Imager Of Sprites And Upper Atmospheric Lightning Research Articles

Transient Luminous Events and Their Relationship to Lightning Strokes Over the Tibetan Plateau and Its Comparison Regions

AbstractThis study investigates the relationship between transient luminous events (TLEs) and lightning strokes, and the characteristics of TLE‐producing thunderstorms over the Tibetan Plateau (TP), and compares them to those over the Yangtze‐River Delta (YRD) and East China Sea (ECS) where at the same latitude during 2005–2015. The data were collected using the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) and the World Wide Lightning Location Network (WWLLN) observations. Elves, sprites and halo (abbreviated as ESHs) were dominantly detected over the southeastern TP (∼88%), mostly in August and September (∼81%). Different from the southeastern TP, the detected sprites and sprite‐to‐lightning stroke ratios over the YRD were larger in spring than those in summer and autumn. Halos were frequently observed in August over all study regions. Blue jets were only detected over the YRD. The density of TLEs over the southeastern TP was three times smaller than those over the YRD and ECS, while the density of elves over the southeastern TP was slightly larger than that over the YRD. The average energy of TLE‐related lightning strokes based on WWLLN was found to be larger over the southeastern TP compared to the YRD and ECS. The ESH‐producing clouds over the southeastern TP had a larger scale with a lower lightning frequency than those over the YRD.

An assessment of whistlers generated from tree-like gigantic jets

The characteristics of whistlers generated from an observed gigantic jet (GJ) are assessed and possible locations to detect these waves are deduced. Modeling is based on disturbances in the electric field, as measured by NCKU ELF/VLF station, associated with a tree-like GJ event over typhoon Lionrock. The power spectrum of GJ differs from that of common cloud-to-ground lightning; therefore, this study also investigates differences between GJ-generated signals and common lightning-generated whistlers. Detectability is evaluated by considering the absorption of amplitudes resulted from collisional damping associated with the propagation of generated waves. Our results show that in the ionosphere the waves are subject to greater attenuation as the frequency increases; however, a reversal occurs at lower frequencies of a few hundred Hz. The calculated waveforms show that the whistlers generated by the tree-like GJs are preceded by small fluctuations at high frequencies generated by the initiating lightning. Overall, the amplitudes increase with the passage of time; however, they are more randomly-distributed over time for whistlers generated from common lightning. The amplitudes decrease again when lower-frequency components below a few hundred Hz arrive. The amplitudes drop to the order of 1 mV/m as the waves propagate in the ionosphere, which puts them within a range detectable by the instruments on most satellites. Based on the locations of tree-like GJ events observed by ISUAL (Imager of Sprites and Upper Atmospheric Lightning), regions of the western and southeastern Pacific Ocean, as well as northern Africa region are the most likely locations to detect these whistlers.

Sprite Distribution of Different Polarities From ISUAL Observations With Machine Learning Method

AbstractThe morphological features of sprites are closely related to the polarity of their causative lightning strokes. Using the machine learning method, we develop a model with an accuracy of 93.8% to identify the polarity of sprite‐producing cloud‐to‐ground (CG) lightning strokes for events recorded during the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission. Approximately 17% of the sprites are identified to be produced by negative CG lightning strokes. The global distribution of the polarity of sprite‐producing CG lightning strokes suggests that the ratio of sprites produced by negative CG lightning strokes relative to sprites produced by positive ones varies with latitude and sea‐land distribution. Sprites produced by negative CG lightning strokes appear to be generated in the tropical regions below 20° latitude and the oceanic area. Moreover, the proportion of sprites produced by negative CG lightning strokes over Africa and North America are much smaller than that over the rest of the continents and the sea.

Global Distribution and Spectral Features of Intense Lightning by the ISUAL Experiment

AbstractDuring the 12 years of the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment onboard the Taiwanese FORMOSAT‐2 satellite (2004/07/01–2016/6/20), 297,349 transient events were recorded, but only 48,414 events (16.3%) were manually classified as transient luminous events (TLEs). Since the main target of ISUAL is to observe TLEs, only the top ∼0.82% of intense lightning can meet the trigger threshold, which means the ISUAL experiment can also serve as a space‐borne intense lightning survey. An autonomous lightning identification algorithm was developed in this work to screen and locate lightning flashes in images, and 292,248 intense lightning discharges were identified successfully. Combining the statistical analysis of lightning events and unique spectral observations of ISUAL, the global distribution of intense lightning with spectral characteristics was constructed. The intense lightning occurrence rate of ISUAL was 1.29 fl s−1, and the land‐to‐ocean ratio 1.27. Intense lightning is most active near the equator, and the bimodal pattern peaks in April and September, different from the typical lightning behavior that occurs in the northern hemisphere during the summertime from July to August. In addition, the spectral statistics show that more than 95% of intense lightning is accompanied by nitrogen band emissions (1PN2, 2PN2, 1NN2+). Far‐ultraviolet photons mainly come from the radiation processes in the mesosphere to the ionosphere, and its radiation flux may correlate with lightning energies. Further, a significant 777.4 nm background enhancement is recognized in South America, which may affect the trigger of space‐borne 777.4 nm lightning surveys.

Retrieval of Airglow Emission Rates in Analytical Form for Limb‐Viewing Satellite Observations at Low Latitudes

AbstractThe observation strategy of the limb‐viewing Imager of Sprites and Upper Atmospheric Lightning (ISUAL) payload aboard the FORMOSAT‐2 spacecraft inspires our development of a method to retrieve the volume emission rates from an array of airglow radiance measurements. The retrieval can be performed simply by evaluation of an analytical formula. The method can be applied to radiation at any wavelength, and its utilities are demonstrated in a reconstructed observation event of the 630.0‐nm OI airglow based on model‐generated emission rates. The simulated observation shows that radiance measurements may underestimate the heights of airglow locations by as much as 20 km. But application of the method to retrieve the emission rate profiles can recover the airglow locations with good accuracy, on average missing the height of the peak emission rate by only 2 km or so, corresponding to only 1–2 grid points in the calculations. The retrieval of airglow locations remains quite accurate even when the effect of noise is incorporated into the array of simulated radiance. Accuracy of the retrieved emission rates improves with the range of the field of view in the vertical direction. The method is also able to resolve the interference between the OI and OH emissions in the radiance data, separating the sources of the two types of emissions at distinctly different altitudes in the retrieved emission rate profiles. Overall, consideration of the reasoning behind this retrieval method could be useful for designing future satellite missions that focus on airglow research.

Meteorological and Electrical Conditions of Two Mid‐latitude Thunderstorms Producing Blue Discharges

AbstractBlue discharges are one type of rarely observed upward discharge emerging from the top of thunderstorms. The favorable meteorological conditions and charge structures needed to produce this marvelous phenomenon are still poorly understood. In this study, we report on two midlatitude thunderstorms producing 13 blue discharges observed simultaneously by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) and ground‐based lightning detection array. In these two thunderstorms, blue discharges always occurred in the vicinity of the coldest cloud top (∼195 K) and clustered within a bounded area near the convective surge, leading to the overshooting thundercloud top (reaching about 18 km) into the stratosphere. The parent thunderstorms were normally electrified with a main midlevel negative and an upper positive charge layer centered at about 15 km as inferred from source heights of narrow bipolar events (NBEs). The associated negative NBEs near the tropopause indicated that they are upward positive discharges initiated between the upper positive charge layer and the negative screening layer at the cloud top. In addition, the outbreak of negative NBEs in the altitude range of 16–18 km was observed in conjunction with blue discharges observed from space. This suggests that there exists a strong upper positive layer with an intense convection at this altitude of the thunderstorm. It is inferred that an overshooting thundercloud top with intense convective updraft could lead to a strong and high upper positive charge layer and therefore create favorable charge structures for initiating upward positive blue discharges.

Space‐Based Observation of a Negative Sprite With an Unusual Signature of Associated Sprite Current

AbstractWe report the first observation of an unusual red sprite produced by negative cloud‐to‐ground (CG) lightning from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) in Colombia on August 15, 2012. The impulse charge moment change for this event is approximately −520 C·km, barely reaching the threshold for negative sprite production. However, different from all previously reported observations, this negative sprite contains a distinct “sprite current” feature that is unambiguously identified in the very‐low‐frequency magnetic field recorded at the 3,387 km range in Duke Forest. There is no evidence that this feature was produced by a fast discharging process associated with the causative CG stroke. Instead, the sprite current signal corresponds well to high‐altitude optical emissions associated with the charge flow in the sprite body. The charge moment change caused by the charge transfer in the sprite region was estimated to be about −460 C·km. We attempt to reveal the possible causes of this atypical event from the perspective of the parent thunderstorm and the local ionosphere irregularities, whereas we believe that the most possible reason remains to lie in an extraordinarily long duration (more than 5 ms) of charge transfer to ground following the return stroke.

On negative Sprites and the Polarity Paradox

AbstractEnergetic positive and negative cloud‐to‐ground (CG) flashes are both capable of producing sprites. Negative CGs typically outnumber the positive ones by 10 to 1. However, >99.9 % of reported sprites were found to be initiated by positive CGs—thus the polarity paradox. Here, sprites recorded by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) mission were analyzed along with extremely low‐frequency band magnetic field data to resolve this paradox. Approximately twenty‐five percent of the sprites are found to be associated with negative CG lightning. “Negative” sprites mainly congregate in the latitudinal regions below 20°, while positive sprites scatter up to 50°. The ISUAL negative sprites are evidently beyond the observable ranges of the ground sites reported in previous studies. Hence, the sprite polarity paradox is likely a selection effect of the middle‐ to high‐altitudinal observation sites. The charge moment changes and accompanying transient luminous events of sprites were also examined and found to be polarity dependent.

ISUAL‐Observed Blue Luminous Events: The Associated Sferics

AbstractThe blue luminous events (BLEs) recorded by ISUAL (Imager of Sprites and Upper Atmospheric Lightning) radiate unambiguous middle ultraviolet to blue emissions (230–450 nm) but contain dim red emissions (623–754 nm). The BLE appears to be dot‐like on one ISUAL image with an integration time of 29 ms. A few BLEs develop upward into blue jets/starters or type II gigantic jets (GJs). The associated sferics of the BLEs in the extremely low frequency to very low frequency band and in the low‐frequency band exhibit similar patterns to the narrow bipolar events (NBEs) identified in the very low frequency and low‐frequency band. The ISUAL BLEs are conjectured to be the accompanied light emissions of the NBEs. Both upward and downward propagating current obtained from the associated sferics of the BLEs have been found. The source heights of the six BLEs related to negative NBEs are estimated in the range of 16.2–17.8 km. These six events are suggested to occur between the upper positive charge layer and the negative screen charge layer on the top of the normally electrified thunderstorm. The six blue starters, one blue jet, and one type II GJ are inferred to be positive upward discharges from their associated sferics in the extremely low frequency to very low frequency band. Based on the simultaneous radio and optical observations, a NBE is conjectured to be the initiation discharge with rapidly flowing current within the thunderstorm, while a blue jet/starter or a type II GJ is suggested to be the ensuing discharge with slowly varying current propagating upward from the thunderstorm.

Temporal and radiometric statistics on lightning flashes observed from space with the ISUAL spectrophotometer

AbstractThe Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 satellite was launched in 2004 and records transient luminous events (TLEs). ISUAL is an instrument composed of an imager and a multichannel spectrophotometer which is able to measure the brightness from far ultraviolet to near infrared for TLEs and lightning flashes. Several types of TLEs have been identified, classified, and documented between 2004 and 2014. However, no statistical analyses on lightning have already been realized. In this paper, we focus on lightning flashes which did not induce TLEs. Statistics about brightness or temporal dynamic are presented from fitted waveforms from far ultraviolet to near infrared, after having corrected for atmospheric effects for each spectrophotometer channels. We demonstrate that the lightning/satellite distance and the altitude of the lightning need to be taken into account. The lightning waveforms from each spectrophotometer channel could be modeled as an exponential pulsed function which the shape could be explained by the photon propagation in the cloud. The median values at 777.4 nm are 1.63 × 10−6 W m−2 for the maximum brightness, 5.85 × 104 J for the total energy at the source, 414 µs for the full width half maximum, 620 µs for the full width at quarter of maximum, and 170 µs for the risetime. Results are compared with previous experiments performed from visible to near‐infrared domains. The median temporal variables for near‐infrared waveforms are shorter than the values of the UV waveforms.

Multivariate analysis of dim elves from ISUAL observations

AbstractThe Imager of Sprites and Upper Atmospheric Lightning (ISUAL) on the FORMOSAT‐2 satellite, launched in 2004, records Transient Luminous Events (TLEs). ISUAL has an imager and a spectrophotometer that observe TLEs all over the globe. Among these phenomena, elves are particularly difficult to detect. ISUAL often records events that correspond to significant far ultraviolet (FUV) emissions in the spectrophotometer but have no discernible TLEs in the imager. These FUV events are called “dim” elves. Therefore, it is important to develop mathematical tools to analyze the data to obtain a better evaluation of the number of elves and their occurrence. Multivariate approaches are applied to characterize the unlabeled events. The first approach is the principal component analysis which distinguishes two different groups, one including elves and dim elves. The second approach is the PARallel FACtor analysis which provides a waveform model for each group. These methodologies confirm that FUV signal is the evidence of TLE presence. A crude classification method was then suggested taking into account these results. The proportion of elves, relatively to the considered ISUAL data set, is found to be about 40%. It is similar to previous results and confirms that relatively weak lightning peak current is sufficient to produce elves. This new strategy demonstrates the potential for discriminating between lightning and TLEs without prior knowledge within the selectivity of the FUV spectral band.

Energetics and geographic distribution of elve‐producing discharges

AbstractThe major types of transient luminous events (TLEs) are believed to be directly triggered by cloud‐to‐ground discharges. Intense lightning generally seems to have a higher production efficiency for TLEs, but this observation has not yet been statistically investigated. Two data sets, the upgraded World Wide Lightning Location Network (WWLLN) lightning stroke data and the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) TLE data, were used to investigate the energetics and the geographic distribution of TLE‐producing lightning. The global median energy of the strokes that produced the TLEs is at least an order of magnitude higher than the global median stroke energy for WWLLN lightning in the same data window. Furthermore, the energy distributions of the elve‐producing strokes exhibit no oceanic and land disparity. These results reveal that the elves are indeed triggered by energetic lightning and the production efficiency of elves with respect to the stroke energy of the causative lightning was insensitive to the underlying landform. Analysis of the spatial correlation between the ISUAL elves and the WWLLN lightning reveals that the geographic distribution of the ISUAL elves agreed well with that for the most energetic 10% of the WWLLN lightning strokes, better than for total lighting. We also found that elve occurrence rates in the apparently reduced detection regions behind the Earth's limb may have been greatly underestimated, partially due to the failure in providing triggers to initiate ISUAL recording or the severity of atmospheric attenuation to the elve emissions that may have caused them to be undetected.

Globalization extension of transient luminous events from FORMOSAT-2 observation

Since its launch on 20 May 2004, FORMOSAT-2 (FS2) is still the unique satellite to observe the transient luminous events (TLEs) from orbit. In nearly 9 years from 4 July 2004 to 31 May 2013, the number and distribution of TLEs observed are summarized. The instrument used for the observation of TLEs is called imager of sprites and upper atmospheric lightning (ISUAL). This is the first time the observation results are presented in the International Astronautical Congress (IAC). Due to the orbital characteristics of FORMOSAT-2, unobserved gaps exist between each two orbits. Also, the ISUAL has its detection threshold. In this paper, besides the presentation of above results, globalization extension of TLEs is performed based on FORMOSAT-2׳s observation. Factors considered include number of orbits ISUAL in operations, daytime and night-time, season, non-observed areas, observability of TLEs, limb view hiding, and other factors. An estimated global occurrence rate is 24TLEs/min, but this number is still much less than the possible potential rate of 195. To enhance the results, more instruments and longer observation time are required.

Space programs in Taiwan

Taiwanʼs current and future space programs are briefly introduced in this paper. The National Space Organization (NSPO) in Taiwan has successfully carried out three satellite programs (FORMOSAT-1, 2, &3) since its establishment in 1991. FORMOSAT-1 is a scientific satellite performing three scientific experiments for measuring the density, velocity and temperature of ionospheric plasmas, taking the ocean color image, and conducting Ka-band communication experiments. Equipped with a 2m ground resolution remote sensing instrument, FORMOSAT-2 operates in a sun-synchronous orbit with revisit time equal to one day. This unique feature of the daily revisit capability is significantly useful for post disaster assessment and environmental monitoring. FORMOSAT-2 also carries a scientific payload “Imager of Sprites and Upper Atmospheric Lightning (ISUAL)”. ISUAL provides the worldʼs first long-term satellite observations on the lighting phenomenon in the earthʼs upper atmosphere. FORMOSAT-3 is a constellation of six micro-satellites to collect atmospheric and ionospheric data for weather prediction and for climate, ionosphere, and geodesy research. FORMOSAT-3 has demonstrated the ability to significantly increase the accuracy of weather forecasting by utilizing the GPS Radio Occultation (GPS-RO) technique. Currently, NSPO is pursuing the follow-on space missions of FORMOSAT-5 and FORMOSAT-7. FORMOSAT-5 will be the first to utilize a CMOS detector on a high-resolution earth-observation camera. FORMOSAT-7 is a joint mission of Taiwan/US to deploy a 12-satellite constellation operational system to provide dense and timely GNSS RO data to the global communities for real-time weather forecast as well as space science research.

Observations of non-migrating tides and ionospheric perturbations of O(1D) airglow by ISUAL instrument

In this paper, we present the spatial variations of O(1D) airglow observed by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the FORMOSAT-2 satellite. With a CCD camera and a 630nm filter, ISUAL can measure global atmospheric emissions lying between the heights of 80 and 300km. In days of 3–6 September 2008 and 25–27 February 2009, ISUAL has measured the emissions of O(1D) airglow with results showing strong longitudinal peak-3 and peak-4 structures. The Lomb-Scargle analyses for these two cases show periods of longitudes of 120° and 90° supporting the DE2 and DE3 non-migrating tides. The 630nm emissions are enhanced in equatorial regions and are lying along the equator. Over Africa its intensity can sometimes increase up to 80% relative to other longitudes. The perturbation is so strong that non-migrating tides are erased. A case of bimodal distribution with strong emissions at latitudes in equator and mid-latitude in geographic coordinates was observed.

The emission of oxygen green line and density of O atom determined by using ISUAL and SABER measurements

Abstract. Emissions of the 557.7 nm green line airglow observed by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the FORMOSAT-2 satellite in May and November 2008 are studied here to derive the density distributions of the atomic oxygen by using atmospheric parameters from MSISE-00 model and TIMED (Thermosphere Ionosphere Mesosphere Energetics and Dynamics)/SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) measurements. The May observations were made in 10 days from a fixed orbit of longitude (100° E) with the results showing emission rate and O atom density both peaked at heights of about 90 km over 10° to 20° latitudes in the Northern Hemisphere (NH). In the Southern Hemisphere (SH), the emission rate and density of O atom are both low compared with those in NH. In November, the observations were made as the satellite traveled over all 14 orbits around the earth, covering all longitudes and latitudes of 25° S–45° N. Strong peaks of emission rates and O atoms are found at heights of about 95 km in the mid-latitudes in both hemispheres. In the equator, the airglow layer has a weaker emission rate but with higher altitude compared with those of mid-latitudes. In the lower and upper mesosphere at heights below 85 km and above 105 km, there are more O atoms in the equatorial regions than in the mid-latitudes. And there is a good correlation between the O atom and the temperature structure. A comparison with O atom distribution derived from OH airglow observed by TIMED/SABER at about the same time shows similar results.

Further investigations of lightning‐induced transient emissions in the OH airglow layer

A previous study of lightning‐induced transient emissions in and below the OH airglow layer using observations by the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) CCD camera onboard the FORMOSAT‐II satellite showed that intensity enhancements occurred more frequently in the OH airglow layer. Here we show the results of new observations made in December 2009 and January 2010 using a narrowband 630 nm filter and spectrophotometer and present further analysis. We estimated the N21P intensity enhancements to be ∼65% and 53% of the total intensity enhancements for the two events we analyzed using ISUAL and the spectrophotometer data in conjunction with a model for emissions of light and VLF perturbations from electromagnetic pulse sources (elves). Our analysis indicates that there is still somewhat considerable intensity enhancement (∼1.25 kR) unaccounted for after the N21P contribution has been removed. Our study suggests that there might be OH emissions in elves and that OH species might also be involved in the lightning‐induced process and contribute to the intensity enhancements that we observed.

OH Airglow and Equatorial Variations Observed by ISUAL Instrument on Board the FORMOSAT 2 Satellite

OH airglow observed by the ISUAL (Imager of Sprites and Upper Atmospheric Lightning) instrument on board the FORMOSAT 2 satellite is reported in this paper. The satellite is sun-synchronous and it returns to the same orbit at the same local time daily. By using this property, we can study the upper atmosphere in detail. With a CCD camera, ISUAL has measured the emission layers of OH Meinel band at 630 nm for several two-week periods in 2004 and 2007 in equatorial regions. ISUAL images are snapshots of the atmosphere 250 km (height) × 1200 km (horizontal distance). These images of OH airglow are analyzed to derive its peak height and latitudinal variations. ISUAL observation is unique in its capability of continuous observation of the upper atmosphere as the satellite travels from south to north along a specific orbit. However, 630 nm filter also measured O(1D) at 200 km, and there are interferences between O(1D) and OH airglows as as observed from a distance in space. We have studied the overlap of two airglows by simulations, and our final analyses show that OH airglow can be correctly derived with its average peak height of 89 ± 2.1 km usually lying within ±10° latitude about the equator. ISUAL data reveal detailed structures of equatorial OH airglow such as the existences of a few secondary maxima within the equatorial regions, and the oscillations of the peak latitudes. These results are discussed and compared with previous reports.

Discharge processes, electric field, and electron energy in ISUAL‐recorded gigantic jets

This article reports the first high time resolution measurements of gigantic jets from the Imager of Sprites and Upper Atmospheric Lightning (ISUAL) experiment. The velocity of the upward propagating fully developed jet stage of the gigantic jets was ∼107 m s−1, which is similar to that observed for downward sprite streamers. Analysis of spectral ratios for the fully developed jet emissions gives a reduced E field of 400–655 Td and average electron energy of 8.5–12.3 eV. These values are higher than those in the sprites but are similar to those predicted by streamer models, which implies the existence of streamer tips in fully developed jets. The gigantic jets studied here all contained two distinct photometric peaks. The first peak is from the fully developed jet, which steadily propagates from the cloud top (∼20 km) to the lower ionosphere at ∼90 km. We suggest that the second photometric peak, which occurs ∼1 ms after the first peak, is from a current wave or potential wave–enhanced emissions that originate at an altitude of ∼50 km and extend toward the cloud top. We propose that the fully developed jet serves as an extension of the local ionosphere and produces a lowered ionosphere boundary. As the attachment processes remove the charges, the boundary of the local ionosphere moves up. The current in the channel persists and its contact point with the ionosphere moves upward, which produces the upward surging trailing jets. Imager and photometer data indicate that the lightning activity associated with the gigantic jets likely is in‐cloud, and thus the initiation of the gigantic jets is not directly associated with cloud‐to‐ground discharges.

Assessment of sprite initiating electric fields and quenching altitude of a1Πg state of N2 using sprite streamer modeling and ISUAL spectrophotometric measurements

In this study, we compare sprite streamer modeling results with Imager of Sprites and Upper Atmospheric Lightning (ISUAL) spectrophotometric data for several sprite events. The model positive streamers are simulated for two representative magnitudes of the quasi‐electrostatic field produced by cloud‐to‐ground lightning discharges, reflecting conditions at 70 km altitude during the initial stage of sprite formation. The intensity ratio of the second positive band system of N2 (2PN2) to the first negative system of N2+ (1NN2+) is obtained separately from the modeling and the ISUAL measurements. The comparison results indicate that the ratio obtained for the streamer developing in an electric field close to the conventional breakdown threshold field Ek agrees with the ISUAL measurements at the very early stage of the sprite development better than for the streamer developing in a field much lower than Ek. This finding supports the sprite theory proposing that sprites are caused by conventional breakdown of air when the lightning field in the upper atmosphere exceeds the local breakdown threshold field, which has also been supported by a recent study by Hu et al. (2007) comparing modeled lightning fields obtained using measured current moments of causative lightning discharges and video observations of sprites. The fact that the early stage emissions from the sprites under study are well explained by the radiation from streamers in strong fields allows the use of ISUAL data to gain additional information on the poorly known quenching altitude of the N2(a1Πg) state, which is responsible for N2 Lyman‐Birge‐Hopfield band system. The results confirm that the 77 km suggested in previous study by Liu and Pasko (2005) is a good estimate for the quenching altitude of N2(a1Πg).