Langmuir Laboratory Research Articles

D Region Electron Density Derived From Sprite Observations

AbstractFour upward propagating streamers in two carrot sprites over northwest Texas were recorded at 100,000 frames per second at 07:46:35 UT, 2 June 2019, from the Langmuir Laboratory near Socorro, New Mexico. The streamers reached velocities of 50–80 × 106 m/s with accelerations up to 25 × 1010 m/s/s. The upward motion ended with the top of the streamers near 90 km altitude. At this time, the streamers reached maximum brightness. The streamer head then dissolved and the brightness decayed exponentially with time constants between 0.078 and 0.097 ms. We interpret the dissolution and decay to be the result of interaction with the bottom of the ionosphere. Assuming the decay to be dictated by electric field relaxation, the ambient D region electron density would be 107 to 108 m−3. The analysis suggests that sprite observations can provide multipoint measurements of the D region ionosphere impacted by powerful lightning capable of triggering sprites.

Triggered Negative Lightning‐Leaders That Propagated Into Thunderstorm Lower Positive Charge

AbstractWhen 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.

Relationship Between Sprite Current and Morphology

AbstractOn June 2 and 3, 2019, 63 sprites were captured from Langmuir Laboratory in central New Mexico. The two storms investigated were located in northwest Texas, 400–800 km away from the observation site. Optical recordings were made with a Phantom V2010 camera operating at up to 100,000 frames per second. Electromagnetic remote sensing of lightning and sprite electric fields was performed with a sensitive slow antenna (LEFA). Data from the Earth Networks Total Lightning Network (ENTLN) were used to locate the sprite parent flashes. The combined information of these three data sets reveals that a staggering fraction of more than half of the sprites observed have a distinguishable electromagnetic signature attributed to currents flowing within the sprite body. Furthermore, these sprite current signatures were unusually large in comparison to previous reports. The sprite electric field changes have roughly half the amplitude of their parent lightning flash's, corresponding to sprite peak currents of 26–58 kA on average. The largest sprites have current moments of up to 2,700 kA km, as inferred from a computationally efficient method to solve Maxwell's equations. Detailed comparison between the sprites' electromagnetic signatures and high‐speed optical recordings show that optically large sprites containing upward streamers (carrots and jellyfish) tend to have larger electrical currents than the ones displaying only downward streamer development (column sprites). Finally, a clear increasing trend in peak current moment is evident with increasing morphological complexity, from columns to carrots to jellyfish sprites.

Charge rearrangement deduced from nearby electric field measurements of an intracloud flash with K‒changes

An intracloud flash near Langmuir Laboratory is analyzed to determine the net rearrangement of charge. The analysis employed data from a balloon borne electric field sensor, or Esonde, that was within a few hundred meters of the lightning channel, data from a similar Esonde on a mountain about 6.4 km from the balloon, and data from the New Mexico Institute of Mining and Technology Lightning Mapping Array (LMA). The recovery of the charge transport required the solution of Poisson's equation over the mountainous terrain surrounding Langmuir Laboratory and the solution of a vastly under‒determined system of equations. The charge movement is analyzed using a new smooth charge transport model that incorporates constraints in the least squares fitting process through the use of penalty terms to smooth the charge movement and prevent data overfitting. The electric field measurements were consistent with about 26% of the negative charge being transported to the end of the channel, 36% deposited along the channel in the positive region, 8% deposited near the start of the channel in the positive region, and 30% deposited in another positive region several kilometers beneath the main channel. The transport of negative charge to a lower positive region occurred during the K‒processes when some negative charge was also deposited along the main channel in the upper positive region. Hence, the charge transport process during the K‒processes amounted to a tripolar charge rearrangement where the charge from the negative region was transported to two distinct positive regions, the positive region along the main channel and a lower positive region beneath the main channel. High altitude, widely scattered LMA sources beyond the end of the main channel could indicate the existence of streamers which transported the end‒of‒channel charge into the surrounding volume. Although the LMA showed the development of two upper channels, the charge transport analysis showed that measurable charge transport only occurred on one of the channels. The channel that did not transport charge was missing the high altitude, widely scattered LMA sources seen at the end of the channel that carried charge.

Charge rearrangement by sprites over a north Texas mesoscale convective system

Charge rearrangement by sprites is analyzed for a mesoscale convective system (MCS) situated in north Texas and east New Mexico on 15 July 2010. During the thunderstorm, electric field data were recorded by the Langmuir Electric Field Array (LEFA), while magnetic field data were recorded by the charge‐moment network near Duke University. A high speed (12500 fps) video system operated at Langmuir Laboratory recorded telescopic images of the sprites. Data from the National Lightning Detection Network (NLDN) show that each sprite was preceded by a series of cloud discharges and cloud‐to‐ground discharges. The triggering event preceding the sprite was typically a positive cloud‐to‐ground (+CG) stroke. For one out of the 10 sprites that were recorded, there was a positive hump in the electric field a few milliseconds after the +CG return stroke. The size and shape of the hump roughly matched the light intensity emitted from the sprite. The electric field hump is fit by a sprite current that originates in the ionosphere and propagates downward, producing the same effect as a downward moving positive current. The integral under the current hump was 23.9 C when the velocity of the current pulse was between 0.25 c and 0.55 c. The large sprite current was followed by impulsive electromagnetic radiation which has not been previously reported and could be a recoil effect similar to what is called a “K‐change” when it is observed in a lightning flash.

VHF lightning mapping observations of a triggered lightning flash

On 3 August 2010 an extensive lightning flash was triggered over Langmuir Laboratory in New Mexico. The upward positive leader propagated into the storm's midlevel negative charge region, extending over a horizontal area of 13 × 13 km and 7.5 km altitude. The storm had a normal‐polarity tripolar charge structure with upper positive charge over midlevel negative charge. Lightning Mapping Array (LMA) observations were used to estimate positive leader velocities along various branches, which were in the range of 1–3 × 104 m s−1, slower than in other studies. The upward positive leader initiated at 3.4 km altitude, but was mapped only above 4.0 km altitude after the onset of retrograde negative breakdown, indicating a change in leader propagation and VHF emissions. The observations suggest that both positive and negative breakdown produce VHF emissions that can be located by time‐of‐arrival systems, and that not all VHF emissions occurring along positive leader channels are associated with retrograde negative breakdown.

Photographic and lightning mapping observations of a blue starter over a New Mexico thunderstorm

[1] On 4 August 2010 a small blue starter was observed over an active thunderstorm in west-central New Mexico. The storm was of normal polarity with upper positive charge over midlevel negative charge. The event was visually observed and photographed, and mapped by the Lightning Mapping Array (LMA) at Langmuir Laboratory. The blue starter occurred during a seven-stroke negative cloud-to-ground (−CG) flash that was detected by the National Lightning Detection Network (NLDN), and data from the LMA show that it initiated between upper positive charge and inferred negative screening charge at the cloud top. Upward positive streamers of the blue starter emerged from the cloud top at 15.2 km altitude and reached a terminal altitude of ∼17 km. The observation confirms that blue starters can be initiated by the electric field changes associated with −CG flashes, which enhance the electric field between upper positive charge and negative screening charge. It also confirms that the upward leaders and streamers of blue starters in normal-polarity storms are of positive polarity.

Charles Bachman Moore (1920–2010)

Charles B. Moore passed away 2 March 2010 at the age of 89, following a long and varied scientific career in meteorology and the atmospheric sciences. He will be remembered best for his substantial contributions in the field of atmospheric electricity and for the students and faculty he guided as chairman of Langmuir Laboratory for Atmospheric Research and professor of physics at the New Mexico Institute of Mining and Technology. He possessed a unique sense of humor and an excellent memory that served as a reservoir of scientific and historical knowledge. Like many of his generation, Charlie's career was profoundly influenced by the Second World War. Following Pearl Harbor, he interrupted his undergraduate studies in chemical engineering at Georgia Institute of Technology to enlist in the Army Air Corps, where he became the chief weather equipment officer in the 10th Weather Squadron, setting up and operating remote meteorological stations behind enemy lines in the China‐Burma‐India theater. He served with distinction alongside Athelstan Spilhaus Sr., who had been one of Charlie's instructors in the Army meteorology program.

Comparison of sprite initiation altitudes between observations and models

[1] Simultaneous analyses of measured sprite initiation altitudes with predicted initiation altitudes from simulations enable an examination of our understanding of the sprite initiation mechanism and the modeling techniques to simulate this mesospheric electrical phenomenon. In this work, we selected a subset of sprites optically observed from Langmuir Laboratory, NM; locations near Las Vegas, NM, in 2007 and near Portales, NM, in 2008; and a Duke University field station. The sprites were observed by high-speed imaging with time resolutions of at least 1 ms and by low light level imagers. Sprite initiation altitudes were determined by triangulation between Langmuir Laboratory and either Portales or Las Vegas, while star field analysis determined the approximate measured initiation altitudes for Duke observations. These video observations were coordinated with electromagnetic field measurements from Yucca Ridge Field Station and Duke University, respectively. With a 2-D finite difference time domain model, we simulated the lightning-driven electric fields and predict the likely altitude of sprite initiation and compare these findings with the measured initiation altitude of each sprite analyzed. Of 20 discrete sprite events analyzed, both the measured and the simulation-predicted initiation altitudes indicate that long-delayed sprites tend to initiate at lower altitude. The average discrepancy between the measurements and the simulation results is 0.35 km with a standard deviation of 3.6 km. This consistency not only confirms previous results about the relationship between sprite initiation altitude and time delay but also helps to develop confidence in the models to reveal the sprite physics.

Free‐running ground‐based photometric array imaging of transient luminous events

We present observations of transient luminous events (TLEs) recorded during a summer TLE observation campaign in 2008 at Langmuir Laboratory near Socorro, New Mexico. The campaign featured observations made by a free‐running, ground‐based multianode photometric array called the Photometric Imager of Precipitated Electron Radiation (PIPER). As a photometric array, PIPER has high (40 μs) temporal resolution and enough spatial resolution (16 anodes per photometer) to make it particularly useful in the study of fast TLEs like elves and halos. As a free‐running instrument, there are no missed detections and no unwanted sampling bias introduced by triggering. As a ground‐based instrument, it can follow individual storms over the course of their lifetime rather than randomly sampling over large numbers of storms as required by space‐based instruments. During the campaign, 143 sprites, 803 elves, and 166 halos were observed over six storms, resulting in averaged elve‐to‐sprite and halo‐to‐sprite occurrence ratios of 5.6:1 and 1.2:1, respectively. There was considerable variability in the elve‐to‐sprite occurrence ratio from storm to storm, ranging from a low of 3.7:1 to a high of 13.4:1. Overall, 78.2% of the elves and 55.4% of the halos were associated with negative cloud‐to‐ground lightning strikes (−CGs); no sprites associated with −CGs were observed. Additionally, 40 events in which pairs of elves occur in rapid succession, events we refer to as elve doublets, were observed in several storms. The duration between elves in the elve doublet events was typically 120 μs. The causative mechanism for these events is still under deliberation.

Three‐dimensional charge structure of a mountain thunderstorm

Lightning charge transport is analyzed for a thunderstorm which occurred on 18 August 2004 near Langmuir Laboratory in New Mexico. The analysis employs wide band measurements of the electric field by a balloon‐borne electric field sonde or Esonde, simultaneous Lightning Mapping Array measurements of VHF pulses emitted during lightning breakdown, and Next Generation Weather Radar data. The thunderstorm was composed of two principal updrafts. In the stronger updraft the positively charged particles reached altitudes up to 14 km, and in the weaker updraft the positive particles reached 11 km altitude. The negatively charged particles generated in the updraft appeared to reach altitudes up to 10 km in the strong updraft and 8 km in the weaker updraft. Just outside the updrafts the positive and negative particles drop sharply; thereafter, they drop down at a nearly linear rate, between 1 and 2 km in altitude per 10 km in horizontal distance. Initially, as the updraft developed, most charge was transported by updraft flashes; later, after about 15 to 20 min, extensive flashes were predominant. Most cloud‐to‐ground (CG) flashes transported negative charge from outside the updraft at 6 km altitude down to ground; however, some strokes of a CG reached into a higher negative charge region closer to the updraft. Nearly 6 times as much charge was transported by intracloud (IC) flashes when compared to CG flashes. The ratio of the average charge transport for an IC flash to the average charge transport for a CG flash was 1.6, while the average generator current associated with the combined updrafts was 2.3 amperes for 40 min.

Sprite initiation altitude measured by triangulation

High time resolution (10,000 frames per second) images of sprites combined with multistation concurrent video recordings have provided data for triangulation of the altitude of the initial sprite onset. The high‐speed images were obtained from the Langmuir Laboratory, New Mexico, during summer campaigns in 2007 and 2008 with video observations from sites at Portales, New Mexico, and Las Vegas, New Mexico. Sprites start with one or more downward‐propagating streamer heads. The triangulated onset altitudes of this initial downward streamer vary between 66 and 89 km. In some sprites the downward streamers are followed a little later by upward‐propagating streamers. The upward streamers start from a lower altitude and existing luminous sprite structures and their triangulated altitudes vary from 64 to 78 km. The downward streamers create C sprite characteristics, while the upward streamers form the broad diffuse tops of carrot sprites. In the sprites analyzed the higher onset altitudes for the downward‐propagating initial streamers were associated with C sprites and the lower with carrot sprites, but our larger data set indicates that this is not generally the case. It appears that the dominant sprite types vary from year to year, indicating that some longer‐lasting environmental parameter, such as mesospheric conductivity and composition or thunderstorm cloud dynamics, may play an important role in determining the types of sprites observed.

On remote sensing of transient luminous events' parent lightning discharges by ELF/VLF wave measurements on board a satellite

First recordings of satellite ELF/VLF waveform data associated with transient luminous event (TLE) observations are reported from the summer 2005 campaign coordinated by Stanford University and Laboratoire de Physique et Chimie de l'Environnement et de l'Espace (LPCE). TLEs are optically observed from the U.S. Langmuir Laboratory, while ELF/VLF waveform data are simultaneously recorded on board the Centre National d'Etudes Spatiales microsatellite DEMETER and on the ground at Langmuir. Analyses of ELF/VLF measurements associated with sprite events observed on 28 July 2005 and 3 August 2005 are presented. Conditions to trace back the wave emissions from the satellite to the source region of the parent lightning discharge are discussed. The main results concern: (1) the identification from a low Earth orbit satellite of the 0+ whistler signatures of the TLE causative lightning; (2) the identification of the propagation characteristics of proton whistlers triggered by the 0+ whistlers of the causative lightning, and the potential use of those characteristics; (3) recognition of the difficulty to observe sprite‐produced ELF bursts in the presence of proton‐whistlers; (4) the use of geographical displays of the average power received by the DEMETER electric field antennas over the U.S. Navy transmitter North West Cape (NWC) located in Western Australia to evaluate VLF transmission cones which explain the presence (28 July events) or the absence (3 August events) of propagation links between sferics observed at ground and 0+ whistlers observed on DEMETER; and (5) owing to electron‐collisions, an optimum transfer of energy from the atmosphere to the ionosphere for waves with k vectors antiparallel, or quasi‐antiparallel, to Earth's magnetic field direction.

Analysis of charge transport during lightning using balloon‐borne electric field sensors and Lightning Mapping Array

Recently, wide band measurements of the electric field near a lightning flash have been obtained by a balloon‐borne electric field sonde or Esonde. This paper develops new techniques for analyzing lightning‐associated charge transport in a thundercloud by using both the Esonde data and simultaneous Lightning Mapping Array (LMA) measurements of VHF pulses emitted during lightning breakdown processes. Innovations in this paper include the following: (1) A filtering procedure is developed to separate the background field associated with instrument rotation and cloud charging processes from the lightning‐induced electric field change. Because of the abrupt change in the signal caused by lightning, standard filtering techniques do not apply. A new mathematical procedure is developed to estimate the background electric field that would have existed if the lightning had not occurred. The estimated background field is subtracted from the measured field to obtain the lightning‐induced field change. (2) Techniques are developed to estimate the charge transport due to lightning. At any instant of time during a cloud‐to‐ground (CG) flash, we estimate the charge transport by a monopole. During an intracloud (IC) flash, we estimate the charge transport by a dipole. Since the location of the monopole and dipole changes with time, they are referred to as a dynamic monopole and a dynamic dipole. The following physical constraints are used to achieve a unique fit: charge conservation during an IC flash, separation (distance between the CG monopole charge center and the ground and separation between IC dipole charge centers both exceed a minimum threshold), location (charge is placed on lightning channel), and likelihood (after a statistical analysis based on instrument uncertainty, highly unlikely charge locations are excluded). To implement the constraint that the charge is located on the lightning channel, we develop a mathematical object called the “pulse graph.” Vertices in the graph are pulse locations obtained from the Lightning Mapping Array. Edges in the graph (that is, the pairs of vertices which are connected by line segments) are obtained by joining, in a systematic way, neighboring vertices. One CG and two IC flashes observed on 18 August 2004 near Langmuir Laboratory are analyzed. In the CG flash, initial strokes drained 12 C charge from an altitude of 5 km, while an intermediate stroke discharged 12 C from a higher charge center at 8 km. For the IC flashes, the current flow lagged behind the channel formation by time intervals on the order of 0.1 s, roughly the same time delay observed for lightning optical signals detected by NASA's Lightning Imaging Sensor.

Possible direct cloud-to-ionosphere current evidenced by sprite-initiated secondary TLEs

[1] Video-rate images of sprites observed from Langmuir Laboratory show slow, upward discharges from the cloud to the mesosphere following the sprite discharge. A single high-time-resolution (1000 fps) measurement of the tip of the upward discharge shows that it terminates at the base of the preceding sprite and persists for a total of ∼50 ms, compared to the sprite lifetime of ∼6 ms. Photometer data show that these events are likely to be predominantly blue. We propose a mechanism by which, in conjunction with intra-cloud activity following the causative CG discharge, charge is moved within the sprite body from the ionosphere to the lower altitude edge of the sprite at ∼50 km; this transfer of charge lowers the upper capacitive plate, whereupon a second breakdown is facilitated between the cloud-top and this charged plate, inducing electrical current between the cloud and the mesosphere and initiating the observed luminous discharges.

Comparing E field changes aloft to lightning mapping data

A newly developed balloon‐borne instrument contains electric and magnetic sensors for determining how lightning alters electric field vectors relative to a coordinate system fixed with the Earth. By combining data from this instrument with results from the compact Lightning Mapping Array (LMA) at Langmuir Laboratory in central New Mexico and the National Lightning Detection Network (NLDN), charge transported to ground by several strokes in a cloud‐to‐ground (CG) lightning flash can be quantified. As the flash progresses, the locations of the charge centers drained by successive strokes are seen to move further from the ground‐strike point. Using this new instrument and two different models to map LMA source points onto charge centers, the charge transported in an intracloud (IC) flash is also estimated. Details of the instrument design and data analysis are also presented.

High‐speed measurements of small‐scale features in sprites: Sizes and lifetimes

We report on the results of observations with a combination of high‐speed and telescopic imaging to capture high spatial resolution images of sprite streamers and beads at high frame rates, revealing the evolution and propagation of these structures on a decameter scale at millisecond and higher resolution. In July and August 2004, sprites were observed from Langmuir Laboratory, in the mountains of New Mexico, using a >1000 frames‐per‐second intensified CCD imager mounted to a Dobsonian reflecting telescope. We present a number of examples of sprite features, along with photometric data on sprites and sprite halos, taken with the Wide‐angle Array for Sprite Photometry (WASP). Results show a variety of structures, including evidence of formation and evolution of both streamers and beads. Examples and statistics presented indicate that most bead structures have sizes of 10–300 m, similar to previous telescopic observations, and endure typically for a few milliseconds to 10 ms, with rare cases of up to 50 ms. Similarly, streamer‐like structures are observed to have diameters of 10–300 m but persist for shorter timescales, typically 2–3 ms, with occasional cases of up to 10 ms. Attempts to measure propagation of streamers indicate that higher frame rates are required in order to observe speeds on the order of previous wide‐field‐of‐view observations and modeling predictions.

Biogenic emissions and ambient concentrations of hydrocarbons, carbonyl compounds and organic acids from ponderosa pine and cottonwood trees at rural and forested sites in Central New Mexico

Abstract Direct emission rates of carbonyl compounds, carboxylic acids and hydrocarbons from Populus fremontil (cottonwood) and Pinus ponderosa (ponderosa pine) trees were studied during the summer of 1997. Ambient air concentrations of these compounds in the vicinity of the sampled trees were also identified and quantified. Study sites were Socorro, NM and Langmuir Laboratory, NM a rural and forested, high mountain site, respectively, located in Central New Mexico. A dynamic branch enclosure method was used to perform the sampling of tree emissions, that are given at standard atmospheric temperature of 303 K, and 1000 μmol m −2 s −1 PAR. Average emission rates of acetic and formic acid, respectively, from cottonwood were 470±540 and 310±300 ng g −1 h −1 and from ponderosa pine were 170±180 and 210±210 ng g −1 h −1 . Formaldehyde and acetaldehyde average emission rates, respectively, from ponderosa pine were 500±400 and 250±190 ng g −1 h −1 , and from cottonwood were 4070±3570 and 1190±1360 ng g −1 h −1 . Cottonwood had an average isoprene emission rate of 9050±10700 ng g −1 h −1 , while ponderosa pine had emission rates of α -pinene and β -pinene of 450±1100 and 520±1050 ng g −1 h −1 , respectively. Total mass emissions of carbon compounds measured from cottonwood were four times larger than from ponderosa pine. Seasonal, diurnal, and temperature dependence of concentrations in ambient air and emission rates from trees are also discussed. Average ambient air concentrations of acetic and formic acid, respectively, were 2.7±3.8 and 0.7±0.9 ppbv for the rural site, and 1.7±2.0 and 0.6±0.5 ppbv for the mountain site. The average range of carbonyl compound concentrations in ambient air was from 0.3 to 3.4 ppbv for various carbonyl compounds with about 60% of the ambient carbonyls consisting of formaldehyde, acetaldehyde and acetone. Isoprene and monoterpene concentrations in ambient air were usually below the detection limit.

Precipitation charge and size measurements inside a New Mexico mountain thunderstorm

We measured the charge and size of precipitation particles with an instrumented free balloon in a convective mountain thunderstorm over Langmuir Laboratory in central New Mexico. Using an instrument that measured precipitation charge from 2 to 220 pC and equivalent particle diameters from 0.6 to 3.8 mm, we deduced that (1) the charge of the main positive charge region is carried by cloud particles, the charge of the main negative charge region is carried by a mixture of negative charge bearing cloud particles and precipitation particles, and the charge of the lower positive charge region is carried almost entirely by precipitation; (2) there was a mixture of both polarities of precipitation charge at nearly all altitudes; (3) there was no relationship between precipitation charge and size; (4) our charge data appear to support the noninductive ice‐ice collisional charging mechanism; and (5) the sign of charge carried by the precipitation reverses with ambient temperature. In comparing these data to previous measurements that were collected in New Mexico mountain thunderstorms with different instrumentation, we found that in most ways our precipitation charge data are similar to the previous measurements. In comparing these data with data that we previously collected in the trailing stratiform regions of mesoscale convective systems, we found that there are substantial differences in the precipitation charge data between these different cloud types.

Elves triggered by positive and negative lightning discharges

Optical flashes in the lower ionosphere due to the transient heating caused by lightning electromagnetic pulses (EMP) are unambiguously identified with the Fly's Eye photometric array. Data from a thunderstorm over Mexico recorded at Langmuir Laboratory on August 27 1997 demonstrate that relatively common negative cloud‐to‐ground lightning is a previously unrecognized major cause of elves. The spatial extent of the transient heating is shown optically to be typically at least 200–700 km laterally, indicating the possibility for accumulation of ionization effects produced by successive flashes within large nighttime thunderstorm systems. One especially bright event suggests that temporal fine‐structure in the causative very low frequency EMP can manifest itself in the photometric record of elves.