Lightning Charge Moment Research Articles

Lightning parameters of sprites and diameter of halos over South Africa

Transient Luminous Events (TLEs) above thunderclouds have been previously associated with variables such as the lightning Charge Moment Change (CMC), charge height, charge transfer, and lightning current rise-time. We show for the first time a comparison of the CMC, rise-time, fall-time, peak electric field, and peak current of the lightning discharges associated with 11 column, 11 carrot, and 18 sprites with halo. We found that carrot sprites are induced by a lightning discharge with CMC, peak electric field, and peak current greater and less than that for column sprites and sprites with halo, respectively. Sprites with a halo are initiated by a lightning discharge with a longer rise-time and fall-time than that for column and carrot sprites. Column sprites top altitude and carrot sprites brightest region altitude positively correlate with lightning rise-time. For carrot sprites top altitude, the results suggest that the electrical breakdown region decreases in altitude for a longer fall-time, greater peak electric field, and greater peak current. For the altitude of the sprites brightest region, column sprites correlate negatively with lightning fall-time, peak electric field, and CMC, and column sprites top altitude also correlates negatively with lightning peak electric field. For sprites with a halo top altitude increased with lightning fall-time and peak current, and sprites with a halo brightest altitude increased with an increase in lightning CMC. Halo diameters correlate positively with lightning fall-time, peak electric field, and peak current. The investigated lightning parameters can be used to identify the initiated sprites morphological type when optics are not available.

The altitude of sprites observed over South Africa

Sprites are mesospheric optical emissions that are mostly produced by large, positive cloud-to-ground lightning discharges. Sprites appear in different morphologies such as carrot, jellyfish and column, and are typically in the altitude range of ~40–100 km above the Earth’s surface. Sprites are a subset of transient luminous events and they contribute to the global electric circuit. South Africa has large convective thunderstorms, which typically occur in the summer months of every year. Peak current, time and geographical position of lightning strokes were obtained from the South African Weather Service. Sprite observations were recorded in South Africa for the first time on 11 January 2016 from Sutherland in the Northern Cape using a night-vision television camera from the South African National Space Agency’s Optical Space Research laboratory. We report the first estimates of the top altitude, and the altitude of maximum brightness, of 48 sprites over South Africa. We found that the average top altitude and the altitude of maximum brightness of sprites are approximately 84.3 km and 69 km, respectively, which is consistent with estimates made elsewhere. We also found a moderately high positive and a weak positive correlation between the top altitude and the altitude of maximum brightness, respectively, of sprites and the lightning stroke charge moment change.
 Significance:
 
 We present the first altitude estimation of sprites observed over Africa.
 The altitude of sprites observed over South Africa is in agreement with observations made elsewhere.
 There is a positive correlation between the top altitude of sprites and the parent lightning charge moment change.
 Sprite maximum brightness is observed near the stratopause.

Impact of lightning on the lower ionosphere of Saturn and possible generation of halos and sprites

We study the effect of lightning on the lower ionosphere of Saturn. A self-consistent one-dimensional model of the electric field and electron density is used to estimate the changes of the local electron and photon emissions. The chemical fingerprint and ion densities are determined using a detailed self-consistent kinetic model. Charge moment change, depth of lightning flashes and their duration are estimated based on the known constraints of saturnian lightning activity. We test two electron density profiles and find that the conservative estimation of lightning charge moment change 104 to 105Ckm could lead to faint halos and possibly sprites if the base of the ionosphere is located at 1000km above the 1bar level; if the base of the ionosphere is located at 600km then only the extreme scenario of a 106Ckm charge moment change could induce considerable ionization, halos and possibly sprites. We found that H3+ ions are rapidly produced from the parent H2+ ions through the fast reaction H2++H2→H3++H, so that H3+ becomes the dominant ion in all the scenarios considered. The resulting light emissions, mostly in the blue and ultraviolet spectral regions, are below the detection threshold of Cassini.

Three years of lightning impulse charge moment change measurements in the United States

We report and analyze 3 years of lightning impulse charge moment change (iCMC) measurements obtained from an automated, real time lightning charge moment change network (CMCN). The CMCN combines U.S. National Lightning Detection Network (NLDN) lightning event geolocations with extremely low frequency (≲1 kHz) data from two stations to provide iCMC measurements across the entire United States. Almost 14 million lightning events were measured in the 3 year period. We present the statistical distributions of iCMC versus polarity and NLDN‐measured peak current, including corrections for the detection efficiency of the CMCN versus peak current. We find a broad distribution of iCMC for a given peak current, implying that these parameters are at best only weakly correlated. Curiously, the mean iCMC does not monotonically increase with peak current, and in fact, drops for positive CG strokes above +150 kA. For all positive strokes, there is a boundary near 20 C km that separates seemingly distinct populations of high and low iCMC strokes. We also explore the geographic distribution of high iCMC lightning strokes. High iCMC positive strokes occur predominantly in the northern midwest portion of the U.S., with a secondary peak over the gulf stream region just off the U.S. east coast. High iCMC negative strokes are also clustered in the midwest, although somewhat south of most of the high iCMC positive strokes. This is a region far from the locations of maximum occurrence of high peak current negative strokes. Based on assumed iCMC thresholds for sprite production, we estimate that approximately 35,000 positive polarity and 350 negative polarity sprites occur per year over the U.S. land and near‐coastal areas. Among other applications, this network is useful for the nowcasting of sprite‐producing storms and storm regions.

Electric fields and electron energies in sprites and temporal evolutions of lightning charge moment

The fundamental electrodynamical coupling processes between lightning and sprites are investigated. By combining the observed spectral data with the Monte Carlo swarm experiments, reduced electric fields and electron energies in sprite streamers and halos are estimated. The obtained fields inside sprite halos (70–97 Td with an analysis error of ±5 Td) are lower than the conventional breakdown field, Ek ∼ 128 Td, indicating a significant reduction of electrons associated with halos while those in sprite streamers (98–380 Td with an error of ±50 Td) are higher than Ek, suggesting that a significant ionization process drives their formation and development. A combined analysis of photometric and electromagnetic data makes it possible to estimate temporal evolutions of lightning charge moment. It is found that lightning discharges with a short time scale (∼1 ms) and a moderate amount of charge moment (∼400 C km) produce discernible halos. On the other hand, lightning discharges with a large amount of charge moment (∼1300 C km) produce streamers regardless of their time scale. The results obtained are comprehensively interpreted with both the conventional breakdown field necessary for the formation of streamers and the electric field necessary for the production of optical emissions of halo which is sensitive to the time scale of the thundercloud field due to the significant reduction of electrons.

Coordinated analysis of delayed sprites with high‐speed images and remote electromagnetic fields

Simultaneous measurements of high‐altitude optical emissions and magnetic fields produced by sprite‐associated lightning discharges enable a close examination of the link between low‐altitude lightning processes and high‐altitude sprite processes. We report results of the coordinated analysis of high‐speed sprite video and wideband magnetic field measurements recorded simultaneously at Yucca Ridge Field Station and Duke University. From June to August 2005, sprites were detected following 67 lightning strokes, all of which had positive polarity. Our data showed that 46% of the 83 discrete sprite events in these sequences initiated more than 10 ms after the lightning return stroke, and we focus on these delayed sprites in this work. All delayed sprites were preceded by continuing current moments that averaged at least 11 kA km between the return stroke and sprites. The total lightning charge moment change at sprite initiation varied from 600 to 18,600 C km, and the minimum value to initiate long‐delayed sprites ranged from 600 for 15 ms delay to 2000 C km for more than 120 ms delay. We numerically simulated electric fields at altitudes above these lightning discharges and found that the maximum normalized electric fields are essentially the same as fields that produce short‐delayed sprites. Both estimated and simulation‐predicted sprite initiation altitudes indicate that long‐delayed sprites generally initiate around 5 km lower than short‐delayed sprites. The simulation results also reveal that slow (5–20 ms) intensifications in continuing current can play a major role in initiating delayed sprites.

Terrestrial gamma ray flashes and lightning discharges

Analysis of ELF/VLF broadband data from Palmer Station, Antarctica indicates that 76% Terrestrial Gamma‐ray Flashes (TGFs) detected on the RHESSI spacecraft occur in association with lightning‐generated radio atmospherics arriving from near the footprint of RHESSI and within a few ms of the TGF. The remaining TGFs are not associated with any radio atmospheric, thus by implication CG lightning. The peak currents of TGF‐associated lightning discharges are often among the most intense from a given storm, with the degree of this association apparently varying between oceanic and land regions. The time‐integrated ELF energy of the associated sferics (and thus the lightning charge moment) exhibit much less tendency to be large. Statistical analysis of the spread in arrival time suggests a ∼2 ms variance due to factors other than geometry and measurement error.

Measurements and implications of the relationship between lightning and terrestrial gamma ray flashes

We report observations and analysis of 30 kHz radio emissions (sferics) from lightning discharges associated with 26 terrestrial gamma ray flashes (TGFs) recorded by the RHESSI satellite over the Caribbean and Americas, between 1500 and 4000 km away from the magnetic field sensors located at Duke University. Thirteen of the TGFs are found to occur within −3/+1 ms of lightning discharges of positive polarity from the direction of the RHESSI subsatellite point, strongly indicating that the TGFs are linked to these discharges. The event timing and sferic direction finding reveals that the discharges occur within a ∼300 km radius circle around the RHESSI subsatellite point. Although the positive polarity of all 13 discharges is consistent with runaway breakdown, the lightning charge moment changes are approximately two orders of magnitude smaller than present high altitude runaway breakdown theory predicts. Implications of these measurements are discussed.

Implications of lightning charge moment changes for sprite initiation

We report impulse lightning charge moment changes (defined as occurring in the first 2 ms after return stroke onset) in all cloud‐to‐ground lightning strokes detected by the National Lightning Detection Network in three storms during which above‐thunderstorm sprite video was recorded. After analyzing strokes that both did and did not produce sprites and carefully accounting for lightning‐sprite delay times, we found that sprite initiation on all three nights is consistent with a sharp charge moment threshold; essentially, all charge moment changes above and none below this threshold produced sprites with short delays (<5 ms) from the source lightning. On two nights this threshold was approximately 600 C km and on the other it was approximately 350 C km. This internight variability is probably due to expected variability in the nighttime mesospheric conductivity, and the thresholds themselves are consistent with predictions of conventional breakdown theory. Additionally, we found only one negative polarity lightning stroke from all three storms that exceeded this threshold, indicating that the rarity of documented sprites produced by negative strokes may be largely explained by the lack of sufficiently big negative strokes in the U.S. High Plains.

Lightning charge moment changes in U.S. High Plains thunderstorms

We report measurements of impulsive (2 ms) lightning charge moment changes in more than 1000 cloud‐to‐ground (CG) return strokes detected by the National Lightning Detection Network in three United States High Plains storms during the Severe Thunderstorm Electrification and Precipitation Study (STEPS) field program of 2000. The positive CG strokes (+CGs) in a mesoscale convective system exhibit an unusual charge moment distribution with a small median and long tail. Analysis suggests the presence of two different classes of +CGs in this MCS, one with small charge moment changes (≤50 C km) and the other containing larger charge moment changes (50–1400 C km). The distributions of negative cloud‐to‐ground stroke charge moment changes are roughly log‐normal in shape with means varying from 17.7 to 36.8 C km. When combined with past measurements these means vary by a factor of 4 between storms, and there is probably not a single distribution that represents well all storms.

Modelling the influence of conductivity profiles on red sprite formation and structure

Strong quasi-electrostatic fields, generated in the mesosphere and lower ionosphere after a lightning discharge by a succeeding redistribution of the induced spatial charges, are considered to be responsible for red sprite generation. Factors considered here as important for sprite occurrence, size and shape are the discharge parameters and conductivity profile. Thundercloud charges are assumed to be of hundreds of Coulombs distributed within layers with a horizontal extent of tens of kilometers as typical for big convective multi-cell systems. Cloud-to-ground positive strokes, characterized by the initial charge and the discharge time parameters, are considered. The thermal breakdown mechanism is considered as responsible for sprite onset. The conditions under which sprites initially occur (i.e., when atmospheric parameters are not disturbed) and their spatial and temporal characteristics are studied, depending on the conductivity profiles. A self-consistent analytical modeling is proposed for this purpose. Maxwell’s equations are applied under quasi-electrostatic conditions, when the magnetic field component is neglected and the electric field is assumed to be important. The features of a lightning discharge as well as of the conductivity profiles (including the slight anisotropy in the lower ionosphere) are taken into account in the model. The conductivity profiles are approximated between 0 and 100 km by stepwise profiles, defined as a layered atmosphere with ∼100 layers. Horizontal conductivity variations occurring at sprite heights (50–90 km) due to electron heating and ionization are represented in the model by step functions defined in each layer. Continuity of the Maxwell current density component normal to each boundary is required. The results obtained show that, for sprite occurrence in daytime conditions, a larger lightning charge moment change of the parent discharge is needed, and the sprites are lower than at night. The conditions (concerning the discharge and conductivity parameters) under which the presence of a ‘ledge’ in the conductivity profile in the lower ionosphere can influence the occurrence of a diffuse upper portion of a sprite, are investigated. It is shown that the electric field is intensified near a conductivity profile ‘ledge’ which leads to sprite formation.

Current moment in sprite-producing lightning

Studies of sprite-producing lightning have revealed much of what we currently know about the mechanisms responsible for this phenomenon. With a combination of new and previous results, we summarize the currently known quantitative characteristics of this special class of lightning. This information has come primarily from the quantitative analysis of the electromagnetic fields produced by distant lightning. The long range of this technique has made it especially powerful, but there are important limitations on what can be measured that are related to the bandwidth of the measurement system. The lightning charge moment change required to initiate sprites varies across a relatively wide range, from approximately 100– 2000 C km . Note that this is not the total charge moment change in sprite producing lightning, which is by definition greater than the initiation threshold. This range is in very good agreement with the predictions of streamer-based sprite modeling. We also summarize the strong evidence, from a variety of sources, in favor of sprite currents as the origin of ELF pulses seen in a significant fraction of sprite events. The largest events show sprite current moment amplitudes of ∼1000 kA km and sprite charge moment changes of at least 1200 C km , and perhaps significantly more. Lastly, we show that delayed sprites are generated from very strong continuing currents (20– 60 kA km ) following a +CG return stroke. In the few cases analyzed, this current generates a charge moment change from 2000 to 6000 C km at the time of sprite initiation, which appears to be consistent with theoretical predictions of larger charge moment changes required for delayed sprite initiation. This current can be detected and analyzed with distant, sensitive ULF magnetic field measurements. Although much is known about sprite-producing lightning, there remain some fundamental yet unanswered questions about the lightning–sprite relationship. We expect that at least some of these will be answered in the relatively near future.

Estimation of sprite occurrences in Central Africa

Extremely low-frequency magnetic field disturbances from intense positive lightning discharges are compared to the convective cloud cover in central Africa, derived from infrared brightness temperatures recorded on board the geostationary satellite Meteosat during April 1998. The mean diurnal variation of the positive lightning charge moment is well correlated with the mean diurnal variation of the cloud cover at ∼ 11.5 km height and constrains the mean lightning channel length. The daily integrated positive cloud to ground charge transfer exhibits a pronounced day to day variability which is well correlated with the cloud cover at ∼ 15.5 km height, related to the charging of the thundercloud ∼ 4 hours prior to the maximum cloud to ground charge transfer. The cloud cover area is used to calculate an effective cloud volume which is related to the cloud to ground charge transfer via the charge density. This charge density is used to determine promising locations for optical sprite observations in the central Congo basin and Cameroon with ∼ 69 estimated sprite occurrences during an average night.

Lightning charge moment changes for the initiation of sprites

The transient ELF(∼50–5000 Hz) magnetic field radiated by lightning discharges across North America was continuously measured at Duke University during the summer of 2000. In total, 881 sprite‐associated lightning discharges over 17 days were analyzed. We report in detail on 76 sprites for which we could reliably determine the lightning charge moment change from the ELF data at the time of sprite onset. The charge moment change for the initiation of a sprite is found to be as low as 120 C km. By folding together the charge moment distributions of sprite‐producing lightning and all positive lightning, we find that the probability of sprite generation for lightning with >1000 C km charge moment change in <6 ms is >90%, while the sprite probability for lightning with <600 C km charge moment change in <6 ms is <10%.

Unusually intense continuing current in lightning produces delayed mesospheric breakdown

Ultra low frequency magnetic field measurements made 500–2000 km from positive lightning discharges show a signature that is consistent with unusually high amplitude cloud‐to‐ground continuing lightning current. The magnitude of this nearly constant current moment is as large as 60 kA km and can last at half this amplitude for longer than 150 ms, thereby moving 640 C or more (assuming a 7 km vertical channel length) to the ground after the return stroke. This total charge transfer is more than an order of magnitude greater than most previously reported continuing currents in positive discharges. Three cases analyzed show this strong continuing current flows before, during, and after sprites that initiate more than 40 ms after the return stroke. Accounting for this continuing current, quantitative analysis shows that the total vertical lightning charge moment changes are large enough to produce mesospheric electrical breakdown and long‐delayed sprites.

Submillisecond resolution lightning currents and sprite development: Observations and implications

We analyze synchronized high speed video images and ELF‐VLF radio emissions from 11 sprite clusters observed on 6 October 1997. Quantitative analysis shows that vertical lightning charge moment changes of 150–1100 C · km occurred before the optical emissions reached their peak with delays of 2–11 ms from the lightning discharge. This threshold unexpectedly decreases with increasing delay from parent lightning to peak emissions. We also find that sprite charge moment change and minimum sprite altitude are not well correlated with the vertical charge moment change in the parent discharge. These observations do not agree well with present sprite generation models, and we suggest that streamer development and horizontal lightning charge motion can play a significant role in sprite generation.