Electromagnetic Radiation of Cloud-to-ground Lightning Flashes Detect-ed by Broadband Receiver BLESKA and Narrowband SAETTA Stations
Abstract
All evolution stages of cloud-to-ground (CG) lightning flashes, both positive (+CG) and negative (-CG), generate electromagnetic radiation, which can be used for their investigation. We focus on the electromagnetic activity immediately following the first return stroke (RS).
We combine measurements of the Broadband Lightning Electromagnetic Signal Keeper Analyzer (BLESKA) and the lightning mapping array (LMA) network SAETTA (Suivi de l'Activité Electrique Tridimensionnelle Totale de l'Atmosphère), capable of detecting sources of narrowband very high frequency (VHF) radiation. From our data collected over the north-western Mediterranean Sea from September to November 2015, we have selected and investigated 17 +CG and 20 -CG flashes.
A nearly continuous VHF radiation during the PB stage and a slow decrease in counts and power of VHF sources in the time interval between the PB pulses and the first RS pulse in both -CG and +CG flashes is seen in SAETTA data. Data from individual SAETTA stations also show that -CG flashes exhibited a decrease in the counts and in the power of VHF radiation sources immediately after the RS pulse.
However, in the case of all inspected +CG flashes, we observed an unexpectedly fast increase in the counts of the VHF radiation sources and their power. At the same time, we observed a visible sequence of bipolar pulses lasting for up to 0.5 ms in the magnetic-field waveforms recorded by BLESKA.
This may be due to a step-like propagation of negative leader inside the thundercloud, emitting electromagnetic radiation in a wide range of frequencies.