Search for: All records

Fast breakdown (FB), a breakdown process composed of systems of high‐velocity streamers, has been observed to precede lightning leader formation and play a critical role in lightning initiation. Vigorous FB events are responsible for the most powerful natural radio emissions on Earth, known as narrow bipolar events (NBEs). In this paper, an improved version of the Griffiths and Phelps (1976,https://doi.org/10.1029/jc081i021p03671) model of streamer breakdown is used alongside supervised machine learning techniques to probe the required electric fields and potentials inside thunderstorms to produce FB and NBEs. Our results show that the electrostatic conditions needed to produceFB observed in New Mexico at 9 km altitude andFB in Florida at 14 km altitude are about the same, each requiring about 100 MV potential difference to propagate 500 m. Additionally, the model illustrates how electric field enhancement ahead of propagating FB can initiate rebounding FB of the opposite polarity.

Cloud‐to‐ground strokes, narrow bipolar events, and energetic in‐cloud pulses are known classes of high peak‐current lightning processes that occur in thunderstorms. Here, we report one more distinct class of high peak‐current events observed exclusively over mountainous terrain, usually above 2,000 m altitude, in the continental Unites States. These events, which we call mountain‐top energetic pulses (MEPs), are bipolar pulses with negative radiated field polarities. MEPs are generated between the high mountain tops and compact overhead thunderclouds. Evidence supports the hypothesis that MEPs are produced by terrain‐initiated upward positive leaders propagating in high electric fields due to the proximity of the low negative charge regions of the thunderstorms. This scenario further suggests the possibility that MEPs are associated with downward terrestrial gamma‐ray flashes, and their high peak currents imply that they may produce elves.

Simultaneous data from two interferometers separated by 16 km and synchronized within 100 ns were collected for a thunderstorm near Langmuir Lab on October 23, 2018. Analysis via triangulation followed by a least squares fit to time of arrival across all six antennae produced a three‐dimensional interferometer (3DINTF) data set. Simultaneous Lightning Mapping Array data enabled an independent calculation of 3DINTF accuracy, yielding a median location uncertainty of 200 m. This is the most accurate verified result to date for a two‐station interferometer. The 3D data allowed profiling the velocity of multiple dart leaders and K leaders that followed the same channel. 3D velocities calculated from the in‐cloud initiation site to ground ranged from 3 × 10⁶to 20 × 10⁶ m/s. Average velocity generally increased with subsequent leaders, consistent with increased conditioning of the channel. Also, all leaders showed a factor of 2–3 decrease in velocity as they proceeded over 15 km of channel. We speculate that the velocity decrease is consistent with energy lost in the reionization of the channel at the leader tip. This paper includes an appendix providing details of the triangulation technique used.

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≃60μs 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≃200μs by a fast6 × 10⁶m/s upward negative breakdown and immediately preceded and accompanied by repeated sequences of fast (10⁷–10⁸m/s) downward then upward streamer events each lasting 10 to 20μs, 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.

In this paper we reconstruct Griffiths and Phelps' seminal model of streamer systems to test if it can reproduce the key observational features of fast positive breakdown. We first confirm that our implementation is accurate by reproducing the original results. The model describes how a system of positive streamers exhibits an initial exponential charge growth, as a function of position or time, which rapidly transitions into a quadratic steady state. The charge growth is accompanied by substantial electric field enhancement near the onset location, creating favorable conditions for lightning initiation. Due to the relatively low conductivity of streamers (effectively zero in this model), the electric field enhancement is created by the charge deposited in the first few meters of propagation, in the scale length where the charge growth transitions from exponential to quadratic. The quadratic growth of charge, combined with conical system expansion, makes the surface charge density of the moving front constant. The resulting electric field ahead of the streamer system remains nearly constant during its propagation, consistent with the observations of fast breakdown, which reveal a nearly constant propagation velocity, independently of discharge polarity. Minimal changes to the model allow for simulation of narrow bipolar events, reproducing very well their characteristic bipolar electric field change waveform. Despite its simplicity, the Griffiths and Phelps model provides valuable physical insights in the relationship between fast positive breakdown and lightning initiation.

This paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high‐frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9–5) × 10⁵. We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5–10 A‐m and 5–20 μC‐m, respectively, the modeling results indicate that a system of 10⁷–10⁸streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

In this paper we report the first close, high‐resolution observations of downward‐directed terrestrial gamma‐ray flashes (TGFs) detected by the large‐area Telescope Array cosmic ray observatory, obtained in conjunction with broadband VHF interferometer and fast electric field change measurements of the parent discharge. The results show that the TGFs occur during strong initial breakdown pulses (IBPs) in the first few milliseconds of negative cloud‐to‐ground and low‐altitude intracloud flashes and that the IBPs are produced by a newly identified streamer‐based discharge process called fast negative breakdown. The observations indicate the relativistic runaway electron avalanches (RREAs) responsible for producing the TGFs are initiated by embedded spark‐like transient conducting events (TCEs) within the fast streamer system and potentially also by individual fast streamers themselves. The TCEs are inferred to be the cause of impulsive sub‐pulses that are characteristic features of classic IBP sferics. Additional development of the avalanches would be facilitated by the enhanced electric field ahead of the advancing front of the fast negative breakdown. In addition to showing the nature of IBPs and their enigmatic sub‐pulses, the observations also provide a possible explanation for the unsolved question of how the streamer to leader transition occurs during the initial negative breakdown, namely, as a result of strong currents flowing in the final stage of successive IBPs, extending backward through both the IBP itself and the negative streamer breakdown preceding the IBP.