The Role of Damped Alfvén Waves in Magnetospheric Accretion Models of Young Stars

We examine the role of Alfvén wave damping in heating the plasma in the magnetic funnels of magnetospheric accretion models of young stars. We study four different damping mechanisms of the Alfvén waves: nonlinear, turbulent, viscous-resistive, and collisional. Two different possible origins for the Alfvén waves are discussed: (1) Alfvén waves generated at the surface of the star by the shock produced by the infalling matter and (2) Alfvén waves generated locally in the funnel by the Kelvin-Helmholtz instability. We find that, in general, the damping lengths are smaller than the tube length. Since thermal conduction in the tube is not efficient, Alfvén waves generated only at the star's surface cannot heat the tube to the temperatures necessary to fit the observations. Only for very low frequency Alfvén waves ~10-5 the ion-cyclotron frequency is the viscous-resistive damping length greater than the tube length. In this case, the Alfvén waves produced at the surface of the star are able to heat the whole tube. Otherwise, local production of Alfvén waves is required to explain the observations. The turbulence level is calculated for different frequencies for optically thin and thick media. We find that turbulent velocites vary greatly for different damping mechanisms, reaching ~100 km s-1 for the collisional damping of low-frequency waves.