Dynamical features and electric field strengths of double layers driven by currents

Abstract

Results from numerical simulations are presented showing the temporal evolution of the potential profiles of double layers in association with the slow temporal fluctuations in the electron current. These fluctuations in our simulations are caused by the electrostatic ion cyclotron waves driven by the current and also by the potential relaxation instability. When the time‐average current density Jo is smaller than the electron thermal current Jth, the formation of a potential dip at the low potential end of a double is a common occurrence even when the conditions for the excitation of ion‐acoustic double layers do not exist. On the other hand, when Jo > Jth the formation of potential dips are associated with interruptions in the current. When the current interruptions are deep causing Je ≃ Jth, where Jth might itself undergo a considerable temporal evolution, the dips are localized. When the current density is large, the double layers tend to form potential humps on their high potential side which are dynamic and may evolve into new double layers. The above features of double layers are seen in different types of numerical simulations ranging from one‐dimensional Vlasov type to two‐and one‐half‐dimensional particle‐in‐cell code. Also, these simulations differ in their initial and boundary conditions. There are some satellite observations on electric fields and on electron beams which indicate the possibility of the formation of potential dips in the parallel potential profiles of the auroral potential drops. Comparisons of the electric field amplitudes from satellite observations with those predicted by two‐and one‐half‐dimensional simulations indicate that the simulations predict electric field strengths of the right order of magnitude.