Effects of Ion Slippage in Earth's Ionosphere and the Plasma Sheet

["Although triggering mechanisms for substorm onsets remain highly controversial, consensus has reached that violation of frozen-in flux condition in the central plasma sheet is required. In this study, we carry out a numerical gedanken experiment to investigate the effects of the violation by assuming ions slip with respect to the magnetic field lines in the late substorm growth phase while electrons remain magnetized, without specifying the microphysics. The simulation results predict (1) a thin arc and a strong westward electrojet associated with downward-upward-downward field-aligned currents and westward-eastward-westward horizontal flows in the ionosphere, which are found to be consistent with a preonset arc observed by the Swarm and the all-sky imager; (2) a rapid creation of a bubble-blob pair in the plasma sheet with a tailward hump of B-z that may lead to tearing instability.","Plain Language Summary Aurora borealis are fascinating phenomena in Earth's high latitude regions. Their various forms, ranging from spirals, rays, streamers, beads, and arcs, change from time to time. One particular structure, as known as the substorm preonset arc, emerges in the sky prior to a very dynamic energy conversion process in near-Earth space. Several mechanisms for the formation of these east-west-aligned arcs have been proposed, and in this study, we propose a different one. Using a numerical simulation, we show that preonset arcs connect with the nightside magnetosphere, where the conservation of entropy is violated. Such a violation creates higher concentrations of charged particles at a larger radial distance, lower concentrations of particles at a smaller radial distance. Significant electric currents arising from that process leads to the precipitation of electrons into the ionosphere, causing aurora light. We also found that a microphysical instability, called tearing, may be triggered in the current sheet, causing further release of magnetic energy. Our simulation results, which are found to be consistent with satellite observations in Earth's ionosphere, shed some light on how substorms progress from the end of growth phase toward onset."]