We present a survey of 64 direct observations of large-amplitude parallel electric fields E_∥ in the upward current region of the southern auroral acceleration zone, obtained by the three-axis electric field experiment on Polar. These E_∥ events range in amplitude from about 25 to 300 mV/m and represent a significant fraction of the total electric field strength (E_∥/E_⊥ ranges from ∼0.25 to O(10)). The E_∥ structures, which tend to occur at the edges of oppositely directed (converging) pairs of perpendicular electric field structures (electrostatic shocks), have transverse (to the magnetic field) widths of ∼1.0–20 km at altitudes ranging from 0.8R_E to 1.5R_E, assuming the structures are stationary. The parallel potential drops associated with these large-amplitude parallel electric fields are highly localized in altitude (e.g., tens of kilometers as opposed to thousands of kilometers). The amplitude of the parallel electric field shows a strong anticorrelation with the plasma density inferred from spacecraft potential measurements. We find no apparent correlation between the amplitude of the parallel electric field and altitudes sampled (between 0.8R_E and 1.5R_E), current density, and K_p, though there is a suggestion that the largest E_∥/E_⊥ ratios are confined to lower altitudes. Taking sampling biases into consideration, we find that the large parallel electric fields occur preferentially at higher values of K_p and within a thin layer centered about 1.28R_E. A detailed analysis favors ambipolar effects over electron inertial effects as an explanation for the parallel electric field signatures.