Hamid K. Rassoul

We present time‐correlated ultra‐high‐speed video camera and electromagnetic field measurements of the attachment processes in a natural negative cloud‐to‐ground stroke. The video camera frame exposure time and pixel resolution were 740 ns and 0.91 m/pixel, respectively. The common streamer zone (CSZ) was first observed 2.52 µs preceding the first frame showing the return stroke (RS) in progress, when the upward and downward leader‐tips were 9.8 m apart. In the next frame, the two leaders were observed to have propagated toward each other within the CSZ, with their tips being 0.91 m apart. Our observations show with unprecedented precision/clarity that (a) the slow front in the field waveform is associated with the CSZ, and (b) the “proper” start of the RS is marked by the onset of the fast transition in the field waveform which occurs at the completion of the attachment processes (when the upward and downward leaders have merged).

We present sub‐microsecond‐scale, high‐speed video camera observations of three negative stepped leaders in cloud‐to‐ground flashes with return‐stroke peak currents (estimated by the U.S. National Lightning Detection Network) of −17, −104, and −228 kA. The camera frame exposure times for these observations were 1.8, 1.0, and 0.74 µs, respectively. The 0.74 µs exposure time is the shortest reported to date. We observed the temporal and spatial evolution of space leaders from their inception to their attachment to the pre‐existing leader channel (PELC). For stepped leaders that led to return strokes having higher peak currents, the space leaders appear to have incepted at farther median two‐dimensional distances from their respective PELC‐attachment points. These median distances were 6.1, 16.6, and 17.6 m, respectively, for the three strokes. Our observations indicate that space leader characteristics are likely influenced by stepped‐leader line‐charge‐density, which is expected to be higher in strokes with higher return‐stroke peak currents.

This study reports on spectroscopy results from a high‐speed optical spectrograph of two naturally occurring lightning return strokes. The two strokes occurred near Melbourne, FL and were from two separate flashes that were about 10 min apart and had National Lightning Detection Network (NLDN) peak currents of −19 and −63 kA. The larger peak current stroke was from a dart leader and was the last stroke in a 5 return stroke flash, while the −19 kA stroke originated from a stepped leader and was the only stroke in that flash. From the flash spectra, the return stroke channel temperature was calculated using the neutral lines of 715.7 nm (OI) and 777.4 nm (OI). In addition to the use of the neutral emission lines, the use of novel instrumentation and image processing techniques allowed the temperature to be calculated for nearly the entire visible channel (several km) and for long durations (several hundred μs). This enables temperature estimates on an unprecedented spatial and temporal scale, which show that the vertical temperature profile is not uniform across the channel. The lower altitudes are significantly hotter than higher altitudes near the time of the return stroke, with temperature gradients along the channel as large as 12,000 K/km. The rate of cooling of the channel is also initially 3–4 times larger at lower altitudes in comparison with the segments at higher altitudes. The stroke with the larger peak current shows larger maximum temperatures, larger temperature gradients along the channel, and also cools quicker.

We report on anomalously long duration (2 ms) count rate bursts following the impact of cosmic ray showers near a 7.62 cm x⊘7.62 cm LaBr₃ scintillation detector at the High Altitude Water Cherenkov array in Mexico, previously described by Stenkin et al. (2001), and termed “neutron bursts.” The largest burst produced 198 counts within 2 ms in our LaBr₃ detector. We simulate the neutron burst albedo flux (that is, secondary emissions from an extensive air shower core impacting the ground), and show that (1) the characteristic spectra and count rates are well explained by neutron absorption in the ground and (2) any cosmic ray secondary that produces neutrons, either through hadron inelastic collisions, or photoneutron production by gamma‐rays, produces the same characteristic spectra. This implies that other natural phenomena that produce downward beams of gamma‐rays, like Terrestrial gamma ray flashes, should produce a similar “neutron burst” signature from the photoneutron reactions occurring in the soil.

This work presents the first simultaneous X‐ray measurement and high‐speed video observation of the propagation of a lightning leader producing X‐rays. As a result, the three‐dimensional leader distance from the X‐ray measurement and, for the first time, the conditions of the preexisting channel during the leader propagation were observed. Although four leaders in this seven‐stroke flash followed the same path to ground, X‐rays were only observed during the leader before the return stroke with the highest peak current. The fact that the other three leaders following the same path to ground did not produce detectable X‐rays confirms the hypothesis that leader line charge density is an important factor that determines X‐ray production. The fact that X‐rays was recorded only when the leader tip was at a certain portion of the lightning channel confirms that the orientation of the leader plays an important role in the detection of X‐rays.

We examined the occurrence characteristics of the lightning observed at the Gaisberg Tower (GBT) in the years 2000 to 2018 and analyzed current waveforms (measured using a 0.25-mΩ shunt) of upward flashes initiated from the tower. During this period, 865 flashes were recorded at the GBT, of which 823 (95%) were upward and 4 (0.5%) were downward. For 18 flashes, the current waveforms were ambiguous and for 20 flashes they were unsuitable for analysis. Of the 823 upward flashes, 651 (79%) were negative, 35 (4.3%) were positive, and 137 (17%) were bipolar. The median initial stage (IS)-current durations in upward negative, positive, and bipolar flashes were 275, 96, and 282 ms, respectively. The median IS-current peaks in these flashes were 1.4, 3.2, and 1.8 kA, respectively. We expanded the traditional classification of bipolar flashes to include five categories. Of the 137 bipolar flashes, 45% were of Type 1S (single reversal of current polarity during IS), 47% of Type 1M (multiple reversals of current polarity during IS), 5.1% of Type 2 [different polarities of current during IS and return stroke (RS)], 1.5% of Type 3 (RSs of different polarities), and 0.73% (one flash) of Type 4 (different polarities of RS and the following continuing current).

Here we report the first observations of gigantic jets (GJs) by the Geostationary Lightning Mapper (GLM) on board the Geostationary Operational Environmental Satellite‐R series. Fourteen GJs produced by Tropical Storm Harvey on 19 August 2017 were observed by both GLM and a ground‐based low‐light‐level camera system. The majority of the GJs produced distinguishable signatures in the GLM data, which include long continuous emissions, large peak flash optical energies, and small lateral propagation distances in comparison with other flashes observed by GLM. For two GJs with the best ground‐based images, each have a single pixel that contains the largest optical energy throughout the duration of the GJ and also coincides with the azimuth of the GJ from the video images. The optical energy of the pixel increases as the GJ propagates upward, reaches its peak when the GJ connects to the ionosphere, and then fades away.

We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 µs) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity.

Observation of linearly polarized He⁺‐band electromagnetic ion cyclotron (EMIC) waves at low L shells is a new, and quite unexpected, result from the Van Allen Probes mission. Here we analyze the two EMIC wave events observed by Van Allen Probes at low L shells and put forward a new‐generation mechanism for the low‐L EMIC waves. Both events were observed at L ∼ 3 but one of them has a discrete spectrum near the O⁺ gyrofrequency and its second harmonic, whereas the second event has a broad spectrum between the O⁺ gyrofrequency and its second harmonic. For both events, the major conclusions of our analysis can be summarized as follows. (1) Only O⁺ causes EMIC wave generation, and instability is driven by the positive derivatives of distribution functions over perpendicular component of velocity. (2) The timing and frequencies of generated waves are in agreement with observations. The generated wave normal angles, however, are highly oblique being in strong disagreement with the minimum variance angles obtained from Fast Fourier transform. (3) The wave step analysis shows that a signal nonstationarity is not a major cause for disagreement between the minimum variance angles and theoretical predictions for normal angles. (4) A superposition of plane sine waves with the same frequency and normal angle but with different azimuthal angles for wave vector around the background magnetic field can reconcile the polarization properties of EMIC waves obtained from Fast Fourier transform and/or the wave step analysis with those predicted by the linear theory of EMIC waves.

Broadband radio frequency (RF) interferometry was first introduced in 1996 for lightning studies. The technique has since been improved significantly and has been increasingly widely used in the lightning community. In addition to the interferometry, we now introduce a broadband polarization capability that simultaneously measures the full polarization state of the corresponding RF sources. Polarization observation provides another level of understanding of the discharge processes. In this paper, we describe the new system and present example observations with such a system. Initial observations of impulsive lightning RF pulses, a K‐event leader, and the initiating process for an intracloud flash are presented. The impulsive pulses were often linearly polarized and are consistent with conventional breakdown processes, whereas some occasional pulses were elliptically polarized and are likely related to relativistic breakdown processes. The K‐event was detected polarized along a section of its channel, and the orientation of the polarization was mostly orthogonal to the channel, apparently due to the preceding charge deposition along the channel. The intracloud flash in this paper was initiated with downward fast positive streamers, and the corresponding signals were linearly polarized, consistent with conventional breakdown processes. The orientations of the polarization were nearly uniformly aligned with the propagation direction of the streamers, indicating a near uniform electric field in the region of the initiating streamers. Finally, an observation of a cosmic‐ray shower is presented.