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Charge separation, lightning and radio emission in low-mass objects

Final Report Summary - LIGHTNING (Charge separation, lightning and radio emission in low-mass objects)

Brown dwarfs and extrasolar planets have chemically rich atmospheres where clouds form. These clouds have a major influence on the chemistry, radiation and atmospheric dynamics, and they therefore determine our chances to find life on planets outside the solar system. Clouds are also know for their lightning activity. How are cloud particles charged in substellar atmospheres? How can charge separation and discharge processes be traced in the atmospheres of M-dwarfs, Brown Dwarfs and planets? How do electric discharges (e.g. lightning) influence the atmospheric chemistry? - These were questions that motivated the research program supported by my ERC starting grant.

We demonstrated that cloud particles can easily charge by collisions in turbulent gases, due to cosmic rays and high-energy radiation, and due to the adsorption of thermal electrons from the gas phase. Cloud particles are not always directly ionised. An increase of the gas-ionisation (like during lightning or cosmic ray ionisation) leads indirectly to cloud particle ionisation.

We investigated the electrostatic and magnetic behaviour of atmospheres of brown dwarfs and exoplanets. We showed magnetic coupling of a substantial atmosphere volume is possible. Combined with a strong magnetic field (typically 1000G for brown dwarfs), a chromosphere and aurorae could form as suggested by radio and X-ray observations of brown dwarfs. Cosmic ray ionisation and discharge processes in clouds will increase the local pool of free electrons in the gas. This work lead to the VLT/HAWKI mini survey for late T-dwarfs and a LOFAR mini-survey for the nearest brown dwarfs.

A statistical study of lightning climatology on Earth and in the solar system allowed us to make first extrapolation for lightning rates in different extrasolar environments. The resultant statistics emphasise that the lighting measurements are very incomplete for all planets and highly under-determined for Saturn, Jupiter, Uranus and Venus because lightning inside clouds is very difficult to observe.

Based on a perturbation analysis of an electromagnetic field in the presence of a flash ionisation, we showed that radio beams from cyclotron maser emission are modulated when passing a flash ionisation like one from a lightning. We presented a 'recipe for observers' in order to search for such lightning fingerprints in cyclotron maser emissions signatures.

Our team has developed a kinetic gas-chemistry (STAND2015) that allows to study a wide range of temperatures for lightning and life in planetary atmospheres. STAND2015 has been used to demonstrate that cosmic rays may open chemical paths leading to the formation of complex biomolecules. STAND2015 was used to predict the observation of the HCN molecule on the exoplanet HAT-P-11b as result of lightning, and the chemical effect of lightning in the early Earth atmosphere.

Our team is a major driver for detailed cloud modelling in 3D radiative global circulation model based on our expertise in kinetic cloud formation. Together with our collaborators, we presented the first 3D atmosphere model that consistently predicts cloud properties for extrasolar planets. This technology jump was applied to the highly irradiated planet HD189733b which for the first time allowed to predict dynamic cloud structures based on detailed modelling of cloud formation processes. We were able to manifest that the local velocity fields in extrasolar planets are large enough to ionised cloud particles and support locally high gas-ionisation through Alfven ionisation. The model results will allow us to study where in the atmosphere of HD18977b lightning or other charge processes can occur.

The ERC starting grant allowed our team to pursue high-risk research in several research fields (lightning, lightning chemistry, atmospheric ionisation, 3D cloud-forming globally circulating atmospheres) which else-wise would have been impossible. We believe to have achieved real breakthroughs in many if of not all projects that we carried out.