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
Recent measurements of narrow bipolar lightning events (NBEs) by very high frequency (VHF) radio interferometer have resolved the dynamic development of this special lightning process with submicrosecond time resolution, and showed that the fast positive breakdown (FPB) process is responsible for initiating at least some lightning flashes. In this study, with a newly built and deployed VHF interferometer system, we analyzed 31 intracloud lightning flash initiation events during three thunderstorms at short range from the interferometer. These events separate into two distinct classes that can be identified based on the time scale and the occurrence contexts of the first detectable VHF emissions from the flash. One class has features completely consistent with previously reported FPB events and is associated with continuous VHF emissions of 10–20 μs duration. Downward motion of the FPB region centroid merged continuously into the development of the subsequent upward negative leaders. But the majority of the lightning flashes analyzed began with ultrashort, submicrosecond duration, isolated pulses of VHF emission with no identifiable FPB signatures between these pulses and the leader development. These short VHF pulses begin typically a few hundred microseconds before the upward leader developes and are located at the same position where the leader eventually begins. We suggest that the FPB process is responsible for initiating some but not all lightning flashes, and the extremely short pulse‐like VHF emissions play a role in initiating those flashes without any FPB process.
more » « less- NSF-PAR ID:
- 10459916
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Atmospheres
- Volume:
- 124
- Issue:
- 6
- ISSN:
- 2169-897X
- Page Range / eLocation ID:
- p. 2994-3004
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
more » « less
Abstract ≃ 60μ s 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≃ 200μ s by a fast6 × 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 20μ s, 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. -
more » « less
Abstract 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.
-
more » « less
Abstract A positive cloud‐to‐ground (+CG) lightning flash containing a single stroke with a peak current of approximately +310 kA followed by a long continuing current triggered seven upward lightning flashes from tall structures. The flashes were observed on 4 June 2016 at the Tall Object Lightning Observatory in Guangzhou, Guangdong Province, China. The optical and electric field characteristics of these flashes were analyzed using synchronized two‐station data from two high‐speed video cameras, one total‐sky lightning channel imager, two lightning channel imagers, and two sets of slow and fast electric field measuring systems. Three upward flashes were initiated sequentially in the field of view of high‐speed video cameras. One of them was initiated approximately 0.35 ms after the return stroke of +CG flash from the Canton Tower, the tallest structure within a 12‐km radius of the +CG flash, while the other two upward flashes were initiated from two other, more distant tall objects, approximately 18 ms after the +CG flash stroke. The initiation of the latter two upward flashes could be caused by the combined effect of the return stroke of +CG flash, its associated continuing current, and K process in the cloud. Each of these three upward flashes contained multiple downward leader/upward return stroke sequences, with the first leader/return stroke sequence of the second and third flashes occurring only after the completion of the last leader/return stroke sequence of the preceding flash. The total number of strokes in the three upward flashes was 13, and they occurred over approximately 1.5 s.
-
Abstract. Deployed on the mountainous island of Corsica for thunderstormmonitoring purposes in the Mediterranean Basin, SAETTA is a network of 12 LMA(Lightning Mapping Array, designed by New Mexico Tech, USA) stations thatallows the 3-D mapping of very high-frequency (VHF) radiation emitted by cloud discharges in the60–66 MHz band. It works at high temporal (∼40 ns in each 80 µs time window) and spatial (tens of meters at best) resolutionwithin a range of about 350 km. Originally deployed in May 2014, SAETTA wascommissioned during the summer and autumn seasons and has now been permanentlyoperational since April 2016 until at least the end of 2020. We firstevaluate the performances of SAETTA through the radial, azimuthal, andaltitude errors of VHF source localization with the theoretical model ofThomas et al. (2004). We also compute on a 240 km × 240 km domainthe minimum altitude at which a VHF source can be detected by at least sixstations by taking into account the masking effect of the relief. We thenreport the 3-year observations on the same domain in terms of number oflightning days per square kilometer (i.e., total number of days during whichlightning has been detected in a given 1 km square pixel) and in terms oflightning days integrated across the domain. The lightning activity is firstmaximum in June because of daytime convection driven by solar energy input,but concentrates on a specific hot spot in July just above the intersectionof the three main valleys. This hot spot is probably due to the low-levelconvergence of moist air fluxes from sea breezes channeled by the threevalleys. Lightning activity increases again in September due to numeroussmall thunderstorms above the sea and to some high-precipitation events.Finally we report lightning observations of unusual high-altitude dischargesassociated with the mesoscale convective system of 8 June 2015. Most ofthem are small discharges on top of an intense convective core duringconvective surges. They are considered in the flash classification of Thomaset al. (2003) to be small–isolated and short–isolated flashes. The other high-altitude discharges, much less numerous, are long-range flashes that developthrough the stratiform region and suddenly undergo upward propagationstowards an uppermost thin layer of charge. This latter observation isapparently consistent with the recent conceptual model of Dye and Bansemer (2019) that explains such an upper-level layer of charge in the stratiformregion by the development of a non-riming ice collisional charging in amesoscale updraft.more » « less
-
more » « less
Abstract When the electric field below a thunderstorm or other electrified cloud is around 10 kV/m, it is sometimes possible to initiate (“trigger”) an upward‐propagating lightning‐leader by launching a rocket that uncoils a wire from the ground. The triggered leader propagates upward from the tip of the wire lifted by the rocket. When the channel is hot enough, a flash is visible. Triggering is common when the leader carries positive charge, but not when it carries negative charge. This article is about four flashes consisting of triggered negative leaders that branched into low‐altitude regions of positive cloud charge over Langmuir Laboratory in central New Mexico. Measurements of current and the locations of leader channels are available for three of the four flashes. Some current pulses at the ground for Flash 2 originated at negative leader steps more than 3 km away, which is a greater distance than has been reported from video measurements. Flashes 3 and 4 propagated only into thunderstorm lower positive charge, and the average lightning‐charge densities inside the volumes occupied by these two flashes are remarkably close. Our best estimate of density for Flashes 3 and 4 lies between −4.2 and −1.8 C/km3, which is compatible with the large spread in cloud‐charge densities derived from instruments carried on airplanes or balloons into low positive regions in thunderstorms.
