This paper reports Cluster high-altitude observations above the acceleration region of a typical monopolar electric field event at the plasma sheet boundary. The observations reveal both characteristics of waves and signatures of electrostatic acceleration. The observations can be interpreted in terms of low-frequency transverse magnetic waves on the auroral interface. A reconstruction of the spatial structure across the interface, effectively correcting for the slow oscillations of the interface, allows a proper determination of the electric potential and of the upward and downward currents. Because of the long wave period, the auroral system can be understood in terms of a quasi-electrostatic model. In this model, the precipitating plasma sheet particles induce a perpendicular heating of cold ionospheric H+ and O+ ions. As these ions escape from the ionosphere, their perpendicular motion is converted into parallel motion in the divergent geomagnetic field as a result of the conservation of magnetic moment. These ions acquire an additional parallel energy through acceleration by the magnetic-field-aligned electric field in the auroral acceleration region. They also may gain substantial perpendicular energy due to the perpendicular electric field, as the equation image × equation image drift increases with altitude. Both types of acceleration are typical electrostatic phenomena.