The Physical Mechanism of Lightning HF/VHF Bursts with Weak Sferics

Recent multi-band radio frequency (RF) observations have clearly shown that bursts of HF and VHF radiation with only weak or even no accompanying sferic pulses (i.e., weak lower-frequency radiation) can be generated during thunderstorms. Examples of such RF bursts include the microsecond-long radiation from the initiating breakdown of precursor events (Rison et al., Nat. Commun., 7, 10721, 2016) and narrow (e.g., <0.5 microseconds) VHF pulses that are the initiating events of a large fraction of lightning and that also occur during lightning formation (Marshall et al., Atmos. Res., 217, 10, 2019; Lyu et al., JGR, 124, 2994, 2019). Interestingly, long lasting (~1 s) bursts of VHF radiation, termed continual radio frequency (CRF), without strong sferic signatures, have also been observed during the onset of explosive volcanic eruptions (e.g., Behnke et al., JGR, 123, 4157, 2018).

In this talk, we investigate a physical mechanism for production of such HF and VHF bursts without significant lower-frequency radiation. As shown by recent studies (Liu et al., JGR , 124, 10134, 2019; Liu et al., PRL, 125, 025101, 2020), a large ensemble of streamers that all propagate in the same direction can explain the elevated spectral magnitude in the HF and VHF bands such as the spectrum of narrow bipolar events (NBEs). Even for NBEs, the spectral magnitude in the high frequency range is still much smaller than that of lower frequencies. However, if the streamers are allowed to propagate in random directions instead, the radiation intensity is only decreased slightly in the HF and VHF bands, but is greatly reduced in lower frequencies and in fact negligible near zero frequencies. We present the theory of the electromagnetic radiation from an ensemble of streamers that propagate in random directions. Numerical simulations are also presented to validate the developed theory. Combined with the observation of lightning initiated by a narrow VHF pulse, our results imply that lightning initiation may begin from a highly localized strong field region in thunderstorms, where many streamers are initiated and propagate in random directions. Finally, our results also support the conclusion of Behnke et al. (2018) that CRF is produced by streamer-like discharges.