Response of Different Ion Species to Local Magnetic Dipolarization Inside Geosynchronous Orbit

Motoba, T.; Ohtani, S.; Gkioulidou, M.; Ukhorskiy, A. Y.; Mitchell, D. G.; Takahashi, Kazue; Lanzerotti, L. J.; Kletzing, C. A.; Wygant, J. R.

doi:10.1029/2018ja025557

Cited by 17 publications

(25 citation statements)

References 55 publications

(98 reference statements)

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“…The peak probability, 0.93 events per day, takes place at 21 < MLT < 22 and 5 < R XY < 6. Recently, Motoba et al (2018) independently searched the same RBSP data set for sharp dipolarization events and found that the distribution of events skews slightly, but noticeably, more toward midnight than our result, which is presumably because their criterion about the magnetic inclination (>30°) is less…”

Section: Data and Event Selectioncontrasting

confidence: 71%

“…Second, the correlation between particle injections and local dipolarizations has been known for a long time for geosynchronous altitude (e.g., Erickson et al, ; Sauvaud & Winckler, ), and it can also be statistically confirmed for dipolarizations in the inner magnetosphere (e.g., Motoba et al, ) although there seems to be a certain type of transient (~1‐min) dipolarization, which also tend to take place in the premidnight sector but often without clear particle injections (Liu et al, ). The correlation between dipolarizations and injections is often interpreted as if the injection is an effect of dipolarization; that is, local plasma is energized and transported by the electric field associated with dipolarization.…”

Section: Discussionmentioning

confidence: 97%

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Spatial Development of the Dipolarization Region in the Inner Magnetosphere

et al. 2018

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The present study examines dipolarization events observed by the Van Allen Probes within 5.8 RE from Earth. It is found that the probability of occurrence is significantly higher in the dusk‐to‐midnight sector than in the midnight‐to‐dawn sector, and it deceases sharply earthward. A comparison with observations made at nearby satellites shows that dipolarization signatures are often highly correlated (c.c. >0.8) within 1 hr in magnetic local time (MLT) and 1 RE in RXY, and the dipolarization region expands earthward and westward in the dusk‐to‐midnight sector. The westward expansion velocity is estimated at 0.4 hr (in MLT) per minute, or 60 km/s, which is consistent with the previously reported result for geosynchronous dipolarization. The earthward expansion is apparently less systematic than the westward expansion. Its velocity is estimated at 50 km/s (0.5 RE/min), comparable to the westward expansion velocity, but it is suggested that the earthward expansion slows down as the dipolarization region approaches Earth, and it eventually stops. This idea is consistent with the earthward reduction of the occurrence probability of dipolarization events. Whereas this earthward expansion is difficult to explain with the conventional wedge current system, it may be understood in terms of a current system with two wedges, one with the R1 polarity outside and the other with the R2 polarity closer to Earth. For such a current system the region of dipolarization is confined in radial distance between the two wedge currents, and it is considered to expand earthward as the R2‐sense wedge moves earthward along with injected plasma.

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Section: Data and Event Selectioncontrasting

confidence: 71%

Section: Discussionmentioning

confidence: 97%

Spatial Development of the Dipolarization Region in the Inner Magnetosphere

et al. 2018

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“…This is consistent with the present result. Motoba et al () recently examined ion injections inside geosynchronous altitude, and also found that particle behavior is strongly governed by changes in the magnetic field.…”

Section: Summary and Discussionmentioning

confidence: 99%

Proton and Electron Injection Path at Geosynchronous Altitude

et al. 2019

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Using tens to hundreds of keV proton and electron flux measurements and simultaneous magnetic field measurements from three Geostationary Operational Environmental Satellites [GOES-13 (75°W), GOES-14 (105°W), and GOES-15 (135°W)], we investigate proton and electron injections and their relationship to the substorm current wedge at geosynchronous altitude. Proton and electron injection processes occur only in the initial formation of the substorm current wedge, the width of which is less than 2 hr in local time in the premidnight region, for moderate substorms. Proton injections are closely related to the formation of a substorm current wedge at geosynchronous altitude, and thus the onset of a substorm, even before local dipolarization in the magnetic field. Proton injections take place only under the western upward field-aligned currents of the current wedge. Electron injections in the energy range of <100 keV are tightly coupled with local dipolarization in the magnetic field, and these take place mostly in the central region of the current wedge, extending in the region under the western upward field-aligned currents. Variations in the magnetic field during substorms have also been studied at geosynchronous altitude (e.g.,

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“…The twin Van Allen Probes spacecraft (Mauk et al, ), which are well‐equipped with scientific instruments and have been in operation since August 2012, provide an opportunity to significantly improve understanding of the physical properties of dipolarizations inside GEO and their relation to energetic particles. To the best of our knowledge, only the following three studies, Nosé et al (), Liu et al (), and Motoba et al (, hereinafter referred to as Paper 1), have statistically investigated the dipolarization‐injection relationship inside GEO using Van Allen Probes data. Nosé et al (), who examined 74 dipolarization events that occurred during the first tail season 2012–2013, found (1) that on average magnetic dipolarizations are accompanied by the stronger flux enhancement of oxygen (O) ions at 20–50 keV than hydrogen (H) and (2) that low‐energy (0.1–5 keV) O ion fluxes are enhanced with energy dispersion only near the field‐aligned directions.…”

Section: Introductionmentioning

confidence: 99%

Pitch Angle Dependence of Electron and Ion Flux Changes During Local Magnetic Dipolarization Inside Geosynchronous Orbit

et al. 2020

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In this study we have statistically examined flux changes of 1–1,000‐keV electrons, and hydrogen, helium, and oxygen ions during local magnetic dipolarization inside geosynchronous orbit, with a focus on their pitch angle dependence. Using 144 dipolarization events that were selected in a previous study with Van Allen Probes observations in 2012–2016, we have performed a superposed epoch analysis of differential flux changes after the dipolarization onset for each species. On average the electron flux increases primarily around pitch angle (α) = 90° at >80 keV and almost isotropically at 10–50 keV. The electron flux at <5 keV increases at α = 0° and 180° but slightly decreases (or remains unchanged) at α = 90°. On the other hand, the ion flux at >80 keV increases around α = 90°, while at <30 keV it decreases nearly independent of pitch angle. Only the low‐energy (<5 keV) helium and oxygen ion flux changes indicate a strong field‐aligned enhancement, which could be attributed to their outflows from the topside ionosphere. After the dipolarization onset, >5‐keV electron and >30‐keV ion fluxes exhibit a perpendicular anisotropy, which is most pronounced for 50–200‐keV electrons. Both distributions become more isotropic with decreasing energy. A noticeable field‐aligned anisotropy is seen for <5‐keV ion fluxes. These statistical pitch angle‐dependent electron and ion properties during dipolarizations may be explained by combining various processes, including adiabatic acceleration and/or transport, wave‐particle interactions, and ion outflow.

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Response of Different Ion Species to Local Magnetic Dipolarization Inside Geosynchronous Orbit

Cited by 17 publications

References 55 publications

Spatial Development of the Dipolarization Region in the Inner Magnetosphere

Spatial Development of the Dipolarization Region in the Inner Magnetosphere

Proton and Electron Injection Path at Geosynchronous Altitude

Pitch Angle Dependence of Electron and Ion Flux Changes During Local Magnetic Dipolarization Inside Geosynchronous Orbit

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