Fanchao Lyu

The source altitude, electric current, and intrinsic brightness of terrestrial gamma ray flashes

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 10 18 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.

Lightning leader altitude progression in terrestrial gamma‐ray flashes

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.

Insights into high peak current in‐cloud lightning events during thunderstorms

We investigated National Lightning Detection Network reports and lightning radio waveforms in a 44 day observation period to analyze the in-cloud (IC) events producing currents above 200 kA. The results show that there are two distinct classes of IC lightning events with very high peak currents: the well-known narrow bipolar events, and a previously unreported type that we call energetic in-cloud pulses (EIPs). Their temporal and spatial context shows that EIPs are generated from existing negative polarity leaders that are propagating usually upward but sometimes downward. The nearly identical characteristics of EIPs and some previously reported terrestrial gamma ray flashes (TGFs) indicate a likely connection between the two, which further suggests the possibility of downward directed TGFs. These very high peak current IC events also suggest the association of EIPs with ionospheric perturbations and optical emissions known as elves.

A low‐frequency near‐field interferometric‐TOA 3‐D Lightning Mapping Array

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.

Imaging lightning intracloud initial stepped leaders by low‐frequency interferometric lightning mapping array

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.

Sprite produced by consecutive impulse charge transfers following a negative stroke: Observation and simulation

On the morning of 5 June 2013, two cameras of the SpriteCam network concurrently captured a red sprite with diffuse halo over a mesoscale convective system (MCS) passing the panhandle area of Oklahoma. This sprite was produced by a negative cloud-to-ground (CG) stroke with peak current of −103 kA in a manner different from previous observations in several aspects. First of all, the causative stroke of sprite is located by the National Lightning Detection Network (NLDN) in the trailing stratiform of MCS, instead of the deep convection typically for negative sprites. Second, the sprite-producing stroke was likely the first stroke of a multistroke negative CG flash (with ≥6 CG strokes) whose evolution was mainly confined in the lower part of thunderstorm; although the parent flash of sprite might contain relatively long in-cloud evolution prior to the first stroke, there is no evidence that the negative leader had propagated into the upper positive region of thundercloud as typically observed for the sprite-producing/class negative CG strokes. Third, as shown by the simulation with a two-dimensional full-wave electrodynamic model, although the impulse charge moment change (−190 C km) produced by the main stroke was not sufficient to induce conventional breakdown in the mesosphere, a second impulse charge transfer occurred with ~2 ms delay to cause a substantial charge transfer (−290 C km) so that the overall charge moment change (−480 C km) exceeded the threshold for sprite production; this is a scenario different from the typical case discussed by Li et al. (2012). As for the source of the second current pulse that played a critical role to produce the sprite, it could be an M component whose charge source was at least 9 km horizontally displaced from the main stroke or a negative CG stroke (with weak peak current for the return stroke) that was not detected by the NLDN.

Analysis of lightning strokes associated with sprites observed by ISUAL in the vicinity of North America

We examined the broadband (< 1 Hz to 30 kHz) lightning sferics associated with 395 sprites observed near North America by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT-2 satellite in a 12-year period from 2004 2015. Our analysis indicates that the ISUAL dataset contains a significant fraction (69, or ~18%) of negative sprites, which were predominantly (> 80%) observed over oceanic and coastal thunderstorms mostly in tropical areas. The mean and median of impulse charge moment change (iCMC) associated with positive (negative) sprites are +346 C km (-709 C km) and +280 C km (-649 C km), respectively. The morphology and parent lightning properties (e.g., typically with high peak currents > -80 kA and large iCMCs > -300 C km) of negative sprites observed by the ISUAL are generally consistent with that documented in ground-based observations, but the ISUAL dataset does imply that sprites are sometimes produced by negative strokes with sub-critical iCMCs (less than -300 C km). Consequently, the future survey of global occurrence of sprites is desired to be based on complementary ground and space-borne observations. Article history: Received 29 February 2016 Revised 26 March 2017 Accepted 31 March 2017

Observations of Blue Discharges Associated With Negative Narrow Bipolar Events in Active Deep Convection

On August 19, 2012, the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) onboard the FORMOSAT-2 satellite captured a sequence of seven blue discharges within one minute that emanated from a parent thunderstorm over Lake Taihu in East China. The analysis of lightning activity produced in the thunderstorm indicates that at least six of these events occurred in association with negative NBEs that were concurrent with the blue discharge by less than 1 ms, and negative CGs occurred within 6 s before each blue discharge, which is in agreement with the modeling presented by Krehbiel et al. [2008]. Therefore, the frequent occurrence of negative CGs could provide the favorable condition for the production of blue discharges, and negative NBEs are probably the initial event of blue discharges. The detection of negative NBEs might provide a convenient approach to detect the occurrence of blue discharges as lightning bolt shooting upward from the top of energetic thunderstorms.

Ground detection of terrestrial gamma ray flashes from distant radio signals: Ground Detection of TGFs

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.