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  1. Free, publicly-accessible full text available November 1, 2023
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  4. Abstract

    Auroral observations were first to identify the substorm, and later used to propose that substorm onset is triggered in the inner plasma sheet (equatorward portion of the auroral oval) by an intrusion of low entropy plasma comprising plasma sheet flow channels. Longitudinal localization makes the intruding flow channels difficult to observe with spacecraft. However, they are detectable in the ionosphere via the broader, two‐dimensional coverage by radars. Line‐of‐sight radar flow measurements have provided considerable support for the onset proposal. Here we use two‐dimensional, ionospheric flow maps for further testing. Since these maps are derived without the smoothing from global fits typically used for global convection maps, their spatial resolution is significantly improved, allowing representation of localized spatial structures. These maps show channels of enhanced ionospheric flow intruding to the time and location of substorm onset. We also see evidence that these intruding flows enter the plasma sheet from the polar cap, and that azimuthal spread of the reduced entropy plasma in the inner plasma sheet contributes to azimuthal onset spreading after initial onset. Identified events with appropriate radar data remain limited, but we have found no exceptions to consistency with flow channel triggering. Thus, these analyses strongly support the proposal that substorm onset is due to the intrusion of new plasma to the onset region. The lower entropy of the new plasma likely changes the entropy distribution of inner plasma sheet, a change possibly important for the substorm onset instability seen via the growing waves that demarcate substorm auroral onset.

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  5. Abstract

    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 ofBzthat may lead to tearing instability.

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