Stellar activity

Abstract

The atmospheres of late-type stars are site of strong magnetic fields responsible for their nonradiative heating. The stellar magnetic activity phenomena are in many cases analogous to those observed on the Sun, but very different scenarios from the solar case are also observed. Magnetic fields drive hot plasma upward, where it radiates in faculae and plages, leaving dark spots in the photosphere at the foot points of the flux tubes. Occasionally, magnetic fields reconnect and produce magnetic free energy and particles, in form of flares, and coronal mass ejections. Avalanches of low-energy flares – nanoflares – are considered the most viable sources of heating and sustaining of the outer atmospheric layers: Chromosphere, transition region, and corona. Magnetic activity is produced by some form of dynamo at work in the stellar interior, whose basic ingredient is the coexistence of a convective zone under the stellar surface and the presence of a differential rotation regime. The whole magnetic activity phenomenology is highly variable in time. Active regions grow and evolve in days to weeks, but are also site of transient phenomena – e.g., flares – evolving on time scales of few seconds to several hours. The global level of magnetic activity changes in time following cycles as it happens for the Sun and decreases with age. The time scale of stellar variability, the amount of involved energy, and the quality of the involved photons are the key observables that can be used as tools for understanding the observed phenomena. This chapter is dedicated to the illustration of the main diagnostics for stellar activity, the characteristics of active regions, and the properties of the chromospheres, transition regions, coronae, flares, and solar-like stellar winds. Finally, we discuss how stellar activity affects exoplanets detection and characterization and the impact of stellar magnetic activity in the formation and evolution of planetary atmospheres.