We have used the LOw-Frequency ARray (LOFAR) to search for the growing tip of an intra-cloud (IC) positive leader. Even with our most sensitive beamforming method, where we coherently add the signals ...of about 170 antenna pairs, we were not able to detect any emission from the tip. Instead, we put constraints on the emissivity of very-high frequency (VHF) radiation from the tip at 0.5 pJ/MHz at 60 MHz, integrated over 100 ns. The limit is independent on whether this emission is in the form of short pulses or continuously radiating. The non-observation of VHF radiation from intra-cloud positive leaders implies that they proceed in an extremely gradual process, which is in sharp contrast with the observations of other parts of a lightning discharge.
We report on a terrestrial gamma ray flash (TGF) that occurred on 15 August 2014 coincident with an altitude‐triggered lightning at the International Center for Lightning Research and Testing (ICLRT) ...in North Central Florida. The TGF was observed by a ground‐level network of gamma ray, close electric field, distant magnetic field, Lightning Mapping Array (LMA), optical, and radar measurements. Simultaneous gamma ray and LMA data indicate that the upward positive leader of the triggered lightning flash induced relativistic runaway electron avalanches when the leader tip was at about 3.5 km altitude, resulting in the observed TGF. Channel luminosity and electric field data show that there was an initial continuous current (ICC) pulse in the lightning channel to ground during the time of the TGF. Modeling of the observed ICC pulse electric fields measured at close range (100–200 m) indicates that the ICC pulse current had both a slow and fast component (full widths at half maximum of 235 μs and 59 μs) and that the fast component was more or less coincident with the TGF, suggesting a physical association between the relativistic runaway electron avalanches and the ICC pulse observed at ground. Our ICC pulse model reproduces moderately well the measured close electric fields at the ICLRT as well as three independent magnetic field measurements made about 250 km away. Radar and LMA data suggest that there was negative charge near the region in which the TGF was initiated.
Key Points
Best documented TGF observed at ground
Second TGF induced by triggered lightning
An ICC pulse occurred simultaneously (within 20 μs) of the TGF
We report the first observations of negative intracloud (IC) dart‐stepped leaders accompanied by regular trains of microsecond‐scale pulses, simultaneously detected by shielded broadband magnetic ...loop antennas and the radio telescope Low Frequency Array (LOFAR). Four investigated pulse trains occurred during complicated IC flashes on 18 June 2021, when heavy thunderstorms hit the Netherlands. The pulses within the trains are unipolar, a few microseconds wide, and with an average inter‐pulse interval of 5–7 μs. The broadband pulses perfectly match energetic, regularly distributed, and relatively isolated bursts of very high frequency sources localized by LOFAR. All trains were generated by negative dart‐stepped leaders propagating at a lower speed than usual dart leaders. They followed channels of previous leaders occurring within the same flash several tens of milliseconds before the reported observations. The physical mechanism remains unclear as to why we observe dart‐stepped leaders, which show mostly regular stepping, emitting energetic microsecond‐scale pulses.
Plain Language Summary
Lightning phenomena inside thunderclouds can be explored using their electromagnetic radiation. To study these processes at small temporal and spatial scales, we combine broadband magnetic loop antennas with the Low Frequency Array (LOFAR) radio telescope. Measurements of broadband antennas acquired during a severe Dutch thunderstorm showed pulse sequences composed of tens microsecond‐scale unipolar pulses, which were surprisingly regularly distributed. Such regular pulse trains have been rarely reported from previous observations. When we thoroughly lined up the timestamps of both simultaneously measuring observational systems, we found that the regular broadband pulses perfectly match with localized isolated bursts of energetic very high frequency radiation detected by LOFAR. The 3D mapping of the radio sources of these bursts allowed us to place the investigated events into the context of the parent intracloud (IC) lightning flash. The results revealed negative IC dart leaders, which propagated along the preconditioned channels originally formed by previous positive or negative IC leaders. Some of these dart leaders then exhibited unusual stepping manifested by the observed regular pulses. We assume that a favorable combination of the conductivity of preexisting lightning channels and the strength of the ambient electric field inside thunderclouds might be needed to trigger this unusual stepping.
Key Points
We observed intracloud negative dart‐stepped leaders producing regular trains of broadband electromagnetic microsecond‐scale pulses
Very High Frequency sources follow channels of previous leaders occurring within the same flash tens of milliseconds before the reported observations
Conductivity of decaying channels and strength of the ambient electric field might act together to trigger this unusual stepping process
We have used the LOw‐Frequency ARray (LOFAR) to image a few lightning flashes during a particularly severe thunderstorm. The images show an exceptional amount of VHF activity at altitudes above ...10 km. Much of this is in the form of small‐scale discharges, not exceeding a few hundred meter, occurring seemingly randomly around the centers of active storm cells. To emphasize the incidental nature of these small‐scale discharges or sparks we refer to them as “sparkles.” A detailed investigation shows evidence that these sparkles are indicative of positive leader channels and that they are equivalent to the needle activity seen around positive leader tracks at lower altitudes.
Plain Language Summary
At the height of the tropopause in very active lightning cells many seemingly unrelated small discharges have been observed. Using the LOw‐Frequency ARray radio telescope, mainly intended for astronomical observations, we have imaged these structures in unprecedented detail and found strong evidence that these are negative discharges that form around a network of positive leaders.
Key Points
In the tops of severe thunderstorms many seemingly isolated sources for VHF radiation are observed
The VHF sources in these tops are small negative discharges, conjectured to be related to an extensive positive leader structure
The charge‐layer structure in these tops is mixed
Ultra-high-energy (UHE) cosmic neutrinos interacting with the Moon’s regolith generate particle showers that emit Askaryan radiation. This radiation can be observed from the Earth using ground-based ...radio telescopes like LOFAR. We simulate the effective detection aperture for UHE neutrinos hitting the Moon. Under the same assumptions, results from this work are in good agreement with previous analytic parameterizations and Monte Carlo codes. The dependence of the effective detection aperture on the observing parameters, such as observing frequency and minimum detection threshold, and lunar characteristics like surface topography have been studied. Using a Monte Carlo simulation, we find that the detectable neutrino energy threshold is lowered when we include a realistic treatment of the inelasticity, transmission coefficient, and surface roughness. Lunar surface roughness at large scales enhances the total aperture for higher observation frequencies (
ν
≥
1
GHz
) but has no significant effect on the LOFAR aperture. However, roughness at scales small compared to the wavelength reduces the aperture at all frequencies.
Now more than ever animal studies have the potential to test hypotheses regarding how cognition evolves. Comparative psychologists have developed new techniques to probe the cognitive mechanisms ...underlying animal behavior, and they have become increasingly skillful at adapting methodologies to test multiple species. Meanwhile, evolutionary biologists have generated quantitative approaches to investigate the phylogenetic distribution and function of phenotypic traits, including cognition. In particular, phylogenetic methods can quantitatively (1) test whether specific cognitive abilities are correlated with life history (e.g., lifespan), morphology (e.g., brain size), or socio-ecological variables (e.g., social system), (2) measure how strongly phylogenetic relatedness predicts the distribution of cognitive skills across species, and (3) estimate the ancestral state of a given cognitive trait using measures of cognitive performance from extant species. Phylogenetic methods can also be used to guide the selection of species comparisons that offer the strongest tests of a priori predictions of cognitive evolutionary hypotheses (i.e., phylogenetic targeting). Here, we explain how an integration of comparative psychology and evolutionary biology will answer a host of questions regarding the phylogenetic distribution and history of cognitive traits, as well as the evolutionary processes that drove their evolution.
We report on ultra‐slowly propagating discharge events with speeds in the range 1–13 km/s, much lower than any known lightning process. The propagation speeds of these discharges are orders of ...magnitude slower than leader or streamer speeds, but faster than the ion drift speed. For one particular event, a lightning leader forms about 40 ms later within 50 m of the discharge, likely within the same high field region. A second slow event forms 9 ms prior to the initiation, and leads into the negative leader. Most slow events appear to not be directly involved with lightning initiation. This suggests that the classic streamer cascade model of initiation is not always a definitive process. In this work we describe these discharge events displaying unique behavior, their relation to common lightning discharges, and their implications for lightning initiation.
Plain Language Summary
While lightning is generally a very fast process, here we report on ultra‐slow discharges which may be a new and unexpected method of lightning initiation. These discharges travel at uncharacteristically low speeds and are observed in conjunction with lightning initiation in two cases, while in three different cases they are not. This indicates that these events are also evidence of failed lightning leader formation, which complicates the current understanding of how lightning initiates. Additionally, the velocity of these events is slow enough that in principle the propagation can be observed by the unaided eye ‐ challenging the colloquial notion of “fast as lightning.”
Key Points
The ultra‐slowly propagating events travel at speeds at least an order of magnitude slower than the slowest positive leaders
In one observed case, the slow propagation led directly into the formation of a lightning leader
In most cases, these discharges are not connected with lightning initiation
Abstract
The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of ...kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km
$$^2$$
2
) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket.
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