David M. Suszcynsky

Photometric measurements in the SPRITES ’95 & ’96 campaigns of nitrogen second positive (399.8 nm) and first negative (427.8 nm) emissions

Abstract We have obtained blue photometric measurements of the N 2 second positive 399.8 nm and the N + 2 first negative 427.8 nm emission from sprites, elves and lightning, along with supporting video images. The pulse width and intensity results for sprites are consistent with those of Suszcynsky et al. (1998) . The red emission from sprites has been independently and unambiguously identified by Hampton et al. (1996) and Mende et al. (1995) as the nitrogen first positive band. The source has been attributed to electron impact excitation from low energy electrons (≈1 eV) in the sprite. The short pulse width of the 427.8 nm and 399.8 nm photometer time traces obtained in this investigation are probably not from the same source that produces the red emission. The results reported here indicate an initial energetic ionizing event sufficient to ionize and excite nitrogen followed by secondary electron processes which give rise to the dominant red emission. The photometer results for elves are consistent with the EMP mechanism suggested by Inan et al. (1996) . The photometer traces obtained for lightning indicate emissions consistent with a ‘continuing current’ as the charge redistributes within the thunderstorm cloud. We find that the ratio of the intensity of the 399.8 nm N 2 (2P) emission to that of 427.8 nm N + 2 (1N) emission can be used to discriminate among sprites, elves and lightning.

Ground-based search for X rays generated by thunderstorms and lightning

A series of ground-based experiments have been performed to investigate gamma ray/X ray count rate increases that are associated with thunderstorm and lightning activity. NaI scintillation detectors were configured in a long-timescale channel to monitor long-term count rate variations (on timescales of 1–104 s) as a function of time, pressure, temperature, humidity, electric field, rainfall, and general thunderstorm activity and in a short-timescale channel that was triggered by an electric field change meter to monitor correlations of individual counts (on timescales of 10−7–10−2 s) with particular phases of a lightning flash. Long-timescale results typically show count rate increases of a few percent to as much as 100% above background levels during thunderstorm activity and are likely due primarily to gamma ray emissions from radon daughter-ion decay as the daughter ions are precipitated to the ground by rainfall. The production of bremsstrahlung X rays by a thunderstorm runaway electron mechanism cannot be ruled out by this data, but the data indicate that if the mechanism is in operation, the bremsstrahlung flux at the ground is at best intermittent and/or barely discernible above background levels. Short-timescale results do not show any evidence for the production of X rays by individual lightning flashes. However, these results are inconclusive since only 10 cases of lightning flashes within a 1 km distance were recorded. The results of the overall study are compared to previous studies which claim positive correlations of count rate increases with thunderstorm and lightning activity. They are also discussed in terms of runaway electron acceleration in thunderstorm electric fields and in terms of the runaway air breakdown mechanism for lightning initiation. A review of the physics of and previous studies of X ray emissions from thunderstorms and lightning is presented in the introduction.

Gamma‐ray emissions observed in a thunderstorm anvil

Balloon-borne gamma-ray and electric-field-change instruments were launched into a daytime summer thunderstorm to evaluate a new experimental design to test hypotheses for the production of transient luminous events (TLE) (eg. sprites, and blue jets) in the mesosphere. While ascending, the instrument triggered many times on the signals from the electric-field-change instrument, recording the gamma-ray background at those times. A greater than three-fold increase in the gamma-ray flux was observed as the balloon descended through a thunderstorm anvil where a strong electric field was suspected to be present. These observations suggest that gamma-ray production in thunderstorms may not be as uncommon as previously believed.

Optical observations of terrestrial lightning by the FORTE satellite photodiode detector

We review data from observations of terrestrial lightning obtained by the FORTE satellite between September 1997 and January 2000. A silicon photodiode detector (PDD) records the intensity-time history of transient optical events occurring within its 80° circular field of view. This field of view corresponds to a circle on the Earths surface having an approximate diameter of 1200 km. We describe the instrument, present examples of the data, explain how the data are screened for false triggers, and review, within the context of previous measurements, the general statistics of peak irradiance, pulse width, and energy associated with the data. We compare the FORTE data with National Lightning Detection Network (NLDN) reported cloud-to-ground (CG) strokes and find that the PDD detection efficiency for these CG strokes is ∼6%. Moreover, we infer that FORTE preferentially detects the in-cloud portion of optical lightning signals. Events having inferred peak powers between 108 and 1012 W and optical energy outputs between 103 and 109 J are observed. From a population of nearly 700,000 events we find that the median peak power and median detected optical energy at the source are estimated to be ∼1×109 W and 4.5×105 J, respectively. These values of source peak power and energy are comparable to previous space-based measurements and consistent with aircraft-based and ground-based measurements. The observed median effective pulse width is about 590 microseconds. Further, the pulse widths for CG strokes, reported by NLDN, are inversely proportional to pulse peak power.

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.

Simulations of lightning optical waveforms as seen through clouds by satellites

We present three-dimensional simulations of photon transport through clouds, specifically designed to address the characteristics and detection of optical lightning waveforms collected by satellites. The model uses a Monte Carlo approach, in which discrete photons are advanced by a standard time step through a distribution of scattering water droplets, whose size and number density distributions are variable. The model is different from previous work, in that it considers both finite and infinite cloud geometries and simulates sources of emission with arbitrary spatiotemporal properties. The model outputs are designed to be directly comparable to data obtained by the FORTE satellite photodiode detector, which records optical waveforms for lightning events with 15 μs resolution. The model treats the light propagation through clouds having a variety of shapes, sizes, and optical depths and constructs the delayed/dispersed/attenuated light curve as seen from arbitrary locations outside of the cloud. We compare the simplest case results to previous models and to data from the FORTE satellite and consider certain special cases such as the signal received from impulses occurring below the cloud. We find that the shape of the cloud and the position of the event within the cloud, rather than the motion or extent of the event itself, are the greatest determinants of the resultant distribution of photons in the sky. We also find that the position of the event within the cloud can be as large a determinant in the apparent attenuation of the signal as the cloud optical depth. We find that the class of FORTE optical waveforms with durations ≥1 ms cannot be accounted for by photon scattering alone, but rather, the intrinsic source duration must itself be quite long, which is not the case for return strokes.

Coordinated observations of optical lightning from space using the FORTE photodiode detector and CCD imager

This paper presents an overview of the coordinated observation of optical lightning from space using the photodiode detector (PDD) and CCD-based imager known as the Lightning Location System (LLS) aboard the Fast On-Orbit Recording of Transient Events (FORTE) satellite. PDD/LLS coincidence statistics are presented and show that both the detected energy density and the detected peak irradiance of optical lightning events are proportional to the number of LLS pixels (pixel multiplicity) which are activated during the event. The inference is that LLS pixel multiplicity is more a function of the detected intensity and horizontal extent of the optical event rather than a direct indicator of the degree of scattering. PDD/LLS event coincidence is also used to improve upon traditional recurrence/clustering algorithms that discriminate against false LLS events due to energetic particles and glint. Energy density measurements of coincident events show that about 4% of the optical energy detected by the broadband PDD appears in the narrowband LLS. This is in general agreement with ground-based measurements and with assumptions incorporated into the design of current and planned CCD-imaging sensors.

Katrina and Rita were lit up with lightning

Hurricanes generally produce very little lightning activity compared to other noncyclonic storms, and lightning is especially sparse in the eye wall and inner regions within tens of kilometers surrounding the eye [Molinari et al., 1994, 1999]. (The eye wall is the wall of clouds that encircles the eye of the hurricane.) Lightning can sometimes be detected in the outer, spiral rainbands, but the lightning occurrence rate varies significantly from hurricane to hurricane as well as within an individual hurricanes lifetime. Hurricanes Katrina and Rita hit the U.S. Gulf coasts of Louisiana, Mississippi, and Texas, and their distinctions were not just limited to their tremendous intensity and damage caused. They also differed from typical hurricanes in their lightning production rate.

Coincident radio frequency and optical emissions from lightning, observed with the FORTE satellite

We present long optical and radio frequency (RF) time series of lightning events observed with the FORTE satellite in January 2000. Each record contains multiple RF and optical impulses. We use the RF signatures to identify the general type of discharge for each impulse according to the discrimination techniques described by Suszcynsky et al. (2000) and reviewed herein. We see a large number of paired, impulsive events in the RF which allow us to study the heights within clouds of several events. We also see that the rate of RF/optical coincidence depends on the type of discharge: nearly 100% of VHF signals from first negative return strokes have an associated optical signal, whereas a mere 50% of impulsive intracloud events appear to have an optical counterpart. While the RF signals from ground strokes clearly coincide with simple optical signals in almost all cases, the intracloud lightning often shows nearly continuous, complicated RF and optical emissions which do not cleanly correlate with one another. The RF and optical pulses do not show a well-defined relationship of intensities, for any lightning type. The observed delay between the RF and optical pulses we interpret as mainly an effect of the scattering experienced by the light as it traverses the cloud. For intracloud lightning, we find no evidence of an intrinsic delay at the source between the onset of the RF and optical signals. Impulsive in-cloud RF events are seen to occur on average every 0.9 ms during a flash.

Secondary electron yields of solar system ices

The secondary electron yields of H2O, CO2, NH3 (ammonia), and CH3OH (methanol) ices have been measured as a function of electron beam energy in the 2- to 30-keV energy range. The ices were produced on a liquid-nitrogen-cooled cold finger and transferred under vacuum to a scanning electron microscope where the yield measurements were made. The imaging capabilities of the scanning electron microscope provide a means of correlating the yield measurements with the morphology of the ices and are also used to monitor charging effects. The yields were determined by measuring the amplified current from a secondary electron detector and calibrating this current signal with the amplified current signal from samples of metals with known secondary electron yields. Each of the measured yields is found to decrease with an increase in energy in the 2- to 30-keV range. Estimates are given for the maximum secondary electron yield Ymax of each ice and the energy at which this maximum yield occurs. Implications for the charging of solar system ice grains are discussed.