Lightning events are very spectacular phenomena on Earth. They are one of those beautiful plays of nature, which interest both scientists and non-scientists equally. But can lightning occur on objects outside our Solar System? The St Andrews student Rachel Bailey studies the “scales that large-scale gas discharges can develop, what atmospheric volume might be affected, and what amount of energy may be deposited into the atmospheres of brown dwarfs and planets” under the supervision of Dr Christiane Helling. This study was performed in collaboration with Gabriella Hodosán, Camile Bilger and Craig Stark.
Atmospheric electrical discharges (like lightning) are observed not only on Earth. Lightning on Jupiter was observed both in optical and radio wavelengths of the electromagnetic spectrum. In the late ‘70s early ‘80s the Voyager 1 spacecraft recorded impulsive events in the radio band which were called SEDs or Saturnian Electrostatic Discharges. These events were identified as lightning discharges, although the optical confirmation did not come until 2009. Electromagnetic signatures associated with lightning activity were also detected on Uranus and Neptune by Voyager 2.
Not all processes involved in lightning are known in detail. The simplest idea is the following: first, a strong electric field needs to be present for long enough. This electric field, or potential difference, builds up by various ionisation processes. Processes like particle encounters with cosmic rays would make the electrons break away from their ‘parent’ atom or molecule, which leads to the formation of negative and positive ions and the atmospheric gas becomes conductive. Second, a large-scale separation of these charges over large enough distances is needed. One process causing large-scale charge separation is gravitational settling (also known as‘rain’). If the distance between the charged cloud layers is large enough, the electric field can grow so strong that it exceeds a threshold, which results in the acceleration of electrons to very high energies. These energetic electrons will ionise their surroundings by freeing more electrons resulting in an avalanche of high-energy electrons and, as a final step, a lightning discharge. This process is called runaway breakdown.
Other interesting phenomena related to lightning activity are Sprites. Sprites appear above thunderclouds as extended red discharges, right after a lightning strike. These luminous events on Earth can be observed from space.
It is very likely that lightning also occurs outside the Solar System. Both exoplanets and brown dwarfs host clouds, which are made of minerals or gemstones. Why is it important to know how extended and energetic lightning events are on extrasolar bodies of interest? First, lightning affects the local chemistry of the atmosphere creating molecules that otherwise would not appear, such as prebiotic molecules responsible for the origin of life on the young Earth. On the other hand it is of interest to know whether the discharge energy is large enough to be detectable from Earth.
Atmospheres of cool objects (brown dwarfs and gas giant planets with global temperatures between 500 and 2700 K) are cold and dense enough for mineral clouds to condense. In this paper we adopt the idea that a large-scale discharge is initiated by an electron avalanche that develops into a streamer (electrically conducting channel). We apply our discharge-propagation model to one-dimensional Drift-Phoenix atmosphere models, which provide information about the local gas temperature, pressure and chemical composition.
Our analysis shows that the electrical breakdown can occur inside the cloud layer (lightning) and/or above the cloud layer (sprite). From these locations the discharge propagates through the atmosphere while subsequent branches appear until it reaches a minimum diameter and the discharge terminates.
Our results show that a lightning strike reaches longer distances in a brown dwarf than in an exoplanet, which means it affects larger atmospheric volumes in the former than in the latter. The total energy that dissipates from one such event is less then 1012 J. (For comparison, on Jupiter and Saturn this value is around 1012-1013 J while on Earth it is ~108-109 J.) This energy causes an increase in the local gas temperature, which results in changes in the local chemistry as well. In the paper we showed the increase of small carbohydrate molecules such as CH and CH2.
Considering mineral clouds, the closest alternatives on Earth we can investigate are volcano plumes, which are composed of small silicate ash particles. After explosive eruptions, volcano plumes host lightning activity that is orders of magnitudes larger than in a common thundercloud on Earth. Taking these arguments into account it is suggested that we provided a lower limit of the dissipation energy and that, in reality lightning can be stronger and more frequent on fast rotating extrasolar objects.
For more details check out the original paper on ADS:
Also look at the University press release of the paper:
Christiane Helling gives a press conferenc on 30 April, 2014 (9 am):
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