On the Asymmetry Between Upward and Downward Field‐Aligned Currents Interacting With the Ionosphere

The paper presents results from the numerical study of the magnetosphere‐ionosphere interactions driven by the large‐scale electric field in the magnetically conjugate, high‐latitude regions of northern and southern hemispheres. Simulations of the two‐fluid MHD model demonstrate that these interactions can lead to a generation of a system of small‐scale, intense field‐aligned currents with a significant difference in size and amplitude between the upward and downward currents. In particular, in both hemispheres, the downward currents (where the electrons are flowing from the ionosphere) become more narrow and intense than the adjacent upward currents. At high latitudes, the field‐aligned currents are closely related to the discrete auroral arcs. The fact that this mechanism produces very narrow and intense downward currents embedded into the broader upward current regions makes it relevant to the explanation of the “black” auroral arcs appearing as narrow, dark strips embedded in the broad luminous background. Key Points The nonlinear stage of the magnetosphere‐ionosphere interactions conducted by the electromagnetic ULF waves at high altitudes is investigated We demonstrate that MI interactions can generate FAC system with significantly different upward and downward currents The ionospheric parameters leading to the formation of the “black” aurora have been identified