Broadband VLF measurements of lightning-induced ionospheric perturbations
TL;DR: In this paper, the authors detect and measure the D region ionospheric disturbances caused by the strong lightning flash by analyzing the broadband VLF spectrum from lightning that occurred just before and after a nearby intense lightning discharge.
Abstract: [1] Very low frequency (VLF) electromagnetic pulses radiated by lightning are an effective tool for probing the D region ionosphere. We detect and measure the D region ionospheric disturbances caused by the strong lightning flash by analyzing the broadband VLF spectrum from lightning that occurred just before and after a nearby intense lightning discharge. Comparing the measured electron density changes to those from previous measurements and the theoretical expectations, we find the detected perturbations are consistent with the theoretically predicted ionization changes produced directly by the lightning electromagnetic pulse.
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TL;DR: A review of the development of ELF and VLF measurements, both from a historical point of view and from the view of their relationship to optical and other observations of ionospheric effects of lightning discharges is provided in this paper.
Abstract: [1] Extremely low frequency (ELF) and very low frequency (VLF) observations have formed the cornerstone of measurement and interpretation of effects of lightning discharges on the overlying upper atmospheric regions, as well as near‐Earth space. ELF (0.3–3 kHz) and VLF (3–30 kHz) wave energy released by lightning discharges is often the agent of modification of the lower ionospheric medium that results in the conductivity changes and the excitation of optical emissions that constitute transient luminous events (TLEs). In addition, the resultant ionospheric changes are best (and often uniquely) observable as perturbations of subionospherically propagating VLF signals. In fact, some of the earliest evidence for direct disturbances of the lower ionosphere in association with lightning discharges was obtained in the course of the study of such VLF perturbations. Measurements of the detailed ELF and VLF waveforms of parent lightning discharges that produce TLEs and terrestrial gamma ray flashes (TGFs) have also been very fruitful, often revealing properties of such discharges that maximize ionospheric effects, such as generation of intense electromagnetic pulses (EMPs) or removal of large quantities of charge. In this paper, we provide a review of the development of ELF and VLF measurements, both from a historical point of view and from the point of view of their relationship to optical and other observations of ionospheric effects of lightning discharges.
152 citations
TL;DR: In this paper, the authors analyzed wideband electric field waveforms of 265 first and 349 subsequent return strokes in negative natural lightning and found that the first and subsequent return-stroke waveforms at 50 to 330 km exhibit an opposite polarity overshoot.
Abstract: [1] We analyzed wideband electric field waveforms of 265 first and 349 subsequent return strokes in negative natural lightning. The distances ranged from 10 to 330 km. Evolution of first- and subsequent-stroke field waveforms as a function of distance is examined. Statistics on the following field waveform parameters are given: initial electric field peak, opposite-polarity overshoot, ratio of the initial electric field peak to the opposite polarity overshoot, zero-to-peak risetime, initial half-cycle duration, and opposite polarity overshoot duration. The overwhelming majority of both first and subsequent return-stroke field waveforms at 50 to 330 km exhibit an opposite polarity overshoot. At distances greater than 100 km, electric field waveforms, recorded under primarily daytime conditions, tend to be oscillatory. Using finite difference time domain modeling, we interpreted the initial positive half-cycle and the opposite-polarity overshoot as the ground wave and the second positive half-cycle as the one-hop ionospheric reflection. The observed difference in arrival times of these two waves for subsequent strokes is considerably smaller than for first strokes, suggesting that the first-stroke electromagnetic field caused a descent of the ionospheric D-layer. We speculate that there may be cumulative effect of multiple strokes in lowering the ionospheric reflection height. Return-stroke peak currents estimated from the empirical formula, I = 1.5–0.037DE (where I is considered negative and in kA, E is the electric field peak considered positive and in V/m, and D is distance in km), are compared to those reported by the NLDN.
89 citations
TL;DR: In this article, a massive γ-ray flare from SGR 1806-20 created a massive disturbance in the daytime lower ionosphere, as evidenced by unusually large changes in amplitude/phase of subionospherically propagating VLF signals.
Abstract: [1] The giant γ-ray flare from SGR 1806-20 created a massive disturbance in the daytime lower ionosphere, as evidenced by unusually large changes in amplitude/phase of subionospherically propagating VLF signals. The perturbations of the 21.4 kHz NPM (Lualualei, Hawaii) signal observed at PA (Palmer Station, Antarctica) correspond to electron densities increasing by a factor of ∼100 to ∼103 cm−3 at ∼60 km and ≳1000 to ∼10 cm−3 at ∼30 km altitude. Enhanced conductivity produced by flare onset endured for >1 hour, the time scale determined by mutual neutralization. A brief (∼100 ms) low frequency (∼3 to 6 kHz) emission is also observed during the flare onset.
86 citations
Cites background from "Broadband VLF measurements of light..."
...... transient disturbances of the nighttime lower ionosphere (� 40 to 90 km altitude), resulting from high energy auroral precipitation [e.g., Potemra and Rosenbert, 1973; Cummer et al., 1997], lightning-induced electron precipitation [e.g., Inan and Carpenter 1987], electromagnetic and quasielectrostatic coupling produced by lightning discharges (e.g., sprites and elves) [Inan et al., 1996; Moore et al., 2003; Haldoupis et al., 2004; Cheng and ......
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TL;DR: In this article, the authors used lightning signals from a distant mesoscale storm to probe the lower ionosphere above a small tropospheric thunderstorm revealing a reduction in ionospheric electron density in response to lightning discharges.
Abstract: Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km. The use of lightning signals from a distant mesoscale storm to probe the lower ionosphere above a small tropospheric thunderstorm reveals a reduction in ionospheric electron density in response to lightning discharges in the small storm. Tropospheric thunderstorms have been reported to disturb the lower ionosphere, at altitudes of 65–90 km, by convective atmospheric gravity waves1,2,3,4,5 and by electric field changes produced by lightning discharges6,7,8,9,10,11,12,13,14,15. Theoretical simulations suggest that lightning electric fields enhance electron attachment to O2 and reduce electron density in the lower ionosphere7,8. Owing to the low electron density in the lower ionosphere, active probing of its electron distribution is difficult16,17, and the various perturbative effects are poorly understood. However, it is now possible to probe the lower ionosphere in a spatially and temporally resolved manner by using remotely detected time waveforms of lightning radio signals4,5,18,19. Here we report such observations of the night-time ionosphere above a small thunderstorm. We find that electron density in the lower ionosphere decreased in response to lightning discharges. The extent of the reduction is closely related in time and space to the rate of lightning discharges, supporting the idea that the enhanced electron attachment is responsible for the reduction. We conclude that ionospheric electron density variations corresponding to lightning discharges should be considered in future simulations of the ionosphere and the initiation of sprite discharges.
86 citations
TL;DR: In this article, the ionospheric D region was probed by measuring the high-power broadband very low frequency (VLF) signals launched by lightning and propagating in the Earth ionosphere waveguide.
Abstract: [1] Significant temporal variability of the nighttime D region is well known, but its inaccessibility means that the time scales, magnitudes, and sources for that variability are not well understood. We probed the ionospheric D region by measuring the high‐power broadband very low frequency (VLF) signals launched by lightning and propagating in the Earth‐ionosphere waveguide. We analyzed broadband sferic data of July and August 2005 recorded by our sensors located near Duke University by comparing measured sferic spectra to model results and extracted the height of an assumed exponential electron density profile for each measurement. The measured nighttime D region electron density profile heights showedlarge temporal variations ofseveral kilometers onsomenights andrelatively stable behaviors on others. The measured hourly average heights in 260 h ranged between 82.0 and 87.2 km, with a mean value of 84.9 km and a standard deviation of 1.1 km. The maximum variation in the 5 h period was around just above 4.0 km and the maximum variation in the 1 h period was around 1.3 km, with sharper gradients observed over shorter time periods. We also observed spatial variability as large as 2.0 km over 5° latitudes on some nights and no spatial variability on others. On some nights, the temporal variability exhibited a close correlation with the occurrence rate of lightning discharges under the probed region. This suggests that the direct energy coupling between lightning discharges and lower ionosphere can be a significant source of the D region variability. However, on other nights, the measured height temporal variability showed weak to no correlation with local lightning, displaced lightning (as would be expected for lightning‐induced electron precipitation), or geomagnetic activities. These measurements suggest that nighttime D region variability may be driven by many sources.
73 citations
References
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TL;DR: In this paper, the authors show that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium).
Abstract: Quasi-electrostatic (QE) fields that temporarily exist at high altitudes following the sudden removal (e.g., by a lightning discharge) of thundercloud charge at low altitudes lead to ambient electron heating (up to ∼5 eV average energy), ionization of neutrals, and excitation of optical emissions in the mesosphere/lower ionosphere. Model calculations predict the possibility of significant (several orders of magnitude) modification of the lower ionospheric conductivity in the form of depletions of electron density due to dissociative attachment to O2 molecules and/or in the form of enhancements of electron density due to breakdown ionization. Results indicate that the optical emission intensities of the 1st positive band of N2 corresponding to fast (∼ 1 ms) removal of 100–300 C of thundercloud charge from 10 km altitude are in good agreement with observations of the upper part (“head” and “hair” [Sentman et al., 1995]) of the sprites. The typical region of brightest optical emission has horizontal and vertical dimensions ∼10 km, centered at altitudes 70 km and is interpreted as the head of the sprite. The model also shows the formation of low intensity glow (“hair”) above this region due to the excitation of optical emissions at altitudes ∼ 85 km during ∼ 500 μs at the initial stage of the lightning discharge. Comparison of the optical emission intensities of the 1st and 2nd positive bands of N2, Meinel and 1st negative bands of , and 1st negative band of demonstrates that the 1st positive band of N2 is the dominating optical emission in the altitude range around ∼70 km, which accounts for the observed red color of sprites, in excellent agreement with recent spectroscopic observations of sprites. Results indicate that the optical emission levels are predominantly defined by the lightning discharge duration and the conductivity properties of the atmosphere/lower ionosphere (i.e., relaxation time of electric field in the conducting medium). The model demonstrates that for low ambient conductivities the lightning discharge duration can be significantly extended with no loss in production of optical emissions. The peak intensity of optical emissions is determined primarily by the value of the removed thundercloud charge and its altitude. The preexisting inhomogeneities in the mesospheric conductivity and the neutral density may contribute to the formation of a vertically striated fine structure of sprites and explain why sprites often repeatedly occur in the same place in the sky as well as their clustering. Comparison of the model results for different types of lightning discharges indicates that positive cloud to ground discharges lead to the largest electric fields and optical emissions at ionospheric altitudes since they are associated with the removal of larger amounts of charge from higher altitudes.
434 citations
TL;DR: In this article, a multichannel high-speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Field Station (40040'N, 104o.56'W) in Colorado as part of the SPRITES'95 campaign from 15 June to August 6, 1995.
Abstract: Observations of optical phenomena at. high alti- tude a, bove thunderstorms using a multichannel high-speed photometer and image intensified CCD cameras were carried out at Yucca Ridge Field Station (40040 ' N, 104o.56 ' W), Colorado as part of the SPRITES'95 campaign from 15 June to August 6, 1995. These newneasurements indicate that diffuse optical flashes with a duration of < I ms and a hori- zontal scale of-.- 100-300 km occur at 75-105 km altitude in the lower ionosphere just after the onset of cloud-to-ground lightning discharges, but preceding the onset of sprites. Here we designate these events as 'alves" to distinguish them from 'i'ed sprites" . This finding is consistent with the production of diffuse optical emissions due to the heating of the lower ionosphere by electromagnetic pulses generated by lightning discharges as suggested by several authors.
287 citations
"Broadband VLF measurements of light..." refers background or result in this paper
...…lightningdischarge.Comparing themeasured electrondensity changes to those from previous measurements and the theoretical expectations, we find the detected perturbations are consistent with the theoretically predicted ionization changes produced directly by the lightning electromagnetic pulse....
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...…due to the heating of ionospheric electrons by the electromagnetic pulse (EMP) from lightning (in a manner similar to that which produce fast optical emissions called elves [Fukunishi et al., 1996]) with the disturbance expanding to radial distances of up to 150 km [Inan et al., 1996a]....
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TL;DR: The US National Lightning Detection Network/sup TM/ (NLDN) as mentioned in this paper is a system that senses the electromagnetic fields that are radiated by individual return strokes in cloud-to-ground (CG) flashes.
Abstract: Lightning is a significant cause of interruptions or damage in almost every electrical or electronic system that is exposed to thunderstorms. The problem is particularly severe for electric power utilities that have exposed assets covering large areas. We summarize the basic properties of cloud-to-ground (CG) lightning, the primary hazard to structures on the ground, and then we discuss methods of detecting and locating such discharges. We describe the US National Lightning Detection Network/sup TM/ (NLDN), a system that senses the electromagnetic fields that are radiated by individual return strokes in CG flashes. This network provides data on the time of such strokes, their location and polarity and an estimate of the peak current. We discuss the network detection efficiency and location accuracy and some of the limitations that are inherent in any detection system that operates with a finite number of sensors with fixed trigger thresholds. We also discuss how NLDN data have benefited utilities by providing lightning warnings in real time and information on whether CG strokes are the cause of faults, documenting the response of fixed assets that are exposed to lightning, and quantifying the effectiveness of lightning protection systems. We conclude with some general observations on the use of lightning data by power utilities and we provide some guidelines on the uncertainties in lightning parameters that are acceptable in the industry.
246 citations
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01 Jan 1961
230 citations
"Broadband VLF measurements of light..." refers methods in this paper
...To apply the technique of Cummer et al. [1998] to search for small ( 100 km in extent) and short-lived (tens of seconds duration) ionospheric perturbations, a number of specific conditions must be met....
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...This model solves the time-harmonic propagation problem using mode theory [Budden, 1961], in which the fields at a distance from the source are described as a sum of independently propagating waveguide modes....
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TL;DR: In this article, the authors used high-speed video recordings along with theoretical modeling to elucidate the optical signatures of elves and sprites, and showed that a brief diffuse flash sometimes observed to accompany or precede more structured sprites in standard speed video is a normal component of sprite electrical breakdown and is due entirely to the quasi-electrostatic thundercloud field (sprites), rather than the lightning electromagnetic pulse (elves).
Abstract: Confusion in the interpretation of standard-speed video observations of optical flashes above intense cloud-to-ground lightning discharges has persisted for a number of years. New high-speed (3000 frames per second) image-intensified video recordings are used along with theoretical modeling to elucidate the optical signatures of elves and sprites. In particular, a brief diffuse flash sometimes observed to accompany or precede more structured sprites in standard-speed video is shown to be a normal component of sprite electrical breakdown and to be due entirely to the quasi-electrostatic thundercloud field (sprites), rather than the lightning electromagnetic pulse (elves). These “sprite halos” are expected to be produced by large charge moment changes occurring over relatively short timescales (∼1 ms), in accordance with their altitude extent of ∼70 to 85 km. The relatively short duration of this upper, diffuse component of sprites makes it difficult to detect and to discriminate from elves and Rayleigh-scattered light using normal-speed video systems. Modeled photometric array signatures of elves and sprites are contrasted and shown to be consistent with observations. Ionization in the diffuse portion of sprites may be a cause of VLF scattering phenomena known as early/fast VLF events.
221 citations
Additional excerpts
...According to Barrington-Leigh et al. [2001] , the QE mechanism requires many hundreds of C km of charge moment changes to produce significant ionization around 70 km altitudes....
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