Radio Interferometer Observations of an Energetic in‐Cloud Pulse Reveal Large Currents Generated by Relativistic Discharges

• Published in: Journal of geophysical research. Atmospheres 2020-10, Vol.125 (20), p.n/a
• Main Authors: Tilles, Julia N., Krehbiel, Paul R., Stanley, Mark A., Rison, William, Liu, Ningyu, Lyu, Fanchao, Cummer, Steven A., Dwyer, Joseph R., Senay, Seda, Edens, Harald, Fan, Xiangpeng, Brown, Robert G., Wilson, Jennifer
• Format: Article
• Language: English
• Subjects: broadband VHF interferometer; Discharge; energetic in‐cloud pulse (EIP); Gamma ray flashes; Gamma rays; Geophysics; high peak current; Intracloud discharges; Lightning; Lightning activity; Lightning flashes; narrow bipolar event (NBE); Polarity; Radio; Radio interferometers; relativistic discharge; Relativistic effects; terrestrial gamma‐ray flash (TGF); Thunderstorms
• ISSN: 2169-897X; 2169-8996
• DOI: 10.1029/2020JD032603

The production mechanism for terrestrial gamma ray flashes (TGFs) is not entirely understood, and details of the corresponding lightning activity and thunderstorm charge structure have yet to be fully characterized. Here we examine sub‐microsecond VHF (14–88 MHz) radio interferometer observations of a 247‐kA peak‐current EIP, or energetic in‐cloud pulse, a reliable radio signature of a subset of TGFs. The EIP consisted of three high‐amplitude sferic pulses lasting ≃60 μs in total, which peaked during the second (main) pulse. The EIP occurred during a normal‐polarity intracloud lightning flash that was highly unusual, in that the initial upward negative leader was particularly fast propagating and discharged a highly concentrated region of upper‐positive storm charge. The flash was initiated by a high‐power (46 kW) narrow bipolar event (NBE), and the EIP occurred about 3 ms later after ≃3 km upward flash development. The EIP was preceded ≃200 μs by a fast 6 × 106 m/s upward negative breakdown and immediately preceded and accompanied by repeated sequences of fast (107–108 m/s) downward then upward streamer events each lasting 10 to 20 μs, which repeatedly discharged a large volume of positive charge. Although the repeated streamer sequences appeared to be a characteristic feature of the EIP and were presumably involved in initiating it, the EIP sferic evolved independently of VHF‐producing activity, supporting the idea that the sferic was produced by relativistic discharge currents. Moreover, the relativistic currents during the main sferic pulse initiated a strong NBE‐like event comparable in VHF power (115 kW) to the highest‐power NBEs. Key Points Simultaneous broadband radio and interferometer observations show the detailed discharge processes involved in producing an EIP The VHF radiation during the EIP is produced by fast streamer events and is uncorrelated with the large‐amplitude sferic of the EIP The relativistic discharges of the EIP are preceded and initiated by the fast streamer events in a compact region of strong positive charge