Using high‐speed video cameras operating with framing rates of 20 and 50 kiloframes per second, we captured an attachment process during a positive cloud‐to‐ground flash, which demonstrates the ...connection of the negative connecting leader (NCL) to the lateral surface of the downward positive leader (DPL) for the first time. When the NCL was initiated, the tip of the DPL had passed the initiation position of the NCL for about 50 m. A common streamer zone (CSZ) was observed when the three‐dimensional distance between the NCL tip and the lateral surface of DPL was about 30 m. It is remarkable to note that a luminous segment (space stem/leader) with a length of about 7 m was captured within the CSZ during the attachment process. The connection between the NCL tip and the lateral surface of the DPL was caused by the development of the CSZ and its inner space leader.
Plain Language Summary
In the lightning attachment process, the leader connecting behavior is an interesting topic. In the attachment process of a negative cloud‐to‐ground (CG) lightning flash, the “Tip to the lateral surface” connection type has been widely observed, and researchers have carried out a series of studies and discussions on the characteristics and the physical mechanisms of the leader connecting behavior. However, is there also a “Tip to the lateral surface” connecting behavior in the attachment process of the positive CG lightning flash? In this study, we analyzed the attachment process in a positive CG lightning flash that struck on a super‐high building, and focused on the development characteristics of the negative connecting leader and the downward positive leader, as well as the connecting behavior between them. The results allow one to see the detail of leader connecting behavior needed for improving our understanding of lightning attachment process mechanisms.
Key Points
A rare positive cloud‐to‐ground lightning flash terminated on a 530‐m‐high structure was observed by multi‐stations simultaneously
The attachment process of negative connecting leader to the lateral surface of downward positive leader was presented
A space stem/leader‐like luminous segment formed within the common streamer zone was observed
By comparing the source position of pulses during initial leaders and the paths of subsequent dart leaders from typical bilevel intracloud (IC) flashes, we demonstrate a new method to image the ...detailed three‐dimensional (3‐D) structure and stepping dynamics of IC initial leaders. Using this approach, we show that nearly half of the initial breakdown pulses are not involved in the main channel extension but instead originate in nonpropagating branches. The primary upward but significantly tilted initial leader channels propagated with measured mean/median 3‐D step length of 295/265 m and mean/median step interval of 1.05/0.6 ms. The structure and dynamics of IC initial upward negative leaders are distinct from those of highly branched cloud‐to‐ground downward negative leaders. We suggest that the tilted initial leader channel and electric field could affect the observation on leader‐associated high‐energy radiation phenomena, such as the position‐dependent observation of terrestrial gamma ray flashes from space‐based detectors.
Key Points
New LF method to image the 3‐D structure and dynamics of intracloud initial upward negative leader
First LF technique to distinguish initial pulses from main channel and from nonpropagating branches
Tilted initial leader channel with average 3‐D step length of ~295 m and step interval of 1.05 ms
Do terrestrial gamma‐ray flashes (TGFs) produce their own radio signatures? To explore this question, we analyze TGF data from the Fermi Gamma‐ray Burst Monitor, independent lightning geolocation ...data from the National Lightning Detection Network, and low‐frequency (LF) magnetic field waveforms, to determine the relationship between TGF generation and LF waveforms. LF waveforms associated with six TGFs are found to contain a clear and isolated slow pulse (~80‐μs duration) within a sequence of multiple fast pulses (<10‐μs risetime). We find that the slow LF pulse is produced simultaneously with the observed gamma rays, with an uncertainty as small as 7 μs. Simultaneity implies a consistent TGF source altitude range of approximately 10–15 km, which is consistent with previous estimates. These findings provide important evidence that the slow LF pulse, when observed, is associated with TGF production and perhaps produced by the electron acceleration itself.
Key Points
Approximately 10% of TGFs are simultaneous with a distinct and isolated slow low frequency radio pulse
Simultaneity implies that a TGF source altitude range of 10‐15 km is consistent with the analyzed examples
The consistency of the TGF and radio time scales indicate that the radio pulse is produced directly by the electron acceleration in the TGF production process
Atmospheric electricity is composed of a series of electric phenomena in the atmosphere. Significant advances in atmospheric electricity research conducted in China have been achieved in recent ...years. In this paper, the research progress on atmospheric electricity achieved in China during 2019–22 is reviewed focusing on the following aspects: (1) lightning detection and location techniques, (2) thunderstorm electricity, (3) lightning forecasting methods and techniques, (4) physical processes of lightning discharge, (5) high energy emissions and effects of thunderstorms on the upper atmosphere, and (6) the effect of aerosol on lightning.
Radio emissions continue to provide insight into the production of terrestrial gamma ray flashes (TGFs) by thunderstorms, including the critical question of the conditions under which they are ...generated. We have identified several TGF‐associated lightning radio emissions in which the altitudes of in‐cloud lightning leader pulses that precede and follow the TGF can be measured. We combine these with high absolute timing accuracy TGF observations from the Fermi satellite to determine the development of the lightning channel before, during, and after the TGF production. All of these TGFs were produced several milliseconds after the leader had initiated and when the leaders reached 1–2 km in length. After the TGFs, the leaders all continued to ascend for several more kilometers with no dramatic change in their characteristics, although they all exhibited high average velocities of 0.8–1.0 × 106 m/s. Implications in the context of TGF models are discussed. These results paint the first clear picture of the lightning processes that occur before, during, and after TGF production.
Key Points
TGFs are produced when a lightning leader is near the midpoint of its ascent
Lightning leaders associated with TGFs are fast moving but otherwise ordinary
Why TGFs are not produced at other times in the leader ascent is not clear
Many details of how thunderstorms generate terrestrial gamma ray flashes (TGFs) and other forms of high‐energy radiation remain uncertain, including the basic question of where they are produced. We ...exploit the association of distinct low‐frequency radio emissions with generation of terrestrial gamma ray flashes (TGFs) to directly measure for the first time the TGF source altitude. Analysis of two events reveals source altitudes of 11.8 ± 0.4 km and 11.9 ± 0.9 km. This places the source region in the interior of the thunderstorm between the two main charge layers and implies an intrinsic TGF brightness of approximately 1018 runaway electrons. The electric current in this nontraditional lightning process is found to be strong enough to drive nonlinear effects in the ionosphere, and in one case is comparable to the highest peak current lightning processes on the planet.
Key Points
TGFs are produced near 12 km between the charge layers of a thunderstormTGFs contain approximately 1018 runaway electronsThe electric current in the TGF generation process can be very strong
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.
Plain Language Summary
Several distinct classes of high peak‐current lightning processes are known to occur in thunderstorms, such as cloud‐to‐ground (CG) strokes, narrow bipolar events (NBEs), and the recently revealed energetic in‐cloud pulses (EIPs). Here, we report on another type of high‐amplitude event that is generated almost always over mountainous terrain in the western United States. We term them mountain‐top energetic pulses, or MEPs, which produce only radiated fields of negative polarity. The low‐frequency radio waveform characteristics, associated lightning flash structures, as well as weather radar images of the storms suggest that MEPs occur during the propagation of upward, terrain‐initiated, positive polarity lightning leaders. MEPs seem to require several conditions, namely a high ground altitude, a relatively short distance between the ground and overhead cloud, and terrain that can enhance the local electric field. MEP‐producing processes are very different from CGs and NBEs but are similar to EIPs. It is known that positive EIPs are always or almost always associated with upward‐propagating terrestrial gamma‐ray flashes (TGFs). We conjecture that MEPs may be associated with downward‐propagating TGFs. And because of the transient high‐amplitude‐radiated electromagnetic fields, MEPs may also produce ionospheric optical emissions in the form of elves.
Key Points
Document a type of highly energetic, negative field polarity lightning processes in the United States only over mountain areas (>2,800 MSL altitude)
Measurements indicate that these pulses occur during the upward propagation of terrain‐initiated positive leaders
Their similarity to other processes suggests that they may be associated with terrestrial gamma‐ray flashes and elves
We report on the development of an easily deployable LF near‐field interferometric‐time of arrival (TOA) 3‐D Lightning Mapping Array applied to imaging of entire lightning flashes. An interferometric ...cross‐correlation technique is applied in our system to compute windowed two‐sensor time differences with submicrosecond time resolution before TOA is used for source location. Compared to previously reported LF lightning location systems, our system captures many more LF sources. This is due mainly to the improved mapping of continuous lightning processes by using this type of hybrid interferometry/TOA processing method. We show with five station measurements that the array detects and maps different lightning processes, such as stepped and dart leaders, during both in‐cloud and cloud‐to‐ground flashes. Lightning images mapped by our LF system are remarkably similar to those created by VHF mapping systems, which may suggest some special links between LF and VHF emission during lightning processes.
Key Points
An interferometric‐TOA LF 3‐D Lightning Mapping Array was designed and appliedMapped and measured stepped and dart leaders from both IC and CG flashesImaged entire LF structure of different flashes which is similar as VHF maps
Terrestrial gamma ray flashes (TGFs) are brief bursts of energetic gammy‐ray photons generated during thunderstorms, which have been detected almost exclusively by satellite‐based instruments. Here ...we present three lines of evidence which includes the three out of three simultaneously observed pairs, the same occurrence contexts, and the consistent estimated occurrence rate, which indicate a direct relationship between a subset of TGFs and a class of energetic radio signal easily detectable by ground‐based sensors. This connection indicates that these gamma ray and radio emissions are two views of the same phenomenon and further enable detection of these TGFs from ground distant radio signals alone. Besides dramatically increasing the detection rate of TGFs, this ground detection approach can identify TGFs in continental and coastal areas that are at latitudes too high for present TGF‐detecting satellites and will provide more insights into the mechanism of TGF production.
Key Points
+EIPs and a subset of TGFs are intrinsically linked, and they are the two views of the same phenomenon
+EIPs can serve as a proxy for the ground detection of TGFs from distant electromagnetic radio signals
More measurements can be done on +EIP‐TGFs on land and thus deepen the insight into TGFs
The analysis of impulse charge moment change (iCMC) for the parent lightning strokes of 497 halos observed by the Imager of Sprites and Upper Atmospheric Lightning near North America in 2004–2015 ...indicates that the majority were produced by negative cloud‐to‐ground strokes predominantly produced in oceanic and coastal thunderstorms. Positive halos are almost always accompanied by sprites, and negative sprites are usually associated with halos. There are limited observations of positive pure halos with supercritical iCMCs (> +320 C km), but there are many negative pure halos with supercritical iCMCs (> −500 C km), suggesting a critical role of impulse charge transfer duration in the formation of streamers. The halo‐producing threshold of lightning strength does not considerably depend on the polarity. Due to the dependence on the timescale of impulse charge transfer for streamer development, many negative cloud‐to‐ground strokes with iCMCs exceeding the threshold for sprite production actually produce halos instead.
Plain Language Summary
Our previous analysis has shown some features for the parent lightning strokes of red sprites observed by the Imager of Sprites and Upper Atmospheric Lightning in the vicinity of North America during a 12‐year period from 2004 to 2015. In this paper, we extend the analysis to 497 halos observed by Imager of Sprites and Upper Atmospheric Lightning in the same region and during the same time period. The results show that the majority (75%) of halos were produced by negative cloud‐to‐ground strokes, which are predominantly produced in oceanic thunderstorms. The duration of impulse charge transfer plays a critical role in the formation of sprite streamers. There are only very limited observations of positive pure halos with supercritical iCMCs (> +320 C km) for positive sprites, whereas there are many negative pure halos with supercritical iCMCs (> −500 C km) for negative sprites. Due to the dependence on the timescale of impulse charge transfer for the streamer development, many negative cloud‐to‐ground strokes with impulse charge transfer exceeding the threshold for sprite production actually produce halos instead.
Key Points
The vast majority of halos observed by ISUAL are produced by negative cloud‐to‐ground strokes predominantly over oceans
The production of halo and sprite observed by ISUAL is dominated by negative and positive cloud‐to‐ground strokes, respectively
Threshold of lightning in terms of impulse charge moment change for halo production does not exhibit a dependence on lightning polarity
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