Sprite Initiation Research Articles

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.

Comment on “Computer simulations on sprite initiation for realistic lightning models with higher‐frequency surges” by T. Asano, T. Suzuki, Y. Hiraki, E. Mareev, M. G. Cho, and M. Hayakawa

Comment on “Computer simulations on sprite initiation for realistic lightning models with higher‐frequency surges” by T. Asano, T. Suzuki, Y. Hiraki, E. Mareev, M. G. Cho, and M. Hayakawa

Reply to comment by L. Z. S. Campos and M. M. F. Saba on “Computer simulations on sprite initiation for realistic lightning models with higher-frequency surges”

Reply to comment by L. Z. S. Campos and M. M. F. Saba on “Computer simulations on sprite initiation for realistic lightning models with higher-frequency surges”

Three-dimensional EM computer simulation on sprite initiation above a horizontal lightning discharge

The purpose of this paper is to study the three-dimensional (3D) effects of the source current and the electromagnetic (EM) pulse on the distribution of upper atmospheric electric field and ionization created. A new lightning model has been employed; i.e., we have included a horizontal channel in addition to the conventional vertical channel. The effects of the horizontal lightning channel are summarized as follows: (1) the effect of a vertical channel plays a fundamental role in the whole view of sprite initiation; (2) the position of a sprite is shifted from the position of its parent vertical channel in response to the length of a horizontal channel; and (3) we observe very fine structures with local maxima and minima in the “reduced” electric field (the electric field divided by the neutral gas density). These theoretical inferences are discussed and compared with the observational facts so far reported (such as lateral shift of sprites, morphological difference of sprites, etc.).

A study of the morphology of winter sprites in the Hokuriku area of Japan in relation to cloud charge height

Continuous observations of sprites in the Hokuriku area of Japan were performed from two optical sites during the three winter periods. The purpose of this observation is to study the major effect in the appearance of sprites and in determining the morphology of sprites (columns or carrots). Detailed analysis is performed based on the estimation of the height of −10 °C at the time of sprite occurrence. When the height of −10 °C is lower than 1800 m, the occurrence of sprites is infrequent, and the dominant shape is column. Then when it is increased (1800–3000 m), a new situation takes place, namely the occurrence of sprites is very enhanced and more spectacular shapes like carrots tend to be frequently observed in addition to column sprites. These sprite characteristics are first compared with those of parent lightning in the Hokuriku area and with our latest computer simulations on sprite initiation.

Computer simulations on sprite initiation for realistic lightning models with higher‐frequency surges

Computer simulations on transient luminous emissions in the mesosphere and lower ionosphere have been performed for realistic lightning modelings with fast‐varying current surges (M components) superimposed on the lightning continuing current (CC). The algorithm used here is an electromagnetic (EM) code, which enables us to estimate self‐consistently the reduced electric field, electron density, conductivity, and luminosity as a function of space and time by solving the Maxwell equations. It is found that M components in the CC with small amplitudes, but with a fast‐varying EM effect, can initiate or enhance the occurrence of sprites. Even for a return stroke (RS) without CC, subsequent high‐frequency current variations (like M components) are found to lead to dramatic changes in the sprite occurrence. The physics underlying these changes is studied by means of, e.g., temporal and spatial variations of luminosity, electron density, and conductivity. As the conclusion, the RS is a fundamental agency for spites, but high‐frequency variations as EM effects exhibit an additional essential influence on sprite occurrence. These computational results are used to offer some useful ideas concerning the unsolved problems of sprites and halos, including polarity asymmetry, long‐delay characteristics, and morphological shapes of sprites.

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.

SUPERCELLS AND SPRITES

Over a decade of monitoring mesospheric transient luminous events (TLEs) above U.S. high plains storms confirmed sprites are almost exclusively associated with positive polarity cloud-to-ground lightning (+CGs). Following C. T. R. Wilson's theory proposed in 1925, only those +CGs lowering large amounts of charge to ground should induce sprites. The key metric, the charge moment change, generally must exceed ~600 C km to initiate the electric breakdown at 75 km, which evolves into the sprite. High plains storms generate the highest percentage, the largest average peak current, and highest density of +CGs in the nation. Various storm types generate +CGs, and especially supercells are often dominated by positive strokes. Few sprites observations above supercells have been obtained (and usually during their decaying phase), while thousands of sprites have been imaged above mesoscale convective system (MCS) stratiform regions and some squall lines. During the 2000 Severe Thunderstorm Electrification and Precipitation Study (STEPS), two supercells were examined. One storm contained >90% +CGs, but none exceeded the sprite charge moment change threshold. A second nocturnal supercell did produce sprites from the last two +CGs of the storm as a stratiform region developed, more favorable for significant continuing currents to follow the +CG return stroke. Unexpectedly, three sprites occurring during the most intense phase of the storm were triggered by unusually intense and impulsive +CGs, which lowered sufficient charge in the return stroke alone for sprite initiation. Such +CGs, and thus sprites, are probably relatively rare events during the supercell mature stage.

Computer simulations on the initiation and morphological difference of Japan winter and summer sprites

Two‐dimensional (axisymmetric) computer simulations (electromagnetic code) have been performed to study the initiation and morphological difference (summer, carrots, and winter, columns) of sprites for simulating Japan summer and winter sprites. By changing the physical parameters of a parent lightning, we have found the following findings by looking at the spatial‐temporal distribution of reduced electric field, etc. (1) There are three important factors (the height where the charge is removed (ds), charge transfer (Q), and lightning current risetime (τ)) in the initiation of sprites and their morphological differences. (2) For the initiation of sprites, the charge transfer (Q) should exceed a certain value for possible charge heights (giving us a threshold of charge moment change (Qds) of the order of ∼120–200 C·km), with a combination of small risetime of lightning current waveform. (3) Further, the height for positive charge is much higher than that for negative charge in a typical lightning configuration, which is the essential factor in determining the morphological difference of sprites in summer and winter. Positive charges for summer are located at a much higher altitude than those in winter in Japan, which might result in carrot‐type in summer and columnar‐type in winter even for the same positive polarity. (4) A combinational effect of (Ids) and (Qds) is important for having sprites. Finally, the present computer results on the initiation of sprites for Japanese lightning have been compared extensively to the well‐documented properties of summer continental sprites and future subjects to study have been suggested.

Testing sprite initiation theory using lightning measurements and modeled electromagnetic fields

Previous research has shown that the statistical measurements of charge moment changes in sprite‐producing lightning are in general agreement with the predictions based on the conventional breakdown theory for sprite initiation in the mesosphere. Measurements have progressed to the point where a detailed, event‐level quantitative comparison between the measurements and predictions could more rigorously test the existing theories by estimating the electric fields above the thunderstorm clouds responsible for sprite initiation. We selected for this analysis a set of sprite events from the summer of 2004 whose initiation times are well bounded. Then we measured the current moments and charge moment changes of the parent lightning discharges using the radiated electromagnetic fields recorded by our extremely low frequency/ultra low frequency lightning remote sensing systems. The measured current moments were then used as the input of a two‐dimensional cylindrical full‐wave finite‐difference time‐domain model for lightning‐generated electromagnetic field simulations. We compared the simulated mesospheric electric fields to the threshold electric field for conventional breakdown to see whether, according to theory, conventional breakdown (and thus a sprite) would be initiated by that electric field and at what altitude. By analyzing sprites that are both short and long delayed from the source lightning strokes, we compared measurements and theory across a wide range of timescales. The results show that for bright, short‐delayed sprites, the measurement‐inferred mesospheric electric field agrees within 20% with the threshold electric field for conventional breakdown. However, for long delayed sprite events and dimmer sprites, the measurement‐inferred mesospheric electric field for sprite initiation is somewhat below the threshold for conventional breakdown.

Are lightning M components capable of initiating sprites and sprite halos?

The role of the M component mode of charge transfer to ground in lightning discharges in initiating sprites and sprite halos is examined. M components (surges superimposed on lightning continuing currents) serve to enhance the electric field at high altitudes and, as a result, may increase the probability of sprite (halo) initiation. It appears that occurrence of an M component shifts electric field maximum from the axis of the vertical lightning channel and therefore increases the likelihood of initiation of sprites displaced from the channel axis. Since M components follow return strokes after a time interval of a few milliseconds or more, they may be primary producers of so‐called delayed sprites.

Theoretical criterion of charge moment change by lightning for initiation of sprites

We demonstrate that the timescale of charge removal by lightning is an essential parameter for the criterion of sprites along with the charge moment. Sprites can be initiated under small charge moment changes of several hundreds of Coulomb kilometers if the discharge time is sufficiently short as hundreds of microseconds, whereas they cannot be done even with 1000 C km if the discharge time is in the order of milliseconds. We generalize the criterion as an analytical expression of charge moment and discharge time and discuss its sensitivity to the ambient plasma conductivity based on the result of the classical electrostatic model simulation. Our result can explain almost the critical values of charge moment for sprite initiation recently derived from analysis of lightning‐induced electromagnetic pulses. We deduce from our criterion that charge moment changes necessary for halo are smaller than those for streamers because the former can be initiated without significant ionization. Considering the role of charge moment and discharge time, we discuss further the condition for initiation of various classes such as a halo and a cluster of streamers with or without halo.

Predicting lightning‐driven quasi‐electrostatic fields at sprite altitudes using in situ measurements and a numerical model

Data from the largest electric field change (>140 V/m) above 30 km ever published, correlated with two positive cloud‐to‐ground (+CG) strokes, are compared with an axi‐symmetric quasi‐electrostatic field (QSF) model developed by the authors and based on the model of Pasko et al. (1997a). Using the best fit parameters of this comparison, the electric field change everywhere in the stratosphere and mesosphere is predicted and compared to various electrical breakdown thresholds needed for the initiation and/or growth of sprites.

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.

Global sprite occurrence locations and rates derived from triangulation of transient Schumann resonance events

We estimated locations and rates of sprite occurrences on a global scale using 1–100 Hz ELF magnetic field waveform data obtained at Syowa station in Antarctica and Onagawa observatory in Japan. From the ELF data obtained in a period between June 19, 2001 and January 20, 2002, we identified 715,500 events of transient Schumann resonances (SRs). The locations and polarizations of these parent cloud‐to‐ground (CG) discharges were determined by a triangulation method. The charge moments of these CG discharges were calculated with a normal mode expansion model of SR waves. From these results and the empirical sprite initiation probability reported by Hu et al. [2002], the global occurrence rate of sprites is estimated to be about 720 events/day on average. It is also found that the active regions of sprite occurrences are located in North and South America, Africa and South‐East Asia.

Imaging of elves, halos and sprite initiation at [formula omitted] time resolution

Elves, halos and sprites were observed during August 1999 with a 1 ms high speed imager. The higher time resolution compared to conventional television cameras (17 or 20 ms ) allowed excellent images of the three phenomena temporally separate from each other to be obtained. Analysis of images of elves and halos indicates that the causal lightning-generated electromagnetic pulse and quasi-electro static fields are homogeneous and any small-scale (sub- 10 km ) structure, if visible, is most likely due to a structured atmosphere. Observations of sprites initiated to the side of a halo, without a halo, and from beads left over from a previous sprite, respectively, all suggest sub-pixel (<0.5 km) background structures in atmospheric pressure or composition as being the dominant factors in determining the sprite “seed” location, or site of sprite initiation.

Reply to comment on “Imaging of elves, halos and sprite initiation at [formula omitted] time resolution”

Reply to comment on “Imaging of elves, halos and sprite initiation at [formula omitted] time resolution”

Comment on “Imaging of elves, halos and sprite initiation at [formula omitted] time resolution”

Comment on “Imaging of elves, halos and sprite initiation at [formula omitted] time resolution”

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.

Multi‐color photometric measurements of ionization and energies in sprites

Recent time‐resolved multi‐color photometric data obtained on one class of lightning‐related transient upper‐atmospheric electromagnetic events called sprites have confirmed an impulsive ionization emission during the sprite initiation. Data have also been obtained on some sprites which do not exhibit observable tendrils and which exhibit ionization emission that, if present, is below our detection limit. This suggests that some sprite events exhibit strong ionization while others do not. These results indicate that conditions causing sprite optical emissions are highly variable.