An auroral ionosphere is generally incompressive and non-uniform medium with anisotropic conductivities. Compressibility may occur, however, following the onset of field line dipolarization. This behavior can happen when; (1) Westward directing electric fields transmitted from the dipolarization region accumulate both electrons and ions in equatorward latitudes in F region. (2) The mobility difference of electrons and ions in E region produces electrostatic potential in a quasi-neutral condition, positive in higher latitudes and negative in lower latitudes. (3) Density modulation in F region excites ion acoustic wave propagating along the field lines towards the magnetosphere. (4) The ion acoustic wave stops in the ionosphere for about 4 min because of a low phase velocity (~ 1.6 km/s). During this compressive interval, density accumulation in equatorward latitudes expands upstream to form a poleward expansion of auroras analogous to upstream propagation of a shock in traffic flow on crowded roads. Electrostatic potential produced in the E region generates field-aligned currents and closing Pedersen currents to retain electrostatic potential in a quasi-neutral ionosphere. The ion acoustic wave produces upward electric fields along the field lines in accordance with the Boltzmann relation which contributed to the ion upflow at topside ionosphere.