PROCESO DE CIERRE DE OBRA

In principle, lightning, or lightning-like discharges, can occur in any environment which is capable of hosting charged seed particles and producing large-scale charge separation. Though this is most prominent for planetary-like atmospheres, such as that of exoplanets and brown dwarfs, lightning-like discharge events have been suggested to occur in other, less obvious environments, like protoplanetary disks (e.g. Desch & Cuzzi, 2000; Nuth et al., 2012), pulsars (Katz, 2017), and black holes (e.g. Aleksi´c et al., 2014; Eisenacher Glawion et al., 2015). In this section I will summarize studies focusing on exoplanetary and brown dwarf lightning. Though, lightning has not been detected from outside the Solar System, the theory of "exo-lightning" formation is a developing field, which suggest that in the future observation of lightning on exoplanets and brown dwarfs might be possible. For further information on atmospheric electrification (inside and) outside the Solar System seeHelling et al.(2016a).

Exoplanets analysed through transit spectroscopy are observed to have clouds in their at- mospheres, most likely made of silicate particles (e.g.Kreidberg et al.,2014;Sing et al., 2009,

2013,2015). These findings are supported by kinetic cloud models as inHelling et al.(2008a,

2011b,a). Various authors demonstrated that atmospheric circulation leads to the formation of zonal jets and local vortices as known from Jupiter and Saturn (e.g.Dobbs-Dixon et al., 2012;

Dobbs-Dixon & Agol, 2013;Mayne et al., 2014). E.g. Zhang & Showman(2014) showed that strong internal heating and weak radiative dissipation results in the formation of large-scale jets.

4.2. Lightning beyond the Solar System - exoplanets and brown dwarfs Lee et al.(2015) modelled local and global cloud patterns on the planet HD 189733b, a tidally locked hot Jupiter orbiting a K star. Their dust opacity, grain size distribution and albedo maps indicate that cloud properties change significantly from dayside to night side forming a spot-like cloud pattern driven by a latitudinal wind jet around the equator. As we have seen in the previous sections, such dynamic environments host lightning activity in the Solar System. The charging of extrasolar atmospheres has been suggested to be the result of particle collisional ionization (Helling et al.,2011a,2013a), cosmic ray ionization (Rimmer & Helling,2013), and ionization due to the internal (Rodríguez-Barrera et al., 2015) and external (Batygin et al.,2011) heating of the object. Other processes, such as Alfven ionization, may also contribute to the production of charged particles in magnetic environments (Stark et al.,2013).

Helling et al. (2011b) studied electron avalanches initiated by dust collision-induced, local charge-inequilibrium in dust clouds of brown dwarfs. They argued that thermal₋and dust colli- sional processes alone will result in a globally neutral atmosphere. However, stochastic ionization in such atmospheres may occur on a short time scale, resulting in enough free electrons to form electron avalanches and eventually intra-cloud discharges.Helling et al.(2013b) estimated elec- tric breakdown characteristics in dusty atmospheres and found that the breakdown field depends on the local gas-phase chemistry, the effective temperature, and the primordial gas-phase metal- licity. They found that charged particles will gravitationally settle resulting in large-scale charge separation. They suggest that different discharge processes will dominate at different atmospheric pressures, such as small-scale sparks at higher pressures and gas densities, and large-scale dis- charges near and above cloud tops. The critical electric field in such dusty atmospheres varies between 10−7and 107V cm−1, and the critical number of charges per dust surface per cm3varies between < 1.6×1014 and 1.6×104 C (Helling et al., 2013b). Bailey et al. (2014) modelled large-scale discharges and derived discharge properties using scaling-laws for giant gas planets and brown dwarfs. The properties they obtained include the breakdown field, the minimum num- ber of charges needed to overcome the electric breakdown, the initiation height of the discharge, the total discharge length, the total energy dissipated (based on a simple scaling law), and the total discharge volume. I will further discuss the findings ofBailey et al. (2014) in Chapter 8.

Rimmer & Helling (2016) introduced a chemical kinetics network for extrasolar atmospheres, which can be used to study the chemical affects of lightning discharges in various atmospheric compositions. Rimmer et al.(2016) used the same chemical network to study lightning-induced chemistry in super-Earth atmospheres, and found that amino acids are produced if the atmo-

chemical changes. Hodosán et al.(2017) estimated radiated energies and radio powers of light- ning in certain giant gas planetary and brown dwarf atmospheres, with input parameters taken fromBailey et al.(2014), which is also presented in Chapter8.

To date, only a handful of studies have presented estimates of detectability of extrasolar light- ning.Zarka et al.(2012) analysed the possibility of detecting lightning emission from extrasolar planets, by up-scaling the radio emission observed from Jupiter and Saturn. They concluded that flashes 105 times stronger than Jovian or Saturnian lightning from a distance of 10 pc, with a bandwidth of 1₋10 MHz, integration time of 10₋60 min would be possible to detect. Although, as they point it out, propagation effects will strongly affect the radio emission below a few MHz.

Vorgul & Helling(2016) modelled the conductivity in extrasolar atmospheres caused by flash ion- ization processes, such as lightning, and found that such events have an effect on the signature of electron cyclotron maser emission (CME) causing a pulse-like amplification of the signal. They suggest that, if such signatures are observed in CME, one could infer the properties of underlying flash events in the atmosphere. The newest research conducted regarding observability of "exo- lightning" is part of this thesis. Hodosán et al.(2016b) estimated lightning occurrence on the exoplanet HAT-P-11b based on previous radio observations carried out byLecavelier des Etangs et al.(2013). The extended version of this work is discussed in Chapter6. Hodosán et al.(2016a) carried out a first statistical study of lightning occurrence on extrasolar planets based on observed occurrence of lightning in the Solar System. This lightning climatology study is discussed in Chap- ter5. In an on-going study, I also estimate observability of lightning radio signatures based on the results ofHodosán et al.(2017, also in Chapter8, Chapter9).