Electromagnetic and electrostatic emissions from a thin electron beam in space plasma

Abstract

Electromagnetic and electrostatic emissions from a spatially confined thin electron beam in a space plasma is studied via electromagnetic particle simulations. We assumed a top-hat ion density model to compensate an excess charge induced by an electron beam injection to identify an effect of current injection from a charge injection effect. At the onset of an electron beam injection, impulsive electromagnetic waves are radiated, propagating away from the electron beam. The simulation system is a two-dimensional x-y plane with both a static magnetic field and an electron beam drift in the y-direction. Boundary conditions are open in the x-direction and periodic in the y-direction. The electromagnetic waves can propagate without reflection at the x-boundaries where a masking method is used to absorb outgoing waves. An instability similar to the conventional two-stream instability in a uniform beam-plasma system develops after the initial response, and beam electrons are diffused. During this quasi-steady period, both electromagnetic and electrostatic waves are excited through the Landau resonance with beam electrons. Electrostatic emissions are confined to a region close to the electron beam, while the electromagnetic emissions propagate away from the beam depending on wave normal angles relative to a static magnetic field. Spatial configuration of the electromagnetic wave spectra shows a close resemblance to a funnel shaped VLF emissions observed in the Spacelab-2 electron beam experiment.