Dana R. Moudry

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.

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.

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).

New evidence for the brightness and ionization of blue starters and blue jets

Blue jets and blue starters are partially ionized luminous cones of primarily blue light that propagate upward out of the top of thunderstorms at speeds of order 100 km s−1. Blue jets propagate up ∼40 km, but blue starters, which resemble blue jets, terminate abruptly after only a few kilometers of upward travel. Theories on the origin of blue jets have proposed that they are due to either positive or negative streamers or runaway electrons. Quantitative analysis of new multi-instrument observations of a blue starter from an aircraft during the Energetics of Upper Atmospheric Excitation by Lightning, 1998 (EXL98) campaign of July 1998, shows that the ionization accounts for ∼3% of the observed intensity. Quantitative analysis of a remarkable color photograph of a blue jet taken from Reunion Island in the Indian Ocean shows that the minimum optical energy deposition was ∼0.5 MJ. The same photograph shows details of streamers never before seen.

N2(B3Πg) and N2+(A2Πu) vibrational distributions observed in sprites

Abstract A pair of spectra taken simultaneously by two different ground-based instruments has been analyzed by our group. As with previous observations, the spectra are composed primarily of the N 2 first positive group (1PG) ( B 3 Π g − A 3 Σ u + ). In a previous study, we compared the N 2 ( B ) vibrational distributions from the spectral analysis with those resulting from a time-dependent kinetic model of N 2 triplet excited state populations. Both spectra reflect emission between 50 and 60 km . The higher-altitude spectrum is primarily 1PG but also shows the presence of features which appear to be N 2 + Meinel ( A 2 Π u − X 2 Σ g + ). The lower-altitude spectrum shows little or none of the apparent Meinel emission but has an N 2 ( B ) vibrational distribution similar to ones observed in laboratory afterglows. In this paper we discuss the apparent presence of the Meinel emission and present the observed N 2 ( B ) vibrational distributions.

Mesospheric sprite current triangulation

A network of three time-synchronized high-precision induction coil magnetometers is installed in North America to measure sprite-associated lightning flash waveforms in the frequency range 0.1–1000 Hz during the Energetics of Upper Atmosphere Excitation by Lightning, 1998, sprite campaign in July 1998. Simultaneous intensified video observations on board an aircraft are used to investigate 16 sprites with long time delays >33.33 ms relative to the parent lightning discharge reported by the National Lightning Detection Network. Three different long-delayed sprite-associated waveforms can be distinguished: 38% do not exhibit any significant magnetic intensity variation, 25% exhibit slow variations ∼100 ms, and 25% exhibit short pulses ∼4 ms. The source locations of the sprite-associated short pulses are triangulated by use of arrival time difference analysis. One source location exhibits a substantial spatial displacement ∼60 km relative to the parent lightning discharge, in agreement with the azimuths of sprite luminosity edges determined from the corresponding background star field of the video observations on board the aircraft. It is concluded from the temporal and spatial coincidence of the secondary short pulse and the sprite luminosity that this particular sprite is associated with current in the mesosphere.

Near-ultraviolet and blue spectral observations of sprites in the 320–460 nm region: N2 (2PG) emissions

Abstract : A near-ultraviolet (NUV) spectrograph (320-460 nm) was flown on the EXL98 aircraft sprite observation campaign during July 1998. In this wavelength range video rate (60 fields/sec) spectrographic observations found the NUV/blue emissions to be predominantly N2(2PG). The negligible level of N+2 (1NG) present in the spectrum is confirmed by observations of a co-aligned, narrowly filtered 427.8 nm imager and is in agreement with previous ground-based filtered photometer observations. The synthetic spectral fit to the observations indicates a characteristic energy of 1.8 eV, in agreement with our other NUV observations.

Sprite and elve electrodynamics

Abstract Flight 3 of the Sprites99 balloon campaign flew from 00:39:32 to 11:12:00 UTC on 21 August 1999, launched from Ottumwa, Iowa. Three axis electric and magnetic field and ground-based low light level TV observations from three sites were made of more than 68 transient luminous events (TLEs) in the middle atmosphere above thunderstorms in South Dakota and Kansas. At least four TLEs were recorded by two or more stations, including sprites at 0955:36, 0541:58 and 0546:10 UTC and a sprite halo at 0746:35. An event triggered on-board memory, sampling 10 quantities at a rate of 50 kHz per channel for 160 ms per event, recorded bursts for some of the sprites and elves. At the range of the balloon from the lightning and TLE (∼400 km), the vertical electric field perturbation is roughly simulataneous with the light emission. The presence of a radial component of some magnetic pulses suggests that signal propagation was not entirely in the TM mode.

Effects of lightning on the middle and upper atmosphere: Some new results

Abstract Recent observations of the effects of lightning on the middle and upper atmosphere have revealed several new dynamical effects of sprites and yielded initial estimates of the total internal energy, including energy residing in nonradiative vibrational states. High speed images (1000 fps) of sprite development and decay show evidence of modification of the local atmosphere by sprites, which may in turn affect the form of subsequent sprites occurring in the same region. Small, relatively long lived (∼100s ms) statinary “balls” are often observed within the decaying regions of some sprite tendrils. Calculations of diffusion time scales for the balls are roughly consistent with thermal diffusion, but the relatively long lifetime of the balls relative to nearby tendrils suggests that other processes, perhaps chemical in nature, are operative to extend the lifetime of the balls. Other contrasting features within sprites include an invariably spatially diffuse top and highly structured tendril bottoms. Following the suggestion of Williams [2000], we test as a possibly useful plasma parameter the number of electrons per electron-neutral collision volume for being potentially able to account for the clear separation between the highly structured tendril lower region and the diffuse upper regions of sprites. Models of spectrographic measurements of sprites obtained in 1998 sugges that substantial energy resides in nonradiating vibrational states of both molecular nitrogen and oxygen, in addition to the energy associated with the N2(1PG) optical emissions. Estimates of the internal energy of sprites are ∼1 GJ for a large event.