A statistical study of large‐amplitude parallel electric fields in the upward current region of the auroral acceleration region

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 0.8 R E to 1.5 R 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 0.8 R E and 1.5 R 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 1.28 R E . A detailed analysis favors ambipolar effects over electron inertial effects as an explanation for the parallel electric field signatures.