MICELA, GiuseppinaGiuseppinaMICELA2021-01-132021-01-132018978-3-319-55332-0http://hdl.handle.net/20.500.12386/29729In this chapter the relevance of stellar coronal activity in determining the physical conditions of planetary atmospheres is discussed. We still lack a comprehensive, self-consistent picture of the role of stellar activity, during the star lifespan, in determining and shaping the evolution of planetary atmospheres, after the circumstellar disk has been cleared out, but many efforts in this direction are today ongoing. Here we focus on high-energy radiation since it penetrates deeply the atmosphere ionizing and heating the gas, thus affecting its chemistry with consequences very different from those of optical or UV radiation. Stellar activity is inhomogeneous and variable; flares in particular can be very frequent and intense in young and dM stars. Depending on their duty cycle, they may drive the atmospheric gas toward different chemical regimes.STAMPAenBook part10.1007/978-3-319-55333-7_192-s2.0-85086424115https://link.springer.com/referenceworkentry/10.1007%2F978-3-319-55333-7_192018haex.bookE..19MFIS/05 - ASTRONOMIA E ASTROFISICA