Stroke VHF Electromagnetic Activity in North-Western Mediterranean Cloud-to-Ground Lightning Flashes

We investigate properties of the electromagnetic activity following the first lightning return stroke (RS), using concurrent observations from the SLAVIA (Shielded Loop Antenna with a Versatile Integrated Amplifier) sensor, the lightning mapping array (LMA) SAETTA (Suivi de l'Activité Electrique Tridimensionnelle Totale de l'Atmosphère) and Météorage LF network in the Corsica region. From the data collected between September and December 2015, we selected 66 negative cloud-to-ground (-CG) and 26 positive cloud-to-ground (+CG) lightning flashes in the north-western Mediterranean region. In the SAETTA data, we observe a decrease of the Very High Frequency (VHF) radiation rate and the VHF power as recorded within a typical 80-µs time window at the LMA stations, immediately after the RS pulse in 59 -CG flashes. Contrastingly, we show that all examined +CG flashes exhibit a rapid increase of the VHF radiation rate and the VHF power immediately after the RS. We suggest a possible explanation of this phenomenon by considering step-like propagation of a negative part of bidirectional leader starting at the top end of the positive lightning channel inside the thundercloud, emitting electromagnetic radiation across a broad frequency spectrum.