We report intensified high speed video observations of two mesospheric transient luminous events acquired at 5000 and 7200 frames per second. Downward streamers appear to initiate either spontaneously or from brightening inhomogeneities at the bottom of a halo, and branch as they propagate downward. Simultaneously, a brighter column expands upward and downward from the initiation point. This expansion is usually followed by the development of bright upward propagating streamers that originate from the bottom of the expanding bright column and that terminate in diffuse emissions. The lower portions of these upward streamers are typically brighter and more persistent and form the bright core of the sprite. A new phenomenon is observed in which the tips of downward‐moving sprite streamers are attracted to and, in some cases, collide with adjacent streamer channels. The points of streamer collision appear to become long‐persisting sprite beads, which have been suggested previously to affect mesospheric chemistry. Other persistent beads appear to form spontaneously on the downward streamer channels near the lower edge of the bright upper portion of the sprite.

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.

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.

Historically, the process of transient luminous event (TLE) detection has required an alert human observer on a low light level television (LLTV) monitor, either in real time or playback. The 1999 Sprites Balloon Campaign payloads had all-sky upward looking photometers not sensitive to events below the balloons. The photometer data was examined at the time of National Lightning Detection Network (NLDN) lightening strokes to find TLEs that were missed visually. 3602 events were analyzed in 4.1 h of storm time. Threshold current moments of ∼50 kA km for the positive cloud to ground (+CG) TLEs and ∼−5 kA km for negative cloud to ground (−CG) TLEs are found.

A balloon campaign was conducted in summer, 1999, to measure the stratospheric electromagnetic fields associated with sprites. This paper will summarize some of the salient results of this work. The balloon payloads were instrumented with electric field detectors, magnetometers, an upward looking photometer, and other instruments. Ground observations for detection of sprites included low light level TV (LLTV) observations from three sites, Jelm Mt, WY, Bear Mt, SD, and Yucca Ridge, CO. The disagreements between models and these data will be discussed. We have used the photometer data to find TLEs by checking the trace at the times of cloud to ground (CG) strokes reported by the US National Lightning Detection Network (NLDN). In total numbers, the number of −CG transient luminous events (TLEs) (presumably all halos) predominates over the number of +CG TLEs. On the positive side, the TLE events show a current moment threshold. Inclusion of the −CG events that are not seen from the ground raises mesospheric power input estimates by a factor of ∼5–7. Including the mesospheric effect of CGs not associated with TLEs increases this estimate by a factor of 2.

A balloon campaign was conducted in summer, 1999, to measure the stratospheric electromagnetic fields associated with sprites. Ground observations for detection of sprites included low light level TV (LLTV) observations from three sites. Flight 1 flew from Palestine, Texas at 01:14:31 UTC to 09:45:00 UTC on 07/06/1999. Flight 3 of the campaign flew from Ottumwa, Iowa at 00:39:32 UTC to 11:12:00 UTC on 08/21/99. During flight 3, 26 sprite halos associated with positive cloud‐to‐ground (+CG) strokes and 17 −CG sprite halos were observed. Of these, 22 +CG and 12 −CG sprite halos were observed by the ground observatories. Seven of the +CG and all 17 −CG halos were not followed by sprites. Next the balloon data were examined during and after the times of the recorded NLDN strokes during 4.1 hours of data. An additional 88 −CG TLEs were found in the flight 3 data and 56 TLEs (7 +CG, 49 −CG) were found in the flight 1 data. It appears that −CG TLEs, mostly spriteless halos, occurred 5–7 times more often than the +CG TLEs. The halo appears to be a fundamental mesospheric response to lightning.

Waveform monitoring of ELF radio signals in the frequency range of 1–400Hz have been carried out on a routine basis at Syowa station (69.0°S, 39.6°E in geographic coordinates), Antarctica since February, 2000. The main purpose of these observations is to monitor global lightning activity and to locate lightning-induced sprites and elves. The ELF observation system consisting of two search coil sensors (geomagnetic north–south (H) and east–west (D) sensors) was installed at a remote unmanned observatory in West Ongul Island located 5km southwest from Syowa station. As a back up system, the same system was installed near Syowa station in East Ongul Island. Signals from these sensors were digitally sampled at 1000Hz with a GPS time code. On July 4, 2000 during the STEPS (Severe Thunderstorm Electrification and Precipitation Studies) 2000 campaign carried out over the Great Plains in the US, 57 sprite events were observed from Yucca Ridge Field Station (40.7°N, 104.9°W), Colorado, and 53 out of these sprite events had one-to-one correspondence to transient Schumann resonances (SR) detected at Syowa station. The waveforms of these SR are characterized by sharp initial pulses and following damped oscillations. The great circles representing the propagation paths are determined from the Lissojous plots of the H and D magnetic field data of the transient SR. It has been demonstrated that the minimum distance between the great circles and the locations of causative cloud-to-ground (CG) discharges is ∼240km on average. It is thus concluded that the method to determine the propagation paths from Lissajous plots is extremely accurate when we use the Syowa ELF waveform data. Consequently, it would be possible to globally triangulate sprite-inducing CG locations by setting up at least one more observation site with the same system.

Abstract During May–July 2000, the Severe Thunderstorm Electrification and Precipitation Study (STEPS) occurred in the High Plains, near the Colorado–Kansas border. STEPS aimed to achieve a better understanding of the interactions between kinematics, precipitation, and electrification in severe thunderstorms. Specific scientific objectives included 1) understanding the apparent major differences in precipitation output from super-cells that have led to them being classified as low precipitation (LP), classic or medium precipitation, and high precipitation; 2) understanding lightning formation and behavior in storms, and how lightning differs among storm types, particularly to better understand the mechanisms by which storms produce predominantly positive cloud-to-ground (CG) lightning; and 3) verifying and improving microphysical interpretations from polarimetric radar. The project involved the use of a multiple-Doppler polarimetric radar network, as well as a time-of-arrival very high frequency (VHF) lig...

A balloon campaign was conducted in summer, 1999, to measure the stratospheric electromagnetic fields associated with sprites. The balloon payloads were instrumented with electric field detectors, magnetometers, an upward looking photometer, and other instruments. Ground observations for detection of sprites included low light level TV (LLTV) observations from three sites, Jelm Mt., Wyoming, Bear Mt., South Dakota, and Yucca Ridge, Colorado. Flight 3 of the campaign flew from Ottumwa, Iowa at 00:39:32 UTC to 11:12:00 UTC on 08/21/99. A sprite at 0955:36.980 UTC produced a vertical electric field perturbation of ∼0.275 V/m that was similar in time profile to the light emission. There was also a positive azimuthal magnetic pulse of ∼3 nT.

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%.

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.

In order to investigate the spatial and temporal variations of sprites and elves and their spectral structures, we have carried out photometric observations during the SPRITES' 97 campaign using two multi-anode array photometers (MAPs). Each MAP has 5 fields of view arrayed in vertical and a time resolution of 52 μs, which enables us to detect the rapid vertical motion of sprites/elves. Since the emissions of sprites and elves mainly consist of the 1st and 2nd positive bands of N2, the intensity ratio of these bands gives us information on the energy distribution of electrons which excite N2 molecules via collision processes. Thus, one of the MAP instrument with an optical sharp cut filter was used to measure only the N2 1st positive band emissions in the wavelength range of 560 – 800 nm, while the otehr MAP without a filter was used to measure both the N2 1st and 2nd positive band emissions in the range 350 – 800 nm. Comparing the data from these two MAPs, we estimated the relative ratio of the 1st / 2nd positive bands of N2. During this campaign, we obtained 66 events data of sprites or elves with the MAPs. It is found that over the entire region of the head of column-shaped sprites the relative ratio of the 1st / 2nd positive bands of N2 is small at the initial phase of the luminosity enhancements lasting only about 1 ms. In the second luminosity enhancement occurring 1 – 2 ms after the initial phase, the relative ratio of the 1st / 2nd positive bands decreases in the lower part of the head. These facts imply that electrons which excite N2 molecules have higher energy at the initial phase over the entire head, while the energy of electrons is high only at lower altitude in the second enhancement.

Video and photometric observations of a meteor-triggered "jet" event in association with the occurrence of a sprite were collected during the SPRITES '98 campaign. The event raises interest in the question of possible meteoric triggering of upper atmospheric transients as originally suggested by Muller [1995]. The event consisted of three stages: (1) the observation of a moderately bright meteor, (2) the development of a sprite in the immediate vicinity of the meteor as the meteor reached no lower than similar to 70 km altitude, and (3) a slower-forming jet of luminosity that appeared during the late stages of the sprite and propagated back up the ionization trail of the meteor. The event is analyzed in terms of its geometry, its relevance to the meteor, and the implications to existing theories for sprite formation.

Using identical observed meteorology for lateral boundary conditions, the Regional Atmospheric Modeling System was integrated for July‐August 1973 for south Florida. Three experiments were performed—one using the observed 1973 landscape, another the 1993 landscape, and the third the 1900 landscape, when the region was close to its natural state. Over the 2-month period, there was a 9% decrease in rainfall averaged over south Florida with the 1973 landscape and an 11% decrease with the 1993 landscape, as compared with the model results when the 1900 landscape is used. The limited available observations of trends in summer rainfall over this region are consistent with these trends.