Cross-current instability generated by pick-up ions in the environment of a large spacecraft in low Earth orbit

Measurements from the space shuttle flights have revealed that a large spacecraft in a low Earth orbit is accompanied by an extensive gas cloud, which is primarily made up of water. The charge exchange between the water molecule and the ionospheric O/sup +/ ions produces a water ion beam traversing downstream of the spacecraft. The authors present results from a study on the linear and nonlinear behaviors of the waves generated by the interaction of the water ion beams with the ionospheric plasma. Since velocity distribution function is key to the understanding of the wave generation process, they have performed a test particle simulation to determine the nature of H/sub 2/O/sup +/ ions velocity distribution function. The simulations show that at, the time scales shorter than the ion-cyclotron period /spl tau//sub c/, the distribution function can be described by a beam. On the other hand, when the time scales are larger than /spl tau//sub c/, a ring distribution forms. A brief description of the linear instabilities driven by an ion beam streaming across a magnetic field in a plasma is presented. In order to study the nonlinear evolution of the waves, the author has performed 2.5-dimensional particle-in-cell (PIC) simulations which show that transient amplitudes of the waves can be up to a few Volts/m at the lower-hybrid frequency. The frequency spectrum of the waves from the simulation shows that the water ion beam produces a broadband electric field noise extending in frequency up to a few times the ion plasma frequency. The simulations also reveal an enhanced electric field noise level near the electron cyclotron harmonics.