The conjugate sprites project is a valiant first attempt to detect evidence of energetic electron beams produced via avalanche acceleration of MeV electrons by intense quasi-static fields produced immediately after positive cloud-to-ground discharges. The runaway electron acceleration process has recently been suggested as the physical mechanism by which terrestrial gamma-ray flashes are produced [for background and references, see: LEHTINEN THESIS]. The presence of intense quasi-static fields during the tens of milliseconds following intense lightning discharges is evidenced by the observations of gigantic luminous high altitude optical emissions known as sprites [for references and discussion see: BARRINGTON-LEIGH THESIS]. Sprites are optical flashes in the mesosphere above thunderstorms generated by quasi-electrostatic electric fields in the mesosphere following primarily positive cloud-to-ground (+CG) lightning discharges. While anecdotal reports of sprites date back to the 18th century, it is only within the last decade that sprites have been observed and studied by the scientific community. Figure 1 illustrates the relationship between sprites and other types of high-altitude optical emissions recently identified to the thunderstorm below.

Several theories have been proposed for the generation of sprites. Common for them all are that free electrons somehow are created and accelerated in the electric field to energies that allow collisional excitation of atmospheric neutral constituents. They can be categorized according to the energy of the free electrons driving the process. It ranges from a few eV needed for excitation of 1st positive bands, over a few tens of eV needed to create classical avalanche ionization, to relativistic energies, where the breakdown electric field threshold is a factor 10 smaller than for classical breakdown [for extensive discussion and references, see: LEHTINEN THESIS].

While the relative importance of the above processes is unknown, there are indications, that relativistic electrons in fact are generated in sprites. The evidence is still incidental, and relates to bursts of gamma-rays from the atmosphere above active thunderstorm regions observed by the Compton Gamma Ray Observatory. The gamma-ray emissions are believed to be produced via brehmsstrahlung from relativistic electrons driven upward above thunderstorm cells. Because gamma-ray emissions are absorbed in the denser lower atmosphere, they are difficult to detect, even from a spacecraft in low earth orbit.

However, theoretical models indicate that beams of relativistic electrons, if generated, may travel upward into the magnetosphere along magnetic field lines and to the magnetically conjugate atmosphere, where a portion of the electrons will precipitate, generating bursts of optical emissions [see Lehtinen et al., in press, at Journal of Geophysical Research]. Thus observations of optical emissions in the upper atmosphere at conjugate points provides an attractive means of establishing the presence of such electron beams. With this background, researchers from the United States, Europe and South Africa have organized the Conjugate Sprites Campaign, to detect, for the first time, evidence of relativistic electron beams generated above thunderstorms in connection with sprites or and/or intense positive lightning discharges. The experiment involves video observations of sprites in Europe together with simultaneous photometric observations of optical emissions expected to be produced in the conjugate region by the downcoming (precipitating) energetic electrons after they have traversed along the field line. Discovery of such beams would have important implications concerning the sources of energetic radiation belt electrons in the inner magnetosphere as well as leading to better understanding of the electrodynamics of the mesosphere above active thunderstorm regions.

Since the first accidental observations over the central plains of the United States, sprites have been over over Japan, Australia, New Zealand and South America, from both space and ground. Most recently, sprites were observed for the first time over Europe [Allin and Neubert, 2000] during the EuroSprite2000 campaign. Optical observations were taken by a low-light-level video camera from the French astronomical observatory Observatoire Midi-Pyrenees (OMP) located in the Pyrenees. The primary objective was to establish if sprites are generated over Europe and to study the thunderstorm characteristics needed for sprite generation. During the one month campaign, sprites were observed two nights over the Alps and one night over South-Eastern France in connection with cold fronts moving in from the Atlantic. Local weather conditions allowed observations approximately half of the nights. Over the Alp region, one night gave 16 sprites, with 1 sprite per 7 min on the average, and the other 12 sprites with 1 per 3 min. For the South-Eastern France area, the numbers are 12 sprites and 1 per 5 min. The weather conditions were almost identical during the 3 nights, with the active area forming on the front-side of the cold front.

The region where sprites were observed in 2000 is magnetically conjugate to South Africa. Relativistic electrons generated in connection with intense positive lightning discharges which produce sprites over southern Europe are expected to travel along magnetic field lines to the conjugate region, where some precipitate into the atmosphere, and other will mirror back. Those that precipitate will generate optical emissions, just as downcoming auroral electrons at higher latitudes. One team will identify sprites in southern Europe, while another will look for optical signatures from relativistic electrons precipitating over South Africa. This experiment is most conveniently performed between Europe and South Africa because the conjugate regions both are over land. The conjugate regions to the US plains, where sprites are observed quite commonly, lies in the South Pacific. Figure 2 shows a map of the magnetic latitude and longitude. The location of last years sprite observations were in the Alp region and southern France which are magnetically conjugate to South Africa. It can be seen from the various displays that South African Astronomical Observatory (SAAO) is in an excellent location for observations.

During the campaign, scientists from Stanford/Lockheed-Martin and Danish Meteorological Institute will be located at the OMP equipped with instrumentation to identify time and approximate location of sprites. The team will be in contact with the colleagues from Stanford and South Africa located at the SAAO, advising them on the coordinates for relativistic electron precipitation.

The Conjugate Sprites Campaign will be carried out during July 15, 2001 and August 15, 2001 where most thunderstorm activity occurs in the region of interest.

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