Generation of Kinetic Alfven Waves by Beam-Plasma Interaction in Nonuniform Plasma

Ion beams are frequently observed in the magnetospheric boundary layers. This work reports the generation of kinetic Alfvén waves (KAWs) by ion beam-plasma interaction in a nonuniform plasma boundary layer using a two-dimensional hybrid simulation. The bulk velocity of the ion beam is assumed to be parallel to the ambient magnetic field B0, with a thermal spread in the velocity space. As a result of the beam-plasma interaction, strong shear Alfvén waves as well as fast mode compressional waves are first generated on the side of the boundary layer with a high density and thus a low Alfvén speed (VA). These waves mainly propagate along the background magnetic field. Later, Alfvén waves are also present inside the boundary layer with a continuous spectrum. As the perpendicular wave vector k⊥ increases with time, large-amplitude, short wavelength KAWs with k⊥>>k|| clearly form in the boundary layer. Reflected waves are also found on the high density side of the boundary. The physical mechanism for the generation of KAWs is discussed.