In a number of barium injection experiments in the ionosphere, field-aligned structures have been observed to develop on the ion-cyclotron time scale. These structures, have been attributed to plamsa jetting across the ambient magnetic field and have previously been analyzed in the electrostatic limit. The electrostatic analysis is reasonable for experiments in which the directed ion kinetic energy per unit volume is much smaller than the magnetic field pressure; however, for higher altitude experiments in which the ambient magnetic field strength is weaker, justification for the electrostatic treatment fails and the full electromagnetic dispersion relation must be examined. It is demonstrated that electromagnetic effects inhibit the growth of the linear instability considered to be the source of prompt striations in the earlier experiments. Consistent with electrostatic analysis, cold background plasma and thermal spread in the ion loss cone distribution function tend to stabilize the instability in the full electromagnetic treatment. The theory suggest that prompt striations in the magnetosphere can form from ion jets only if the plasma density is sufficiently tenuous so that electromagnetic stabilization is not complete.