Double layers and density cavitons in downward current region of aurora

The nonlinear coupling equations of low-frequency potential, high-frequency modulation field and density disturbance in strong turbulent plasmas were numerically calculated in the static limit case. The evolutions of double layers and density cavitons were obtained. The results show that the double layer in downward current region of aurora is formed by localized drop in density owing to collapsed high frequency field. The structure is a double-double layer and a nonlinear entity: caviton. The ramp potential of the double layer is in range of 14 to 40 V, and the thickness of the structure is between 100 and 200 m, roughly tens of Debye lengths; the maximum density perturbation rate is up to more than 80%, which are all in harmony with the measurements by the Fast Auroral Snapshot satellite