Manh D. Tran

Infrared (IR) luminosity of lightning channel in the 3–5 μm range usually persisted throughout the entire interstroke interval, which is in contrast to the simultaneously recorded visible (0.4–0.8 μm) luminosity that always decayed to an undetectable level prior to a subsequent return stroke pulse. A longer visible luminosity period at the end of flash tended to be associated with a longer IR afterglow period following the decay of visible luminosity (and by inference current) to an undetectable level. At the end of flash, the IR luminosity persisted up to about 1 s, and the median IR afterglow duration was a factor of 10 longer than the median visible luminosity duration. The IR luminosity often exhibited a hump when the visible luminosity was monotonically decaying or undetectable, with the corresponding channel temperature being likely around 3400 K.

Our basic knowledge of downward positive lightning leaders is incomplete due to their rarity and limited ability of VHF mapping systems to image positive streamers. Here, using high‐speed optical records and wideband electric field and magnetic field derivative signatures, we examine in detail the development of a descending positive leader, which extended intermittently via alternating branching at altitudes of 4.2 to 1.9 km and involved luminosity transients separated by millisecond‐scale quiet intervals. We show that the transients (a) are mostly initiated in previously created but already decayed branches, at a distance of the order of 100 m above the branch lower extremity, (b) extend bidirectionally with negative charge moving up, (c) establish a temporary (1 ms or so) steady‐current connection to the negative part of the overall bidirectional leader tree, and (d) exhibit brightening accompanied by new breakdowns at the positive leader end. One of the transients unexpectedly resulted in a negative cloud‐to‐ground discharge. Both positive and negative ends of the transients extended at speeds of 10⁶–10⁷ m/s, while the overall positive leader extension speed was as low as 10³–10⁴ m/s. Wideband electric field signatures of the transients were similar to K‐changes, with their millisecond‐ and microsecond‐scale features being associated with the steady current and new breakdowns, respectively. For transients with both ends visible in our optical records, charge transfers and average currents were estimated to be typically a few hundreds of millicoulombs and some hundreds of amperes, respectively.

This review covers selected results of recent observations of lightning discharges performed across the entire electromagnetic spectrum (radiofrequency, optical, and energetic radiation) at the Lightning Observatory in Gainesville, Florida. The most important results include (a) characterization of the preliminary-breakdown, stepped-leader, and return-stroke processes in high-intensity (⩾50 kA) negative lightning discharges, (b) the first high-speed video images of bidirectional leader that made contact with the ground and produced a return stroke, (c) discovery of negative stepped leader branches colliding with the lateral surface of neighboring branches of the same leader, (d) new data on the occurrence context and properties of compact intracloud discharges, and (e) observation of a terrestrial gamma-ray flash that occurred during a bipolar cloud-to-ground lightning discharge. The results serve to improve our understanding of the physics of lightning with important implications for lightning modeling, lightning protection, and high-energy atmospheric physics studies.

Using visible‐range and infrared (3–5 µm) high‐speed video cameras, we observed collisions of adjacent branches in downward negative stepped leaders. Typically, a lagging (chasing) branch (CB) approached a leading branch (LB) from aside at about 90° angle and connected to the lateral surface of the LB within some tens of meters or less of its tip. We infer that collisions can be facilitated by the attracting force of upward moving positive‐charge wave associated with stepping at the leading branch tip. Outcomes of branch collisions differ. The chasing branch may be absorbed by the LB, rebound, or temporarily bridge two branches. It appears that a heavily branched negative stepped leader creates a highly structured and rapidly changing electric field pattern inside the volume it occupies. We observed abrupt changes in the direction of branch extension, suggesting that the direction of local electric field can differ significantly from the ambient.

Using visible‐range and infrared (3–5 μm) high‐speed video cameras, we observed luminosity transients that reilluminated decayed branches of two close (2 to 4 km) negative stepped leaders in Florida. Leader branches were energized via stepping at their tips and, as a result, were most heated near their lower ends, with the hotter sections being connected via cooler sections to the trunk. In the modeling of lightning leaders, usually a single tip is considered. In contrast, in the present study, many (up to 30 per major branch) tips were active at the same time, forming a network‐like structure with a descending multitip “ionization front” whose transverse dimensions were of the order of hundreds of meters. The front exhibited alternating stepping, with each step necessarily generating a positive charge wave traveling from the leader tip up along the channel, like a mini return stroke. We inferred that the step‐related waves can cause luminosity transients in the remnants of decayed negative branches at higher altitudes. Such reactivated branches, in turn, may facilitate further leader stepping at lower altitudes, as first reported by Stolzenburg et al. (2015, https://doi.org/10.1002/2014JD022933). The reactivation process is likely to involve multiple steps, as evidenced by a large number of active tips (some tens per 50‐μs frame) and corresponding electric field pulses occurring at time intervals of 2 μs or less. Additionally, our observations suggest that a transient in one decayed branch can trigger (or assist with triggering of) a transient in another branch.

Mallick et al. (2012, https://doi.org/10.1029/2012JD017555) discovered that subsequent‐stroke leaders in natural negative lightning could be more prolific producers of hard X‐rays and gamma rays than the first‐stroke leader in the same flash. However, they had no optical records to confirm that their subsequent leaders followed the same path to ground as the first leader, as opposed to forging a new path to ground through cold air. In this paper, we present new observations, including optical data, showing that a second stroke produced more detectable X‐ray pulses than the first stroke, with both strokes following the same channel to ground. Additionally, we present data for the fifth stroke from a different flash, which show the occurrence of X‐ray emission at the onset of the common streamer zone between the hot channels of the downward negative dart‐stepped leader and upward positive connecting leader. However, there were no detectable X‐rays associated with negative leader steps.

Using the high‐speed optical and electric field records, in conjunction with Earth Networks Total Lightning Network and radar data, we examined in detail the morphology and evolution of an upward negative flash containing six downward leader/upward return stroke sequences terminated on a 257‐m tower in Florida. The upward flash was induced (triggered) by a single‐stroke 50‐kA +CG that occurred about 45 km from the tower. The in‐cloud part of +CG was optically detected to extend toward the tower and appeared to stop at a height of about 3 km above the tower top. The six leader‐return‐stroke sequences were each initiated by a bidirectional leader utilizing the remnants of branches created during the initial stage. Electric field signatures of bidirectional leaders were similar to K‐changes. The upper end of the return‐stroke channel in all six cases exhibited branching and appeared to extend to higher altitudes or/and move closer to Lightning Observatory in Gainesville with increasing stroke order.

An advanced nonlinear and nonuniform distributed circuit (RLCG) model of lightning M‐component has been developed. The model accounts for the variation of the series resistance R of M‐component channel due to its heating by the transient current and its subsequent cooling, longitudinal voltage drop along the channel due to the background continuing current, ohmic losses in the channel corona sheath (represented by shunt conductance G), and variation of series inductance L and shunt capacitance C of the channel with height above ground. The model was tested against the channel‐base current and corresponding close electric fields measured for seven M‐components in negative lightning triggered using the rocket‐and‐wire technique. Detailed sensitivity analysis was performed for one M‐component. The influences of height‐varying series inductance and shunt capacitance and the length of in‐cloud channel (representing the excitation source) on the computed current and field waveforms were found to be relatively insignificant, while the influences of ohmic losses in the channel corona sheath and voltage drop along the grounded channel were significant. The effects of background continuing current level and grounding resistance were significant for M‐field, but not for M‐current. Model‐predicted overall power and current profiles below the cloud base are consistent with the observed M‐component luminosity profiles and are drastically different from the observed downward leader/upward return stroke profiles. The characteristic feature of M‐components, the time shift between the current onset and close electric field peak (essentially absent for leader/return stroke sequences), was well reproduced by our model.

The lumped‐voltage‐source model proposed by Baba and Rakov [2005b] for studying the interaction of lightning with tall objects was used to examine the origin of earlier zero‐crossings observed in electric field signatures produced by lightning strikes to towers. Different return‐stroke models of transmission‐line type were used and model parameters were varied in wide ranges. Lightning channel was assumed to be straight and vertical. It was found that the observed narrow field signatures cannot be reproduced by traditional models and require a narrower input current waveform or/and its faster decay with height. Contribution to the total electric field peak from a tower whose height exceeds 100 m or so is greater than that from the lightning channel. At distances of 2 to 50 km, the electric field signature due to the tower current was found to be bipolar, while that due to the lightning‐channel current was unipolar. The narrow bipolar electric field waveforms produced by lightning striking the 257‐m tower in Florida were reproduced using two approaches. In the first one, we employed as input a typical channel‐base current waveform and the MTLE (exponential current decay with height) model with a very small decay height constant. In the second approach, we used a narrow pulse followed by a steady‐level tail as the input current waveform and the MTLL (linear current decay with height) model. In both approaches, the computed electric field waveforms matched well with the corresponding measured waveforms, at least for the initial half‐cycle and opposite‐polarity overshoot.

Synchronized high-speed (124 or 210 kiloframes per second) video images and wideband electromagnetic field records of the attachment process were obtained for 4 negative strokes in natural lightning at the Lightning Observatory in Gainesville, Florida. The majority of imaged upward connecting leaders (UCLs) and upward unconnected leaders, inferred to be mostly initiated from trees, exhibited a pulsating behavior (brightening/fading cycles). UCLs, whose maximum extent ranged from 11 to 25 m, propagated in virgin air at speeds ranging from 1.8 × 10⁵ to 6.0 × 10⁵ m/s with a mean of 3.4 × 10⁵ m/s. Within about 100 m of the ground, the ratio of speeds of the downward negative leader and the corresponding positive UCL was about 3–4 for 2 events and 0.5 for 1 event. The breakthrough phase (final jump) was imaged for 2 events. The initial length of the common streamer zone (CSZ) was estimated to be about 30–40 m. For 2 events, speeds of positive and negative leaders developing toward each other inside the CSZ were found to be between 2.4 × 10⁶ and 3.7 × 10⁶ m/s. For 1 event, opposite polarity leaders were observed to accelerate inside the CSZ. The current at the end of the breakthrough phase, lasting on average 4.7 μs, was estimated to be approximately one-half of the overall current peak. Thus, about one-half of the current peak traditionally attributed to the return-stroke process is actually associated with two leaders extending toward each other to collision inside the CSZ.