The Onset of a Substorm and the Mating Instability. Issue 10 (19th October 2021)

• Record Type: Journal Article
• Authors: Haerendel, Gerhard; Frey, Harald
• Is Part Of: Journal of geophysical research. Volume 126:Issue 10(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-10-19
• Subjects: substorm onset -- growth phase arc -- auroral streamer -- low‐entropy bubble -- current sheet mating -- mating instability; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA029492
• Languages: English
• ISSNs: 2169-9380

Abstract

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 RE . 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 Plain Language Summary: The substorm begins with two independent events of common origin. The first one is normally the brightening of the growth phase arc, which is the trace of a hot plasma layer forming at the inner border of the tail. The second one is due to flow bursts, emerging from reconnection in the tail, arriving at the inner edge of the tail, the very reason for the substorm. The common origin is the thinning of the tail current sheet due to the stretching by the solar wind. The paper is devoted to the understanding of the first event. As observed about 10 years ago, the brightening of the growth phase arc with beadlike structures and growth of instability is often triggered by a weak arc, an auroral streamer, arriving from high latitudes and lining up with the growth phase arc. The paper proposes that the trigger process involves the formation of a new current circuit between the two arcs by the mating of the neighboring current sheets, which involves a non‐MHD process. It generates the sudden appearance and motion of the beads and constitutes a channel for the outflow of internal energy of the high‐energy plasma. Key Points: Growth phase arc brightening and instability and reconnection in the near‐Earth tail are completely separate processes Auroral streamers may be low‐entropy content bubbles with a flow channel attached and manifested by an Alfvénic arc Mating of two unconnected current sheets by a non‐MHD process creates a channel for outflow of energy from the high‐beta plasma layer