Nonlinear evolution of Alfven waves and fast waves in magnetar magnetospheres

Young and active neutron stars like magnetars may experience star quakes that can launch kHz Alfven waves into the magnetosphere. An Alfven wave packet propagating along the closed field lines undergoes several interesting nonlinear processes: (1) Alfven waves propagating to the outer magnetosphere may break and form an ejecta that accelerates away from the star. This process may be a viable mechanism to produce the simultaneous X-ray bursts and fast radio bursts from the galactic magnetar SGR 1935+2154. (2) The Alfven waves bouncing back and forth in the inner magnetosphere can convert to fast magnetosonic waves that spherically expands out. The fast waves can also become nonlinear before escaping the magnetosphere, producing shocks or regions with E>B that efficiently dissipate the fast wave energy, powering observable emission. In this talk, I’ll present our systematic study of all these processes and discuss their observational consequences.