Evolution of Asymmetrically Displaced Footpoints During Substorms

Ohma, Anders; Østgaard, Nikolai; Reistad, Jone Peter; Tenfjord, P.; Laundal, K. M.; Snekvik, K.; Haaland, Stein; Fillingim, M. O.

doi:10.1029/2018ja025869

Cited by 24 publications

(49 citation statements)

References 84 publications

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“…Reduced asymmetry is also frequently observed in conjugate auroral images during substorm expansion phase (Ohma et al, ; Østgaard et al, , Østgaard, Humberset, et al, , ). Ohma et al () showed that this was associated with an enhancement of the tail reconnection rate. A more symmetric system is also consistent with the observations of Owen et al (), who studied the orientation of the distant plasma sheet boundary layer.…”

Section: Discussionmentioning

confidence: 87%

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

et al. 2019

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In this study we use high-quality convection data from the Electron Drift Instrument on board Cluster to investigate how near-Earth tail activity affects the average convection pattern in the magnetotail lobes when the interplanetary magnetic field has a dominating east-west (B y ) component. Two different proxies have been used to represent different levels of reconnection in the near-Earth tail: The value of the AL index and the substorm phases identified by the Substorm Onsets and Phases from Indices of the Electrojet algorithm. We find that the convection changes from a dominantly Y GSM direction, but opposite in the two hemispheres, to a flow oriented more toward the plasma sheet, as the north-south component of the convection increases when reconnection enhances in the near Earth tail. This result is consistent with recent observations of the convection in the ionosphere, which suggest that the nightside convection pattern becomes more north-south symmetric when tail reconnection increases. This is also supported by simultaneous auroral observations from the two hemispheres, which shows that conjugate auroral features become more symmetric during substorm expansion phase. The reduced asymmetry implies that the asymmetric pressure balance in the lobes is altered during periods with strong reconnection in the near-Earth tail.

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Section: Discussionmentioning

confidence: 87%

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

et al. 2019

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“…Further SSTDM analysis may include not only more events and substorm regimes but also an improvement of the binning space . In particular, the IMF B y component may modify the dayside reconnection and hence the energy input into the magnetosphere, penetrate the magnetotail, causing its tilt, and substantially modify the whole substorm picture(e.g., Ohma et al, , and refs. therein).…”

Section: Discussionmentioning

confidence: 94%

Signatures of Nonideal Plasma Evolution During Substorms Obtained by Mining Multimission Magnetometer Data

et al. 2019

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Substorm-type evolution of the Earth's magnetosphere is investigated by mining more than two decades of spaceborne magnetometer data from multiple missions including the first two years (2016)(2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X-lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic field B z . X-lines are found to form mainly beyond 20 R E , but for strong driving, with the solar wind electric field exceeding ∼ 5mV/m, they may come closer. For substorms with weaker driving, X-lines may be preceded by redistribution of the magnetic flux in the tailward B z gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 R E , comparable to the thermal ions gyroradius, and at the same time, as long as 15 R E . Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects. Plain Language SummaryThe sun emits a steam of charged particles called the solar wind that flows past the Earth interacting with the planet's dipole magnetic field. This stretches the dipolar magnetic field away from the sun on the nightside of the planet storing energy in the stretched field. Once every few hours, this stretched configuration suddenly becomes more dipolar bringing particles and magnetic flux closer to the planet and powering aurora in the polar regions. During these processes, termed substorms, the gas of charged particles, protons, and electrons trapped by the dipole and known as plasma, behaves largely as a perfectly conducting fluid. However, only deviations from this ideal conducting plasma behavior can explain the substorm mechanisms. We mine two decades of spacecraft magnetometer data from multiple missions to form swarms of thousands of synthetic probes. They help reveal effects of nonideal plasma evolution during substorms, which cannot be captured by direct in situ observations because of their extreme paucity.However, more than half a century ago, while examining the magnetospheric convection cycle, Dungey (1961) concluded that there are regions with topological singularities of the magnetic field, where ideal MHD

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“…Yet when one specifically looks for these B y ‐related flows in the tail, for example, Pitkänen et al (2016), the reconnection event must have already taken place for the flows to be observed and thus the control is clear. Importantly, we also note that too much tail activity, particularly substorms, can inhibit the asymmetry observed in ionospheric flows (e.g., Ohma et al, 2018, 2019; Reistad et al, 2018) and so we have attempted to address this by filtering by Kp and AL in the SuperDARN plots.…”

Section: Discussionmentioning

confidence: 99%

Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF B_y Reversals

et al. 2020

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Past studies have demonstrated that the interplanetary magnetic field (IMF) B y component introduces asymmetries in the magnetosphere-ionosphere (M-I) system, though the exact timings involved are still unclear with two distinct mechanisms proposed. In this study, we statistically analyze convective flows from three regions of the M-I system: the magnetospheric lobes, the plasma sheet, and the ionosphere. We perform superposed epoch analyses on the convective flows in response to reversals in the IMF B y orientation, to determine the flow response timescales of these regions. We find that the lobes respond quickly and reconfigure to the new IMF B y state within 30-40 min. The plasma sheet flows, however, do not show a clear response to the IMF B y reversal, at least within 4 hr postreversal. The ionospheric data, measured by the Super Dual Auroral Radar Network (SuperDARN), match their counterpart magnetospheric flows, with clear and prompt responses at ≥75 • magnetic latitude (MLAT) but a less pronounced response at 60-70 MLAT. We discuss the potential implication of these results on the mechanisms for introducing the IMF B y component into the M-I system.

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Evolution of Asymmetrically Displaced Footpoints During Substorms

Cited by 24 publications

References 84 publications

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Signatures of Nonideal Plasma Evolution During Substorms Obtained by Mining Multimission Magnetometer Data

Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF B_y Reversals

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Evolution of Asymmetrically Displaced Footpoints During Substorms

Cited by 24 publications

References 84 publications

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Observations of Asymmetric Lobe Convection for Weak and Strong Tail Activity

Signatures of Nonideal Plasma Evolution During Substorms Obtained by Mining Multimission Magnetometer Data

Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF By Reversals

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Convection in the Magnetosphere‐Ionosphere System: A Multimission Survey of Its Response to IMF B_y Reversals