Test particle simulations of the effect of moving DLs on ion outflow in the auroral downward-current region

Abstract

Test particle simulations of ion outflow in the auroral downward‐current region are carried out to investigate the effect of a series of highly localized, moving double layers. Double layers (DLs) have been observed in the downward‐current region, but may not be the only source of electron acceleration. However, when present, they may dramatically affect ion outflow. In the simulation the double layers (DLs) move anti‐earthward along the magnetic field (B) at the ion‐acoustic speed and are assigned a perpendicular ion heating profile along B that moves with them. The DL speed and the associated ion heating profile are based on in situ observations and dynamic simulations. This configuration greatly changes the mechanism of the ion outflow in the downward‐current region from the classical ion‐pressure cooker picture. In the moving DL model, a majority of out‐flowing ions do not pass through the DL. Instead, the ions tend to be pushed in front of a DL or trapped between two DLs which enforce outward motion and result in a significant increase in net number and energy fluxes of upgoing ions. The ion distributions from the simulations compare well to FAST observations as a function of the relative distance from the DL position along B.