The relationship between electrostatic shocks and kinetic Alfvén waves

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 Alfvén 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 Alfvén 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 Alfvén 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 Alfvénic value, and thus may appear as electrostatic.