Electric fields parallel to the magnetic field play a major role in the transport of mass, momentum, and energy in the auroral zone. In this paper we explore which terms of the steady state electron momentum equation are consistent with the large-amplitude, macroscopic parallel electric fields (~25 mV/m to 300 mV/m) measured by the electric field instrument on Polar in the upward current part of the auroral acceleration region. From a detailed analysis of eight events we found evidence suggesting that the large-amplitude, macroscopic parallel electric fields are ambipolar in character; namely, they are supported by electron pressure gradients present at the interface separating the relatively cold, dense ionospheric plasma from the hot, tenuous magnetospheric plasma in the auroral density cavity. These structures do not appear to be explained by purely propagating Alfv¿n waves. These large-amplitude, macroscopic parallel electric fields are likely to be a sheath field that forms at the boundary separating a high and low density plasma, similar to that encountered in laboratory experimental devices.