Powerful electric currents have been detected in discharges between thunderclouds and the upper atmosphere. Carried by gigantic jets, they are a new factor in the model of the Earth's electrical and chemical environment.
Although cloud-to-ground lightning is a familiar disruption in the modern electronic world, lightning formed above the clouds is also an important factor in what is known as the global circuit of atmospheric electricity. Radio atmospherics are natural electromagnetic emissions from lightning discharges and can propagate thousands of kilometres through the 'waveguide' formed by the Earth's surface and the ionized region of the upper atmosphere, known as the ionosphere. In 1925, the physicist C. T. R. Wilson wrote1: “The discharges above the cloud would doubtless give rise to atmospherics. If, as has been maintained, atmospherics frequently originate in regions of rain unaccompanied by thunder, they may in such cases be due to discharges of this nature.”
Wilson was right: on page 974 of this issue2, Su and colleagues report their observations of several large-scale discharges, branching upwards to an altitude of about 90 km from the top of thunderclouds in the South China Sea; no associated lightning discharges were detected in the underlying thunderstorm. Radio atmospherics recorded at remote locations in Japan and Antarctica, however, showed that there was a significant flow of current that moved several tens of coulombs of negative charge upwards from the thundercloud to the lower ionosphere.
Although eyewitness reports of events such as these — known as 'transient luminous events', or TLEs — have been recorded for more than a century, the first image of one was captured3 only in 1989, serendipitously during a test of a low-light television camera. Since then, several different types of TLEs above thunderclouds have been documented and classified4,5,6 (Fig. 1, overleaf). They seem to be fairly common in most regions of the globe, appearing over North, Central and South America, Europe, Australia, Japan and China. TLEs are known to be associated with large volumes of ionization, creating electrical paths through the atmosphere7 and causing significant perturbations of long-range communication signals8. But the effects of this ionization and its associated currents on the Earth's electrical and chemical environment are not fully understood.
In a simplified picture of the global electrical circuit, the Earth's surface and the conducting atmosphere above it can be imagined as plates of a giant spherical capacitor, with a potential difference of about 300,000 volts between them9. There are many components contributing to the balance of potential between the plates, but two are critical: thunderstorms, of which there are about 2,000 globally at any given time and which act as batteries charging the capacitor; and fair-weather regions, in which the capacitor can discharge continuously through the weakly conducting atmosphere, with a global leakage current of about 1 kiloampere.
The upper plate of the capacitor is not confined to a single level, but rather is distributed through the atmosphere, reflecting changes in atmospheric conductivity and the fair-weather electric field (so that the density of the leakage current in fair-weather regions remains roughly constant with altitude). In fact, most of the 300,000-volt potential drop between the capacitor plates happens within a few tens of kilometres of the Earth's surface. For instance, 'blue jets' — TLEs that terminate at altitudes of around 40 km (Fig. 1) — probably move some charge to the upper plate of the capacitor. But no associated radio atmospherics have been detected for these events, probably because they take much longer to develop than the more impulsive jets reported by Su and colleagues.
Atmospherics of the strength recorded by Su et al. have previously been observed only in conjunction with the most powerful cloud-to-ground lightning discharges and the TLEs triggered by them, known as 'sprites'10. The authors admit that there may be a slight chance that the atmospherics they detected were associated with cloud-to-ground lightning discharges in the underlying thunderstorm — but then these discharges must have been repeatedly missed by the local lightning detection network, which is unlikely. However, it is clear that the events observed by Su et al. are very different from sprites, which typically start at altitudes of about 70 km and propagate downwards: the gigantic jets seen by Su and colleagues branch upwards from thunderclouds, spreading to a diameter of about 40 km at an altitude of 85–90 km (Fig. 1).
The ionization created by a gigantic jet is likely to have a significant chemical effect on that volume of atmosphere. In fact, the occurrence and dynamics of many TLEs, including those observed by Su et al., closely resemble the behaviour of 'streamers' — miniature needle-shaped filaments of ionization, commonly observed when an electric field is applied to a small volume of relatively un-ionized ambient air at ground pressure. Streamer discharges can lead to significant power losses on high-voltage transmission lines and can damage insulating materials; a streamer plasma of hot electrons embedded in cooler air is a good source of highly reactive species for use in the chemical treatment of hazardous and toxic pollutants11. Because streamer filaments have high electric fields around their tips, streamer plasmas can easily generate electrons with sufficient energies to dissociate atmospheric oxygen molecules. The dissociation initiates a chain of reactions that leads to the formation of ozone in air (this process has been used for industrial ozone production for more than a century11).
As atmospheric pressure is much lower at ionospheric altitudes than at the Earth's surface, streamers that would have diameters of a fraction of a millimetre at ground level instead appear as channels of glowing plasma that are many kilometres long and a hundred metres in diameter — easily observable above thunderclouds by low-light imaging systems deployed hundreds of kilometres away12. High-altitude streamers also have the ability to produce highly active chemical species and can effectively 'treat' thousands of cubic kilometres of atmosphere. The branching observed in atmospheric TLE discharges, including those documented by Su and colleagues, is also known in ground-level streamers (and is recognized as an important parameter for effective chemical treatment of large gas volumes13). So the known chemical impact of streamers may be a good indication that TLEs noticeably affect the chemistry of the atmosphere.
This field is in its infancy, and it remains to be seen how important the electrical and chemical effects of the gigantic jets and other TLEs are for our planet. The large quantities of negative charge transported by these jets, discharging the atmospheric capacitor, may have a strong influence on the voltages and currents in the global electric circuit. The events seen by Su et al.2 seem to be a property of oceanic thunderstorms, and a global survey of these and other types of TLEs is planned, using Earth-orbiting sensors6,14. Knowing how frequently these events occur will help us to understand their contribution to the global electrical circuit.
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