Shear Flow‐Interchange Instability in Nightside Magnetotail as Proposed Cause of Auroral Beads as a Signature of Substorm Onset

Abstract

A geometric wedge model of the near-Earth nightside plasma sheet is used to derive a wave equation for low-frequency shear flow-interchange waves which transmit ⃗ E × ⃗ B sheared zonal flows along magnetic flux tubes toward the ionosphere. Discrepancies with the wave equation result used in Kalmoni et al. (2015, https://doi.org/10.1002/2015JA021470) for shear flow-ballooning instability are discussed. The shear flow-interchange instability appears to be responsible for substorm onset. The wedge wave equation is used to compute rough expressions for dispersion relations and local growth rates in the midnight region of the nightside magnetotail where the instability develops, forming the auroral beads characteristic of geomagnetic substorm onset. Stability analysis for the shear flow-interchange modes demonstrates that nonlinear analysis is necessary for quantitatively accurate results and determines the spatial scale on which the instability varies.

Are Dawn Storms Jupiter's Auroral Substorms?

“…At Earth, such reconnection closes the magnetic field lines open to the solar wind in the magnetotail, while at Jupiter, reconnection is internally driven (Ge et al., 2010; Kronberg et al., 2005; Vogt et al., 2019; Woch et al., 2002) and is expected to take place on closed field lines. In the middle magnetosphere, various plasma instabilities may occur, such as ballooning instability (Hameiri et al., 1991; Kalmoni et al., 2018; Oberhagemann & Mann, 2020), cross‐field current instability (Lui et al., 1991), shear flow ballooning (Viñas & Madden, 1986) or shear flow‐interchange instability (Derr et al., 2020). Since the magnetic field lines in Jupiter's outer magnetosphere are also highly stretched, and the magnetosphere consists of more energetic ions than the Earth's magnetotail, many plasma instabilities identified in Earth's magnetotail would likely take place in Jupiter's outer magnetosphere.…”

The Onset of a Substorm and the Mating Instability

“…Both the fast rise time of the beads and the narrow gap hint in the same direction, a non-MHD process must be at work. For instance, a shear-flow interchange instability like proposed by Derr et al (2020) or the ballooning-interchange instability of Sorathia et al (2020), both involving flux tubes, would have growth times of at least twice the Alfvénic propagation time to the ionosphere, that is, more than one minute. As suggested by Kalmoni et al (2015), Nishimura et al (2016), and in the earlier literature (see Section 2), it must be some sort of kinetic ballooning instability, but proceeding locally in the hot plasma layer.…”

“…In their work on shear‐flow interchange instability, Derr et al. (2020) show growth phase simulations with the RCM‐E model clearly exhibiting beta values up to 40 in the central near‐Earth plasma sheet. Coroniti and Pritchett (2014) related the quiet evening arc and in continuation the growth phase arc to the current and pressure distribution in the near‐Earth tail.…”

“…However, more and more investigations, such as Kalmoni et al (2015), Nishimura et al (2016), andLui (2020) come to the same conclusions. With respect to the instability process as such, an inspection of the auroral images and theoretical work has strongly promoted the conviction that some sort of ballooning instability, preferentially related to shear flows, must be at work (Derr et al, 2020;Kalmoni et al, 2015;Ohtani & Tamao, 1993;Pritchett & Coroniti, 2010;Roux et al, 1991;Voronkov et al, 1997). Current disruption (Lui, 1991;Lui et al, 1990) or cross-field current instability (Kalmoni et al, 2015;Lui, 2020) have been proposed as alternate processes, while Haerendel (2010) interpreted the growing structures as resulting from the inflow from a collapsing current sheet.…”

“…The initial brightening of the growth phase arc is characterized by the appearance of a fast moving wavy structure, so‐called beads, which quickly develop into larger‐scale bright folds, protrusions, or vortexes (Donovan et al., 2006; Dubyagin et al., 2003). Obviously, a nonlinear instability is proceeding, which has been commonly interpreted as a kind of ballooning instability (Derr et al., 2020; Kalmoni et al., 2015; Pritchett & Coroniti, 2010; Ohtani & Tamao, 1993; Roux et al., 1991; Voronkov et al., 1997). In recognition of these manifestations and interpretations of the initial brightening, we are using for it synonymously the terms “onset arc breakup” and “onset arc instability”.…”