[1] Observations on the night of 21 July 2003 of the ionospheric effects of a thunderstorm in central France are reported. From 0200 to 0315 UT, a camera system in the Pyrenees Mountains captured 28 sprites, triggered by +CG lightning as observed by the French METEORAGE lightning detection system. A narrowband VLF receiver located on Crete, at $2200 km southeast of the storm, observed subionospheric VLF signals from six ground-based transmitters. The amplitude of one of the VLF signals, originating at a transmitter located $150 km west of the storm and passing through the storm region, exhibited rapid onset perturbations occurring in a nearly one-to-one relationship with the optical sprites. These ''early'' VLF events are consistent with a process of narrow-angle forward scattering from a volume of enhanced ionization above the storm with lateral sizes larger than the VLF radio wavelength. The many +CG and ÀCG discharges that did not produce sprites were also found to not be associated with detectable VLF amplitude perturbations, even though some of these discharges reached relatively large peak currents. The rapid onsets of several of the sprite-related VLF perturbations were followed by relatively long onset durations, ranging from $0.5 to 2.5 s, indicating that these events were early but not ''fast.'' These ''early/slow'' events may suggest a slow process of ionization build-up in the lower ionosphere, following intense lightning discharges that also lead to sprites. A limited number of early VLF perturbation events were also associated with whistler-induced electron precipitation events, or classic Trimpi perturbations, undoubtedly produced by the precipitation of electrons due to whistlermode waves injected into the magnetosphere by the same lightning flash that led to the production of the sprite.

During the Northern Hemisphere summer of 2003 a sprite observation campaign was conducted from Southern Europe and the magnetically conjugate region in South Africa. The campaign brought together a wide variety of instrumentation to investigate the effects of sprites on the mesosphere, and to search for signatures of the relativistic electron acceleration process in the magnetically conjugate hemisphere. Measurements in Europe included optical video imaging from a remote-controlled, semi-automatic camera system located at the Observatoire du Pic du Midi in the Pyre´ne´es mountains in Southern France, infrasound observations from France, and ULF-HF electromagnetic observations from a number of locations. The measurements in South Africa included observations of optical emissions and VLF electromagnetic waves. The campaign was successful, with more than 100 sprites observed during 10 storms. The paper will give an overview of the campaign and some results. They include: (1) the first clear identification of infrasound from sprites, allowing independent (of optical observations) estimates of the energy input to the mesosphere, (2) the first detection of sprites triggered by intra-cloud lightning, a result that underscores the need for considering the complete thunderstorm source field resulting from both cloud-to-ground discharges and the intra-cloud discharges feeding them, and (3) a clear one-to-one relationship with sprites and early VLF events, suggesting that VLF ground transmitter signals can be an important tool for estimating ionisation and relaxation of ionised structures in sprites. No signatures of relativistic electrons were identified during the campaign. r

During the summer of 2003, complementary measurements were taken in Europe during the Sprite2003 campaign to study sprites and associated processes. On July 21, 28 sprites observed by light‐sensitive optical cameras located at the Observatoire du Pic du Midi in the Pyrenees mountains were found to correlate with chirp‐like signals of several tenth to few minutes duration, measured by an infrasound station at Flers about 400 km from the thunderstorm. The sprite activity identified by the infrasound signature continued past sunrise into the daytime when optical systems cannot be used, documenting the occurrence of daytime sprites. Using an acoustic wave propagation model, the observed propagation delays, frequency dispersion, and duration of the infrasound bursts are found to be consistent with source altitudes at 60–80 km with horizontal dimensions equal to the optical width of the sprites. The chirp‐like dispersion is a result of the propagation properties in the earth‐thermosphere waveguide.

Abstract.Results are presented from the first European campaign for observation of sprites, conducted during the summer of 2000 from the French astronomical observatory, Observatoire Midi-Pyrdndes. The primary objective was to establish if sprites are generated over Europe and to identify the characteristics of the associated thunderstorms. During the one-month campaign local weather conditions allowed observations approximately half of the nights. Sprites were observed two nights over the Alps and one night over southeastern France in connection with cold fronts moving in from the Atlantic. In all, 40 sprites were recorded, including dancing sprites, multiple carrot sprites and csprites. The weather conditions were almost identical during the 3 nights, with the active area forming on the front-side of the cold fronts. The storms are not of the same magnitude as active systems often observed over the North American plains. Even so, sprites seem to be a common occurrence also over Europe.