The plasma double layer is a possible mechanism for the acceleration of high-energy auroral electrons. Some of the basic properties of double layers are deduced from particle-in-cell computer simulations and are related to parallel electric field structures above the auroral regions. We conclude that a large potential difference applied to a finite length plasma will be concentrated in a shocklike localized region instead of occurring over the entire length of the system. The initial stage of the formation of the double layer is dominated by a large-potential pulse. Once the double layer is formed, the entire potential ϕ0 is dropped over a distance LDL?6(eϕ0/kTe)1/2&lgr;D. For the parameters above the auroral region this distance is about 1 km.