The evolution of the U‐shaped double layer at the auroral cavity‐ionosphere boundary in Earth's upward current region

The evolution of the ionosphere‐auroral cavity boundary is studied using a two‐dimensional electrostatic particle‐in‐cell simulation. The boundary is modeled as a double layer which forms by imposing a density cavity as part of the initial conditions. The density cavity consists of a contact discontinuity which separates the cold, dense ionosphere on one side and the hot, tenuous auroral cavity on the other side. The simulation consists of hot electrons, H + and O + antiearthward traveling ion beams, hot magnetospheric H + and cold ionospheric electrons. A U‐shaped double layer is formed by initializing the ion beams with a perpendicular shear in the parallel drift velocity. We show that the strength and obliqueness angle of the evolved double layer depends most strongly on the ratio of the ion beam's gyrofrequency to the plasma frequency, and that the U‐shaped double layer is quasi‐stable on simulation time scales. Finally, we show that the oblique double layer causes perpendicular ion heating in the ion beams and that a ring distribution forms in a weakly magnetized species. However, we find that at high altitudes above the double layer, the dominant effect on the shape of ion phase space and the ion temperature is the turbulence in the auroral cavity, leading to the conclusion that oblique double layers have little lasting effect on the shape and temperature of the ion beams.