[1] The Optical Transient Detector (OTD) is a space-based instrument specifically designed to detect and locate lightning discharges as it orbits the Earth. This instrument is a scientific payload on the MicroLab-1 satellite that was launched into a 70°inclination low Earth orbit in April 1995. Given the orbital trajectory of the satellite, most regions of the Earth are observed by the OTD instrument more than 400 times during a 1 year period, and the average duration of each observation is 2 min. The OTD instrument optically detects lightning flashes that occur within its 1300 Â 1300 km 2 field of view during both day and night conditions. A statistical examination of OTD lightning data reveals that nearly 1.4 billion flashes occur annually over the entire Earth. This annual flash count translates to an average of 44 ± 5 lightning flashes (intracloud and cloud-to-ground combined) occurring around the globe every second, which is well below the traditional estimate of 100 fl s À1 that was derived in 1925 from world thunder day records. The range of uncertainty for the OTD global totals represents primarily the uncertainty (and variability) in the flash detection efficiency of the instrument. The OTD measurements have been used to construct lightning climatology maps that demonstrate the geographical and seasonal distribution of lightning activity for the globe. An analysis of this annual lightning distribution confirms that lightning occurs mainly over land areas, with an average land/ocean ratio of $10:1. The Congo basin, which stands out year-round, shows a peak mean annual flash density of 80 fl km À2 yr À1 in Rwanda, and includes an area of over 3 million km 2 exhibiting flash densities greater than 30 fl km À2 yr À1 (the flash density of central Florida). Lightning is predominant in the northern Atlantic and western Pacific Ocean basins year-round where instability is produced from cold air passing over warm ocean water. Lightning is less frequent in the eastern tropical Pacific and Indian Ocean basins where the air mass is warmer. A dominant Northern Hemisphere summer peak occurs in the annual cycle, and evidence is found for a tropically driven semiannual cycle.

Observations have been made of a new terrestrial phenomenon: brief (-millisecond), intense flashes of gamma rays, observed with space-borne detectors. These flashes must originate at altitudes in the atmosphere above at least 30 km in order to be observable by orbiting detectors aboard the Compton Gamma-Ray Observatory (CGRO). At least a dozen events have been detected over the past 2 years. The photon spectra from the events are very hard and are consistent with bremsstrahlung emission from energetic (MeV) electrons. The most likely origin of these high energy electrons, while speculative at this time, is a rare type of high altitude electrical discharge above thunderstorm regions. 3 / 9 3 0064142https://ntrs.nasa.gov/search.jsp?R=19960001309 2018-05-11T01:44:02+00:00ZWe report here the serendipedous detection of high-energy photons from the Earth's upper atmosphere, observed by the Burst and Transient Source Experiment1 (BATSE) on the CGRO.Their apparent correlation with storm systems leads us to implicate as their cause, electrical discharges from these systems to the stratosphere/ionosphere. Runaway discharges to the ionosphere had been predicted in the early literaturG3 and modeled in detail pre~iously.~ These gamma-ray events may also be related to recently recorded optical discharge phenomena above thunderstorms5 and to other cloud-to-stratosphere discharges that have been reported in the past.6-7The Compton Observatory was launched in April 199 1 to perform observations of celestial gamma-ray sources. The BATSE experiment1 is one of four experiments on the observatory. It serves as an all-sky monitor and has detected over 800 cosmic gamma-ray bursts, several hard xray transients, numerous persistent and pulsed hard x-ray sources and several thousand solar flares. In addition to these celestial sources, on m occasions BATSE has responded to gammaray flashes from the Earth, previously unreported.BATSE consists of an array of eight detector modules located at the corners of the observatory, arranged to provide maximum unobstructed sky coverage. The scintillation detectors are sensitive to photons with energies above 20 keV. It is believed that prior instrumentation and experiments were incapable of detecting this phenomenon for several reasons, or these events were overlooked as being spurious. Most detectors used in high-energy astronomy are collimated and would likely have missed these rare events andor data are not analyzed during Earth-viewing times. Also, the temporal resolution of most experiments would not have been able to respond to these very brief events and would thus have had p r signal-to-noise when sampled with coarser time resolution. The BATSE array of multiple, independent detectors viewing different directions gives us confidence in the reality of these events as opposed to some instrumental or spacecraft effect such as electronic noise. The multiple, wide-field detectors also allow a direction determination to be made for each events The observed counting rate ratios of the detec...

Previous total lightning climatology studies using Tropical Rainfall Measuring Mission (TRMM) Lightning Imaging Sensor (LIS) observations were reported at coarse resolution (0.5°) and employed significant spatial and temporal smoothing to account for sampling limitations of TRMM’s tropical to subtropical low-Earth-orbit coverage. The analysis reported here uses a 16-yr reprocessed dataset to create a very high-resolution (0.1°) climatology with no further spatial averaging. This analysis reveals that Earth’s principal lightning hotspot occurs over Lake Maracaibo in Venezuela, while the highest flash rate density hotspot previously found at the lower 0.5°-resolution sampling was found in the Congo basin in Africa. Lake Maracaibo’s pattern of convergent windflow (mountain–valley, lake, and sea breezes) occurs over the warm lake waters nearly year-round and contributes to nocturnal thunderstorm development 297 days per year on average. These thunderstorms are very localized, and their persistent development anchored in one location accounts for the high flash rate density. Several other inland lakes with similar conditions, that is, deep nocturnal convection driven by locally forced convergent flow over a warm lake surface, are also revealed. Africa is the continent with the most lightning hotspots, followed by Asia, South America, North America, and Australia. A climatological map of the local hour of maximum flash rate density reveals that most oceanic total lightning maxima are related to nocturnal thunderstorms, while continental lightning tends to occur during the afternoon. Most of the principal continental maxima are located near major mountain ranges, revealing the importance of local topography in thunderstorm development.

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[1] It is generally believed that a strong updraft in the mixed-phase region of thunderstorms is required to produce lightning. This is the region where the noninductive charging process is thought to generate most of the storm electrification. Analytic calculations and model results predict that the total lightning frequency is roughly proportional to the product of the downward mass flux of solid precipitation (graupel) and the upward mass flux of ice crystals. Thus far this flux hypothesis has only been tested in a very limited way. Herein we use dual-polarimetric and dual-Doppler radar observations in conjunction with total lightning data collected in Northern Alabama and also Colorado/ Kansas during two field campaigns. These data are utilized to investigate total lightning activity as a function of precipitation and nonprecipitation ice masses and estimates of their fluxes for different storm types in different climate regions. A total of 11 storms, including single cell, multicell, and supercell storms, was analyzed in the two climatologically different regions. Time series of both precipitation and nonprecipitation ice mass estimates above the melting level show a good relationship with total lightning activity for the 11 storms analyzed (correlation coefficients exceed 0.9 and 0.8, respectively). Furthermore, the relationships are relatively invariant between the two climate regions. The correlations between total lightning and the associated product of ice mass fluxes are even higher. These observations provide strong support for the flux hypothesis.

[1] We report the observation with the North Alabama Lightning Mapping Array (LMA) related to a terrestrial gamma-ray flash (TGF) detected by RHESSI on 26 July 2008. The LMA data explicitly show the TGF was produced during the initial development of a compact intracloud (IC) lightning flash between a negative charge region centered at about 8.5 km above sea level (−22°C temperature level) a higher positive region centered at 13 km, both confined to the convective core of an isolated storm in close proximity to the RHESSI footprint. After the occurrence of an LMA source with a high peak power (26 kW), the initial lightning evolution caused an unusually large IC current moment that became detectable 2 ms after the first LMA source and increased for another 2 ms, during which the burst of gamma-rays was produced. This slowly building current moment was most likely associated with the upward leader progression, which produced an uncommonly large IC charge moment change (+90 C·km) in 3 ms while being punctuated by a sequence of fast discharge. These observations suggest that the leader development may be involved in the TGF production.

[1] This study uses TRMM lightning and radar observations to study the fundamental relationship between precipitation ice mass and lightning flash density. The results indicate that the physical assumptions of precipitation-based charging and mixed phase precipitation development are robust and that on a global scale, the relationship between precipitation ice water path and lightning flash density is relatively invariant between land, ocean and coastal regimes. Hence lightning data may be a useful variable for inclusion in combined space borne algorithms designed to retrieve ice water content.

[1] We describe the clustering algorithm used by the Lightning Imaging Sensor (LIS) and the Optical Transient Detector (OTD) for combining the space-based observations of lightning pulse data into events, groups, flashes, and areas. Events are single pixels that exceed the LIS/OTD background level during a single frame (2 ms). Groups are clusters of events that occur within the same frame and in adjacent pixels. Flashes are clusters of groups that occur within 330 ms and either 5.5 km (for LIS) or 16.5 km (for OTD) of each other. Areas are clusters of flashes that occur within 16.5 km of each other. The flash data from LIS/OTD are currently being used for lightning and thunderstorm processes and climatological studies; therefore we test how variations in the algorithms for the event-group and group-flash clustering affect the flash count for a subset of the LIS and OTD data. We divided the subset into areas with low (1-3), medium (4-15), high (16-63), and very high (64+) flash counts to see how changes in the clustering parameters affect the flash rates in these different sizes of areas. We found that as long as the cluster parameters are within about a factor of two of the current values, the overall flash counts do not change by more than about 20%. Therefore the flash clustering algorithm used by the LIS and OTD sensors are robust and create flash rates that are relatively insensitive to reasonable variations in the clustering algorithms.