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Abstract

The paper underlines the view that the appearance of beading and its nonlinear growth in the onset arc occurs independently from the onset of reconnection in the tail at about 20 R_E. Both events follow from an extreme thinning of the central current sheet of the tail at the end of the growth phase. Subsequently, we concentrate on the processes connected with the onset arc breakup. Its origin lies in the instability of a high-beta plasma layer building up at the outer boundary of the dipolar magnetosphere during the substorm growth phase, the growth phase arc (GPA) being the ionospheric trace. The observation of auroral streamers triggering the onset arc instability lets us analyze what is known about auroral streamers with strong support from high-resolution videos of two substorm onsets. We conclude that they may be low-entropy content bubbles with a balanced field-aligned current system, framing a flow channel. However, there are unresolved questions. The visible streamer is identified as an Alfvénic arc. In searching for a mechanism by which a streamer bubble lining up along the GPA can trigger the instability, we are led to the recognition that an entirely new non-MHD process must be at work. Taking also into account the surprising fact that the beads are moving oppositely to the convection in GPA and auroral streamer, we postulate the appearance of a new current system in the gap between the two. What happens can be described as the mating of two current sheets, which were completely separated before. It breaks the stability of the high-beta plasma layer and channels the release and conversion of free internal energy. For this reason, we name the process mating instability. A physical analysis of this process shows consistency with detailed features exhibited by the two videos