Using one-dimensional (1-D) particle-in-cell (PIC) simulations of a diverging magnetic flux tube, the spatial distribution of parallel electric field in response to an applied potential drop is studied. Hot and cold plasma populations as appropriate for the upward current region are included. Simulations start with an initially empty flux tube representing an auroral density cavity. The cavity refills with plasmas flowing from the ionosphere at the bottom and the magnetosphere at the top. The ionospheric cold ions are significantly accelerated by means of plasma expansion processes in the cavity. When an appropriate warm secondary electron population at the ionospheric boundary is included in the simulations, a significant feature of the expanding ionospheric plasma is a rarefaction shock (RFS) near the bottom of the density cavity. Dynamics of the shock and its role in setting up a low-altitude double layer (DL) are examined. Even after the shock's decay due to the heating of the ionospheric cold electrons by wave-particle interactions, electron heating at low altitudes sustains the enhanced role of plasma expansion in upward acceleration of ions. The applied potential drop does localize like that in a strong DL, but only rarely a strong monotonic DL forms; often the DL is highly dynamic and evolves continually via plasma turbulence consisting of both electron and ion timescales. Averaging the fast fluctuations at the electron timescales, the potential distribution across the quasi-static DL yields oscillatory and spiky parallel fields, which evolve with ion-acoustic and ion hole turbulence. We compare our results with observations from satellites.