A strong pulsing nature of negative recoil leaders accompanied by regular trains of microsecond-scale pulses

A presence of regular sequences of microsecond-scale pulses has been occasionally reported in the lightning literature for more than forty years. Due to a fine time resolution of modern electromagnetic receivers, the properties of these pulse trains are now well described. Nevertheless, the conditions for their occurrence are still not understood, and the information needed for their proper modelling is not sufficient. To contribute to this effort, we report for the first time properties of negative recoil stepped leaders accompanied by regular trains of microsecond-scale pulses simultaneously seen by the broadband magnetic loop antenna SLAVIA (Shielded Loop Antenna with a Versatile Integrated Amplifier; 5 kHz-90 MHz), and the radio telescope LOFAR (Low Frequency Array; 30-80MHz). We investigate four pulse trains that occurred during complicated intracloud flashes on 18 June 2021, when heavy thunderstorms hit Netherlands.The pulses within the trains are unipolar, a few microseconds wide with an inter-pulse interval of about ten microseconds. The pulse trains last from 100 µs to 800 µs. After a careful time alignment of both magnetic field and LOFAR time series, we found that the broadband pulses perfectly match with regularly distributed and relatively isolated bursts of VHF sources localized by the LOFAR impulsive imager. All trains were generated by negative recoil stepped leaders propagating downward (two events) or upward (two events) at altitudes between 5.5 km and 8.5 km. Their tracks were formed by positive leaders occurring within the same flash several hundreds of milliseconds previously. The peak powers of VHF sources seen by the LOFAR electric antennas closest to the investigated discharges were about one order of magnitude higher than the power of signals emitted by normal negative leaders. These stepped recoil leaders propagate at a relatively low speed of about 2-5x10^6 m/s, when similar recoil leaders often reach speeds of 10^7 m/s. The velocity and inter-pulse intervals decrease towards the end of trains.We show that observed pulse trains are due to stepping recoil leaders. However, we consider this strong pulsing nature of the examined recoil leaders to be quite unusual. The physical mechanism giving rise to the energetic VHF bursts and accompanying regular microsecond-scale pulses remains unclear.