RF radiation observations of positive cloud‐to‐ground flashes

Abstract

Abstract. During the summers of 1995 and 1996 we conducted broadband HF-UHF and narrowband VHF radio frequency (RF) observations of positive cloud-to-ground (+CG) flashes at Langmuir and Los Alamos laboratories, New Mexico. These observations indicate that positive leaders to ground produce no or very weak radiation from HF to UHF. The broadband system was able to record 2 ms data each time it was triggered. For a +CG the system was usually triggered by the return stroke, and a 1 ms pretrigger period was coincident with the positive leader process. It was commonly observed that no or little radiation was associated with the leader process in the 1 ms pretrigger period. The narrowband VHF system employed a logarithmic power amplifier and recorded one 1 s data each time it was triggered. The narrowband observations show that strong and often continuous radiation occurs at the beginning of the +CGs, but the radiation usually becomes intermittent and impulsive during the last few tens of milliseconds preceding the return strokes. The observations for most of the +CGs also show complete lack of radiation a few ms before the beginning of the return strokes, suggesting that the ongoing downward positive leaders were quiet at VHF, at least during the final few ms. The results of this study for natural positive leaders are in agreement with the results obtained from laboratory gap discharges and rocket-triggered lightning.

Preliminary Analysis of Characteristics of Lightning in Nagqu Area of Qinghai‐Xizang Plateau

“…After the returnstroke, dense radiation record appeared again. The phenomena is accordant with the research results revealed by Shao et al [12] . The radiation of positive leader was further proved to be very weak.…”

Characteristics of downward leader in a positive cloud‐to‐ground lightning flash observed by high‐speed video camera and electric field changes

“…But due to the lack of the HF/VHF radiation measurements from the in-cloud discharge process of this flash, we could not draw the whole picture of the development and structure for this positive CG flash. Using the broadband HF-UHF and narrowband VHF radio frequency (RF) observations, Shao et al [1999] found that the radiation from +CG flashes usually start a few hundred milliseconds before the occurrence of the first return stroke. They also found that the positive leaders of natural +CG flashes produced no or very weak radiation from HF to UHF.…”

“…Saba et al [2006] found that the peak currents larger than 20 kA and continuing currents greater than 40 ms were much more likely to occur in positive strokes than in negative ones. Positive return strokes are often preceded by significant in-cloud discharge or pre-stroke activity [Fuquay, 1982;Shao et al, 1999]. The long pre-stroke activity seems to suggest that a positive discharge to ground can be initiated by a branch of a cloud discharge, or otherwise produced by a cloud discharge [Rakov and Uman, 2003].…”

Fine Structure Signatures in Ground Flashes as a Source for HF Radiation

“…According to the results obtained by the previous researches [5,[7][8][10][11][12], return strokes are the strongest sources of radiation of ground flashes. On the other hand, according to [1,11] preliminary breakdown process has been seen to be strong source below 10 MHz, comparable to return strokes.…”

“…A few studies have been carried out to understand the characteristics of lightning HF radiation [1,[3][4][5][6][7][8][9][10][11][12][13][14]. Two methods have traditionally been used to measure HF spectrum.…”