The finite extent of plasma clouds in the ionosphere imposes a nonzero parallel wave number on plasma modes. The consequent parallel modal currents and modal electric fields have a profound effect on the development of eigenmodes and the possible interaction of the eigenmodes with both the ionosphere and the magnetosphere. Specific consideration is given to the kinetic loss cone instability, which has been postulated to cause the initial growth of prompt striations. It is shown that an enhancement, in the effective electron collision frequency above the value expected from purely classical considerations, produces an increase in the parallel wave number of the fundamental eigenfunction of this instability. This provides a plausible explanation for the observation of prompt striations in the Buaro barium release. The development of eigenmodes implies the transfer of free energy originally residing in the loss cone ions into the parallel electron and ion motion. In this way, the eigenmodes can directly contribute to field-aligned plasma currents and auroral precipitation of particles.