Auroral satellites and sounding rockets frequently observe large electric fields perpendicular to the magnetic field that have a narrow scale length perpendicular to the magnetic field if they are interpreted as spatial structures. These fields have been variously attributed to electrostatic shock structures or to kinetic Alfv¿n waves. These two models can be distinguished by considering the ratio of the magnetic field perturbation to the electric field. This ratio is calculated within the context of the electrostatic approximation, the fully kinetic Alfv¿n wave dispersion relation considered by Lysak and Lotko [1996>, and the cold fluid model including ionospheric reflection presented by Lysak [1991, 1993>. Results for this model show that the ratio of the electric to magnetic field is not always equal to the Alfv¿n speed, especially for structures that are very narrow in the direction perpendicular to the magnetic field. These narrow structures have electric fields that are enhanced with respect to the Alfv¿n value, and thus may appear as electrostatic. ¿ 1998 American Geophysical Union