Transient Luminous Discharges (TLD) produced in the mesosphere and lower ionosphere of the Earth: impact on the ozone chemistry

Transient Luminous Discharges (TLD) are large scale optical events taking place at stratospheric and mesospheric/lower ionospheric altitudes (40 – 90 Km), which are directly connected to the electrical activity in underlying thunderstorms. The possibility of these phenomena was first predicted in 1925 by the Nobel Prize winner C. T. R. Wilson. However, the first image of one TLD was only captured by chance in the USA in 1989. Since then, several different types of TLD above thunderstorms have been documented and classified. Among these, recent results indicate that the so-called SPRITES are weakly ionized plasmas produced upwards following lightning from thunderclouds and lasting very short periods of times (from about 10 ms to 150 ms). It appears from observations using orbiting sensors that TLD occur over most regions of the planet. To date, TLD have been successfully detected from ground and airborne platforms in all continents including Europe. Sprites, considered as weakly ionized plasmas, usually follow intense positive-to-ground lightning discharges. It has been recently demonstrated that Sprites exhibit a highly nonuniform altitude structure characterized by a lower (40-70 Km) streamer-like region and a higher diffuse region (70 – 90 Km). Several theories have been put forward to explain the Sprite phenomenon. They all assume that the quasistatic electric field formed in the stratosphere and mesosphere following a lightning accelerates free seed electrons to energies sufficient to collisionally excite the atmospheric species, but these theories differ in the physical mechanisms involved which relative importance remain nowadays unknown. One of the open questions in Sprite research is whether they directly (or indirectly) influence the dynamics and chemistry of the mesosphere and, perhaps, even the whole Earth’s weather including the ozone cycle. Thus, one active research line nowadays focus in understanding the ability of SPRITE discharges and post-discharge air chemistries to locally produce nitride oxides such as NOx (NO, NO2) and N2O. These oxides are important since they provide catalytic pathways for the destruction of ozone and participate in the oxidation of a number of chemical species in the stratosphere.

In order to clarify the above mentioned issues and other unknowns, an American-Taiwanese consortium designed the first Imager for Sprites and Upper Atmospheric Lightning (ISUAL) scientific instrument launched on board of the FORMOSAT-2 satellite on May 2004. >From that moment, fresh ISUAL measurements, controlled by the American and Japanese-Taiwanese scientific communities, are carried out. Among the very recent results from ISUAL in the open literature, it is very interesting to stress the electric field measurements recently published where a clear transition is shown between the values of the electric field of the diffuse and streamer regions of Sprites. These measurements, which are two or three times smaller than available theoretical predictions in sprite streamers, together with predictions of local electron densities of up to 107 cm-3 in the streamer-like region of Sprites are extremely useful as input data for chemistry models. These models are becoming increasingly important in the present and future Sprite research since, despite the growing amount of data available, no clear understanding has been reached on Sprites’ impact on the chemistry and composition of the mesosphere.

Finally, it is interesting to point out that the European research on TLD and, more specifically on Sprites, started very recently (year 1999) and has been leaded by the Danish Space Research Center.

In this presentation I will give an overview of TLD, mention some topics of present interest, planned space missions related to TLD and comment on the possible impact of TLD on the ozone chemistry (and its kinetic modelling) of the upper atmosphere, an issue of special interest to me.