A thunderstorm may look different when seen from above, says S.Ananthanarayanan.

The face of Mother Nature, when she rages with rain, lightning and thunder, can be fearsome when seen from the earth. It can also be picturesque. But what is it like behind the face that we can see?

In a paper in the journal, Science, Torsten Neubert, Nikolai Østgaard, Victor Reglero, Olivier Chanrion, Matthias Heumesser, Krystallia Dimitriadou, Freddy Christiansen, Carl Budtz-Jørgensen, Irfan Kuvvetli, Lundgaard Rasmussen, Andrey Mezentsev, Martino Marisaldi, Kjetil Ullaland, Georgi Genov, Shiming Yang, Pavlo Kochkin, Javier Navarro-Gonzalez, Paul H. Connell, Chris J. Eyles, from Technical University of Denmark, University of Bergen, Norway, University of Valencia, Spain and National Institute for Astrophysics, Bologna, Italy, go into aspects of thunderstorms that are best seen from the fine weather there is well above the storm. Aircraft cannot usually fly high enough to get there, and need to stay away. But the authors of the paper make use of satellite observations, and find that high voltage lightning, during a thunderstorm, powers ultraviolet emissions and even gamma rays, up in the ionosphere.

Thunderstorms, which are also known as electric storms, are marked by lightning and thunder, usually accompanied by winds and rain. They occur in clouds called cumulonimbus, which form in the lower part of the atmosphere (where the temperature falls as one goes higher) but grow vertically, as high as 20 km. Due to processes within clouds, there is separation of particles into lighter, positively charged particles, and heavier, negatively charged particles. The lighter, positively charged part migrates to the top of the cloud, while the lower end is negatively charged.

Very high charges can build up, with differences in millions of Volts, and there is electric discharge – from different parts of a cloud, from one cloud to another or from a cloud to the ground. These discharges are the strokes of lightning and they represent huge transfer of electrical energy. Very high temperatures are reached and the rapid expansion of air is heard as thunderbolts. The electrical discharge leads to ionization, or the splitting of atoms of air into positive and negative parts, and chemical reactions, and the discharge can interfere with radio communication.

While the visual effects of this activity are spectacular, what goes on within the kilometers-high pile of cloud, and then the upper reaches of the atmosphere above the cloud, are hidden from view. There had been isolated references, for example by explorers atop high mountains, above the clouds, who saw flashes of light that rose upwards from the clouds below. Much later, in the 1920s that Scottish physicist C. R. T. Wilson theorised that electrical breakdown should occur in the atmosphere high above large thunderstorms. Highflying aircraft also noticed electrical discharge above clouds but it was only in 1989 that weak luminous flashes above a thunderstorm, at the same time as a cloud to ground lightning stroke, were photographed.

These flashes are reddish-orange flames with hanging tendrils, or circular doughnut shapes, or greenish-blue streaks, many kilometers across and rising 50 to 90 km above the earth’s surface. Lasting just a fraction of a second, these Transient Luminous Events (or TLEs) as they are known have been evocatively named jets, sprites and elves. Blue Jets are seen in the Stratosphere, 10 -20 km high, where the atmosphere starts warming as we go higher. They propel upwards, can rise some 12 km above a thundercloud, and are not associated with a downward lightning stroke. And along with them are flashes that are called trolls, gnomes and pixies. Red Sprites are found 25-55 km above the cloud and their tendrils can hang by 25 km. Elves are doughnut shaped flashes over 250 km across and generated by the current that flows during lightning strikes and are the most frequent TLEs.

In 1994, NASA’s Compton Gamma Ray Observatory, in a 450 km orbit around the earth, discovered flashes of gamma rays coming from the earth’s atmosphere. Terrestrial gamma ray flashes (TGFs) are brief (just a few thousandths of a second) emissions of high-energy photons, and they were observed when the satellites passed over thunderstorms. The energy of the photons reached that of gamma rays, which arise from interaction of high-energy cosmic ray particles with the atmosphere or in nuclear reactions. It was suggested that TGFs were arose from high-altitude TLEs, the paper says, but it was found that their source was within thunderstorm clouds.

Now, it is understood that the high-energy photons are emitted when electrons are accelerated by the gigantic electric fields in thunderclouds or lightning flashes. It is a property of electric charges that they emit electromagnetic radiation, or light, when they are accelerated. Atmospheric gamma rays arise when charged cosmic ray particles are deflected when they pass near the nuclei of atoms in the atmosphere, a process called bremsstrahlung, or braking radiation. The conductivity of air comes more from electrons that are set free by cosmic ray particles interacting with the atmosphere. Electrons, being lightweight, experience high acceleration, to speeds near that of light. In the process, they emit gamma rays, apart from setting free other electrons, which join the torrent.

The team writing in Science used the data from the Atmosphere Space Interactions Monitor, which is on board the International Space Station, the orbiting research facility managed by the US, European, Russian, Japanese and Canadian space agencies. The Monitor observed a TGF produced in the initial stage of a lightning flash, east of Indonesia. The monitor has two x-ray and gamma ray detectors, three ultraviolet and optical photometers, and two optical imaging cameras, all pointed directly downward), the paper says.

The cameras observed and recorded the entire sequence of emissions before and during lightning flashes, in the optical, the ultraviolet and the gamma ray regions, in conjunction with atmospheric data, such as cloud altitudes, from ground based facilities. The timing of the optical, the ultraviolet and TGF activity, measured correct to microseconds, showed how the ultraviolet, X-ray and gamma ray emission, and TLEs, like elves, kept pace with the rising lightning current.

“The TGF occurred at the onset of a lightning current pulse that generated an elve, in the early stage of a lightning flash”, the paper says and “The observations provide evidence that there is a connection between TLEs and TGFs,” the paper concludes. A glimpse ‘backstage’ of thunderstorm, and perhaps an indicator of what is going on at the surface of a distant, cloud covered planet, from the upper atmosphere theatrics that are visible to us!

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