Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

Zhao, Shaodong; Fu, S. Y.; Sun, W.; Parks, G. K.; Zhou, X.‐Z.; Pu, Z. Y.; Zhao, Dan; Wu, Tong; Yu, F. B.; Zong, Qiugang

doi:10.1029/2018ja025689

Cited by 8 publications

(9 citation statements)

References 49 publications

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“…The parameters of O + exhibited similar variations as the H + . However, an increase of N O+ ahead of the DF and a decrease of N O+ in the DFB were more gradual than those of N H+ , which is consistent with the features of O + around DF investigated in Zhao et al ().…”

Section: Observationssupporting

confidence: 90%

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Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

Zhao

Sun

et al. 2019

JGR Space Physics

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Cluster observed two intermittent oxygen ion (O + ) flux enhancements with energy dispersions in a dipolarizing flux bundle, which is known as a region of enhanced northward magnetic field (B z ) embedded in the earthward high-speed flow. The flux enhancements of O + show clear pitch angle dependences, which are termed as butterfly distributions. Two corresponding flux enhancements of field-aligned protons (H + ) are also shown in its spectrum, but they are weaker and emerge later (~10 s) than those of O + . Simulation shows that both enhanced ion species are the counterstreaming populations. They originated from the lobe region and were driven into the center plasma sheet by the dawn-dusk electric field (E y ). Backward tracing test-particle simulations reproduce the butterfly O + and the counterstreaming H + distribution. The differences between O + and H + are because of their different gyroradii. The lobe O + can arrive at the magnetic equatorial plane in less than one gyromotion due to its large gyroradius, and O + with a larger field-aligned velocity can arrive at the equatorial plane earlier, leading to the energy and pitch angle dependence. While H + with similar energy can drift into dipolarizing flux bundle through electric field drift (E × B motion) and arrive at the equatorial plane through adiabatic motion, which consequently forms the field-aligned flux enhancements in dipolarizing flux bundle, that is, the B z -dominant region. The simulation further confirms that intermittent increases of E y component can produce the two intermittent flux enhancements, as indicated in the in situ observation.

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Section: Observationssupporting

confidence: 90%

“…In this section, we applied a backward tracing test-particle simulation (Zhao et al, 2018;Zhou et al, 2011Zhou et al, , 2014 to study the features of butterfly O + and counterstreaming H + . In the simulation, the first step is to set an initial ion distributions f (r i , v i , t i ) in a DFB model.…”

Section: Test-particle Simulationsmentioning

confidence: 99%

Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

Zhao

Sun

et al. 2019

JGR Space Physics

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“…S. J. Zhao et al. (2018) showed flux increases in the 25–40 keV energy range after the passage of a DF, decaying more slowly than those of H + ions while S. J. Zhao et al. (2019) found pitch angle‐dependent enhancements at lower, 2–9 keV, energies, described as butterfly distributions, together with bi‐directional field‐aligned H + distributions.…”

Section: Introductionmentioning

confidence: 99%

Acceleration of Oxygen Ions in Dipolarization Events: 1. CPS Distributions

et al. 2021

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Large increases in fluxes of energetic ions from a few tens to hundreds of keV are commonly observed in the magnetotail in association with dipolarization events and substorms. While most of the observations could not distinguish protons from heavier ions, several of the early studies already indicated significant flux increases of energetic O + ions correlated with activity (e.g., Fritz & Wilken, 1976;Ipavich et al., 1984;Möbius et al., 1987). More recently, Kronberg et al. (2015) used seven years of Cluster observations in the plasma sheet to study fluxes of oxygen ions and protons at energies from 274 to 955 keV. Compared with protons, the distribution of energetic oxygen was found to have stronger dawn-dusk asymmetry related to geomagnetic activity. Significant intensity increases at the near-Earth duskside tail were found to be correlated with disturbed geomagnetic conditions, enhanced solar wind dynamic pressure, and southward IMF. They concluded this to indicate effective inductive acceleration mechanisms and a strong duskward drift. Maggiolo and Kistler (2014) also documented significant amounts of O + at 1-40 keV in the plasma sheet correlated with geomagnetic activity and solar EUV flux.A large number of investigations have documented the important, and sometimes dominant, contribution of O + ions to the total pressure in geomagnetic storms or generally active conditions (e.g.,

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“…After encountering the DFB near the equator, SI0 is quickly deflected duskward and accelerated by the DFB electric field E y (see Figure f for the color changes near the DF), until a few seconds later it returns to the unperturbed plasma sheet with a net energy gain. Such a nonadiabatic acceleration process has been well understood as an ion DF reflection in previous studies (Zhao et al, ; Zhou et al, , ). Note that in this case, the reflected SI0 has to gyrate clockwise due to the presence of a finite B z in the ambient plasma sheet, and it quickly returns to the DFB region.…”

Section: Prediction From Simulationsmentioning

confidence: 80%

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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Oxygen Ion Reflection at Earthward Propagating Dipolarization Fronts in the Magnetotail

Cited by 8 publications

References 49 publications

Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

Oxygen Ion Butterfly Distributions Observed in a Magnetotail Dipolarizing Flux Bundle

Acceleration of Oxygen Ions in Dipolarization Events: 1. CPS Distributions

On the Origin of Perpendicular Ion Anisotropy Inside Dipolarizing Flux Bundles

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