VHF Radio Spectrum of a Positive Leader and Implications for Electric Fields. Issue 11 (5th June 2021)

• Record Type: Journal Article
• Authors: Pu, Yunjiao; Cummer, Steven A.; Liu, Ningyu
• Is Part Of: Geophysical research letters. Volume 48:Issue 11(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-06-05
• Subjects: positive lightning -- RF spectra -- VHF interferometry -- X‐ray -- positive leader; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL093145
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Electric fields associated with a developing natural lightning leader are difficult to measure. This work demonstrates a new approach to indirectly probing the electric fields in the streamer zone of a lightning leader. Using a 10–250 MHz broadband lightning interferometer, very high frequency (VHF) radio emissions from the tip of a positive cloud‐to‐ground (CG) leader were measured and localized. We specially use a normalized spectral analysis to avoid the challenge of absolute system calibration to show that the positive leader spectrum exhibits a clear cutoff frequency at 80 MHz. Compared with theoretical predictions, this cutoff frequency corresponds to a streamer growth rate of 1.7 × 10 8 s − 1 and an average electric field of 0.9 times the conventional breakdown field E k in streamer bursts from the positive leader. Implications for the detectability of positive leaders through VHF emissions and for the production of X‐rays by positive leaders are analyzed. Plain Language Summary: Electric fields at the tip of a developing natural lightning leader are very difficult to measure. This work demonstrates a new approach to indirectly probing the electric fields in the streamer zone of a propagating lightning leader using radio frequency (RF) spectra. We use a broadband RF lightning interferometer to measure and locate the very high frequency radio emissions from the tip of a positive cloud‐to‐ground lightning leader channel. Spectral analysis shows that the positive leader spectrum exhibits a clear cutoff frequency at 80 MHz. According to theory, this cutoff frequency is controlled by the internal lightning properties, specifically the streamer acceleration, which in turn is controlled by the electric fields. We can thus find out the electric fields that are responsible for the detected cutoff frequency in the positive leader spectrum. Compared with theoretical predictions, this 80 MHz cutoff frequency corresponds to an average electric field of 0.9 times the conventional breakdown field E k in streamer bursts from the positive leader. The field of 0.9 E k has important implications for the detectability of positive leaders through VHF emissions and for the production of X‐rays by positive leaders. Key Points: A new approach is demonstrated to infer electric fields and streamer properties in natural lightning using 10–250 MHz very high frequency (VHF) spectra An 80 MHz cutoff frequency implies an electric field of 0.9 E k during streamer bursts from a positive cloud‐to‐ground leader Implications of this field for X‐ray production and the detectability of VHF from positive leaders are discussed