Ion acoustic instabilities excited by injection of an electron beam in space

Abstract

It is shown by means of numerical simulations that an injection of a tenuous suprathermal electron beam into a fully ionized plasma from spacecraft can lead to an excitation of ion acoustic waves. The return current carried by the ambient electrons has a drift speed that is much slower than the beam speed, so that these electrons can excite the ion waves via wave‐particle interaction. Numerical simulations confirm the excitation of ion acoustic waves and the acceleration of ions near the spacecraft producing a high‐energy tail.

Waves in the innermost open boundary layer formed by dayside magnetopause reconnection

“…Although the reduced distribution function is not well observed at low‐electron velocities due to the ESA detection limit, it is likely that the reduced distribution function has a peak in the negative velocity range v ∥ <0. This peak in the reduced distribution function, formed by background cold electrons that are oppositely accelerated as a return current of the beam electrons originating from the magnetosheath, is consistent with the one‐dimensional simulation of Okuda and Ashour‐Abdalla [].…”

“…Ion acoustic instability tends to occur when the electron temperature is greater than the ion temperature, thus preventing ion Landau damping. In the present case, the cold proton temperature is lower than the temperature of cold electrons drifting at V ∥ =−450 km/s (Table ); therefore, the instability may be present [ Okuda and Ashour‐Abdalla , ]. The linear growth rate decreases as the propagation angle increases since a higher drift speed is needed to produce resonance with the electrostatic wave, the phase speed of which along the magnetic field is [ Umeda et al , ].…”