Analytical Streamer Model to Explain VHF Radiation from Lightning Related Events

Narrow bipolar events (NBEs) are the very low frequency-low frequency (VLF-LF) signatures of compact intracloud discharges (CIDs). These discharges, with spatial extents of few hundreds of meters, are highly impulsive and powerful sources of very high frequency (VHF) radiation in the Earth's natural environment, often preceding lightning discharges in thunderclouds. The VHF radiation is believed to be emitted by the presence of a system of streamers around the NBE source [Rison et al., 2016, Nat. Comm., 7, 10721]. In this work, an analytical model is developed for effective representation of streamer current for a double-headed exponentially growing streamer with parameters such as the frequency of exponential growth informed from results of past studies [Celestin and Pasko, 2011, JGR, 116, A03315; Kosar et al., 2012, JGR, 117, A08328; Qin and Pasko, 2014, J. Phys., 47, 435202]. The decay of streamer current is attributed to two- and three-body attachment of electrons to neutrals with the latter, when present, violating the scaling laws of streamer properties. Consequently, where three-body attachment is negligible, the model predicts streamers of growing strength and spatial scales, such as those observed in blue jets and gigantic jets. This effect is explained by the suppression of electron losses due to three-body attachment with reducing air pressure. Previous work calculated NBE source currents from electric field observations [Pervez et al., 2024, GRL, 51, e2023GL107789], and a recent study has reported a mechanism based on photoelectric feedback in relativistic electron avalanches to explain the production of the source current generating the VLF-LF signature [Pasko et al., Abstract AE23B-2573, AGU Fall Meeting, 9-13 December 2024, Washington, D.C.]. Once the growing thundercloud electric field passes the threshold for streamer initiation, electrons produced by the photoelectric feedback discharge seed streamers, that eventually produce the VHF signature of a CID. We demonstrate that CIDs at higher altitudes develop during a longer period such that, as confirmed by observations, the spectral content of recorded sferics shifts towards lower frequencies.