Harald E. Edens

Interferometric measurements of VHF radio-frequency signals produced by lightning are one of the most effective techniques for studying lightning breakdown processes, so uncertainty and error analyses of interferometric location results have become important topics. Based on the plane wave approximation of lightning RF signal transmission for interferometric location, a geometric model for the systematic error in the interferometric location due to the wavefront being spherical is developed and evaluated for short baseline interferometers, beginning with orthogonal and equilateral triangle baseline layouts. The symmetry of the baseline layout is shown to be helpful for reducing the systematic error caused by the plane wave approximation in interferometric location using a geometric relationship. Furthermore, a baseline layout scheme with the center of an equilateral triangle as the coordinate origin optimizes the systematic error caused by the plane wave approximation.

Based on the very high-frequency (VHF) pulse signal detected by interferometer (INTF) antennas as the reference unit, a lightning location method based on pulse matching and peak extraction is proposed in this paper. The first key step of this method is to optimize the raw VHF data of the INTF by using the ensemble empirical mode decomposition (EEMD) method to control the quality of the original signal by bandpass filtering to preserve only the relatively high-frequency components of 40-80 MHz. Then, through a combination of main and auxiliary windows, the matching of waveforms from different antennas is realized by means of generalized cross-correlation. In a microscale window (11 ns), the pulse signals are further accurately matched, and the arrival time difference is calculated under threshold and similarity constraints. Finally, the two-dimensional coordinates of the matched pulsed radiation sources are obtained using the nonlinear least squares method. Compared with the results of the traditional ’centroid’ approach, the number of radiation sources obtained with the proposed method is greatly increased. For lightning cases with different discharge intensities, the number of located radiation sources can be increased by a factor of 10-20, and for specific short-duration and rapidly changing discharge processes (such as dart leaders and K events), the number of located radiation sources can be increased by a factor of nearly 100. For INTF operation at 20-88 MHz with a sampling rate of 180 MHz, the analytical resolution of the lightning discharge process can be improved to 10 ns. When the radiation sources are fully located, the widths of the lightning channels can be successfully reconstructed from the density of no-size points representing the located radiation sources in space.

We present a method for classification of the two distinct types of electrical activity that occur during an explosive volcanic eruption: vent discharges and lightning. Vent discharges occur at the onset of an explosion and create a distinctive radio frequency signature called continual radio frequency. Seconds to minutes after the onset of the eruption, lightning begins to occur throughout the eruption column. We use logistic regression to classify a radio frequency impulse as being part of either a lightning flash or a period of continual radio frequency. The classifier uses the number of peaks in the amplitude envelope from 1 ms windows before and after the impulsive very high frequency waveform, with an average accuracy of 97.9%. We propose that this method could be used in an algorithm to determine when explosive eruptions occurred by identification of the distinctive signatures of vent discharges and lightning.

When the electric field below a thunderstorm or other electrified cloud is around 10 kV/m, it is sometimes possible to initiate (“trigger”) an upward‐propagating lightning‐leader by launching a rocket that uncoils a wire from the ground. The triggered leader propagates upward from the tip of the wire lifted by the rocket. When the channel is hot enough, a flash is visible. Triggering is common when the leader carries positive charge, but not when it carries negative charge. This article is about four flashes consisting of triggered negative leaders that branched into low‐altitude regions of positive cloud charge over Langmuir Laboratory in central New Mexico. Measurements of current and the locations of leader channels are available for three of the four flashes. Some current pulses at the ground for Flash 2 originated at negative leader steps more than 3 km away, which is a greater distance than has been reported from video measurements. Flashes 3 and 4 propagated only into thunderstorm lower positive charge, and the average lightning‐charge densities inside the volumes occupied by these two flashes are remarkably close. Our best estimate of density for Flashes 3 and 4 lies between −4.2 and −1.8 C/km³, which is compatible with the large spread in cloud‐charge densities derived from instruments carried on airplanes or balloons into low positive regions in thunderstorms.

We analyze slow electric field change and lightning mapping measurements to provide insight into the characteristics of volcanic lightning and the associated implications on charging processes and the charge structure of a Vulcanian eruption plume. Data were obtained during a multi‐instrumental field campaign at Sakurajima volcano in 2015 when the Showa crater was active. We combine the electric field change and lightning mapping data from one explosive eruption on June 6, 2015 to identify individual flashes. From this, we interpret the flash type and polarity. In addition, the long‐time constant of the electric field change instrument allowed measurement of the quasi‐static field associated with charge separation in the eruption plume. We find that both intracloud and cloud‐to‐ground discharges occurred, and the polarity of cloud‐to‐ground discharges were all negative. The quasi‐static field measurement showed the plume carried a net negative charge. We calculate both the total charge transferred by cloud‐to‐ground discharges and the net charge density of the eruption plume. We find that cloud‐to‐ground discharges transfer an average of −0.41C per flash and the net charge density was −33C/. The percent error is at least 200%, due to uncertainty in the antenna gain. We show that these estimates are consistent with lightning that is 100 m in length. Further, the average flash rate during the first 8 s following the onset of eruption was five flashes per second. After that time, the flash rate abruptly decreased, which may be related to the end of gas‐thrust forcing.

In this study, we analyze the pulse width and spectral content of vent discharges and volcanic lightning flashes. We made measurements of electrical activity with a broadband very high frequency antenna (20–80 MHz) during an explosive eruption of Sakurajima volcano on November 8, 2019. The individual impulses that comprise vent discharges and volcanic lightning were analyzed to determine the fundamental width of the impulses and the rate of fall‐off of their energy spectral density. The results show that vent discharges are more similar to volcanic lightning than they are different. The mean pulse width for both vent discharges and volcanic lightning was 50 ns. Both types of electrical activity had similar spectral content; the average slope of the amplitude spectra was −3.4 for both. Further, examination of the pulse width and spectral slope distributions showed that while the distributions of volcanic lightning and vent discharges are statistically distinct from each other, the distribution of vent discharges is a subset of the distribution of volcanic lightning.

Since their introduction 22 years ago, lightning mapping arrays (LMA) have played a central role in the investigation of lightning physics. Even in recent years with the proliferation of digital interferometers and the introduction of the LOw Frequency ARray (LOFAR) radio telescope, LMAs still play an important role in lightning science. LMA networks use a simple windowing technique that records the highest pulse in either 80 μs or 10 μs fixed windows in order to apply a time-of-arrival location technique. In this work we develop an LMA-emulator that uses lightning data recorded by LOFAR to simulate an LMA, and we use it to test three new styles of pulse windowing. We show that they produce very similar results as the more traditional LMA windowing, implying that LMA lightning mapping results are relatively independent of windowing technique. In addition, each LMA station has its own GPS-conditioned clock. While the timing accuracy of GPS receivers has improved significantly over the years, they still significantly limit the timing measurements of the LMA. Recently, new time-of-arrival techniques have been introduced that can be used to self-calibrate systematic offsets between different receiving stations. Applying this calibration technique to a set of data with 32 ns uncertainty, observed by the Colorado LMA, improves the timing uncertainty to 19 ns. This technique is not limited to LMAs and could be used to help calibrate future multi-station lightning interferometers.

The origin of electrical activity accompanying volcanic ash plumes is an area of heightened interest in volcanology. However, it is unclear how intense an eruption needs to be to produce lightning flashes as opposed to “vent discharges,” which represent the smallest scale of electrical activity. This study targets 97 carefully monitored plumes 55 m/s) were most likely to create vent discharges, whereas lightning flashes occurred in plumes with high volume flux. We identified conditions where volcanic lightning occurred without detectable vent discharges, highlighting their independent source mechanisms. Our results imply that plume dynamics govern the charging for volcanic lightning, while the characteristics of the source explosion control vent discharges.

Since their introduction 22 years ago, lightning mapping arrays (LMA) have played a central role in the investigation of lightning physics. Even in recent years with the proliferation of digital interferometers and the introduction of the LOw Frequency ARray (LOFAR) radio telescope, LMAs still play an important role in lightning science. LMA networks use a simple windowing technique that records the highest pulse in either 80 $\mu$s or 10 $\mu$s fixed windows in order to apply a time-of-arrival location technique. In this work we develop an LMA-emulator that uses lightning data recorded by LOFAR to simulate an LMA, and we use it to test three new styles of pulse windowing. We show that they produce very similar results as the more traditional LMA windowing, implying that LMA lightning mapping results are relatively independent of windowing technique. In addition, each LMA station has its own GPS-conditioned clock. While the timing accuracy of GPS receivers has improved significantly over the years, they still significantly limit the timing measurements of the LMA. Recently, new time-of-arrival techniques have been introduced that can be used to self-calibrate systematic offsets between different receiving stations. Applying this calibration technique to a set of data with 32 ns uncertainty, observed by the Colorado LMA, improves the timing uncertainty to 19 ns. This technique is not limited to LMAs and could be used to help calibrate future multi-station lightning interferometers.

Simultaneous data from two interferometers separated by 16 km and synchronized within 100 ns were collected for a thunderstorm near Langmuir Lab on October 23, 2018. Analysis via triangulation followed by a least squares fit to time of arrival across all six antennae produced a three‐dimensional interferometer (3DINTF) data set. Simultaneous Lightning Mapping Array data enabled an independent calculation of 3DINTF accuracy, yielding a median location uncertainty of 200 m. This is the most accurate verified result to date for a two‐station interferometer. The 3D data allowed profiling the velocity of multiple dart leaders and K leaders that followed the same channel. 3D velocities calculated from the in‐cloud initiation site to ground ranged from 3 × 10⁶ to 20 × 10⁶ m/s. Average velocity generally increased with subsequent leaders, consistent with increased conditioning of the channel. Also, all leaders showed a factor of 2–3 decrease in velocity as they proceeded over 15 km of channel. We speculate that the velocity decrease is consistent with energy lost in the reionization of the channel at the leader tip. This paper includes an appendix providing details of the triangulation technique used.