Substorm current wedge composition by wedgelets

Liu, Jiang; Angelopoulos, V.; Chu, Xiangning; Zhou, X.‐Z.; Yue, Chao

doi:10.1002/2015gl063289

Cited by 69 publications

(97 citation statements)

References 41 publications

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“…That figure shows region 2 sense FACs forming at the leading edge and a dominant region 1 sense current associated with the main part of the bubble, with longitudinal velocity on the earthward side and a region of earthward velocity on the tailward side. This FAC morphology is also consistent with observations of FACs at dipolarization fronts [ Yao et al , ; Sun et al , ; Liu et al , ] which are believed to represent the leading edge of flow bursts. A quantitative evaluation of the motion of the streamer FACs compared to the local ionospheric convection can be used to further examine mechanisms for FAC generation as suggested by numerical simulations and magnetotail observations of dipolarization fronts.…”

Section: Discussionsupporting

confidence: 90%

High‐spatial‐resolution velocity measurements derived using Local Divergence‐Free Fitting of SuperDARN observations

2016

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A new technique for analysis of Super Dual Auroral Radar Network (SuperDARN) line‐of‐sight velocity observations enables resolving plasma convection with unprecedented spatial resolution. The technique, Local Divergence‐Free Fitting (LDFF), can be used to produce maps with a spatial resolution that is determined by the resolution of the observations rather than an arbitrary fit order. Other techniques, which express the potential as a sum of harmonic functions, limit the number of functions in the expansion to the fit order. Doing so imposes a limit on the minimum size of features that will be represented in the results. The LDFF technique is not limited by this constraint. Rather, it is limited by the resolution of the observations and the amount of regularization required by the observed noise level, which generally allows finer‐scale features to be represented. The LDFF technique is described and then applied to a synthetic data set to demonstrate its validity. Then high‐resolution convection maps are presented from an interval during which auroral observations over Alaska showed poleward boundary intensifications (PBIs) and auroral streamers. Overlays of the convection vectors on the auroral images illustrate correspondence between flow features and the auroral luminosity. Detailed comparison between the flows and images showed that the PBIs originated from polar cap boundary arcs that extended away from midnight toward earlier local times. As the arcs extended they were accompanied by enhanced shear flow. The arcs intensified then moved equatorward becoming streamers. As the arcs moved, the region of shear flow followed their motion, indicating a pattern of field‐aligned current associated with the moving arc. The observations are the most comprehensive and detailed known to the authors for such an interval and agree well with the expected plasma flows based upon magnetospheric simulations. Flow vectors generated for the interval by the spherical harmonic fit technique for the most part do not show the direct relationship between the convection and the localized luminosity features nearly as well as the LDFF results. Some of the features found in the LDFF fitting are simply not present in the results from the global fits. The LDFF technique should find application in a variety of studies where high‐spatial‐resolution estimates of plasma flows are required. The example study presented here, which examined the details of flows in the region of auroral arcs, is representative of such problems.

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Section: Discussionsupporting

confidence: 90%

High‐spatial‐resolution velocity measurements derived using Local Divergence‐Free Fitting of SuperDARN observations

2016

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“…The additional ground magnetic field perturbation may come about through small-scale up-down current pairs within MLT sectors that are not resolved by AMPERE. It has been proposed that the substorm current wedge could arise from the summation of a series of elemental currents (Birn & Hesse, 2013;Liu et al, 2015;Lyons et al, 2012Lyons et al, , 2013Rostoker, 1991;Zhang et al, 2011), which may or may not be linked to magnetospheric flows. Forsyth et al (2014) showed that a large-scale substorm current wedge covering 4 hr of MLT had a longitudinal structure consisting of more than thirty~100 km wide north-south aligned current sheets.…”

Section: Time Profiles Of Postonset Facsmentioning

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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“…These current layers have thicknesses estimated to be comparable to plasma sheet proton gyroradius (~500 to ~1,000 km; e.g., Liu et al, ; Runov et al, ; Sun, Fu, Pu, et al, ). They are also associated with kinetic features (e.g., H. S. Fu et al, ; Sergeev et al, ; Sun, Fu, Pu, et al, ) and field‐aligned currents (e.g., Liu et al, , ; Sun et al, , Sun, Fu, Parks, et al, ) as well.…”

Section: Introductionmentioning

confidence: 99%

Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

Zhao

Sun

et al. 2018

JGR Space Physics

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A dipolarization front (DF) is known as the leading edge of an earthward high‐speed flow with a sharp enhancement in the northward magnetic field (Bz). Analysis of an event observed by Cluster shows that the behavior of oxygen ions (O+) around the DF is very different from protons (H+). After the crossing of the DF, the O+ density decreases more gradually than H+. The distance between the density minimum of O+ and the DF layer is ~4 times longer than that of H+, which is close to their gyroradii ratio with the same energy. A flux dropout is observed in the O+ energy spectrum, whose energy dependence indicates that ions with higher energies can reach locations farther tailward of the DF. Similar variations are also seen in studies of 22 events in which a common pattern of ion properties is obtained by performing a superposed epoch analysis. Finally, using backward tracing test‐particle simulations, we reproduce the characteristics of the flux dropout and verify that the time dependence of the dropout is highly correlated with the gyromotion of different energy O+ behind the DF. All these results provide a further understanding of ion dynamics associated with DFs and suggest that the observed O+ ions are reflected within a half gyromotion in the central plasma sheet.

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Substorm current wedge composition by wedgelets

Cited by 69 publications

References 41 publications

High‐spatial‐resolution velocity measurements derived using Local Divergence‐Free Fitting of SuperDARN observations

High‐spatial‐resolution velocity measurements derived using Local Divergence‐Free Fitting of SuperDARN observations

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

Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

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