Simultaneous ground‐satellite observations of meso‐scale auroral arc undulations

Motoba, T.; Hosokawa, K.; Ogawa, Y.; Sato, Natsuo; Kadokura, Akira; Milan, S. E.; Lester, M.

doi:10.1029/2011ja017291

Cited by 8 publications

(10 citation statements)

References 40 publications

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“…Our work has demonstrated for the first time that as a macroscopic coherent process, the ideal MHD ballooning instability is capable of inducing the formation of plasmoids in the magnetotail configuration without relying on any microscopic, kinetic, or turbulent processes. In light of recent evidence found in ground and in situ observations for the presence of ballooning instability in the pre‐onset auroral and plasma sheet structures [ Saito et al , , ; Panov et al , ; Motoba et al , , ], our findings on the ballooning instability‐induced plasmoid formation may indeed provide a solid and practical scheme for ballooning instability in the near‐Earth magnetotail to play a critical role in triggering the substorm onset process.…”

Section: Summary and Discussionsupporting

confidence: 62%

Ballooning instability‐induced plasmoid formation in near‐Earth plasma sheet

Zhu

Raeder

2014

JGR Space Physics

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[1] The formation of plasmoids in the near-Earth magnetotail is believed to be a key element of the substorm onset process. Previous work has identified a new scenario in MHD simulations where the nonlinear evolution of a ballooning instability is able to induce the formation of plasmoids in a generalized Harris sheet with finite normal magnetic component. In present work, we further examine this novel mechanism for plasmoid formation and explore its implications in the context of substorm onset trigger problem. For that purpose, we adopt the generalized Harris sheet as a model proxy to the near-Earth region of magnetotail during the substorm growth phase. In this region the magnetic component normal to the neutral sheet B n is weak but nonzero. The magnetic field lines are closed, and there are no X lines. Simulation results indicate that in the higher Lundquist number regime S & 10 4 , the linear axial tail mode, which is also known as "two-dimensional resistive tearing mode," is stabilized by the finite B n , hence cannot give rise to the formation of X lines or plasmoids by itself. On the other hand, the linear ballooning mode is unstable in the same region and regime, and its nonlinear development leads to the formation of a series of plasmoid structures in the near-Earth and middle magnetotail regions of plasma sheet. This new scenario of plasmoid formation suggests a critical role of ballooning instability in the near-Earth plasma sheet in triggering the onset of a substorm expansion.

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Section: Summary and Discussionsupporting

confidence: 62%

Ballooning instability‐induced plasmoid formation in near‐Earth plasma sheet

Zhu

Raeder

2014

JGR Space Physics

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“…The eastward propagation velocity along the orientation of the auroral form is estimated at 1,200 m/s. Similar eastward propagating auroral undulations were also reported by Motoba et al ().…”

Section: Event Studiessupporting

confidence: 90%

Longitudinal Development of Poleward Boundary Intensifications (PBIs) of Auroral Emission

et al. 2018

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We have investigated the longitudinal propagation and extension of the poleward boundary intensifications (PBIs) of auroral emission by examining four events, Events 1-4, which show different spatiotemporal structures. In Event 1 an auroral form extended both eastward and westward immediately following the arrival of a fast polar cap flow and became dynamic as the polar cap flow enhancement continued. The PBI extended 3 hr in local time in a few minutes, which questions the conventional idea that the PBIs are an ionospheric manifestation of distant reconnection. In Events 2 and 3, an auroral form was already dynamic and was collocated with an upward field-aligned current (FAC), which, along with an adjacent downward FAC, formed a longitudinal flow channel confined near the poleward boundary of the auroral oval. Auroral structures propagated in the direction of this longitudinal convection flow. In Event 4, as a transient westward convection flow arrived, a new auroral form developed and extended also westward but noticeably faster than the convection flow, and it faded as it extended. These results suggest that the longitudinal ionospheric convection plays a critical role in the formation and development of the PBIs. They are consistent with a recently proposed idea that the PBIs are an effect of the ionospheric electrostatic polarization, which deflects the enhanced polar cap flow from equatorward to along the auroral oval at its poleward boundary. The contrast between Events 1 and 4 suggests that morphological differences of the PBIs reflect different durations and intensities of the polar cap flow enhancement.

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“…Azimuthally periodic auroral forms are commonly seen on equatorward arcs prior to substorm breakup [ Murphree et al , 1994 ; Elphinstone et al , 1995 ; Donovan et al , 2007 , 2008 ; Liang et al , 2008 ; Sakaguchi et al , 2009 ; Henderson , 2009 ; Rae et al , 2009a , 2009b ] and undulations in the poleward arc have been reported during a substorm recovery phase [ Motoba et al , 2012a ]. The equatorward arc forms are observed to brighten and expand prior to the auroral breakup, which has been associated with various instabilities in the inner magnetosphere [ Rae et al , 2010 ].…”

Section: Discussionmentioning

confidence: 99%

In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge

Forsyth¹,

Fazakerley²,

Rae³

et al. 2014

JGR Space Physics

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The substorm current wedge (SCW) is a fundamental component of geomagnetic substorms. Models tend to describe the SCW as a simple line current flowing into the ionosphere toward dawn and out of the ionosphere toward dusk, linked by a westward electrojet. We use multispacecraft observations from perigee passes of the Cluster 1 and 4 spacecraft during a substorm on 15 January 2010, in conjunction with ground-based observations, to examine the spatial structuring and temporal variability of the SCW. At this time, the spacecraft traveled east-west azimuthally above the auroral region. We show that the SCW has significant azimuthal substructure on scales of 100 km at altitudes of 4000–7000 km. We identify 26 individual current sheets in the Cluster 4 data and 34 individual current sheets in the Cluster 1 data, with Cluster 1 passing through the SCW 120–240 s after Cluster 4 at 1300–2000 km higher altitude. Both spacecraft observed large-scale regions of net upward and downward field-aligned current, consistent with the large-scale characteristics of the SCW, although sheets of oppositely directed currents were observed within both regions. We show that the majority of these current sheets were closely aligned to a north-south direction, in contrast to the expected east-west orientation of the preonset aurora. Comparing our results with observations of the field-aligned current associated with bursty bulk flows (BBFs), we conclude that significant questions remain for the explanation of SCW structuring by BBF-driven “wedgelets.” Our results therefore represent constraints on future modeling and theoretical frameworks on the generation of the SCW.Key PointsThe substorm current wedge (SCW) has significant azimuthal structureCurrent sheets within the SCW are north-south alignedThe substructure of the SCW raises questions for the proposed wedgelet scenario

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Simultaneous ground‐satellite observations of meso‐scale auroral arc undulations

Cited by 8 publications

References 40 publications

Ballooning instability‐induced plasmoid formation in near‐Earth plasma sheet

Ballooning instability‐induced plasmoid formation in near‐Earth plasma sheet

Longitudinal Development of Poleward Boundary Intensifications (PBIs) of Auroral Emission

In situ spatiotemporal measurements of the detailed azimuthal substructure of the substorm current wedge

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