Characteristics of high‐latitude precursor flows ahead of dipolarization fronts

Li, Jiazheng; Zhou, X.‐Z.; Runov, A.; Angelopoulos, V.; Liu, Jiang; Pan, Dong‐Xiao; Zong, Qiugang

doi:10.1002/2017ja023902

Cited by 5 publications

(5 citation statements)

References 60 publications

(77 reference statements)

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“…If the ion were not captured by the virtual spacecraft, it would have kept moving along the field lines and exited the DFB region in the opposite (Northern) Hemisphere. This type of ion acceleration and DF reflection, also observed and discussed in Eastwood et al () and Li et al (), differs from the near‐equatorial DF reflection process discussed earlier. The near‐field‐aligned ion acceleration and reflection mainly affects parallel‐moving ions that encounter the DFB at high latitudes.…”

Section: Interpretationssupporting

confidence: 49%

“…Note that in the adopted DFB model, the B z perturbation is assumed to be independent of z, which may not be realistic since the DF-associated B z enhancement is often observed to be weaker at larger |z| values. Although the model may be refined to accommodate the z dependence of the B z perturbation (which requires variations of B x and B y in the DF vicinity to satisfy the divergence-free condition of the magnetic field; see Li et al, 2017 equations ( 8)-( 12)), here we decide to retain the simpler model to avoid complications from the secondary, and somewhat artificial, B x and B y variations.…”

Section: Simulationsmentioning

confidence: 99%

“…They also carry a secondary current with earthward and dawnward components in the ambient plasma sheet (Pan et al, ; Zhou, Angelopoulos, Liu, Runov, Li, ), which can be approximately treated as the ion diamagnetic current associated with the plasma compression, to generate the B z dips frequently observed ahead of the DF (Runov et al, ; Schmid et al, ; Yao et al, , ). The DF‐reflected ions can even stream along magnetic field lines, to constitute the characteristic crescent‐shaped ion structures in the plasma sheet boundary layer (Birn, Hesse, et al., ; Li et al, ; Zhou et al, ).…”

Section: Introductionmentioning

confidence: 99%

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On the Acceleration and Anisotropy of Ions Within Magnetotail Dipolarizing Flux Bundles

et al. 2018

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Dipolarizing flux bundles (DFBs), earthward propagating structures with enhanced northward magnetic field Bz, are usually believed to carry a distinctly different plasma population from that in the ambient magnetotail plasma sheet. The ion distribution functions within the DFB, however, have been recently found to be largely controlled by the ion adiabaticity parameter κ in the ambient plasma sheet outside the DFB. According to these observations, the ambient κ values of 2–3 usually correspond to a strong perpendicular anisotropy of suprathermal ions within the DFB, whereas for lower κ values the DFB ions become more isotropic. Here we utilize a simple, test particle model to explore the nature of the anisotropy and its dependence on the ambient κ values. We find that the anisotropy originates from successive ion reflections and reentries to the DFB, during which the ions are consecutively accelerated in the perpendicular direction by the DFB‐associated electric field. This consecutive acceleration may be interrupted, however, when magnetic field lines are highly curved in the ambient plasma sheet. In this case, the ion trajectories become stochastic outside the DFB, which makes the reflected ions less likely to return to the DFB for another cycle of acceleration; as a consequence, the perpendicular ion anisotropy does not appear. Given that the DFB ions are a free energy source for instabilities when they are injected toward Earth, our simple model (that reproduces most observational features on the anisotropic DFB ion distributions) may shed new lights on the coupling process between magnetotail and inner magnetosphere.

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Section: Interpretationssupporting

confidence: 49%

Section: Simulationsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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On the Acceleration and Anisotropy of Ions Within Magnetotail Dipolarizing Flux Bundles

et al. 2018

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“… 2015 ; Li et al. 2017 ). Such populations were found to result from a single encounter and reflection of ambient ions at a DF (Zhou et al.…”

Section: Explosive Magnetotail Dynamicsmentioning

confidence: 99%

Explosive Magnetotail Activity

et al. 2019

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Modes and manifestations of the explosive activity in the Earth’s magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field ; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased , prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.

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“…The lack of multiple DFB reflections and reentries is related to the fact that SI1 encounters the DF at high latitudes: the large B x field at this location enables SI1 to continue its tailward motion (along magnetic field lines; see Figure h) rather than being immediately reflected at the DF. Without multiple reflections, SI1 can only be weakly accelerated during its DF penetration (see Eastwood et al, and Li et al, for DFB ion behavior with considerable field‐aligned velocity components). Moreover, the SI1 trajectory is typical among the ions that can reach vs1, because to reach high latitudes all these ions must have had significant velocity components in the field‐aligned direction.…”

Section: Prediction From Simulationsmentioning

confidence: 99%

On the Origin of Perpendicular Ion Anisotropy Inside Dipolarizing Flux Bundles

Zhou

Runov

et al. 2019

JGR Space Physics

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Perpendicular anisotropy of suprathermal ions, observed inside some of the dipolarizing flux bundles (DFBs) in the magnetotail plasma sheet, have been attributed to successive, betatron-type accelerations during the DFB entry of ambient ions. It has been unclear, however, where and how these ions enter the DFBs. The proposed locations include the DFB flanks where cross-tail drifting ions are picked up, and the DFB leading edge with sharp magnetic field gradient (the dipolarization front, DF). Here we examine the latter scenario, based on a simplistic, test particle approach, to predict the preferred conditions for the appearance of the DFB ion anisotropy. Our model predicts that the ion anisotropy would be stronger at locations closer to the neutral sheet and would appear preferentially in the DFB dawnside and central sectors rather than the duskside sector. We also predict that the ion anisotropy would more likely be observed in DFBs with higher propagation speeds. These properties can be understood in our model by the dawnward drift of ions during their DF penetration (attributed to the large magnetic gradient). To examine these predictions, we carry out a statistical survey based on observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission, to show a clear dependence of the ion anisotropy on spacecraft location and the DFB propagation speed. These findings, therefore, are consistent with the scenario that the perpendicular ion anisotropy originates from the ion acceleration and penetration across sharp DFs.

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Characteristics of high‐latitude precursor flows ahead of dipolarization fronts

Cited by 5 publications

References 60 publications

On the Acceleration and Anisotropy of Ions Within Magnetotail Dipolarizing Flux Bundles

On the Acceleration and Anisotropy of Ions Within Magnetotail Dipolarizing Flux Bundles

Explosive Magnetotail Activity

On the Origin of Perpendicular Ion Anisotropy Inside Dipolarizing Flux Bundles

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