A class of unusual lightning electric field waveforms with very strong high‐frequency radiation

Abstract

The first wideband dE/dt recordings have been obtained for the narrow bipolar pulses previously identified by Le Vine (1980) as “sources of the strongest RF radiation from lightning.” These dE/dt waveforms are dramatically different from those of other known lightning processes. A burst of high‐frequency “noise” is superimposed on the slower bipolar pattern one might expect from the relatively smooth E waveforms. For 18 such pulses from an isolated thunderstorm cell at known range, the mean peak E and dE/dt, range‐normalized to 100 km, were 8.0±5.3 V/m and 20±15 V/m/μs, respectively. Spectral analysis indicates that the sources of these pulses radiate much more strongly than first‐return strokes at frequencies from 10 MHz to at least 50 MHz. Absolutely calibrated power and energy spectra are presented which are reliable from 200 KHz to perhaps 20 MHz. At 18 MHz the narrow pulses appear to contain nearly 16 dB more spectral energy than first return‐stroke waveforms from the same range. Supporting evidence shows that they generally occur as isolated pulses in intracloud flashes but are not associated with K changes or other known phenomena. They can occur in either polarity.

Revisiting "Narrow Bipolar Event" intracloud lightning using the FORTE satellite

“…For this limited dataset that has LASA ground-truth on whether the coincidence is from an NBE, it appears that NBE-coincident VHF events show no significant pattern of proximity to flash neighbors, unlike the non-NBE-coincident VHF events. This would tend to agree with the earliest, pre-LMA reports (Le Vine, 1980;Willett et al, 1989).…”

“…8-11 of the present study confirm that at the high VHF TIPP ERP ranges, say >30 kW, the likelihood of prompt concurrence above the PDD threshold (top-of-cloud peak emission power ∼ 3×10 8 W) falls monotonically versus increasing ERP. We remark that since the earliest report of NBE sferics (Le Vine, 1980), all studies of this phenomenon have observed that NBE sferics are synchronously accompanied by the most intense VHF emissions of any lightning Jacobson and Light, 2003;Nag andRakov, 2009, 2010a, b;Nag et al, 2010;Smith, 1998;Thomas et al, 2001;Willett et al, 1989Willett et al, , 1990Willett and Krider, 2000). Thus, the results of Figs.…”

“…This was proposed in the first reports of NBEs (Le Vine, 1980;Willett et al, 1989), but was somewhat contradicted by later studies based on the Lightning Mapping Array Thomas et al, 2001), which reported NBEs initiating intracloud flashes. Recently, a ground-based study of NBEs (Nag et al, 2010) observed that out of a total of 157 events, 73 % were isolated.…”

“…In early reports of the NBE sferic accompanied by intense high-frequency noise (Le Vine, 1980;Willett et al, 1989), it was observed that NBEs tend to occur in temporal isolation from other lightning processes. However, Lightning Mapper Array (LMA) observations have been reported to implicate some extremely intense VHF intracloud emissions as marking the initiation of at least certain intracloud flashes Thomas et al, 2001).…”

“…It has been inferred since the outset that NBEs had an intracloud origin (Willett et al, 1989). The extremely intense VHF radiation associated with NBEs made it possible for a satellite-borne radio recorder to trigger and record this phenomenon from orbit, with the Blackbeard payload aboard the Alexis satellite.…”