The characteristics of double layers driven by an applied potential drop and by an injected current into a plasma are compared. In the latter case the potential drop across the double layer appears because of the formation of a virtual cathode. The double layers formed by the two mechanisms show striking similarities with regard to their structure, temporal evolution and dynamics. However, in the case of current-driven double layers a large energization of ions trapped in the virtual cathode region is observed. Such a large energization of trapped ions is not seen in the case of the potential-drop driven double layers. The interrelation between the field-aligned currents and potential drops for the euroral plasma is discussed. For current-driven double layers, it is found that the current density Je&agr;ϕ01/2, where ϕ0 is the potential drop.