◍ According to Gabriella, in objects (like gas gaints and brown dwarfs) which are quite different from our Solar System planets, lightning may be 2− 8 orders of magnitude more energetic than Earth lightning, and up to 5 orders of magnitude more energetic than Saturnian and Jovian lightning.
◍ Also, volcanically very active planets, and objects with clouds of similar compositions as volcano plumes, would show the largest lightening flash densities if lightening occurred at the same rate on these planets as it does in volcano plumes on Earth.
◍ She also suggested that sub-solar metallicity atmospheres host more energetic, more powerful lightning flashes.
◍ Furthermore, she detected the radio emission from the direction of Exoplanet HAT-P-11b which she think could possibly be produced by extremely large lightening activity.
◍ She also manifested that low-frequency radio arrays like LOFAR or LWA (Long Wavelength Array), high-precision optical and NIR telescopes, like the ones of the Gemini Observatory, and future space missions such as JWST (James Webb Space Telescope) could all contribute to the future study of extrasolar lightning.
Lightning is a common phenomenon in the Solar System. On Jupiter and Saturn it has been suggested to occur inside the deep water clouds through similar processes known from Earth. The optical and radio emission from the two planets seem to be consistent with higher lightning energy release on the gas giants, than on Earth, with a total energy of ∼ 1012 − 1013 J. Lightning activity on Uranus and Neptune is less studied. Because water clouds form on very high pressures (around 40 bar), it was suggested that on the ice giants lightning occurs in the H2S−NH3 clouds closer to the top of the atmosphere. Lightning on Venus most probably occurs in the form of intra cloud lightning due to the high pressures close to the surface. According to theoretical works, lightning may exist on Mars and Titan as well, however no observations confirmed the theory yet. In extrasolar planets so far dust cloud charging has been modelled (but not yet observed) in more details, and it has been suggested that such mineral clouds will allow large enough charge separation through gravitational settling to overcome the breakdown threshold and produce large-scale discharges.
Now, Gabriella Hodosán in her PhD project determined if lightning on exoplanets and brown dwarfs can be more energetic than it is known from Solar System planets, what are the most promising signatures to look for, and if these “exo-lightning” signatures can be detected from Earth.
She discussed a lightning climatology study of Earth, Jupiter, Saturn, and Venus, based on optical and/or radio measurements. Then applied the obtained lightning statistics to extrasolar planets in order to give a first estimate on lightning occurrence on exoplanets and brown dwarfs. Her results suggested that Volcanically very active planets, and objects with clouds of similar compositions as volcano plumes, would show the largest lightening flash densities if lightening occurred at the same rate on these planets as it does in volcano plumes on Earth.
She have also given first estimate of lightening activity on the exoplanet HAT-P-11b based on observational data. She found that the tentative radio emission detected from the direction of the exoplanet, could be produced by extremely large lightening activity, if the lightening has same energetic properties that we know from Saturn.
Moreover, she studied what the optical emission of “exo-lightning the would be, if it had same statistical and radiating characteristics that we know from the solar system. She estimated optical fluxes and apparent magnitudes of lightening storms on the 3 closest brown dwarfs, Luhman-16, ϵ Indi and SCR 1845–6357, in standard I, V and U bands. Her results suggested that lightening will occur in a large variety of extrasolar objects, with various flash densities, emitted powers and discharge durations. Some of the parameter combinations with the largest optical power output and largest flash densities could favour lightening observations in the optical band, since lightening storms on the investigated brown dwarfs could be as bright as 13–16 magnitude star.
After studying lightning in the Solar System and applying that knowledge to extrasolar planets, she asked how different lightning can be from this picture on exoplanets and brown dwarfs? Is it possible that discharges release more energy and power outside the Solar System than on Solar System planets? The energy released from lightning will determine the strength of the electromagnetic waves emitted from the discharge, and the amount of non-equilibrium species produced in the atmosphere. Ultimately, it will determine whether “exo-lightning” signatures can be observed from Earth or not. She presented a model built from previously tested lightning models, and extended it so that exoplanetary lightning can be studied as well. The simple dipole model calculated the energy and total radio power released by lightning, and connected these estimates to the atmospheric properties through the extension of the discharge and the peak of the current flowing in the discharge channel. She found that due to the very short channels, and the large amount of charges in the channel, very quick discharges occur with very large peak currents in giant gas planets. Moreover, Figure 1 suggests that total dissipated energies can reach as high as 1011 − 1013 J in brown dwarfs (log(g)=5.0) and 1016 − 1017 J in giant gas planets (log(g)=3.0).
“I found that in objects quite different from Solar System planets (log(g)=3.0 and 5.0; Teff = 1500 . . . 2000; [M/H] = 0.0 and -0.3), lightning may be 2− 8 orders of magnitude more energetic than Earth lightning, and up to 5 orders of magnitude more energetic than Saturnian and Jovian lightning.”, told Gabriella, author of the project thesis.
She also found that lightning in giant gas planets, or low-gravity, young, brown dwarfs, with log(g)= 3.0, reaches higher energies than in brown dwarfs with log(g)= 5.0, if the metallicity is sub-solar, however higher surface gravity objects with solar metallicity host more energetic lightning flashes. In general, atmospheres with sub-solar metallicity host stronger flashes than in solar compositions. The released lightning energy and power are less dependent on the bodies’ effective temperature, than on the surface gravity or the chemical composition of the object (Figs 2 and 3).
Furthermore, she applied the results from Bailey et al. (2014) for minimum number of charges Qmin, and an extension of discharge (h) that is measured for discharges in the Solar System. She estimated the energy and power for three different extensions. The results (Fig. 4) suggested that sub-solar metallicity atmospheres host more energetic, more powerful lightning flashes. For the same h, higher surface gravity objects (i.e. Brown dwarfs) host less powerful and energetic flashes than in lower surface gravity objects. The released power and energy slightly decreases with effective temperature. Also, the shorter the discharge channel the higher the released energy and power are. In this case, gaint gas planetary lightening produces higher energy every time. The released radio energy to gas gaints is, Wrad ∼ 108 − 1015 J for gas giant planets, and Wrad ∼ 5 × 104 − 1010 J for brown dwarfs.
“My results suggests that the discharge energy will strongly depend on the process through which the cloud particles are charged and the processes that cause the electrostatic potential to build up.”— said Gabriella, author of the project thesis
As the results of her thesis suggest, a multi-wavelength observational campaign could lead to the first detection of lightning induced signatures on an extrasolar object, most probably a close-by faint brown dwarf. Radio and optical observations simultaneously could produce yet un-seen and un-explained signals, which may be the result of lightning activity. Such observations could be followed up by infrared telescopes, which could detect spectral signatures of enhanced non-equilibrium species produced by lightning in the atmosphere of the observed object. Low-frequency radio arrays like LOFAR or LWA (Long Wavelength Array), high-precision optical and NIR telescopes, like the ones of the Gemini Observatory, and future space missions such as JWST (James Webb Space Telescope) could all contribute to the future study of extrasolar lightning.
Such project would involve the addressing of the following questions: which objects are the best candidates for lightning detection with current technology, and what sensitivity would be needed to observe lightning on an Earth-like planet in the habitable zone? With current radio facilities, what depths can we hope to reach? What radio bands are the most promising for observations? How could the observations be improved by potential space radio-arrays? The data obtained by ground-based telescopes could further provide with information on Earth lightning as a noise-source in the data of extra-terrestrial observations. The outcome of such observational research project would be beneficial and significant no matter whether lightning is detected on an exoplanet or brown dwarf, or not. The detection of “exo-lightning”, combined with planetary lightning observations with, e.g. the Juno spacecraft, would support theoretical works regarding cloud formation, convection, ionization, and electricity in extrasolar atmospheres, or would encourage the further development of such work. Non-detection of lightning would suggest that a better understanding of exo-climates, cloud patterns and theoretical lightning formation is needed, carving the path for future studies.
Reference: Gabriella Hodosán, “Lightning on exoplanets and brown dwarfs: Modelling and detection of lightning signatures throughout the electromagnetic spectrum”,
St Andrews Research Repository, pp. 1-249, 2021. https://research-repository.st-andrews.ac.uk/handle/10023/12079
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