Stan Heckman

Sprites, ELF Transients, and Positive Ground Strokes

In two summertime mesoscale convective systems (MCSs), mesospheric optical sprite phenomena were often coincident with both large-amplitude positive cloud-to-ground lightning and transient Schumann resonance excitations of the entire Earth-ionosphere cavity. These observations, together with earlier studies of MCS electrification, suggest that sprites are triggered when the rapid removal of large quantities of positive charge from an areally extensive charge layer stresses the mesosphere to dielectric breakdown.

Criteria for sprites and elves based on Schumann resonance observations

Ground flashes with positive polarity associated with both sprites and elves excite the Earths Schumann resonances to amplitudes several times greater than the background resonances. Theoretical predictions for dielectric breakdown in the mesosphere are tested using ELF methods to evaluate vertical charge moments of positive ground flashes. Comparisons of the measured time constants for lightning charge transfer with the electrostatic relaxation time at altitudes of nighttime sprite initiation (50–70 km) generally validate the electrostatic assumption in predictions made initially by Wilson [1925]. The measured charge moments (Q dS = 200–2000 C-km) are large in comparison with ordinary negative lightning but are generally insufficient to account for conventional air breakdown at sprite altitudes. The measured charge moments, however, are sufficient to account for electron runaway breakdown, and the long avalanche length in this mechanism also accounts for the exclusive association of sprites with ground flashes of positive polarity. The association of elves with large peak currents (50–200 kA) measured by the National Lightning Detection Network in a band pass beyond the Schumann resonance range is consistent with an electromagnetic pulse mechanism for these events.

Regional Differences in Tropical Lightning Distributions

Abstract Observations from the National Aeronautics and Space Administration Optical Transient Detector (OTD) and Tropical Rainfall Measuring Mission (TRMM)-based Lightning Imaging Sensor (LIS) are analyzed for variability between land and ocean, various geographic regions, and different (objectively defined) convective “regimes.” The bulk of the order-of-magnitude differences between land and ocean regional flash rates are accounted for by differences in storm spacing (density) and/or frequency of occurrence, rather than differences in storm instantaneous flash rates, which only vary by a factor of 2 on average. Regional variability in cell density and cell flash rates closely tracks differences in 85-GHz microwave brightness temperatures. Monotonic relationships are found with the gross moist stability of the tropical atmosphere, a large-scale “adjusted state” parameter. This result strongly suggests that it will be possible, using TRMM observations, to objectively test numerical or theoretical predicti...

A survey of thunderstorm flash rates compared to cloud top height using TRMM satellite data

The relationship between cloud height and lightning activity is examined using data from the Tropical Rainfall Measuring Mission (TRMM) satellite. Coincident data from the precipitation radar (PR) and Lightning Imaging Sensor aboard the TRMM satellite are used to examine whether lightning flash rate is proportional to the fifth power of cloud top height. This study is unique in that (1) the relationship between instantaneous rather than maximum storm height and flash rate is obtained and (2) relatively unbiased full data sets for different locations and seasons over the globe are used. The relationship between thunderstorm height and flash rate is nonlinear with large variance. The overall trend shows that flash rate increases exponentially with storm height. Some tall thunderstorms do not have large flash rates, but the reverse situation never occurs. The fifth power dependency that is derived from scaling laws is not inconsistent with, but not necessarily required by, the observed data.

A Diagnostic Analysis of the Kennedy Space Center LDAR Network. 1; Data Characteristics

An analytic framework is developed in which to analyze climatological VHF (66 MHz) radiation measurements taken by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network. A 19 month noise-filtered sample of LDAR observations is examined using this framework. It is found that the climatological impulsive VHF source density as observed by LDAR falls off ∼10 dB every 71 km of ground range away from the network centroid (a 31 km e-folding scale). The underlying vertical distribution of impulsive VHF sources is approximately normally distributed with a mean altitude of 9 km and a standard deviation of 2.7 km; this implies that the loss of below-horizon sources has a negligible effect on column-integrated source densities within a 200 km ground range. At medium to far ranges, location errors are primarily radial and have a slightly asymmetric distribution whose standard deviation increases as r2. Error moments estimated from observed lightning are significantly higher than those from aircraft-based signal generator or analytic estimates. LDAR bulk flash detection efficiency is predicted to be above 90% to 94–113 km range from the network centroid and to fall below 10% at ranges greater than 200–240 km.

High-speed video and electromagnetic analysis of two natural bipolar cloud-to-ground lightning flashes

High-speed video records of two bipolar cloud-to-ground flashes were analyzed in detail. They both began with a single positive return stroke that was followed by more than one subsequent weak negative stroke. Due to the elevated cloud base height of its parent thunderstorm, the preparatory processes of each subsequent negative stroke were documented optically below cloud base. In the first event (Case 1) it was observed that all four subsequent negative strokes were initiated by recoil leaders that retraced one horizontal channel segment previously ionized by the positive leader. Those recoil leaders connected to the original vertical channel segment and propagated toward ground, producing four subsequent strokes that had the same ground contact point as the original positive discharge. The second event (Case 2), in contrast, presented 15 subsequent strokes that were initiated by recoil leaders that did not reach the original channel of the positive stroke. They diverged vertically toward ground, making contact approximately 11 km away from the original positive strike point. These results constitute the first optical evidence that both single- and multiple-channel bipolar flashes occur as a consequence of recoil leader activity in the branches of the initial positive return stroke. For both events their total channel length increased continuously at a rate of the order of 104 m s−1, comparable to speeds reported for typical positive leaders.

Vertical Development of Lightning Activity Observed by the LDAR System: Lightning Bubbles

Abstract In some Florida thunderstorm cells, impulsive very high frequency (VHF) radiation from lightning channels begins abruptly in a layer that is typically 3–6 km in diameter, 1–3 km tall, and initially located just above the freezing level. In 208 cases described here, 58% of the lightning bubbles ascended with a velocity 11–17 m s–1. Of the 1060 1-h periods with lightning that were examined, approximately 10% had one or more ascending concentrations of lightning activity. Often in summer, as one region ascended, a new lightning bubble would abruptly begin near the freezing level. This subsequent region would be horizontally displaced 1–2 km from the starting point of the previous region. In winter, no more than one ascending region was seen in any one storm. A detailed examination of the structure of lightning associated with the rising layers of lightning activity indicates that these layers were composed of negative leaders, which tend to propagate through positive charge. This suggests that the r...

A diagnostic analysis of the Kennedy Space Center LDAR network: 2. Cross‐sensor studies

An analytical framework is developed in which to analyze climatological VHF (66 MHz) radiation measurements taken by the Kennedy Space Center Lightning Detection and Ranging (LDAR) network. A 19-month noise-filtered sample of LDAR observations is examined using this framework. It is found that the climatological VHF source density as observed by LDAR falls off approximately 10 dB every 71 km of ground range away from the network centroid (a 31 km e-folding scale). From this framework it is inferred that the underlying source distribution is likely inverse exponential in amplitude, with a log-slope of 126-147 mW(sup -1/2). The underlying vertical distribution of VHF sources is approximately normally distributed with a mean altitude of 9 km and a standard deviation of 2.7 km; this implies that the loss of below-horizon sources has a negligible effect on column-integrated source densities within 200 km ground range. At medium to far ranges, location errors are primarily radial and have a slightly asymmetric distribution whose first moment increases as range squared. Error moments estimated from observed lightning are significantly higher than those from aircraft-based signal generator or analytic solution estimates, suggesting that timing errors arising from poor signal identification and discrimination may dominate over timing errors arising from nominal sensor resolution. The VHF source properties of individual LDAR-observed flashes are computed and an analytic expression for flash detection efficiency vs. range is derived. This reveals nearly constant flash detection efficiency to 80-94 km range from the network centroid.

Cross-sensor comparison of the Lightning Imaging Sensor (LIS)

The mapping of the lightning optical pulse detected by the Lightning Imaging Sensor (LIS) is compared with the radiation sources by Lightning Detection and Ranging (LDAR) at KSC and the National Lightning Detection Network (NLDN). Flash-based comparisons are done for the 15 August 1998 case including 122 flashes. The temporal and spatial differences are examined. For ground flash, the time difference of the first LDAR source and first LIS event has a mean of 0.23 s and the total duration of the flash has a mean of 0.28 s, compared to 0.56 s by LDAR. The LIS records the subsequent return stroke or K-change component. For cloud flash, the time difference has a mean of 0.2 s and the total duration of the flash has a mean of 0.38 s, compared to 0.44 s by LDAR. The LIS also records cloud flashes at higher altitude. The location differences are about 4 km for cloud flash and 12 km for ground flash.

Waveform analysis of cloud-to-ground flashes as detected by fast e-field antennas and lightning location systems: On the way to precisely estimate the stroke peak current

BrasilDAT network is an EarthNetworks Total Lightning System (ENTLS) that detects both intra-cloud and cloud-to-ground discharges. Multiple time-of-arrival (TOA) sensors send to central processor the whole waveform of every event detected. The EarthNetworks Lightning Sensor (ENLS) operates in a wide range (from 1Hz to 12MHz) and was designed to reduce system noise and to broaden the frequency range. At present, BrasilDAT is composed of 56 sensors covering 10 States of Brazil: RS, SC, PR, SP, RJ, ES, MG, MS, GO, and BA. We expect to conclude the full network deployment by the end of 2013 reaching up to 75 sensors covering almost all Brazilian regions (except the Amazon basin). This work intends to compare the fast e-field (E-fast) waveforms recorded during CHUVA project campaign for CG strokes that struck at a known location with the waveform recorded by several sensors at different distances from the flash. We know that waveform shapes recorded by different sensors at different distances from the flash have not just different amplitudes, but also different shapes. We know that even waveform shapes recorded by different sensors at the same distance from a flash have different shapes. To better understand these differences, we will predict electric fields at one location from measurements at many other locations and compare our predictions to measurements. We expect this to improve our propagation model and our site calibrations, and we expect it to provide a measurement of one part of the BrasilDAT peak current error.