Double-Layer Driven Plasma Filaments in the Upward Current Region of the Auroral Plasma

Simulations of double layers (DL) were performed using a 2.5-dimensional particle-in-cell code. The simulations included hot and cold plasmas on the low- and high-potential sides of the DL, respectively. This configuration of plasma and polarity of the potential corresponds to the DL, which accelerate electrons downward and ions upward in the auroral plasma. Both electrons and ions in the simulations are strongly magnetized. A persistent feature of the simulations performed for a variety of combinations of the densities and temperatures of the cold and hot plasmas is that the formation of a planar DL is immediately followed by the formation of thin and magnetic-field aligned plasma filaments. The process of filament formation begins just above the DL in the hot plasma permeated by an ion beam accelerated by the DL itself. A preliminary analysis shows that the beam-driven electrostatic ion-cyclotron (EIC) modes filament the DL and the hot plasma as the driven waves propagate upward with the beam velocity. The filamented structures appear in the plasma density, electron and ion energies as well as in parallel and perpendicular electric fields. When the filamented DL rises sufficiently above the ionospheric cold-plasma boundary, the plasma heating below the DL depletes the plasma creating a density cavity. In response to the cavity formation, the filamented DL moves downward filling the cavity with the hot plasma from the top. During the period of this downward motion, the DL tends to regain its initial laminar (planar) feature. As soon as the laminar DL reforms, the EIC waves are triggered again and the above process involving the filament formation reoccurs. The relevance of the recurring process of filament formation driven by a DL to satellite observations will be discussed.