Distribution of Region 1 and 2 currents in the quiet and substorm time plasma sheet from THEMIS observations

Liu, Jiang; Angelopoulos, V.; Chu, Xiangning; McPherron, R. L.

doi:10.1002/2016gl069475

Cited by 10 publications

(17 citation statements)

References 38 publications

(77 reference statements)

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“…The observed ionospheric flow shear is just what is expected for the footprint of a magnetospheric flow shear that can trigger and maintain a KHI that manifests as omega bands. This flow shear appears near the boundary between R1 and R2 currents, whose magnetospheric location is at or a little (<~5 R E ) tailward of the inner edge of the plasma sheet (e.g., Liu et al, ). With the T96 model we can indeed map the flow shears to this downtail region (Table S1).…”

Section: Summary and Discussionsupporting

confidence: 90%

“…The plasma sheet flow's ionospheric footprint should be the eastward flow immediately poleward of the flow shear. This eastward flow is within the range of the R1 current which occupies a large portion of the plasma sheet (Liu et al, ) and can become concentrated during active times (Yang et al, , ). When mapped to the equatorial plane, this flow is azimuthally dawnward and in the order of several tens of km/s to 100 km/s.…”

Section: Summary and Discussionmentioning

confidence: 99%

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Flow Shears at the Poleward Boundary of Omega Bands Observed During Conjunctions of Swarm and THEMIS ASI

Liu

Lyons

Archer

et al. 2018

Geophysical Research Letters

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Omega bands are curved aurora forms that evolve from a quiet arc located along the poleward edge of a diffuse auroral band within the midnight to morningside auroral oval. They usually propagate eastward. Because omega bands are a significant contributor to an active magnetotail, knowledge about their generation is important for understanding tail dynamics. Previous studies have shown that auroral streamers, footprints of fast flows in the tail, can propagate into omega bands. Such events, however, are limited, and it is still unclear whether and how the flows trigger the bands. The ionospheric flows associated with omega bands may provide valuable information on the driving mechanisms of the bands. We examine these flows taking advantage of the conjunctions between the Swarm spacecraft and Time History of Events and Macroscale Interactions during Substorms all‐sky imagers, which allow us to demonstrate the relative location of the flows to the omega bands' bright arcs for the first time. We find that a strong eastward ionospheric flow is consistently present immediately poleward of the omega band's bright arc, resulting in a sharp flow shear near the poleward boundary of the band. This ionospheric flow shear should correspond to a flow shear near the inner edge of the plasma sheet. This plasma sheet shear may drive a Kelvin‐Helmholz instability which then distorts the quiet arc to form omega bands. It seems plausible that the strong eastward flows are driven by streamer‐related fast flows or enhanced convection in the magnetotail.

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Section: Summary and Discussionsupporting

confidence: 90%

Section: Summary and Discussionmentioning

confidence: 99%

Flow Shears at the Poleward Boundary of Omega Bands Observed During Conjunctions of Swarm and THEMIS ASI

Liu

Lyons

Archer

et al. 2018

Geophysical Research Letters

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“…Classically, the substorm current wedge was considered to be a Region 1‐type current system, with the cross‐tail current diverted into the ionosphere on the dawnside and the return current appearing on the duskside (Mcpherron et al, ). More recently, it has been suggested that the large‐scale structure of the substorm current wedge consists of both Region 1‐ and Region 2‐type current systems, with the Region 2‐type current system forming earthward of the Region 1‐type system (Coxon et al, ; Liu et al, ; Ritter & Luhr, ; Sergeev et al, ). Figure shows that the nightside upward and downward FACs increase in both the premidnight and postmidnight sectors, with the upward current increasing more in the premidnight sector and the downward current increasing more in the postmidnight sector.…”

Section: Resultsmentioning

confidence: 99%

“…This forms a new FAC system (Akasofu & Chapman, ; Mcpherron et al, ) or enhances the magnitude of and moves existing FACs (Friedrich & Rostoker, ; Rostoker, ; Rostoker & Friedrich, ). Recent studies have suggested that the substorm current wedge could also include a R 2 ‐type current system (Coxon et al, ; Liu et al, ; Ritter & Luhr, ; Sergeev et al, ). Some 15–30 min after the substorm onset, the currents begin to reduce toward their presubstorm levels in a period known as the recovery phase.…”

Section: Introductionmentioning

confidence: 99%

Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic Deflections During Substorms

Forsyth¹,

Shortt²,

Coxon

et al. 2018

JGR Space Physics

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Field‐aligned currents (FACs), also known as Birkeland currents, are the agents by which energy and momentum are transferred to the ionosphere from the magnetosphere and solar wind. This coupling is enhanced at substorm onset through the formation of the substorm current wedge. Using FAC data from the Active Magnetosphere and Planetary Electrodynamics Response Experiment and substorm expansion phase onsets identified using the Substorm Onsets and Phases from Indices of the Electrojet technique, we examine the Northern Hemisphere FACs in all local time sectors with respect to substorm onset and subdivided by season. Our results show that while there is a strong seasonal dependence on the underlying FACs, the increase in FACs following substorm onset only varies by 10% with season, with substorms increasing the hemispheric FACs by 420 kA on average. Over an hour prior to substorm onset, the dayside currents in the postnoon quadrant increase linearly, whereas the nightside currents show a linear increase starting 20–30 min before onset. After onset, the nightside Region 1, Region 2, and nonlocally closed currents and the SuperMAG AL (SML) index follow the Weimer (1994, https://doi.org/10.1029/93JA02721) model with the same time constants in each season. These results contrast earlier contradictory studies that indicate that substorms are either longer in the summer or decay faster in the summer. Our results imply that, on average, substorm FACs do not change with season but that their relative impact on the coupled magnetosphere‐ionosphere system does due to the changes in the underlying currents.

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“…Their formation ushers in one of the magnetotail's most dramatic and repeatable phenomena, magnetospheric substorms. During the substorm growth phase, both transverse and field‐aligned currents are intensified (see reviews by Baker et al [], Kepko et al [], Ganushkina et al [], and references therein) and well measured by properly configured multispacecraft missions [see, e.g., Artemyev et al , ; Liu et al , , and references therein]. This intensification is accompanied by various transients [e.g., Liu et al , ; Sun et al , ; Birn and Hesse , ; Liu et al , ] or by quasi stationary plasma pressure gradients [e.g., Kepko et al , ; Ganushkina et al , , and references therein].…”

Section: Introductionmentioning

confidence: 99%

Electron currents supporting the near‐Earth magnetotail during current sheet thinning

Artemyev

Angelopoulos

Liu

et al. 2017

Geophysical Research Letters

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Formation of intense, thin current sheets (i.e., current sheet thinning) is a critical process for magnetospheric substorms, but the kinetic physics of this process remains poorly understood. Using a triangular configuration of the three Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft at the end of 2015 we investigate field‐aligned and transverse currents in the magnetotail current sheet around 12 Earth radii downtail. Combining the curlometer technique with direct measurements of ion and electron velocities, we demonstrate that intense, thin current sheets supported by strong electron currents form in this region. Electron field‐aligned currents maximize near the neutral plane Bx∼0, attaining magnitudes of ∼20 nA/m2. Carried by hot (>1 keV) electrons, they generate strong magnetic shear, which contributes up to 20% of the vertical (along the normal direction to the equatorial plane) pressure balance. Electron transverse currents, on the other hand, are carried by the curvature drift of anisotropic, colder (<1 keV) electrons and gradually increase during the current sheet thinning. In the events under consideration the thinning process was abruptly terminated by earthward reconnection fronts which have been previously associated with tail reconnection further downtail. It is likely that the thin current sheet properties described herein are similar to conditions further downtail and are linked to the loss of stability and onset of reconnection there. Our findings are likely applicable to thin current sheets in other geophysical and astrophysical settings.

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Distribution of Region 1 and 2 currents in the quiet and substorm time plasma sheet from THEMIS observations

Cited by 10 publications

References 38 publications

Flow Shears at the Poleward Boundary of Omega Bands Observed During Conjunctions of Swarm and THEMIS ASI

Flow Shears at the Poleward Boundary of Omega Bands Observed During Conjunctions of Swarm and THEMIS ASI

Seasonal and Temporal Variations of Field‐Aligned Currents and Ground Magnetic Deflections During Substorms

Electron currents supporting the near‐Earth magnetotail during current sheet thinning

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