Characteristics of Radio Emissions Associated With Terrestrial Gamma‐Ray Flashes

Abstract

In this study, we analyze 44 terrestrial gamma‐ray flashes (TGFs) detected by the Fermi Gamma‐ray Burst Monitor (GBM) occurring in 2014–2016 in conjunction with data from the U.S. National Lightning Detection Network (NLDN). We examine the characteristics of magnetic field waveforms measured by NLDN sensors for 61 pulses that occurred within 5 ms of the start‐time of the TGF photon flux. For 21 (out of 44) TGFs, the associated NLDN pulse occurred almost simultaneously with (that is, within 200 μs of) the TGF. One TGF had two NLDN pulses within 200 μs. The median absolute time interval between the beginning of these near‐simultaneous pulses and the TGF flux start‐time is 50 μs. We speculate that these RF pulses are signatures of either TGF‐associated relativistic electron avalanches or currents traveling in conducting paths “preconditioned” by TGF‐associated electron beams. Compared to pulses that were not simultaneous with TGFs (but within 5 ms of one), simultaneous pulses had higher median absolute peak current (26 kA versus 11 kA), longer median threshold‐to‐peak rise time (14 μs versus 2.8 μs), and longer median peak‐to‐zero time (15 μs versus 5.5 μs). A majority (77%) of our simultaneous RF pulses had NLDN‐estimated peak currents less than 50 kA indicating that TGF emissions can be associated with moderate‐peak‐amplitude processes. The lightning flash associated with one of the TGFs in our data set was observed by a Lightning Mapping Array, which reported a relatively high‐power source at an altitude of 25 km occurring 101 μs after the GBM‐reported TGF discovery‐bin start‐time.

Low Frequency Radio Pulses Produced by Terrestrial Gamma‐Ray Flashes

“…But the TGF source altitude range of 10–15 km is consistent with all of the events. Note that the TGF source altitude range of 10.6−13.9 km in case 6 is consistent with the LMA VHF source altitudes reported by Mailyan et al () and Lyu et al () for the same event.…”

“…Fermi GBM detected a TGF at 21:23:43.910235 UT on 4 September 2015 (also discussed in Mailyan et al, , and Lyu et al, ). The Fermi geographic footprint was 24.69 °N, −83.06 °E while the satellite altitude was 541.55 km.…”

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

“…In an effort to understand how EIPs are produced and gain insight into TGF generation, we have presented sub-microsecond VHF radio mapping of an EIP, providing over an order of magnitude finer temporal detail than previously reported TGF-related VHF observations (Lu et al, 2010;Lyu et al, 2016Lyu et al, , 2018Mailyan et al, 2018). Given that EIPs can serve as proxy for a subpopulation of TGFs (Cummer et al, 2017;Lyu et al, 2016), and that the observed EIP sferic developed independently from the VHF emissions generally associated with streamer activity (Liu et al, 2019;Rison et al, 2016;Shi et al, 2016Shi et al, , 2019, our study provides strong evidence that the EIP sferic was not produced by conventional lightning processes (i.e., streamers and leaders), but by the relativistic electrons and associated ionization of a TGF-producing discharge.…”

High Peak‐Current Lightning Discharges Associated With Downward Terrestrial Gamma‐Ray Flashes

“…Although the relativistic runaway electron avalanche process (Dwyer, 2003;Gurevich et al, 1992) is widely accepted, additional mechanisms such as the thermal electron runaway model (Carlson et al, 2010;Celestin & Pasko, 2011) or the relativistic feedback processes (Dwyer, 2003(Dwyer, , 2008(Dwyer, , 2012 are thought to be required. Besides gamma-ray observations, ground-based radio-frequency (RF) measurements provide insights into the TGF-producing mechanism (Abbasi et al, 2018;Cummer et al, 2014Cummer et al, , 2015Lu et al, 2010;Lyu et al, 2016;Mailyan et al, 2018). Source location and altitude estimated by RF measurements are useful to investigate the total number of avalanche electrons in TGFs (Dwyer & Cummer, 2013;Cummer et al, 2014Cummer et al, , 2015Mailyan et al, 2016).…”