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
Unintentional local temperature effects can occur during irreversible electroporation (IRE) treatment, especially near the electrodes, and most frequently near the tip. Partial electrical insulation ...of the IRE electrodes could possibly control these temperature effects. This study investigated and visualized the effect of partial electrical insulation applied to the IRE electrodes on the electric field line pattern and temperature gradient. Six designs of (partial) electrical insulation of the electrode tip and/or active needle length (ANL) of the original monopolar 19G IRE electrodes were investigated. A semolina in castor oil model was used to visualize the electric field line pattern in a high-voltage static electric field. An optical method to visualize a change in temperature gradient (color Schlieren) was used to image the temperature development in a polyacrylamide gel. Computational models were used to support the experimental findings. Around the electrode tip, the highest electric field line density and temperature gradient were present. The more insulation was applied to the electrodes, the higher the resistance. Tip and ANL insulation together reduced the active area of and around the electrodes, resulting in a visually enlarged area that showed a change in temperature gradient. Electrically insulating the electrode tip together with an adjustment in IRE parameter settings could potentially reduce the uncontrollable influence of the tip and may improve the predictability of the current pathway development.
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 present a macroscopic calculation of coherent electro-magnetic radiation from air showers initiated by ultra-high energy cosmic rays, based on currents obtained from Monte Carlo simulations of air ...showers in a realistic geo-magnetic field. We can clearly relate the time signal to the time dependence of the currents. We find that the most important contribution to the pulse is related to the time variation of the currents. For showers forming a sufficiently large angle with the magnetic field, the contribution due to the currents induced by the geo-magnetic field is dominant, but neither the charge excess nor the dipole contribution can be neglected. We find a characteristic bipolar signal. In our calculations, we take into account a realistic index of refraction, whose importance depends on the impact parameter and the inclination. Also very important is the role of the positive ions.
We report here on a novel analysis of the complete set of four Stokes parameters that uniquely determine the linear and/or circular polarization of the radio signal for an extensive air shower. The ...observed dependency of the circular polarization on azimuth angle and distance to the shower axis is a clear signature of the interfering contributions from two different radiation mechanisms, a main contribution due to a geomagnetically-induced transverse current and a secondary component due to the build-up of excess charge at the shower front. The data, as measured at LOFAR, agree very well with a calculation from first principles. This opens the possibility to use circular polarization as an investigative tool in the analysis of air shower structure, such as for the determination of atmospheric electric fields.
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.
We have developed a macroscopic description of coherent electromagnetic radiation from air showers initiated by ultra-high-energy cosmic rays due to the presence of the geo-magnetic field. This ...description offers a simple and direct insight in the relation between the properties of the air shower and the time structure of the radio pulse.
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
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