Monday, December 30, 2013

How to Look for Lightning on Brown Dwarfs and Exoplanets

Ionisation in atmospheres of brown dwarfs and extrasolar planets VI: Properties of large-scale discharge events


Bailey et al


Mineral clouds in substellar atmospheres play a special role as a catalyst for a variety of charge processes. If clouds are charged, the surrounding environment becomes electrically activated, and ensembles of charged grains are electrically discharging (e.g. by lightning), which significantly infuences the local chemistry creating conditions similar to those thought responsible for life in early planetary atmospheres. We note that such lightning discharges contribute also to the ionisation state of the atmosphere. We apply scaling laws for electrical discharge processes from laboratory measurements and numerical experiments to Drift-Phoenix model atmosphere results to model the discharge's propagation downwards (as lightning) and upwards (as sprites) through the atmospheric clouds. We evaluate the spatial extent and energetics of lightning discharges. The atmospheric volume affected (e.g. by increase of temperature or electron number) is larger in a brown dwarf atmosphere (108− 1010m3) than in a giant gas planet's (104− 106m3). Our results suggest that the total dissipated energy in one event is greater than 1012 J for all models of initial solar metallicity. First attempts to show the infuence of lightning on the local gas phase indicate an increase of small carbohydrate molecules like CH and CH2 at the expense of CO and CH4. Dust forming molecules are destroyed and the cloud particle properties are frozen-in unless enough time is available for complete evaporation. We summarise instruments potentially suitable to observe lightning on extrasolar objects.

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