Davis D. Sentman

Preliminary results from the Sprites94 Aircraft Campaign: 1. Red sprites

An adapter for an electrical wall fixture such as a wall light switch having a toggle or other actuator, the adapter serving to protrude the switch outside of the wall outlet box in which it is nested to the extent necessary to project the toggle through an opening in a fabric covering spaced from the wall. The standard switch includes a pair of mounting wings each having a slot therein to receive a primary screw for attaching the wing to the outlet box and a secondary screw which normally serves to hold a standard face plate over the switch to cover the outlet box. The adapter is constituted by a cover plate which overlies the outlet box and is provided with spacer elements to place the surface of the plate against the rear of the fabric covering. The cover plate has an opening to accommodate the toggle, a pair of slots which register with the wing slots and a pair of holes which register with the wing holes, whereby the switch may be secured to the rear of the cover plate by the secondary screws and the assembly of the cover plate and switch secured to the outlet box by the primary screws. The fabric covering is sandwiched between the cover plate and a transparent face plate attached thereto, the face plate having an opening therein which registers with an opening in the fabric covering to permit the toggle to project therethrough.

Observations of upper atmospheric optical flashes recorded from an aircraft

Short-duration upper atmospheric optical flashes were recorded on the night of 8 July, 1993 (9 July UT) from the NASA DC-8 Airborne Laboratory flying over the American Midwest. All-sky video images from an intensified silicon intensified target (ISIT) camera revealed 19 upper atmospheric flashes occurring over a period of ∼100 min. The flashes were similar in appearance to previously reported ground and shuttle-based video observations. Detailed analysis of 12 of the events yielded these parameters: (1) duration ≤17 ms; (2) brightness 10–50 kR, roughly that of bright aurorae; (3) terminal heights 30–100 km, with a mean of ∼60 km; (4) horizontal extent 10–50 km; (5) emission volume >1000 km³. The relative frequency of the optical flashes was (6) 1:200–1:400 compared to negative cloud-to-ground discharges and 1:20–1:40 compared to positive cloud-to-ground discharges.

Simultaneous observations of mesospheric gravity waves and sprites generated by a midwestern thunderstorm

from a central excitation region directly above the storm. During the initial stages of the storm outwardly expanding waves possessed a period of � ≈10 min and wavelength � ≈50 km. Over a 1 h interval the waves gradually changed to longer period � ≈11 min and shorter wavelength � ≈40 km. Over the full 2 h observation time, about two dozen bright sprites generated by the underlying thunderstorm were recorded near the center of the outwardly radiating gravity wave pattern. No distinctive OH brightness signatures uniquely associated with the sprites were detected at the level of 2% of the ambient background brightness, establishing an associated upper limit of approximately A T . 0:5 K for a neutral temperature perturbation over the volume of the sprites. The corresponding total thermal energy deposited by the sprite is bounded by these measurements to be less than ∼1 GJ. This value is well above the total energy deposited into the medium by the sprite, estimated by several independent methods to be on the order of ∼1–10 MJ. c

Triangulation of sprites, associated halos and their possible relation to causative lightning and micrometeors

Sprite halos were recently identified as an impulsive but spatially diffuse phenomenon that sometimes occurs just prior to, but distinct from, sprites. The lack of discernible spatial structure and the temporal development sequence in halos differs markedly from the highly structured bodies and tendrils and the complex development sequences of sprites. However, both phenomena are thought to result from an electric field due to charge moment changes usually associated with large positive cloud-to-ground (CG) lightning but also following negative CG flashes. Three-dimensional triangulations of sprites and sprite halos were made between stations in South Dakota and Wyoming in August 1999 during the NASA Sprites99 balloon campaign. Halos were found to have a Gaussian 1/e diameter of ∼66 km and 1/e thickness of ∼4 km. Comparison with the location of the underlying lightning strokes, as recorded by the National Lightning Detection Network (NLDN), confirms that the horizontal position of sprites may be laterally offset by as much as 50 km from the underlying parent lightning discharge, as has been previously reported. The point of maximum apparent brightness for sprite halos occurs at an altitude of ∼78 km, similar to that of sprites. However, unlike sprites, this point tends to be centered directly above the underlying parent lightning discharge, 4.6 ± 2.7 km mean distance from the center of the halo to the NLDN location. This difference in spatial location relative to the underlying lightning suggests that the electrical breakdown associated with discrete sprites may require a random ionizing event such as a micrometeor. In contrast, sprite halos do not appear to require such a random component.

VLF signatures of ionospheric disturbances associated with sprites

VLF perturbations on signals propagating along great-circle-paths (GCP) through electrically active midwest thunderstorms are associated with luminous high altitude glows (referred to as sprites) observed from aircraft or ground. The data constitutes the first evidence that the physical processes leading to sprites also alter the conductivity of the lower ionosphere.

Imaging of elves, halos and sprite initiation at time resolution

Abstract 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 ( km ) background structures in atmospheric pressure or composition as being the dominant factors in determining the sprite “seed” location, or site of sprite initiation.

Observations of ‘Columniform’ sprites

Abstract This paper reports observations of a distinctive form of sprites associated with positive CG flashes carrying currents of 23 or less to about 100 kA in mesoscale thunderstorms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, and show virtually no variation in brightness along their length. Three dimensional triangulation of what we define as a ‘columniform’ sprite (c-sprite) event on the evening of 19 June 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some show faint diffuse ‘hair’ or tendrils extending above and below the column. The sprite columns are nearly vertical, in video imagery. On some evenings, c-sprites are the dominant form of sprite activity above thunderstorms but, on other nights with many sprites, they may not be observed at all. Comparison of c-sprite forms vs National Lightning Detection Network (NLDN) positive cloud-to-ground current, shows a progression from simple thin vertical forms to brighter and more complicated forms. Theoretical explanations which predict the form and vertical structure of the classical sprites do not at present account for these different forms.

Time resolved N2 triplet state vibrational populations and emissions associated with red sprites

Abstract The results of a quasi-electrostatic electron heating model were combined with a time dependent N2 vibrational level population model to simulate the spectral distributions and absolute intensities observed in red sprites. The results include both N2 excited state vibrational level populations and time profiles of excited electronic state emission. Due to the long atmospheric paths associated with red sprite observations, atmospheric attenuation has a strong impact on the observed spectrum. We present model results showing the effect of atmospheric attenuation as a function of wavelength for various conditions relevant to sprite observations. In addition, our model results estimate the variation in the relative intensities of a number of specific N2 emissions in sprites (1PG, 2PG, and VK) in response to changes in observational geometry. A recent sprite spectrum, measured from the Wyoming Infrared Observatory (WIRO) on Jelm Mountain, during July, 1996, has been analyzed and includes N2 1PG bands down to v′ = 1. In addition to N2 1PG, our analysis of this spectrum indicates the presence of spectral features which are attributable to N+2 Meinel emission. However, due to the low intensity in the observed spectrum and experimental uncertainties, the presence of the N+2(A2Πu) should be considered preliminary. The importance of both the populations of the lower levels of the N2(B3Πg) and the N2(B3Πg)/N+2(A2Πg) population ratio in the diagnosis of the electron energies present in red sprites is discussed. While the current spectral analysis yields a vibrational distribution of the N2(B3Πg) which requires an average electron energy of only 1–2 eV, model results do indicate that the populations of the lower levels of the N2(B3Πg) will increase with increases in the electron energy primarily due to cascade. Considering the importance of the populations of the lower vibrational levels, we are beginning to analyze additional sprite spectra, measured at higher resolution, which contain further information on the population of B(v = 1).

Blue starters : Brief upward discharges from an intense Arkansas thunderstorm

This paper documents the first observations of a new stratospheric electrical phenomenon associated with thunderstorms. On the night of 30 June (UT 1 July) 1994, 30 examples of these events, which we have called “blue starters,” were observed in a 6 m 44 s interval above the very energetic Arkansas thunderstorm where blue jets were first observed. The blue starters are distinguished from blue jets by a much lower terminal altitude. They are bright and blue in color, and protrude upward from the cloud top (17–18 km) to a maximum 25.5 km (83,655 ft.) in altitude. All blue starters events were recorded from two small areas near Texarkana, Texas/Arkansas where hail 7.0 cm in diameter was falling. Comparison to cloud-to-ground (CG) lightning flashes revealed: 1. Blue starters were not observed to be coincident with either positive or negative CG flashes, but they do occur in the same general area as negative CG flashes; 2. Cumulative distributions of the negative CG flashes in ±5 s before and after the starter and within a radius of 50 km shows a significant reduction for about 3 s following the event in the two cells where starters and jets were observed. The energy deficit is approximately 109 J. It is possible that blue starters are a short-lived streamer phenomenon.

Statistical analysis of space-time relationships between sprites and lightning

Abstract We present a detailed statistical analysis of the association of 40 sprite events with lightning from the parent thunderstorm. Both temporal and spatial criteria were used to identify the parent cloud-to-ground (CG) lightning. Sprite images were GPS time stamped and their locations triangulated. In contrast to previous reports of nearly one-to-one association of sprites with positive cloud-to-ground (+CG) lightning, 11 events (27%) did not have a +CG recorded by the National Lightning Detection Network (NLDN), and 7 events (17%) had neither NLDN nor very low frequency (VLF) signatures associated with them. A negative cloud-to-ground (−CG) preceded one of these events by 9 ms . As expected for ∼16.7 ms integrated images, none of the sprites without a +CG had any discernible visual characteristic that would distinguish them from “regular positive sprites”. We have calculated the distribution of time intervals (Δt=tsprite−tlightning) for the sprites that had a parent +CG flash registered by the NLDN or VLF systems, and the distribution of distances between the sprite nadir positions and the flash locations registered by the NLDN. The time interval (Δt) distribution had a peak around 10– 20 ms and a mean of 30 ms (total). This distribution is broadly consistent with the characteristic single electron avalanche time scale associated with streamer growth between ∼70 and 85 km . The distribution of the distances (Δs) between the nadir point of sprites and the parent +CGs showed that approximately two-thirds of the sprites occurred within 50 km lateral displacement from the parent +CG. The parent +CG peak current distribution had a maximum at 40– 50 kA and mean of 60 kA , suggesting that high peak currents (I⩾75 kA ) are not a necessary prerequisite for sprites. The peak current distribution for all +CGs of the storm, with a maximum around 10– 20 kA and mean of 27 kA , exhibits a qualitatively different form from the peak current distribution of the parent +CGs producing sprites.