The ACATMOS is the first and, to date, only group in Latin America studying the electrodynamic coupling Atmospheric and Space, Transient Luminous Events signaled by (ELTs), of which the best known are the Sprites, and the High Energy Emissions Storm clouds (EAET) as the Terrestrial Gamma Ray Flashes (FGTS).

Recent discovery of the sprites in 1989 and in 1994 FGTS, phenomena constitute a new area of ??research studied worldwide. The ELTs are observed optical emissions in the upper atmosphere above storm clouds (18-100 km altitude), generated by the electrical activity of these clouds lightning. The high-energy emissions resulting from that same electrical activity are observed from space, with sensors aboard satellites, and soil in the form of gamma rays, X-rays, electron beams, positrons and neutrons, their generation mechanisms still are open.

The ACATMOS was seeded in the Division of Aeronomy INPE DEELUMINOS with project support, funded from 2005 to 2010 by the Program for Young Researchers in Emerging Centers of FAPESP. A total of 4 successful observation campaigns were carried out with the support of this project, over 600 ELTs on storms observed in Brazil, Paraguay and Argentina. At the moment the group participates in the thematic project RAIN, funded by FAPESP. Collect the first sprites and collaboration with RAIN over Rio Grande do Sul early on 18-19/11/2012. We are also developing a project LEONA: Collaborative Network in Latin America for Research Transient Luminous Events Emissions and High Energy Storm.

Transient Luminous Event, TLE, is the generic term attributed to upper atmospheric effects of thunderstorm electrical activity such as sprites, ELVEs, halos, blue jets, gigantic jets and others, associated with lightning discharges (Figure 1). Their optical emissions are observed in the atmosphere directly above thunderstorms using low-light level video cameras located ~150-1000 km from the producing thunderstorm, either on the ground or onboard airplanes, and onboard satellites. The observations are performed at night due to the low luminosity of the phenomena (a few hundred kR to a few MR). They were serendipitously discovered in 1989 when sprites were documented for the first time [Franz et al., 1990].

These observations turned out to be only one of a large class of phenomena and were initially thought to be cloud-to-ionosphere or cloud-to-stratosphere lightning discharges. However, to avoid ascribing physical properties to the events before they were actually known, Sentman et al. [1995a] suggested using an attribute-free name and proposed the neutral term sprite. Following the Franz et al. [1990] discovery, other investigations quickly revealed that sprites are only one of a diverse set of lightning driven optical transients above thunderstorms. In their totality these events span the full vertical extent of the atmosphere from the tropopause (~18 km) to the base of the ionosphere (~100 km). Their existence revealed the electrodynamical coupling of the several atmospheric layers, including the ionized regions, i.e. ionosphere and magnetosphere.


Figure 1 - Illustration in correct scale, using false colors, with the main TLEs known up to date (from TARANIS team).

Sprites are short lived (a few ms to ~100s ms) optical emissions in the Mesosphere above thunderstorms. Initial brightness estimates indicated they were of low luminosity (~100 kR to ~10 MR) based on video observations (30 frames per second – fps), but more recent observations using high-speed cameras (10,000 fps) showed that the brightness of the heads of the streamer plasma channels that form sprites can exceed 1 GR [Stenbaek-Nielsen et al. 2007]. Their red color is predominantly from the first positive bands of N2[Hampton at al., 1996], and the blue in the low portion is due to emissions from the second negative bands of N2. Figure 1 shows the first image of sprites, obtain by Franz et al. [1990], monochromatic, and a color image of sprites showing their real colors, obtained by Sentman et al. [1995a].


Figure 1 - Results that marked the history of the new research area that studies the TLEs. The left image marks the discovery of TLEs [Franz et al., 1990]. The right image is the first color image of sprites, with real colors, marking the beginning of this research area [Sentman et al., 1995a].

Sentman et al. [1995a] performed the first detailed study of these phenomena, which they named. They held a campaign in the central region of United States using monochromatic and color intensified video cameras onboard two aircraft in order to triangulate the sprites geolocation (latitude, longitude and altitude). They recorded the first real color images of sprites and determined their main physical characteristics, i.e. their morphology and physical dimensions. They also determined sprites’ reddish-blue color and the atmospheric region where they occur, providing the first indications of the physical mechanism involved in sprite generation. The work of Sentman et al. [1995a] is a mark that established the investigation of these phenomena as a new scientific research area. Measurements of sprite spectra, performed by Hampton et al. [1996] and partially shown in Figure 2, further revealed that sprites are generated by electron impact ionization and excitation, similarly to auroras.


Figure 2 - The solid line shows the spectrum of a sprite in the region between 600 and 900 nm and the vertical lines show the aurora spectrum. A comparative analysis allowed for the conclusion that the physical mechanism generating sprites is similar to the aurora’s, electron impact ionization and excitation [Hampton et al., 2000].

Sprites are the most spectacular and most studied of the TLEs, they span vertical altitudes of ~30 km to ~100 km, and typically have lateral dimensions of a few 10s m to ~40 km [Gerken et al., 2000; Stenbaek Nielsen et al., 2000]. Sprites are associated with cloud-to-ground (CG) lightning discharges, primarily of positive polarity [Boccippio et al., 1995; Lyons, 1996; São Sabbas et al., 2003]. Currently, the most accepted mechanism for their production is electron impact ionization and excitation, in which ambient electrons in the mesosphere gain enough energy from the transient quasi-electrostatic field of an underlying lightning discharge to ionize the air, creating breakdown, and producing air fluorescence primarily from N2 [Pasko et al., 1997; São Sabbas, 2003]. Recent models focus on the description of the mesospheric plasma streamers that compose the sprites and are expected to develop at the locations where breakdown occurs [Pasko et al, 1998; Raizer et al., 1998; Liu and Pasko, 2004; Luque and Ebert, 2009; da Silva and São Sabbas, 2012].

Halos are horizontal disk-shaped transient optical emissions. Triangulation measurements by Wescott et al. [2001] determined that halos occur centered at ~78 km and exhibit a Gaussian 1/e thickness of ~4 km and 1/e diameter of ~66 km. Halos, which appear red in color and whose initiation mechanism is similar to sprites, can occur as isolated events, as well as preceding or accompanying sprite, therefore are also called sprite-halos. Before halos were identified as a unique type of TLE [Wescott et al., 2001; Barrington-Leigh at al., 2001] they were often (incorrectly) interpreted as ELVEs in imagery data.

Halo diameters varies between 50 and 70 km and their duration is of 2 to 10 ms [Moudry et al., 2003]. They propagate downwards, with an average speed of 3,0 a 6,0×107 m/s, starting from an average altitude of 87 km and going down to an average altitude of 73 km, with the centroid at ~78 km. Figure 1 shows a few examples of halos observed in August 18th, 1999, above a thunderstorm over Nebraska State, USA, by the team of University of Alaska Fairbanks (UAF) Geophysical Institute (GI).


Figure 1 - Examples of halos observed in August 18th, 1999 (monochromatic images). On the third halo one can see the formation of a streamer (indicated by the arrow), a plasma channel that forms the sprites. Extracted from Moudry et al. [2003].

According to the model of Pasko et al. [1997], halos, like sprites, are also produced by quasi-static electric fields (QE) established after the occurrence of positive cloud-to-ground (+CG) lightning. Miyasato et al. [2002], using a vertical photometric arrangement consisting of 16 channels (Figure 2, left), with a resultant field of view of 10,7º×10,7º, observed 89 sprites during several campaigns between 1996 a 1999. Of the 89 sprites observed, 35 presented some diffuse structure. After a detailed analysis they verified that: 26% of the halos were alone, 3% were preceded by an elve, 34% besides being preceded by an elve were followed by sprites, and 37% were followed by sprites without elves. This shows that sprite-halos are as common as, or even more common than, single halos.


Figure 2 - Sequence of events reported by Miyasato et al. [2002]. Left: image of a halo with centroid altitude estimated in ~87 km and a representation of the channel of the photometric arrange. Right: Time evolution of the signal. Extracted from Miyasato et al. [2002].

The acronym ELVE stands for Emission of Light and VLF Perturbation Caused by Electromagnetic Pulse Sources [Fukunishi et al., 1996]. Before halos were identified as a class of TLE, they were often mistaken by elves due to their similar shape in imagery data, even though they are different phenomena. Halos have the same generation mechanism as sprites, electron impact ionization and excitation. Elves, however, as the name says, are produced by the Electromagnetic Pulse (EMP) from cloud-to-ground (CG) lightning, which can be of either polarity [Inan et al., 1997; Chen et al., 2008].

They are characterized by rapid lateral expansion (~3 times the speed of light) of ring-like emissions that are less intense and occur at higher altitudes than halos, at the base of the nighttime ionosphere (~100 km altitude). Elves usually have larger horizontal extension than halos and can reach a few hundred km in diameter. Their optical lifetimes are generally between 0.5 a 5 ms, and their ground apparent luminosity is significantly lower than sprites, when observed from the ground. Figure 1 shows a sequence of images with a faint emission in the central image, which is an elve that preceded the halo shown on the following image.



Figure 1 - Sequence of 1ms images obtained in August 18th, 1999. The first image shows the background atmosphere, the second shows an elve and the third a halo. Extracted from Moudry et al. [2003].

Boeck et al. [1992] identified the first elve above a thunderstorm over the cost of French Guiana in a video tape from an experiment onboard the space shuttle. Only in 1995 there were other successful observations of this phenomenon, which acquired this name after Lyons in this same year Boeck et al. [1998]. Space borne TLE observations performed by the ‘Imager of Sprites and Upper Atmospheric Lightning – ISUAL’ experiment onboard the Taiwanese satellite FORMOSAT-2, shown in Figure 2, reveled that elves are, unexpectedly due to their low apparent luminosity from the ground, the dominant TLE, with an estimated occurrence rate of 35 elves/minute. Sprites and halos have a global occurrence rate estimated in 1 event/minute and only 13 gigantic jets were observed by ISUAL, all in equatorial regions. No blue jet was identified by ISUAL.

ISUAL is the only equipment in orbit so far performing unambiguous observations of TLEs. It covers 45o S to 25o N latitude during the northern summer and 25o S to 45o N latitude during the northern winter [Chen et al., 2008]. Only recently the global TLE distribution started to be known with ISUAL observations. Chen et al. [2008] estimated that the elves impact on the ionospheric total electron content above the active zones to be 5%.


Figure 2 - Global TLE distribution as observed by ISUAL onboard FORMOSAT-2 satellite and global lighting distribution as detected by the OTD and LIS lightning imagers onboard satellites. Notice that ISUAL is turned off when over most of South America due to the South Atlantic Magnetic Anomaly -SAMA. The few events displayed were collected during the initial tests of the equipment and in collaboration with our 2005 TLE campaign. Extracted from Chen et al. [2008].

The first major study of sprites began in the Sprites94 campaign [Sentman et al., 1995], it was in the Sprites94 campaign that the phenomenon received its name. Furthermore, another phenomenon started to be studied from that moment, the blue jets [Wescott et al., 1995]. Blue jets, as the name implies, are jets of bluish color, of ~ 500 kR intensity, escaping the tops of the thunderclouds toward the ionosphere, and lasting ~ 200-300 ms. Westcott et al. [1995] recorded 18 events during the Sprites94campaign. From the data they were able to conclude that the jets have terminal altitude of 40-50 km, that the angle of the "cone" formed by the jet is about 15 ° and that the propagation speed (upwards) is approximately 100 km/h. Figure 1 (left) shows a photograph of a blue jet captured by the Australian photographer Peter Jarver. There are a number of models in the literature that describe the mechanism of generation of blue jets, such as Pasko et al. [1996a], based on the hypothesis that jets blue are positive streamers, propagating upwards, generated by quasi-electrostatic fields. This model predicts the essential characteristics of blue jets: geometry, speed, brightness and terminal altitude. Pasko et al. [1996a] proposed that positive streamers can be generated by the rapid growth of charge on the top of the thundercloud, ie with no association with lightning. This agrees with the observational evidence of the association between blue jets and hail precipitation [Wescott et al ., 1995; and Westcott et al., 1996]. An extension of this model is that proposed by Pasko and George [2002], based on three-dimensional fractal geometry.

The fractal model of Pasko and George [2002] predicts the physical structure, terminal altitude, etc, and agrees with Wescott et al. [1998] experimental evidence that the bluish color of the jets is due to the emissions of the second negative band of ionized molecular nitrogen (N2 +). Figure 1 (right) illustrates Pasko and George [2002] model.


Figure 1 - Left: photograph of a blue jet, extracted from Lyons et al. [2003]. Right: representation of the blue jet initiation model of Pasko and George [2002]. There are two phenomena similar to blue jets reported in the literature: "blue starters" and "gigantic jets" (Figure 2). Blue starters have characteristics similar to blue jets, but are smaller. The average blue starter terminal altitude is about 20 km, half the altitude of a blue jet. When the terminals altitudes distribution of jets and starters were compared, a bimodal distribution, rather than a continuous distribution, was found, which confirms the idea that the blue starters are a different phenomenon, probably related to the initial phase of the jets [Wescott et al., 1996]. Pasko and George [2002] model provides a way to distinguish jets from starters, based on the amount of charge accumulated at the top of the cloud. The gigantic jets also have similar physical characteristics to blue jets, but have incredibly higher altitudes terminals, about 70 km altitude. It is possible that gigantic jets are tropical phenomena due to the low conductivity in these latitudes, allowing the quasi-electrostatic field of the cloud to reach higher altitudes [Pasko et al., 2002].


Figure 2 - Left: black and white image of a blue starter extracted from Wescott et al. [1996]. Right: artificially colored image of a gigantic jet, extracted from Lyons et al. [2003].

Since the dawn of mankind we feel intrigued by how solar and extra-terrestrial energy sources affect our planet. The process by which these energy fluxes interact with the Earth's magnetic field, affecting the outermost layer of the planet, the magnetosphere, and is transferred "downwards" to the Ionosphere, and consequently to the lower neutral atmospheric layers, have captivated researchers for several decades and have become well-established research areas.

In the 60s, the first "upwards" transport mechanism of the energy produced in the lower atmospheric layer, the Troposphere, was discovered in the form of gravity waves. The meteorological thunderstorm systems, a byproduct of solar energy, are the most significant sources of gravity waves, which transport mechanical energy. Recently, in 1989, another "upwards" transport process of thunderstorm energy, this time electrical, was discovered a few years later and called sprite. Since then, new types of electrical transport of the energy to the upper atmosphere and space, associated with thunderstorm, have been discovered every year. They can be assembled into two categories: Transient Luminous Events - TLEs; and High Energy Emissions from Thunderstorm - HEET, such as Terrestrial Gamma Ray Flashes - TGFs, X-rays emissions, electron, positron and neutron beams , observed from satellites, with instrumentation onboard aircraft (Figure 1), and from the ground.


Figure 1 On the right, global distribution of lightning observed by OTD (Optical Transient Detector) onboard a satellite (courtesy Dr. Hugh Christian, MSFC / USA). In the middle, global distribution of TLEs observed by ISUAL (courtesy Dr. Stephen Mende, UCB / USA). Left, global distribution of TGFs observed by RHESSIS (courtesy Dr. David Smith, UCSC / USA). The distribution of lightning, TLEs and TGFs are underestimated or unavailable in South America due to the shutdown of the instrumentation as the satellites pass through the South Atlantic Magnetic Anomaly (SAMA), region of high precipitation of energetic particles in Brazil.

Satellite observations of TLEs and TGFS across the Earth have shown that they are global phenomena with unknown implications in the total energy balance of the planet. The group at the University of Alaska Fairbanks - UAF, USA, pioneered in exploring this global aspect, performing airplane campaigns in Central and South America in 1995. In 2002 we documented the first events from the ground and aircraft over Brazil [Pinto et al., 2004]. Currently, the new observations from space with MEIDEX experiment from the space shuttle Columbia in 2003 [Yair et al., 2004], and the ISUAL imager onboard the Taiwanese satellite FORMOSAT-2, enables the mapping of TLE occurrence rate in the planet. The TLEs signal the continuous occurrence of complex mechanisms of energy exchange in the middle/upper atmosphere, involving the coupling of various geophysical processes such as gravity waves, atmospheric chemistry, plasma physics, interactions between the ionosphere and neutral atmosphere and possible magnetospheric effects. The high-energy emissions by mechanisms associated with thunderstorms, still poorly understood, show the earth's capacity to transform the high extra-terrestrial energy, received continuously, in life on the planet's surface and in low energy processes fairly well understood, and re- send into space pulses with energies comparable to supernova explosions, as the tens of MeV TGFS observed by RHESSIS satellite, for example.

Our country is one of the regions with most intense thunderstorm storm activity worldwide [Zipser et al., 2006], and potentially one of the world regions with higher production of TLEs and TGFS. The total energy deposited in the Mesosphere (~ 40-90 km) by a sprite, the most important of TLEs, is estimated at 10.1 MJ [Sentman et al., 2003]. Photometric measurements determined that the optical emissions are less than 1% of the total, most of the energy is deposited in excited electronic states or non-radiative vibrational and rotational states (i.e. that do not emit light) of N2 and O2, which can initiate chain reactions or catalytic cycles through interactions with minority species that otherwise would not occur. Using a model with dozens of atmospheric species, Sentman et al. [2000] estimated an increase of several orders of magnitude in the density of negative ions, negativos CO3- and CO4- and of the positive hydrated H+(H2O)3 e H+(H2O)2 above thunderstorms (at ~70 km) following a single flash. The effects, which may persist for 10s - 1000s, would be significant above a thunderstorm active for several hours. Armstrong et al. [2001] reported significant increases in the density of NO, a catalytic of the ozone cycle at 60-80 km altitude, modeling ~ 9000 reactions with ~ 400 species, and at ~ 35 km, using satellite observations of UARS HALOE. They estimated that the local production of NO by sprites is more than an order of magnitude larger than the currently accepted global mechanism.

Thousands of sprites/TLEs have been documented in the United States and hundreds have been documented in other countries and continents. Around 800 ELTs (mainly sprites) were documented in Brazil during the six scientific campaigns, international, in 2002-2003, 2005-2008, and 2012, performed up to date. The global rate of sprite occurrence is unknown, but is estimated at 200-25000 sprites/day. The uncertainty, over two orders of magnitude, directly reflects the uncertainty in the rate of energy deposition in the middle/upper atmosphere, ~2x108 – 2.5x1011 J/day, demonstrating the importance of TLEs in the dynamics of the atmospheric system and in the composition of the middle/upper global atmosphere due to the chemical processes generated by them.

In Brazil these effects would be accentuated due to the high incidence of thunderstorms in the country. The presence of the SAMA, currently over the national territory would generate still other studies, completely new, only possible in our region. All this demonstrates the importance of establishing this new scientific research area in Brazil, started in 1999 by the group coordinator, and developed by ACATMOS.

The ACATMOS group was nucleated in the Aeronomy Division (DAE), Coordination of Space and Atmospheric Sciences (CEA), of the National Institute for Space Research – INPE, in Brazil, via the DEELUMINOS project, funded by FAPESP’s Program to Support Young Researchers in Emerging Centers, in the period from 2005 to 2010. The group is currently developing the initial stages of LEONA project, and is awaiting funding approval from FAPESP for its full development. The project, currently under review by FAPESP, aims to create the Collaborative Network in Latin America to Investigate Transient Luminous Events (TLEs) and High Energy Emissions from Thunderstorms (HEET), LEONA. The summary is below.

The group collaborates with FAPESP thematic project CHUVA, Cloud processes of The main precipitation systems in Brazil: A contribution to cloud resolving modeling and to the GPM (Global Precipitation Measurement), in progress. We performed the first ELT observations related to this project in November 2012, on Southern Brazil. The group also collaborates with the French project COBRAT, Coupled Observations from Balloon Related to ASIM and TARANIS, under evaluation by CNES, to perform observations of ELTs and EAETs onboard long and short balloons together with the micro-satellite TARANIS and the experiment onboard the International Space Station ASIM. The launch of the space experiments and balloon flights are planned for 2017.

The ACATMOS group was nucleated in the Aeronomy Division (DAE), Coordination of Space and Atmospheric Sciences (CEA), of INPE via the DEELUMINOS project, funded by FAPESP’s Program to Support Young Researchers in Emerging Centers, in the period from 2005 to 2010. Below is the original summary of the proposal approved by FAPESP.

INVESTIGATION TITLE:

Electromagnetic Energy Deposition in the Upper Atmosphere Signaled by Sprites and other Transient Luminous Effects

PRINCIPAL INVESTIGATOR AND INSTITUTION:

Dr. Fernanda São Sabbas Aeronomy Division National Institute for Space Research – INPE São José dos Campos, Brazil

SUMMARY:

This project aims to implement in Brazil the new area of ​​research that investigates the deposition of electromagnetic energy by electric and electromagnetic fields induced by lightning in the region of Mesosphere-Thermosphere-Ionosphere (MTI). This energy deposition, discovered in 1989, is signaled by the occurrence of Sprites and other Transient Luminous Effects (TLEs) in MTI. Theoretical and observational studies of these processes and their relationship with other phenomena in MTI will be conducted. Work involving computer simulations and observational campaigns in Brazil and abroad in collaboration with researchers from Brazil and abroad are planned, transferring knowledge to the country and strengthening collaboration with countries that carry out research in this area. Sprites are the most significant of these events, because they are the most frequent, intense and complex, occupying the entire region between ~ 40-90 km altitude and reaching ~ 80 km in diameter.

Their optical emissions (N2 1PG) represent less than 1% of the total energy deposited in MTI by a sprite, estimated at ~ 1-10 MJ. Most of it is deposited in excited, vibrational and rotational non-radiative states, triggers of electrochemical reactions generating various chemical species that can have a significant contribution to the overall composition of the middle/upper atmosphere. South America is the region with the second highest occurrence rate of thunderstorm systems in the world, so the implementation of this new area of ​​research in Brazil is strategic not only for the country but for the development of the area worldwide. The PI is the first and only Brazilian with a PhD in the subject, having started this research in the country with her Master thesis, completed at INPE in 1999. With the PhD completed in 2003 under the advisory of the scientist who named the phenomenon Sprite, she gained important experience to the success of the first campaign in Brazil to observe the phenomenon in 2002. Her goal is to insert Brazil in the international scientific community of the area, producing cutting-edge knowledge and conducting research excellence.

The group is currently developing the initial stages of LEONA project, and is awaiting funding approval from FAPESP for its full development. The project, currently under review by FAPESP, aims to create the Collaborative Network in Latin America to Investigate Transient Luminous Events (TLEs) and High Energy Emissions from Thunderstorms (HEET), LEONA. The summary is below.

INVESTIGATION TITLE:

Transient Luminous Event and Thunderstorm High Energy Emission Collaborative Network in Latin America

PRINCIPAL INVESTIGATOR AND INSTITUTION:

Dr. Fernanda São Sabbas Aeronomy Division National Institute for Space Research – INPE São José dos Campos, Brazil

SUMMARY:

This project has the goal of establishing the Collaborative Network LEONA, to study the electrodynamical coupling of the atmospheric layers signaled by Transient Luminous Events - TLEs and High Energy Emissions from Thunderstorms (HEETs). We will develop and install a remotely controlled network of cameras to perform TLE observations in different locations in South America and one neutron detector in southern Brazil. The camera network will allow building a continuous data set of the phenomena studied in this continent. The first two prototype observation units are already installed, in Brazil and Peru. We expect to determine the TLE geographic distribution, occurrence rate, morphology, and possible coupling with other geophysical phenomena in South America, such as the South Atlantic Magnetic Anomaly SAMA.

We also expect to study HEETs in a region of intense electrical activity, measuring thunderstorm neutron bursts for the first time in South America, and therefore addressing one hot open question in Physics since their physical generation mechanism is completely unknown. Additionally, a neutron detector has many more applications, such as to study solar modulation and other atmospheric effects. Using an intensified high-speed camera for TLE observation during 2 campaigns we expect to be able to determine several parameters of the spatial-temporal development of the TLEs observed. The camera was acquired via the FAPESP project DEELUMINOS (2005-2010), which also nucleated our research group ACATMOS, i.e. Atmospheric and Space Electrodynamical Coupling. LEONA will nucleate this research in other institutions in Brazil and other countries in South America, providing continuity for this important research in our region.

The camera network will be a unique tool to perform consistent long term TLE observation, and in fact is the only way to accumulate a data set for a climatological study of South America, since satellite instrumentation turns off in this region to avoid damages due to the energetic particle precipitation in the SAMA. Thus this project is not only a potential benchmark in TLE and HEET research by creating a collaborative network in Latin America and nucleating this research locally, it is also strategic since LEONA’s camera network will be able to provide extremely valuable information to fill up this satellite gap. Plus, we will also attempt to make simultaneously observations of HEETs, TLEs and lightning for the first time ever.

Sprites are large-scale electrical discharges that occur high above thunderstorm clouds, or cumulonimbus, giving rise to a quite varied range of visual shapes flickering in the night sky. They are triggered by the discharges of positive lightning between an underlying thundercloud and the ground. Sprites appear as luminous reddish-orange flashes. They often occur in clusters within the altitude range 50–90 km above the Earth's surface. Sporadic visual reports of sprites go back at least to 1886, but they were first photographed on July 6, 1989 by scientists from the University of Minnesota and have subsequently been captured in video recordings many thousands of times. Sprites are sometimes inaccurately called upper-atmospheric lightning. However, sprites are cold plasma phenomena that lack the hot channel temperatures of tropospheric lightning, so they are more akin to fluorescent tube discharges than to lightning discharges.

Red Sprites and Blue Jets:
 http://elf.gi.alaska.edu/


FMA Research Inc.:
 http://www.fma-research.com/


Stanford VLF group:
 http://vlf.stanford.edu/


ASSIM:
  http://www.space.dtu.dk/English/Research/Projects/ASIM.aspx


TARANIS micro-satellite:
 http://smsc.cnes.fr/TARANIS/
 http://taranis.latmos.ipsl.fr/



COBRAT Project:
 http://www.electricstorms.net/wiki/COBRAT_BALLOON_CAMPAIGN

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