Magnetospheric processes at cool stars and exoplanets

Magnetospheric processes and phenomena have been studied in great detail at Solar System planets. This thesis investigates the occurrence of these processes at cool stars and exoplanets, and the generation and detectability of magnetospheric radio emission via the electron-cyclotronmaser instability. Three detailed studies are presented within this thesis. The first study explores magnetospherically-driven radio emission at ultracool dwarfs (UCDs), using an analytic model of Jupiter’s middle magnetosphere to explain a component of radio emission observed from a fraction of fast-rotating UCDs. Key parameters governing the generation of auroral radio emission for both open and closed magnetospheres are investigated, and it is found that a broad range of these parameters are capable of producing emission consistent with the observed luminosity. The next study examines the detectability of radio emission from M-dwarf exoplanetary systems generated through sub-Alfvénic interaction between the exoplanet and stellar wind. Radio flux densities are estimated for three promising case studies, TRAPPIST-1, Proxima Centauri, and NGTS-1, finding that sporadic bursts of radio emission are feasibly detectable with currently available telescopes. A wider survey of exoplanet-hosting M-dwarfs identifies eleven further systems with potentially detectable radio flux densities >10 uJy. The final study uses a numerical magneto hydrodynamic model to investigate the interaction between Sun-like stars and magnetised hot Jupiters. The emergent field-aligned current system is modelled, and auroral radio power is calculated in post-processing. Simulations are run to study the effect of interplanetary magnetic field strength and ionospheric Pedersen conductance. Radio emission is also investigated as a function of orbital distance from the host star, finding a relation between distance and radio power consistent with previous analytic studies of the same systems.