Ion beam driven electrostatic shocks

Electrostatic shock waves have been detected during the injection of an artificial xenon ion beam into the collisionless ionospheric plasma. The beam was directed quasi-perpendicular to the ambient magnetic field. The ion source was situated on a small ejectable payload. The properties of the plasma/ion beam interaction were recorded on the main payload as a function of the separation distance to the ion source. The shock structure develops at distances where the apparent azimuthal speed of the beam fan is comparable to the phase velocity of ion acoustic waves sustained by background O+ ions. The shock is characterized by a decrease in the local density up to about 10% of the background density, strong shear motion visible in the convection electric field, field-aligned potential drops of the order of several tens of millivolts, increased electron temperatures, and fluxes of reflected ions. Probably suprathermal electron fluxes along the magnetic field were also present, but these have not been measured. The Mach number is determined to M = 1.09, and thus the shock is weak and turbulent as a result of ion acoustic turbulence which has been observed throughout the shock region. It is suggested that this turbulence is driven by an ion-ion instability excited by reflected background ions. Estimates of the number density of reflected ions show that roughly 50% of the background ion population may be reflected from the shock potential. On a larger spatial scale these reflected ions may be responsible for the excitation of the observed lower hybrid and ion cyclotron harmonic waves in front of the leading edge of the beam. No shock has been found on the trailing edge of the beam, suggesting that different physical conditions apply there.

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