Future space missions aiming at the accurate measurement of cold plasmas and DC to very low frequency electric fields will require that the surface potential of their conductive surfaces be actively controlled to be near the ambient plasma potential. To do this, an active ion emission instrument will be flown aboard the upcoming Cluster, Geotail, and Interball satellites that involves the emission of an indium ion beam. To understand the beam-plasma interaction that can occur for an indium ion beam injected from the satellite into magnetospheric and solar wind plasma, a linear theory parameter search has been carried out to investigate whether particular combinations of ambient plasma parameters and emission modes exist such that plasma instabilities may be excited. A restricted plasma model in the vicinity of spacecraft was used with an indium beam drifting with respect to an ion-electron background plasma. Since the satellites will fly through different space plasma regions, the linear theory analysis was carried out using background parameters for solar wind, lobe, plasma sheet boundary layer, and central plasma sheet conditions. It was found that a beam-driven lower hybrid type instability can be excited with maximum wave growth for beam emission and wave propagation transverse to the ambient magnetic field. The stability threshold as a function of the beam emission current, which can be varied during instrument operation, has been found for the various background plasma regions. The instability has the lowest current threshold in the central plasma sheet, and the highest in the lobe. The emission current determines the beam temperature, thus by varying the emission current above or below the threshold determined here, the instability can, in principle, be turned on and off, and the effects on other instruments on the satellite can be evaluated. ¿ American Geophysical Union 1992